Research Progress on Alloying of High Chromium Cast Iron—Austenite Stabilizing Elements and Modifying Elements

Abstract

High chromium cast iron (HCCI) is widely used in the manufacturing of equipment parts in the fields of mining, cement, electric power, metallurgy, the chemical industry, and paper-making because of its excellent wear and corrosion resistance. Although the microstructure and properties of HCCI can be modified by controlling the casting and heat treatment process, alloying is still the most basic and important method to improve the properties of HCCI. There are about 14 common alloying elements in HCCI, among which nickel, copper, and manganese are typical austenite stabilizing elements, which can increase austenite content and matrix electrode potential. The addition of elements such as silicon, nitrogen, boron, and rare earth (RE) is often small, but it has a significant effect on tailoring the microstructure, thereby improving wear resistance and impact toughness. It was thought that after years of development, the research on the role of the above elements in HCCI was relatively complete, but in the past 5 to 10 years, there has been a lot of new research progress. Moreover, the current development situation of HCCI is still relatively extensive, and there are still many problems regarding the alloying of HCCI to be further studied and solved. In this paper, the research results of austenitic stabilizing elements and modifying elements in HCCI are reviewed. The existing forms, distribution law of these elements in HCCI, and their effects on the microstructure, hardness, wear resistance, and corrosion resistance of HCCI are summarized. Combined with the current research situation, the future research and development direction of HCCI alloying is prospected.

1. Introduction

High chromium cast iron is a white cast iron containing 1.8~3.6 wt.% C, 11~30 wt.% Cr, and a small amount of Mn, Ni, Mo, Cu, and other alloying elements, which can be regarded as a kind of carbide (mainly M₇C₃) distributed in the iron matrix (austenite, ferrite, or martensite) in situ composite [1,2]. Hypereutectic high chromium cast iron (HHCCI) is a kind of high chromium cast iron whose carbon content exceeds the eutectic point, and during the solidification process, the primary carbide (usually M₇C₃-type) is crystallized first, and then the eutectic transformation occurs. Consequently, HHCCI generally exhibits higher hardness and more excellent wear resistance than hypoeutectic or eutectic HCCIs.

M₇C₃ carbide has high hardness (HV1300~1800) and presents hexagonal rod or strip discontinuously distributed in the matrix; the matrix is a continuous phase, and then the damage effect of carbide on the matrix is greatly reduced, providing the fundamental conditions for enhancing the impact toughness and wear resistance simultaneously. The matrix microstructure of HCCI can be regulated by heat treatment to apply to different wear conditions [2]. The matrix microstructure of commonly used HCCI is a mixture of martensite and austenite, which is characterized by high hardness and high toughness. In addition, the HCCI matrix has a high Cr content, high electrode potential, and excellent corrosion resistance [3,4,5]. The good wear and corrosion resistance of HCCI means that it is mainly applied in the manufacturing of machinery and equipment parts in mining, cement, electric power, metallurgy, the chemical industry, and the paper-making industry [6].

The wear resistance of HCCI is affected by many factors, including internal factors (type, morphology, size, content, distribution of carbides and matrix microstructure type and relative content, etc.) and external factors (wear form, stress condition, medium, temperature, etc.) [2]. However, in terms of internal factors, the good wear resistance of HCCI is mainly related to M₇C₃ carbides with higher hardness, although the toughness of the matrix also has an impact on the wear resistance of the material [7]. There are mainly two types of carbides in HCCI: M₇C₃ and M₂₃C₆. With the increase in the Cr content, the type of carbide changes from M₇C₃ to M₂₃C₆. The hardness of M₇C₃ carbides is higher than that of M₂₃C₆, which is more conducive to enhancing the wear resistance. The corrosion resistance of HCCI mainly depends on the chromium content and the type of matrix. In the carbides of HCCI, the electrode potential of M₂₃C₆ is the lowest, and the electrode potential of austenite is the highest in the matrix. When the chromium content is high, the formation of M₂₃C₆ reduces the potential difference in the electrochemical corrosion of HCCI and improves the corrosion resistance. In addition, the increase in the total chromium content increases the chromium content in the matrix, which helps the formation and repair of dense passivation film and improves the corrosion resistance of HCCI. Because the electrode potential of austenite and carbide is the most similar, in order to improve the corrosion resistance, it is hoped that a single austenite structure will be obtained.

The increase in the carbide volume fraction is beneficial to improve the wear resistance of HCCI. However, the large volume fraction of carbides can reduce toughness, leading to high crack sensitivity and premature component failure, especially under high-speed impact [8]. This requires HCCI to have high wear resistance and good impact toughness and other mechanical properties. The mechanical properties of HCCI mainly depend on the content, type of eutectic carbides, and matrix microstructure. In order to obtain higher toughness, it is necessary to limit the volume fraction of eutectic carbides and avoid the formation of coarse carbides. In addition, the austenite matrix is more conducive to the improvement in the impact toughness of HCCI than martensite [9]. In general, the impact toughness of HCCI can be significantly improved by a uniform and fine microstructure. At present, the properties of HCCI are optimized by adjusting the carbide and matrix microstructure through alloying, controlling the solidification process, modification treatment, plastic deformation, heat treatment, and other methods [2].

According to the relevant provisions of the Chinese national standard GB/T 8263-2010, the main brands of HCCI in China include BTMCr12-DT, BTMCr12-GT, BTMCr15, BTMCr20, BTMCr26, etc. Their chemical compositions are shown in

Table 1. Main brands and corresponding chemical compositions of HCCI in China (wt.%).

Brands	C	Cr	Si	Mn	Mo	Ni	Cu	S	P
BTMCr12-DT	1.1–2.0	11.0–14.0	≤1.5	≤2.0	≤3.0	≤2.5	≤1.2	≤0.06	≤0.06
BTMCr12-GT	2.0–3.6	11.0–14.0	≤1.5	≤2.0	≤3.0	≤2.5	≤1.2	≤0.06	≤0.06
BTMCr15	2.0–3.6	14.0–18.0	≤1.2	≤2.0	≤3.0	≤2.5	≤1.2	≤0.06	≤0.06
BTMCr20	2.0–3.3	18.0–23.0	≤1.2	≤2.0	≤3.0	≤2.5	≤1.2	≤0.06	≤0.06
BTMCr26	2.0–3.3	23.0–30.0	≤1.2	≤2.0	≤3.0	≤2.5	≤1.2	≤0.06	≤0.06

. The national standard allows for the addition of trace amounts of B and RE and other alloying elements in these HCCI. In the actual production process, factories often design new HCCI according to the specific application conditions, on the basis of the national standard brand compositions, by appropriately adjusting the alloy composition and adding a small amount of other alloying elements so as to meet the practical requirements.

Alloying is an effective method to enhance the properties of HCCI. In the composition design of HCCI, it is necessary to clarify the existence and function of various alloying elements in HCCI, and it is also necessary to understand the scope of the application of various alloying elements and other limiting factors. Alloying elements generally exist in the microstructure of HCCI in solid solution state, chemical combination state, adsorption state, and free state. For example, elements such as Ni, Cu, and Mn exist in the martensite or austenite matrix mainly in the form of replacement solution. Nb, Mo, V, W, Ti, and other elements have a large difference in electronegativity with carbon, so it is easy to combine them with carbon to form carbides. The Fe and C atoms of carbides in HCCI can be replaced by alloying elements with similar properties to form alloy carbides. When a few alloying elements exceed their solid solubility in the solid solution, they will be precipitated from the solid solution and distributed in the matrix as dispersed particles. For example, this phenomenon can occur in both Cu and Pb elements in martensite. In addition, some alloying elements can be adsorbed on the surface of a certain phase [10].

Since the study of HCCI began in the 1960s, its alloying has been an important research direction [2]. After more than 60 years of development, both the types and functions of the main alloying elements in HCCI have been basically clarified. However, there are still some problems in the application of HCCI, such as its high cost, the mismatch of wear and corrosion resistance, and low impact toughness, which pose a new challenge to the study of the alloying of HCCI. In this paper, the latest research progress of austenite stabilizing elements and modifying elements in HCCI and their effects are reviewed, and the challenges suffered by the alloying of HCCI are analyzed so as to prospect the key research directions in the future and provide some references for further improving the comprehensive properties and reducing the cost of HCCI by the alloying method.

2. Austenite Stabilizing Elements—Ni, Mn, and Cu

2.1. Ni Alloying

Ni is insoluble in carbides, and almost all dissolve into the matrix. Ni improves hardenability and can reduce the martensitic transition temperature M_S. Ni is an austenite stable element, which can increase the content of austenite in HCCI, and its addition is beneficial to improve the toughness of HCCI. Ni can strengthen and repair the passivation film on the surface of HCCI and improve the electrode potential of the matrix so as to enhance the corrosion resistance of HCCI. In addition, Ni is also a typical graphitizing element which is enriched at the interface between graphite and the iron matrix [11,12].

Inthidech et al. [13,14] studied the change in the microhardness of hypoeutectic HCCI during heat treatment and found that Ni reduced the microhardness of HCCI in solid solution state (held at 1000 and 1100 °C for 90 min with fan air-cooling). This is mainly because Ni can effectively increase the austenite content of HCCI in solid solution. Moreover, secondary hardening can be achieved by the aging treatment of HCCI in the solid solution state (the solid solution is held at 300~600 °C for 2 h and then air-cooled), and the degree of secondary hardening is proportional to the residual austenite in the HCCI in the solid solution state. Yang et al. [15] studied the effects of Ni, Mo, and Cu on the microstructure and properties of Cr26 HCCI by the orthogonal test and found that the single alloying element that has the most significant influence on the hardness and impact toughness of HCCI is Ni.

Si et al. [16] recently found that with the increase in Ni content, the austenite content of HCCI increased, and the martensite content decreased. The primary dendrites were coarsened, while the eutectic microstructures were refined (see Figure 1). In the high-temperature oxidation resistance test, with the improvement in Ni content, the oxidation weight gain of HCCI decreased and the thickness of the oxide layer also decreased. In the high-temperature thermal fatigue test, the length of the main crack decreased with the increase in the Ni content. The thermal fatigue resistance of HCCI containing 3.05 wt.% Ni is 36% greater than that of HCCI containing 0.65 wt.% Ni.

The results in [17,18] show that Ni can improve the matrix electrode potential of HCCI and obviously improve its corrosion and wear resistance. The corrosion and wear resistance of HCCI mainly depends on its mechanical properties. For further improving the corrosion and wear resistance of HCCI, Li et al. [19] increased its hardness and carbide volume fraction by adding trace Ni element. When the Ni content is 0.09 wt.%, the corrosion resistance of HCCI is the best. The calculation results indicate that Ni atoms can reduce the interfacial energy between the matrix and carbide, which reduces the nucleation activation energy of carbide, thereby increasing the nucleation rate and forming finer primary carbide. This is beneficial to reduce the stress concentration, reduce the possibility of carbide breakage during corrosion wear, and thus improve the corrosion and wear resistance of HCCI.

It has been shown that Ni is an austenite stabilizing element, which can effectively enhance the toughness and corrosion resistance of HCCI. However, Ni is also a graphitizing element, so in order to avoid graphitization, the Ni content in HCCI cannot be too high. Although the influence of Ni element on the heat and corrosion resistance of HCCI has been studied, the research is still in the initial stage, and it is necessary to carry out more systematic and in-depth research on the related mechanisms.

2.2. Mn Alloying

Mn is soluble in both carbide and matrix. Mn is an austenite stabilizing element, and its combination with Mo can obviously improve the hardenability of HCCI, but Mn strongly reduces the M_S point, and the content in HCCI is generally controlled below 2 wt.%. HCCI contains an appropriate amount of Mn element, which is beneficial to improve the strength and toughness of castings and improve the wear resistance.

Xu et al. [20] studied the effects of Mn and B elements on the microstructure, hardness, and wear resistance of the HCCI cladding layer by arc cladding on the surface of Q345 steel plate and found that the hardness of the cladding layer increased by 10.64% when 0.7 wt.% Mn was added alone compared with that of the HCCI cladding layer; while the addition of 1.20 wt.% B was continued, the surface hardness of the cladding layer was 43.05% higher than that of the HCCI cladding layer. This indicates that the hardness of HCCI can be improved to some extent by the appropriate Mn element, but the increasing range is limited.

Bouhamla et al. [21] studied the effects of Mn and Mo elements on the microstructure, secondary precipitate shape, and wear resistance of HCCI and revealed that compared with 0.5 wt.% Mn + 0.5 wt.% Mo, the addition of 3 wt.% Mn is conducive to the precipitation of larger and closer polygonal secondary carbides. The addition of Mn element will enlarge the range of the eutectic transition region and decrease the maximum temperature of the eutectic transition.

Hadji et al. [22] studied the effect of Mn (1.27~3.66 wt.%) on the microstructure and wear resistance of HCCI (Cr content is 15 wt.%) and found that with the increase in Mn content, the content of residual austenite and M₇C₃ carbide in HCCI increased. In addition, the microhardness of the matrix and M₇C₃ carbide also increases correspondingly, which is mainly due to the generation of secondary precipitates in the matrix after heat treatment (annealed at 980 °C for 15 h and then air-cooled to room temperature; subsequently tempered at 250 °C for 242.5 min followed by air-cooling to ambient temperature) and the solid solution strengthening of M₇C₃ carbide by the Mn element. Therefore, the increase in the Mn content contributes to the improvement in the wear resistance of HCCI.

Duan [23,24] performed a systematic study on the effect of Mn on the solidification process and solid-state phase transformation of HCCI. It was found that Mn decreased the liquidus and eutectic temperature and narrowed the solidification temperature range. Mn reduced the transition temperature of austenite to pearlite and increased the saturation solubility of C and Cr in austenite, thus greatly increasing the stability of austenite. With the increase in Mn content, the residual austenite content of HCCI increased and the hardness decreased accordingly. However, Mn has no obvious effect on carbides in HCCI. Mn increased the hardenability of the alloy, improved the quenching hardness of large-section castings, and increased the residual austenite volume fraction of small-section castings. It is very effective to improve the hardenability of HCCI with Mn, but its addition amount should be commensurate with the section size of the casting.

Stott et al. [25] also found that the addition of Mn was harmful to the high-temperature oxidation resistance of HCCI and would increase the growth rate of oxide film and its probability of failure. This is due to the rapid diffusion of Mn through the Cr₂O₃ oxide film under high-temperature conditions, thus forming the MnCr₂O₄ spinel oxide layer on the outer surface.

To sum up, Mn is a typical austenite stabilizing element, so the austenite content of HCCI can be improved by increasing the Mn content, thus improving its toughness, but this reduces the hardness and wear resistance of HCCI to some extent. On the other hand, Mn can improve the hardenability of HCCI and can improve the hardness and wear resistance of HCCI by promoting the precipitation of polygonal secondary carbides during the tempering process. Therefore, it is necessary to adjust the Mn content and select the appropriate heat treatment regime according to the application requirements so as to obtain HCCI with matching wear resistance and toughness. However, at present, the research on the effect of Mn on the wear resistance and impact toughness of HCCI is not extensive enough, and there is a lack of studies on the influence of Mn on the corrosion resistance of HCCI, so relevant research can be strengthened in the future.

2.3. Cu Alloying

Cu is similar to Ni; it is insoluble in carbide, completely soluble in the Fe matrix (mainly austenite), and it has an obvious solid solution strengthening effect. Cu increases the hardenability but decreases the martensitic transition temperature M_S, which increases the residual austenite content. Cu is a kind of austenite stabilization element, but its solubility in austenite is not high, at about 2 wt.%. Cu is also a graphitization element. Cu can strengthen and repair passivation film and improve the corrosion resistance of HCCI.

Sarkar et al. [26] studied the effect of the addition of Cu on the microstructure and hardness of as-cast HCCI and heat-treated HCCI (held at 400 °C for 0.5 h + held at 600 °C for 0.5 h + held at 1050 °C for 4 h; the heat treatment process was continuous without interval cooling) and found that for as-cast HCCI, the proportion of fine rod-like M₇C₃ carbides was higher in samples with a higher Cu content, while the content of blocky carbides was higher in samples with a lower Cu content. The heat-treated samples with a different Cu content were mainly composed of residual austenite, martensite, eutectic, and secondary carbides, in which the content of carbide and martensite decreased with the increase in the Cu content, while the residual austenite had an opposite variation trend with Cu content. Whether using as-cast or heat-treated HCCI, samples with a higher Cu content had lower hardness than those with a lower Cu content, and heat-treated samples tended to have higher hardness values than as-cast ones.

Guerra et al. [27] investigated the effect of Cu content on secondary precipitated carbides in HCCI and found that the addition of Cu led to the formation of more discontinuous eutectic carbides under the as-cast condition. After heat treatment, the number of precipitated carbides increased when the heat treatment temperature was lower. After heat treatment at 850 °C, the secondary carbides of the sample with a higher Cu content were smaller and distributed more evenly. The volume fraction of secondary carbide increased with the decrease in the heat treatment temperature and the improvement in holding time. The volume fraction of residual austenite increased with the increase in the heat treatment temperature. Therefore, when heat-treated at 1050 °C, austenite remained stable, and carbide precipitation was little. In addition, when the holding time was longer, the eutectic carbide dissolved. Due to the reduction in secondary carbides and the lower degree of transition from austenite to martensite, the microhardness decreased with the increase in the heat treatment temperature.

In order to further enhance the wear resistance of HHCCI, Gong et al. [28] systematically studied the effect of Cu on the microstructure and mechanical properties of HHCCI and found that the carbide size of as-cast HHCCI decreased with the increase in the Cu content. When the Cu content was 0.5 wt.%, the carbide size was the smallest (average size 12.68 ± 0.69 μm, as shown in Figure 2). With the increase in the Cu content, the austenite content of as-cast HHCCI increased gradually, and the martensitic phase transformation was inhibited. The hardness of as-cast HHCCI decreased with the increase in the Cu content. However, after heat treatment at 1050 °C, as secondary carbides precipitated from austenite, austenite became unstable and transformed into martensite, and the hardness of HHCCI was significantly improved compared with the as-cast state (see Figure 3a,b). With the improvement in the Cu content, the wear loss of HHCCI after annealing at 1050 °C decreased correspondingly, and the wear resistance increased continuously. As the Cu content improved to 1 wt.%, the wear resistance of HHCCI was the best, which was 2.6 times that of Cu-free HHCCI, but as the Cu content continued to increase, the wear resistance of HHCCI was no longer significantly improved (see Figure 3c,d). The increase in the Cu content promoted the precipitation of secondary carbides and made the interface between α-Fe and secondary M₂₃C₆ carbides a semi-coherent interface. By the first-principle calculation, it was found that a small number of Cu atoms tend to be adsorbed on the M₇C₃ (0001) crystal plane, thus playing a role in refining carbides. The solid solution strengthening effect of the Cu atom on the matrix was also revealed.

Gong et al. [29] also investigated the effect of Cu element on the corrosion resistance of HHCCI using the first-principles calculation method, and the results showed that under the acidic condition of pH = 1, the predicted corrosion current density of the Cu-containing HHCCI matrix (2.19 × 10⁻⁵ A cm²) was lower than that of the copper-free HHCCI matrix (4.41 × 10⁻⁵ A cm²). In a neutral environment of pH = 7, HHCCI’s corrosion resistance was better than that in acidic conditions. The predicted corrosion current density of the Cu-containing HHCCI matrix (4.39 × 10⁻¹⁰ A cm²) was also lower than that of the copper-free HHCCI matrix (7.98 × 10⁻¹⁰ A cm²), indicating that the corrosion resistance of the Cu-containing HHCCI matrix is better than that of the copper-free HHCCI matrix from the point of view of the first-principles calculation. Combined with the results of electrochemical experiments, it is shown that the corrosion potential of HHCCI containing 1.5 wt.% Cu was remarkably higher than that of HHCCI without copper in both acidic solution (PH = 1) and neutral solution (PH = 7). The changing tendency of the electrochemical test results was consistent with the predicted tendency of the calculated polarization curve, which reveals that copper alloying can enhance the corrosion resistance of HHCCI.

In summary, Cu element can significantly refine the carbide of HCCI; it is a typical austenite stabilizing element and inhibits martensitic transformation. Therefore, increasing the Cu content reduces the hardness of HCCI to some extent. However, after heat treatment, a large number of secondary carbides are precipitated from HCCI containing Cu, which can greatly increase the hardness of HCCI and thus enhance the wear resistance. In addition, under neutral or acidic conditions, Cu can increase the corrosion potential of the matrix and reduce the corrosion current density, thus effectively improving the corrosion resistance of HCCI. Although it has been found that Cu can facilitate the precipitation of secondary carbides in the heat treatment process of HCCI, and these carbides can effectively improve the hardness and wear resistance of HCCI, there is still a lack of research on the strengthening and toughening mechanism of these fine carbides and whether they are conducive to the improvement in corrosion resistance. In addition, in order to further play the role of Cu in improving corrosion resistance, it is necessary to study the corrosion mechanism of Cu-containing HCCI in different corrosion media by the experimental method on the basis of computer simulation.

3. Modifying Elements—B, Si, N, and RE Elements

The modifying elements in this research refer to the alloying elements that mainly act as an inoculant, except the carbide-forming elements in HCCI, which have a significant effect on the microstructure and properties of HCCI, mainly including B, Si, N, and RE elements and so on.

3.1. B Alloying

B is soluble in both matrix and carbide, which can improve the hardness of the matrix and carbide, as well as the wear resistance of HCCI. B can reduce the initial temperature of the eutectic transformation and improve the hardenability of HCCI. In addition, it also has the effect of refining carbide and improving the morphology and distribution of carbide.

Kaleicheva et al. [30] studied the effect of B content (0.18~1.25 wt.%) on the microstructure, mechanical properties, and wear resistance of Fe-3.1C-13.1Cr-1.1Mo HCCI and found that with the increase in the boron content, the microstructure of HCCI changed from hypoeutectic to hypereutectic, and the impact toughness gradually decreased; however, the carbide content of HCCI continued to increase, and its hardness also increased correspondingly, from 53 HRC to 59 HRC. When the B content is 0.18 wt.%, HCCI had the best dry wear resistance. When the B content is greater than 0.18 wt.%, the dry wear resistance of HCCI decreased with the increase in the B content. It was found that the wear resistance of B-containing HCCI increased significantly after solid solution (held at 950 °C for 25 min and air-cooling) + aging (held at 235 °C for 1 h) heat treatment.

Zhang et al. [31] studied the effect of B on the microstructure and properties of the casting infiltration layer of HCCI and found that M₂B borides were formed in the microstructure of the casting infiltration layer after adding a trace amount of B element. With the increase in the B content, the eutectic microstructures were gradually refined, the M₇C₃ carbide content decreased, M₂B-type borides increased, and the hardness of the casting infiltration layer gradually increased. When the B content was 0.72 wt.%, the hardness of the casting infiltration layer reached 1190 HV. Meanwhile, it was found that quenching and low-temperature tempering heat treatment can make the eutectic carbide and boride accumulate and grow and promote the secondary phase precipitation, which is conducive to the improvement in hardness.

Purba et al. [32] studied the effect of the B content (0~3.0 wt.%) on the abrasive wear performance of HCCI and found that with the addition of B element, the microstructure of HCCI containing B was mainly composed of the martensitic matrix and a variety of boron carbides. These boron carbides include M₇(C, B)₃, M₂(C, B), and M₂₃(C, B)₆. With the increase in B content, the volume fraction of M₇(C, B)₃ increased continuously, while the volume fraction of M₂(C, B) and M₂₃(C, B)₆ increased first and then decreased. When the B content is 3.0 wt.%, the morphology of M₂(C, B) changes from fishbone to sheet. Under the condition of low load (73.5 N), B could effectively improve the wear resistance of HCCI due to the increase in the volume fraction of high hardness M₂(C, B). However, under the condition of high load (196 N), the addition of B reduced the wear resistance of HCCI because the fracture toughness of M₇(C, B)₃ was reduced. This is mainly because under low load conditions, the wear resistance of B-containing HCCI is mainly determined by the matrix hardness and the volume fraction of M₂(C, B), while under high load conditions, the wear resistance of B-containing HCCI is mainly determined by the fracture toughness of M₇(C, B)₃.

Bedolla-Jacuinde et al. [33] systematically studied the effects of B addition (0~1.197 wt.%) on the microstructure, hardness, and wear resistance of the Fe-17Cr-3C-1Ni-1Mo alloy. It was found that the addition of B could significantly increase the volume fraction of carbide (from 27.1% to 53.84%) and martensite in HCCI and promote the transformation from M₇C₃ eutectic carbide to M₂₃(C, B)₆ carbide (see Figure 4). The hardness of HCCI increased with the increase in the B content due to the increase in the carbide volume fraction and the strengthening of the iron matrix (reaching a maximum of 767 HV at high B content).

In addition, the destabilization treatment caused the secondary hardening of B-containing HCCI; however, the hardening effect became less significant as the B content increased (see Figure 5a). The wear resistance of as-cast HCCI containing B increased with the improvement in the B content under both high and low load conditions. Similarly, for the destabilized HCCI with higher hardness, when the B content was less than 0.598 wt.%, it exhibited better wear resistance than the as-cast alloy, and its wear resistance deteriorated with increasing load. For the heat-treated HCCI with a B content of 1.197 wt.%, the amount of load applied in the wear test significantly affected the wear loss (see Figure 5b); the alloy had the lowest wear loss compared to the other B-containing HCCIs in the test process at a load of 54 N and had significantly higher wear loss compared to the lower B alloy under the same heat treatment condition at a load of 130 N. This is due to the cracking of blocky carbides after heat treatment, which promoted the carbides to fall off during high-load testing. This study shows that the hardness and wear resistance of HCCI can be effectively improved by adding B up to 1.197 wt.%, thus eliminating the need for destabilizing heat treatment.

Guo [34] and Kou [35] et al. studied the influence of different B contents on the microstructure and properties of Cr26 and Cr25 HCCIs and found that the addition of B increased the compound content and formed boron carbide. With the increase in the B content, the martensitic content of as-cast HCCI increased, and the eutectic carbides changed from long strips with sharp edges to discontinuous and isolated globular or plate-like morphology, and the microstructures were obviously refined and evenly distributed. When the addition of B is 0.3 wt.%, the comprehensive properties of the alloy were the best.

Lu et al. [36] found that the addition of a minor amount of B element could significantly enhance the corrosion and wear resistance of HCCI. The added B element was distributed in both carbide and matrix, and the hardness of carbide increased obviously. Adding a small amount of B could effectively increase the carbide content. With the increase in the hardness and volume fraction of carbide, the corrosion and wear resistance of B-containing HCCI in sandy slurry could be significantly improved. The addition of B element often sacrificed the impact toughness to some extent while increasing the hardness of HCCI. The addition of B element could decrease the corrosion potential of HCCI slightly, which might be related to the increase in the activity of B-containing carbides and matrix. In addition, the formation of a large number of carbides will consume the Cr element in the matrix, which has a negative impact on the passivation ability of the HCCI surface in the corrosive medium. Wang et al. [37] found that the morphology of carbide had a great influence on the erosion wear performance of HCCI, and the erosion wear performance was good when the carbide was small and dispersed.

Huang et al. [38] studied the effect of B content on the microstructure and properties of 4C-17Cr (wt.%) semi-solid HHCCI and found that with the increase in B content, the martensite content increased correspondingly; the hardness and impact toughness of the samples increased and decreased, respectively, and the variation range of both decreased as the B content increased to a certain extent.

Correa et al. [39] studied the effect of a trace addition of B (no more than 195 ppm) on the microstructure and sliding wear of directionally solidified HCCI; it was indicated that B was mainly distributed in carbide and segregated at the austenite/carbide interface during solidification but less precipitated in the form of borides. The addition of B element reduced the initial solidification temperature of HCCI and made the solidification temperature range narrower, thus reducing the volume fraction of the primary matrix phase and promoting the refinement of the microstructure by shifting the chemical composition of the alloy to the eutectic direction. With the increase in B content, the volume fraction of carbide increased; however, no increase in carbide density was observed.

The eutectic carbides in the directionally solidified HCCI were arranged in the direction of heat flow, and the microstructure of the B-containing HCCI was finer. The destabilization heat treatment of directionally solidified HCCI was conducted at 950 °C. The heat-treated microstructure was mainly composed of the martensitic matrix and secondary carbides distributed within it, which made the heat-treated alloy obtain higher strength. Compared with as-cast B-free HCCI with the orientation of eutectic carbide perpendicular to the wear surface, the destabilized B-containing HCCI with the orientation of eutectic carbide parallel to the wear surface had higher wear resistance.

Zeytin et al. [40] found that the secondary carbides of B-free HCCI were mainly acicular Fe-rich M₃C carbides, while the secondary carbides of HCCI with B (0.5 wt.%) were granular Fe₂₃(C, B)₆ carbides. The relatively low temperature of the destabilization heat treatment could obtain better hardness and wear resistance.

In conclusion, B can be distributed in carbide and matrix at the same time and is enriched at the austenite/carbide interface during solidification, reducing the initial solidification temperature and decreasing the solidification temperature range, effectively increasing the carbide content and refining microstructures. Therefore, an appropriate addition of B can effectively enhance the hardness and wear resistance of HCCI. However, the addition of B elements causes some damage to the impact toughness and corrosion resistance of HCCI, and the relevant mechanism is still unclear. Therefore, it is necessary to conduct more systematic and in-depth research on these problems.

3.2. Si Alloying

Si has a greater affinity with O than Mn and Cr and can play a role in deoxidation in smelting. Si is distributed in the iron matrix in a solid solution state, and the solid solution strengthening effect on the iron matrix is greater than that of Mn, Cr, Ni, Mo, W, and V elements. Si reduces the austenite region, reduces the stability of austenite, and contributes to the formation of pearlite. Si can reduce the temperature range of the eutectic reaction of HCCI, refines the eutectic carbide effectively, and improves the morphology of eutectic carbide. The addition of an appropriate amount of Si element can promote the martensitic transformation of the matrix and improve the hardness and wear resistance of HCCI.

As early as nearly 30 years ago, Powell et al. [41] used the EBSD method to study the effect of the addition of Si on the crystal orientation and morphology of carbides in HCCI and found that compared with low-silicon HCCI (0.1 wt.%), the continuity of eutectic carbide (Cr, Fe)₇C₃ in HCCI with 1.3 wt.% Si content was worse. It is indicated that the addition of Si content is beneficial to reduce the continuity of eutectic carbide in HCCI, which is helpful to improve fracture toughness.

Bedolla-Jacuinde et al. [42] studied the effect of silicon content (1~5 wt.%) on the microstructure and wear resistance of HCCI (16.8 wt.%Cr). The wear experiment adopted dry sliding wear and applied a load range of 42~238 N and found that Si could refine the dendrite structure. The volume fraction of eutectic carbide was increased. When the Si content exceeded 3.0 wt.%, the austenite matrix tended to change into the pearlite matrix instead of martensite. At low loads (42 and 91 N), all HCCIs exhibited similar wear rates (3 × 10⁻⁴~4 × 10⁻⁴ mm³/m), which were related to the formation of thin Fe₂O₃ oxide films (~3 μm) and fine debris, as well as small deformation depths (~7 μm) below the wear surface. Under a higher load condition, the wear loss was closely related to the microstructure and was associated with thicker Fe₂O₃ and Fe₃O₄ oxide films and a greater deformation depth. HCCI with a Si content of 2 wt.% had the best wear resistance (wear rate of 7 × 10⁻⁴ mm³/m) thanks to the formation of a fine microstructure and thicker oxide film. In contrast, HCCI with a Si content of 5 wt.% exhibited the worst wear resistance, with a wear rate of 14 × 10⁻⁴ mm³/m, mainly due to its pearlitic matrix. In addition, a linear relationship was observed between the depth of carbide fracture and the wear rate.

Bedolla-Jacuinde et al. [43] also analyzed the effect of different Si additions (0~5.0 wt.%) on the microstructure of HCCI (2.56 wt.% C, 16.8 wt.% Cr). It was found that Si had a greater effect on the microstructure by reducing the stability of the austenite matrix, thus promoting the precipitation of eutectic carbide. The presence of Si reduced the dissolution amount of Cr and C in austenite, resulting in the volume fraction of secondary carbide that precipitates in the matrix decreasing with the increase in the Si content after heat treatment. When the Si content was higher than 3 wt.%, pearlite was more easily formed than martensite. When the Si content was low (≤3 wt.%), the matrix microstructure of HCCI was mainly composed of martensite with secondary carbides and a small amount of residual austenite. The ferritic matrix was produced when the silicon content was higher. The microhardness of austenite and the macrohardness of eutectic carbide were increased by increasing the volume fraction of Si through solution strengthening. When the silicon content was 2 wt.%, the fracture toughness of HCCI was the best, which was mainly related to the microstructural refinement (the reduction in secondary dendritic spacing), as well as the finer, more circular, and more isolated carbides; meanwhile, the volume fraction of austenite was still high (65%), and the eutectic carbide content was not significantly increased. The fracture toughness values of all the as-cast HCCIs was higher than those of heat-treated cast irons, mainly because the matrix of the as-cast HCCI is austenite, while the heat-treated HCCI is martensite.

Recently, Gong et al. [44] studied the effect of Si alloying and heat treatment on the microstructure and properties of Fe-35Cr-4C-xSi (x = 0.5, 1.2, 1.9, 2.6) HHCCI and found that with the increase in Si content, the matrix microstructure of HCCI changed from martensite to ferrite. The analysis of the phase diagram showed that with increased Si content, the initial temperature of the austenite phase transformation increased, the austenite phase region decreased, and the ferrite phase region expanded. It is noteworthy that Si atoms and C atoms repel each other in the iron matrix, and Si atoms reduce the diffusion coefficient of C atoms in the iron matrix. After ultra-high-temperature treatment (quenching at 1150 °C), the matrix microstructure of high silicon and HCCI (1.9 wt.% Si) was transformed from ferrite to martensite. With the increase in Si content, the corrosion and wear resistance of HCCI first increased and then decreased. The 1.9Si HCCI after ultra-high-temperature heat treatment showed the best corrosion and wear resistance under both acidic and alkaline corrosion and wear conditions, and the alkali corrosion and wear resistance of Si-containing HCCI were stronger.

Chen et al. [45] found that Si was only soluble in the matrix, and the Si content was greater in the matrix near the edge of the primary carbide. When the content of Si increased from 0.5 wt.% to 2.6 wt.%, the primary carbide in HHCCI was obviously refined (approximately from 67 μm to 36 μm on average). With the increase in Si content, the volume fraction of carbide increased from 52.4% to 63.9%, the volume fraction of austenite decreased from 45.8 to 2.0%, and the matrix microstructure gradually changed from γ-Fe to α-Fe. After quenching at 1050 °C, a large number of secondary carbides were precipitated from the matrix of HHCCI. With the increase in Si content, the amount of precipitated secondary carbides decreased obviously. After quenching at 1050 °C, the hardness of HHCCI was obviously improved, but the hardness of high-silicon (1.9Si and 2.6Si) samples was not obviously changed. Enhancing quenching temperature and increasing holding time could remarkably enhance the hardness of high-silicon samples. The wear resistance of HHCCI increased first and then decreased with the increase in Si content, and the best wear resistance occurred when the Si content was about 1.9 wt.%.

Wu et al. [46] studied the effect of Si content on the microstructure, hardness, and wear resistance of Cr10 cast iron and found that with the increase in Si content, the Cr content in the matrix decreased, the content of eutectic carbide increased, the carbide size was refined, and the hardness also increased. After oil quenching and low-temperature tempering, both the hardness and wear resistance of the surface of cast iron increased with the increase in Si content. However, with the increase in Si content, the critical cooling rate of cast iron gradually increased, and the hardenability decreased. The dry-sliding reciprocating friction and wear test showed that compared with 0.8Si cast iron, the average wear volume of 2.7Si cast iron was only 0.1061 mm³, and the wear loss was reduced by 26.5%.

Shi et al. [47] found that when the content of other constituent elements was similar, the chemical composition of as-sintered HCCI changed from hypoeutectic to hypereutectic with the increase in Si content (from 1.48 wt.% to 3.23 wt.%). Si atoms can be solid-dissolved in austenite and reduce the solubility of C and Cr atoms in austenite, thus affecting the stability of austenite.

Lai et al. [48] also found that with the improvement in Si content from 0.5 wt.% to 1.5 wt.%, eutectic carbides were significantly refined and increased, and the transformation of the austenite matrix to pearlite was observed. The refinement of the eutectic microstructure was related with the decrease in eutectic temperature. After the destabilization heat treatment (950 °C for 3 h), a large number of secondary carbides were precipitated from the matrix of HCCI with a Si content of 1.5 wt.%, which was related to the large amount of C and Cr elements being confined in the matrix before the destabilization heat treatment. The precipitation of numerous secondary carbides effectively improved the tensile strength, impact toughness, hardness, and wear resistance of HCCI. When the load was increased from 20 N to 100 N, the degree of crushing of carbide was higher, and the cracks extended deeper from the wear surface. Carbide fracture was more serious in alloys with a Si content of 0.5 wt.%. The increase in Si content increased the density of secondary carbides, strengthened the matrix distributed between eutectic carbides, and provided a strong support for improving the cracking resistance of carbides.

As mentioned above, a lot of studies have been conducted on the solidification behavior, microstructure, mechanical properties, and wear resistance of Si-alloyed HCCI. It was found that Si is mainly distributed in the iron matrix, which significantly reduces the stability of austenite, effectively refines eutectic carbide, and effectively improves the hardness and wear resistance of HCCI with an appropriate addition of Si. However, there are still many problems in the mechanism of Si-alloyed HCCI, including the growth and refinement mechanisms of carbide, toughening mechanism, acid–alkali corrosion resistance mechanism, and so on, which have a guiding significance for promoting the application of Si-alloyed HCCI.

3.3. N Alloying

N is mainly distributed in the iron matrix and plays a solid solution strengthening role. It is a typical austenite stabilization element which can significantly expand the austenite phase region and can replace part of the precious nickel element under certain circumstances. N can also effectively refine the carbide and grain size of HCCI and improve the impact toughness. N can improve the electrode potential of HCCI matrix, reduce the electrode potential difference between the iron matrix and carbide, and enrich the interface between the iron matrix and oxide film, as well as be on the active surface of metal, slow down electrochemical corrosion, and play a role in improving the corrosion resistance of HCCI.

The author in [9] previously systematically studied the effect of N on the microstructure, Rockwell hardness, impact toughness, and corrosion resistance of Fe-25Cr-2.2C-0.8Ti (wt.%) hypoeutectic HCCI and found that with the addition of N element (0.028 wt.%, 0.075 wt.%, 0.2 wt.%), Ti(C, N) precipitates were generated in HCCI and dispersed in M₇C₃ carbide and austenite matrix. With the improvement in N content, the microstructures of cast irons were obviously refined, especially for the M₇C₃ carbides, mainly because Ti(C, N) precipitates can act as a heterogeneous nucleation site for M₇C₃ carbides. When the N content increased from 0.028 wt.% to 0.2 wt.%, the Rockwell hardness of HCCI increased from 47 HRC to 49 HRC. When the N content was 0.075 wt.%, the impact toughness of the sample reached a maximum of 20.79 J/cm², which was nearly 80% higher than that of the sample with a N content of 0.028 wt.% (11.80 J/cm²) (see Figure 6).

A galvanic cell was constructed by combining an HCCI test piece with a copper electrode in a corrosive medium (PH = 4, 4 wt.%NaCl(SO₄²⁻) solution), where the HCCI test piece was a negative electrode and the copper electrode was a positive electrode. The passivation curves of HCCI with different N contents of 0.028 wt.%, 0.075 wt.%, and 0.2 wt.% were measured by ammeter (see Figure 7a).

As shown in Figure 7a, the passivation curves of HCCI first rise sharply and then slowly decline as time goes on, presenting a peak current; with the increase in N content, the maximum current of passivation curves gradually decreased, and the maximum currents were measured as 2.42 mA, 1.48 mA, and 1.44 mA successively from large to small. As indicated by the passivation curves, the HCCI with a N content of 0.028 wt.% did not reach the stable stage until 7 min; that is, it had not reached a stable passivation state, while the HCCIs with a N content of 0.075 wt.% and 0.2 wt.% had reached the stable passivation state. Meanwhile, the passivation process of the 0.2 wt.%N alloys was slightly shorter than that of the 0.075 wt.%N alloy, and their passivation curves basically coincided after the end of the passivation process, indicating that they have the same passivation state. In conclusion, the corrosion resistance of HCCI increased with the increase in N content; N can not only increase the electrode potential of the iron matrix but also improve the repair capability of the passivation film of HCCI.

According to the peak current of the passivation curves (2.42 mA, 1.48 mA, and 1.44 mA, respectively), the corrosion rates of HCCI with different N contents can be calculated as 13.7, 8.4, and 8.1 mm/a, respectively. Then, the relationship between the N content and corrosion rate of HCCI is shown in Figure 7b. With the increase in N content, the corrosion rate of HCCI showed a decreasing trend, and when the N content increased from 0.028 wt.% to 0.075 wt.%, the corrosion rate of HCCI dropped sharply, and when the N content was greater than 0.075 wt.%, the corrosion rate tended to be stable and decreased little. Therefore, for HCCI working in acid mortar, its N content should be greater than 0.075 wt.%.

By observing the microscopic corrosion morphology of N-containing HCCI after the passivation curve test (see Figure 8), the reasons for N improving corrosion resistance can be further analyzed. With the increase in N content, the contrast in secondary electron mode between carbide and matrix decreased, which indicates that the corrosion resistance of HCCI increases with the increase in N content. This is because the secondary electronic photos are imaged according to the different levels in height of various characteristic structures on the surface of the object. The reduction in contrast between carbide and matrix indicates that the height difference between the matrix surface and the carbide surface is reduced; that is, less matrix was corroded during the passivation curve test. In Figure 8c,d, it can be observed that there are obvious passivation products on the matrix of HCCI, and more passivation products are seen in Figure 8d. It is indicated that with the increase in N content, the formation and self-healing ability of the passivation film also increase, which is consistent with the results obtained from the analysis of passivation curves. It can also be seen from the figures that the corrosion of N-containing HCCI is bulk corrosion, the matrix of HCCI is uniformly corroded, and the carbide is almost not corroded. It can be concluded that in order to improve the corrosion resistance of HCCI, the electrode potential of the matrix should be increased at first, and then the formation ability of the passivation film should also be improved.

Gu et al. [49] investigated the effect of different quenching temperatures on the microstructure evolution, hardness, and wear resistance of N-containing HHCCI and found that with the increase in quenching temperature, the primary carbides changed from hexagonal prisms to irregular shapes, and the eutectic carbides changed from daisy-like and strip-like to large blocks and tended to aggregate and grow. Secondary carbides changed from precipitation to redissolution; the precipitated secondary carbides had two forms, namely blocky and short-rod-like, among which the blocky M₇C₃ was mainly distributed around the primary carbides and eutectic carbides, while the short-rod-like M₂₃C₆ tended to precipitate in the center area away from the eutectic carbide. With the increase in quenching temperature, both the hardness and wear resistance of HHCCI showed a trend of increasing first and then decreasing. The hardness of the sample was the highest at 1050 °C, reaching 66.0 HRC, and the wear resistance was also the best.

Liu et al. [50] found that for HHCCIS, with the increase in N content, the primary carbide was significantly refined, the austenite content increased, and dispersed granular Cr₂N was precipitated. After quenching at 1000 °C, short-rod-like and blocky secondary carbides were precipitated in the matrix, and the matrix was transformed into martensite + residual austenite. With the increase in N content, the corrosion resistance of HHCCIs was improved, among which 0.3 wt.% N HHCCI had the best corrosion resistance, mainly because the addition of N element could improve the electrode potential of the matrix, but the excessive addition of N element would lead to the formation of a large amount of Cr₂N, resulting in the formation of a Cr-poor zone in the matrix, thereby reducing the corrosion resistance. With the increase in N content, the wear resistance of HHCCI improved, and the HHCCI with 0.15 wt.% N had the best wear resistance, which was 1.57 times that of HHCCI without N. Ovcharenko et al. [51] studied the effect of N on the microstructure and properties of HCCI (2~4 wt.%C, 30 wt.%Cr, 2.4 wt.%Mn, 0.6 wt.%Si) and found that N was mainly soluble in the iron matrix and partially distributed in (Fe, Cr)₇C₃ carbide. N alloying can stabilize austenite in HCCI, and the heat treatment of HCCI containing N can form the austenite/martensite matrix with higher hardness. N has a limited effect on improving the strength and wear resistance of HCCI but can obviously improve the corrosion resistance of HCCI.

Gong et al. [52] studied the effect of a modification treatment on the primary carbide refinement and wear resistance of the 35%Cr-4.0%C HHCCI and found that the size of the primary carbide of M₇C₃ was refined from 13.25 ± 0.72 μm to 11.67 ± 0.33 μm by adding N element under the effect of adsorption refinement. When N + Ti + V + RE-Mg was added, the combined effects of adsorption refinement and heterogeneous nucleation refinement significantly reduced the carbide size to 5.35 ± 0.15 mm. The refinement of M₇C₃ primary carbides can be attributed to two aspects: (1) the refinement resulted from the heterogeneous nucleation of TiC and VC; (2) N, Ti, and V atoms combine with the preferred growth surface—(0001) M₇C₃ crystal plane to refine the primary carbide through adsorption. The compound-modified HHCCI had excellent wear resistance. Compared to the unmodified sample, the wear resistance was increased by 75% under the condition of dry sliding wear, while the wear resistance was increased by 98.1% under corrosive wear conditions.

In order to improve the abrasive wear resistance of HCCI, Xu et al. [53] prepared a new HCCI-VN HCCI with 5.0 wt.% V instead of 5.0 wt.% Cr in a traditional HCCI with a Cr content of 20 wt.% and by adding 0.1 wt.% N and then studied the wear resistance of HCCI-VN under different abrasive particle sizes and load conditions. It was found that the microstructure of HCCI-VN contained not only Cr and Mo carbides (M₇C₃, M₂₃C₆, M₂C), but also many V(C, N) carbides, which were distributed in the matrix composed of martensite and austenite. The new HCCI-VN alloy had excellent wear resistance, and the relative wear resistance of HCCI-VN was 2.2~9.1 times that of HCCI containing 23 wt.%Cr under different abrasive particle sizes and wear loads. The excellent wear resistance of HCCI-VN is mainly due to the high hardness of V(C, N), and the combination of multi-scale and multi-type Cr and Mo carbides can effectively resist abrasive scratches.

In order to enhance the erosion and wear properties of HCCI, Xu et al. [54] prepared a novel HCCI (HCCI-N) containing 0.38 wt.% N by using a vacuum induction positive pressure melting furnace under 0.4 MPa N partial pressure based on the pseudo-binary phase diagram of the (Fe-27Cr-2.2C)-N alloy system. Both the microstructure and erosion wear behavior of the HCCI-N in alkaline mortar were studied. It was found that HCCI-N exhibited martensitic and austenitic matrix with multi-scale and multi-type carbides, including eutectic M₇C₃ (~20 μm), secondary M₂₃C₆ (~2 μm), and Cr₂C (~1 μm). The M₇C₃–ferrite–martensite sandwich structure was generated at the interface of eutectic M₇C₃. N was principally distributed in the matrix. The Cr₂C $\left(\overline{1}011\right)$ crystal plane is coherent with the (110) crystal plane of martensite. Erosion wear is the result of the interaction of corrosion and wear. Under different test conditions, the pure wear rate of HCCI-N accounted for 71~93% of the total erosion wear rate, which indicates that the wear is mainly caused by the mechanical action of alkaline mortar. However, the synergistic rate of wear and corrosion is also important to the total erosion wear rate, and the synergistic rate of the corrosion and wear of HCCI-N accounted for 7% to 29% of the total erosion wear rate, while the corresponding proportion of HCCI was 13% to 31%. Under all the test conditions, the erosion resistance of HCCI-N was better than that of HCCI, and its optimal wear resistance was 1.34 times that of HCCI (see Figure 9 in detail). The N and Cr elements that dissolved in the matrix improved the corrosion resistance and reduced the synergistic rate of corrosion and wear. Multi-scale carbides strengthened the matrix, thereby improving the mechanical wear resistance. The combined effect of these two factors makes HCCI-N superior to erosion wear resistance.

Lu et al. [55,56] studied the erosion wear behavior of HCCI with a high N content (0.39 wt.%) in salt mortar, water mortar, and acid mortar and found that the actual phase composition of HCCI with a high N content was martensite + austenite + Cr₂(C, N) + M₇C₃ + M₂₃C₆. The relative wear resistance of high N HCCI could reach 1.48 times that of HCCI. The erosion wear patterns of salt mortar were mainly plowing, indentation, large chips, and the removal of the extrusion lip; the erosion wear patterns of water mortar were plowing, indentation, and large chips; and the erosion wear patterns of acid mortar were galvanic corrosion and carbide fracture. High N HCCI had better mechanical properties and corrosion resistance than HCCI, which reduced the synergistic effect of corrosion and wear. In the erosion wear process of salt mortar, the pure wear rate of high N HCCI accounted for 65~90% of the erosion wear rate, indicating that mechanical action is the main cause of material wear. The potentiodynamic polarization curve test showed that with the increase in the NaCl concentration, the current density of high N HCCI increased more slowly than that of HCCI. Therefore, high N HCCI had a lower erosion wear rate.

Ibrahim et al. [57] studied the effect of four different N additions (0.026~0.15 wt.%) on the as-cast microstructure, hardness, impact toughness, and wear resistance of high chromium iron containing Nb. It was found that with the increase in N content, the columnar austenite dendrites transformed into equiaxed crystals gradually, and the size of eutectic M₇C₃ carbide decreased continuously. The N element preferentially accumulated in the NbC particle, replacing part of the C to form Nb(C, N) or NbN. These nitride or carbonitride particles were harder than the matrix and eutectic carbides, so the hardness of Nb-containing HCCI increased with the increase in N content. When the N content was 0.093 wt.%, HCCI obtained the best toughness (12 J), which was 33% higher than that of 0.026 wt.%N HCCI. With the increase in the addition of N, the wear resistance of the alloy increased. When the N content was 0.15 wt.%, HCCI had the best wear resistance. Under low load (30 N), the wear resistance could be increased by 47%, and under high load (60 N), the wear resistance could be increased by 17.5%.

Wang et al. [58] studied the effect of N additions (0.09 wt.% and 0.19 wt.%) on the microstructure and abrasive impact wear properties of the Fe-Cr-C-Ti-Nb high chromium hardfacing alloy. It was found that Fe-Cr-C-Ti-Nb high chromium hardfacing alloys with different N contents were mainly composed of (Cr, Fe)₇C₃ primary carbide, eutectic (Cr, Fe)₇C₃, austenite, and (Ti, Nb)(C, N) precipitates. With the improvement in N content in the Fe-Cr-C-Ti-Nb high chromium hardfacing alloy, the primary carbide was obviously refined. Meanwhile, the hardness of (Cr, Fe)₇C₃ carbide improved slightly from 18.7 GPa to 21.3 GPa, and the hardness of (Ti, Nb)(C, N) increased slightly from 27.0 GPa to 27.6 GPa. The volume loss of the hardfacing alloy during wear was reduced from 72 mm³ to 39 mm³. In the abrasive impact wear test, the coarse primary carbide broke off from the matrix in the hardfacing alloy without the addition of N, and its abrasive impact wear performance was low. However, the refined (Cr, Fe)₇C₃ carbide in the nitrogenous hardfacing alloy would bend in the wear test, which improved the abrasive impact wear performance. The lattice mismatch degree between (Cr, Fe)₇C₃ and (Ti, Nb)(C, N) is 6.15%, indicating that (Ti, Nb)(C, N) can be used as the heterogeneous nucleation site of (Cr, Fe)₇C₃ primary carbides. Therefore, the coarse primary (Cr, Fe)₇C₃ was refined by N alloying to improve the impact wear performance of the abrasive.

As mentioned above, a large number of studies have been conducted on the microstructure, mechanical properties, abrasive wear resistance, erosion wear resistance, and corrosion resistance of N-alloyed HCCI. It was found that N is mainly distributed in the iron matrix, which significantly improves the austenite stability and effectively refined the microstructure. The addition of a high N content can effectively enhance the wear resistance and corrosion resistance of HCCI. However, for N-alloyed HCCI, there are still many mechanism problems to be further studied, including the microstructure evolution under different heat treatment regimes, toughening mechanism, acid and alkali corrosion resistance mechanism, etc. In addition, at present, N tends to be combined with Ti, V, Nb, and other carbide-forming elements for the combination alloying of HCCI. However, the interaction mechanism of different elements in the combination alloying process needs to be further studied, which has guiding significance for promoting the application of N-alloyed HCCI.

3.4. RE Modification

RE is one of the most commonly used inoculants in the production of HCCI [59]. Due to RE elements owning characteristics of high activity and difficult purification, researchers usually used mixed RE or compound RE to modify HCCI and rarely used pure RE oxides or metals. Among them, the mixed RE mainly refers to the mixture of a variety of RE oxides, and the compound RE refers to the master alloy containing RE. Researchers mainly used light REs (cerium, lanthanum, and other RE elements) to modify HCCI.

Radulovic et al. [60] found that RE elements such as cerium, lanthanum, and neodymium could change the microstructure of 18 wt.%Cr white cast iron and improve its mechanical properties. The best contents of cerium, lanthanum, and neodymium in HCCI are 0.13~0.26 wt.%. Yang [61] found that after adding 0.3 wt.% of mixed RE (cerium > 45%), the wear resistance of HCCI was increased by 23.6%, and the impact stripping resistance was increased by nearly two times, but the excessive addition of RE worsened the performance of HCCI. Guo et al. [62] found that with the addition of RE elements, the grain size of HCCI was obviously refined, and the shape of carbide changed. When the RE was 0~0.3 wt.%, the high-temperature oxidation resistance of HCCI increased with the increase in RE content, but when the content of RE exceeded 0.3 wt.%, the high-temperature oxidation resistance decreased.

Guo et al. [63] found that the proper addition of RE and Al elements could significantly reduce the volume fraction of residual austenite in the as-cast microstructure of W-containing HCCI, significantly refine the grain size of primary austenite, and change the shape of carbide from thick lath to thin sheet, rose- and feather-like, and also improve the hardness, wear resistance, and impact toughness of cast iron. Guo et al. [64] also studied the effect of RE, V, Ti, and B on the microstructure and properties of 3 wt.% Mo HCCI under as-cast and heat treatment conditions and found that with the increase in RE content, both the primary austenite and carbide of HCCI were significantly refined. The hardness, wear resistance, and impact toughness of HCCI were obviously improved by the compound modification of RE, V, Ti, and B. Chen et al. [65] found that the addition of RE-Si-Fe was conducive to refining the morphology of M₇C₃-type carbides in HCCI hardfacing alloys. With the addition of RE-Si-Fe, the wear resistance of the HCCI hardfacing alloy increased at first and then decreased, and the wear resistance was the best when the addition of RE-Si-Fe was 1.0 wt.%. In the surfacing welding metallurgy reaction, a series of RE inclusions could be formed, which had a good interface match with M₇C₃-type carbides and could become the effective heterogeneous nucleation sites of M₇C₃-type carbides; their formation promoted the refinement of M₇C₃-type carbides and thus improved the wear resistance of the hardfacing alloy.

Hao et al. [66] found that adding RE oxides (cerium and lanthanum oxide content > 98%) to the flux-cored welding wire can make the carbide in HCCI hardfacing metal refined and spheroidized. According to the results of the lattice mismatch calculation, the lattice mismatch between the $\left(10\overline{1}0\right)$ crystal plane of Ce₂O₂S and the (0001) crystal plane of M₇C₃ is 6.2%, indicating that Ce₂O₂S has medium effectiveness as the heterogeneous nuclei of M₇C₃, thus refining the carbide.

Hou et al. [67] found that RE nanoparticles or inoculants had little effect on the phase composition of HCCI (composed of M₇C₃ carbide + α-Fe). However, with the increase in RE nanoparticles, the morphology of M₇C₃ carbide changed significantly, from long strip to granular or island-like. When the content of RE nanoparticles was 0.4 wt.%, the microstructure of HCCI was significantly refined. When the content of RE nanoparticles was 0.4 wt.%, the impact toughness of HCCI increased by 36.4%. At this time, the erosion wear rate of HCCI was the smallest, 0.32 × 10⁻³ g/mm², which was 51.5% higher than that without adding any RE nanoparticles. Gou et al. [68] found that RE oxide nano-additives (mainly composed of cerium oxide) improved the hardness and fracture toughness of the primary carbide of the Fe-Cr-C-B hardfacing alloy. The hardness of the primary carbide in the hardfacing alloy with 0.288 wt.% RE oxides was the highest, at 1716.54HV_0.2. The fracture toughness of the primary carbide in the hardfacing alloy with the addition of 0.432 wt.% RE oxides was 9.27 MPa·m^1/2, which was 51.7% higher than that without adding RE oxides. The erosion rate of the hardfacing alloy was 31.8% lower than that of the hardfacing alloy without RE oxides. Wang et al. [69] found that with the increase in the nano-additive of RE oxide (mainly cerium oxide), the volume fraction of primary carbide in the HCCI hardfacing alloy first increased and then decreased and achieved the maximum value (about 22.2 vol.% on average) when the addition of RE oxide was 0.288 wt.%, signifying an increase of 27% compared with the hardfacing alloy without adding RE oxides. The hardness of the hardfacing alloy without the addition of RE oxides was 61.5 HRC, while the hardness of the hardfacing alloy with the addition of RE oxides at 0.288 wt.% was 64.5 HRC; correspondingly, the wear rate was reduced by 21.2% compared with that of the hardfacing alloy without the addition of RE oxides.

Quyen et al. [70] studied the effect of RE on M₇C₃ eutectic carbides in 13 wt.%Cr HCCI and found that with the increase in RE content, the microstructure of M₇C₃ was gradually refined and distributed more evenly. When the carbide is fine and evenly distributed, it will help to improve the performance of HCCI, especially the impact strength and wear resistance. In the presence of RE elements, RE elements reacted with oxygen to form La₂O₃ and Ce₂O₃, which could act as effective heterogeneous nucleation site for the austenite phase, thus obtaining a finer austenite phase. With the increase in RE content, the grain size of primary austenite decreased, and the size of M₇C₃ eutectic carbide gradually decreased, and the distribution became more uniform.

Dojka et al. [71] studied the effect of ferrotitanium and mixed RE as inoculants on the primary crystallization and wear resistance of HCCI and found that the crystallization temperature and wear resistance of the modified sample were higher than that of the unmodified sample under the condition of a slow cooling rate. A large addition of Ti element would lead to TiC agglomeration, which is harmful to wear resistance and toughness; therefore, adding different inoculation elements, such as RE, should be encouraged so as to improve the performance of the alloy without producing an additional agglomeration phase. TiC and RE compounds (Ce₂O₂S, etc.) are effective inoculants of M₇C₃ carbides, and RE compounds can also be used as substrates for TiC crystallization.

Guo et al. [72] studied the effect of a modification treatment with a different content of RE magnesium on the microstructure, hardness, and wear resistance of Cr26 HCCI and found that the addition of RE magnesium changed the formation and growing environment of carbides, and the morphology of carbides changed from long sheet-like and strip-like distribution to clumping and approximately spherical distribution, reducing the separated degree of the matrix. When the content of RE magnesium was 0.6 wt.%, the microstructure and carbide distribution reached the best state. The main function of RE is to purify the alloy liquid and refine the as-cast microstructure of the alloy. The main function of magnesium is deoxidation, desulfurization, purification of iron liquid, and the reduction in inclusions. The wear morphology were mainly plowing, curls, pits, and a few microcracks. The main mechanisms of wear were cutting wear, multiple plastic deformation, and chiseling wear.

In addition to the use of mixed RE or compound RE to modify HCCI, with the progress of metallurgical technology, researchers also try to use pure RE elements or oxides to modify HCCI so as to study the effect of different RE elements on the microstructure and properties of HCCI. At present, relevant research is mainly focused on the following three elements: cerium, lanthanum, and yttrium.

3.4.1. Ce Modification

Qu et al. [73] studied the effect of cerium (purity 99.9%) on the as-cast microstructure of HHCCI (4.0 wt.%C, 20.0 wt.%Cr) and found that the addition of appropriate cerium could refine and spheroidize the primary carbides of M₇C₃, and Ce₂O₂S was found in the primary carbides. It could act as the heterogeneous nucleation site of M₇C₃ primary carbide, thereby improving the nucleation rate of primary carbide and increasing the refinement degree of primary carbide.

Zhou et al. [74] systematically studied the refinement mechanism of the microstructure and carbide of the Fe-24 wt.%Cr-4.1 wt.%C hardfacing alloy when 0~4 wt.% cerium dioxide was added and found that the Fe-Cr-C hardfacing alloy was chiefly composed of martensite, residual austenite, M₇C₃, and MC carbides. The average size of the primary M₇C₃ carbide decreased with the increase in the addition of cerium dioxide and reached the minimum when the addition was 2 wt.%. The addition of cerium dioxide could effectively improve the wear resistance of the hardfacing alloy. When the addition was 2 wt.%, the wear resistance of the hardfacing alloy reached the best level, which was in good agreement with the carbide refinement results. The XRD analysis showed that the main type of formed RE inclusions was Ce₂O₂S. These RE inclusions precipitated before the M₇C₃ carbide and served as the heterogeneous nucleation sites of M₇C₃ in the welding process, thus effectively refining M₇C₃ carbide and improving the wear resistance.

Zhi et al. [75] studied the effect of Ce modification on the microstructure and properties of HHCCI containing 4.0 wt.%C and 20.0 wt.%Cr and found that the primary carbide of M₇C₃ was significantly refined after adding Ce to the alloy melt. Ce₂S₃ was found in primary M₇C₃ carbides as a heterogeneous nucleation site for M₇C₃ carbides. The impact toughness of the sample modified by 0.5 wt.% Ce was 50% higher than that of the sample without Ce modification. The hardness of the Ce-modified alloy was slightly higher than that of the unmodified alloy (see Figure 10).

Wang et al. [76] studied the microstructure and properties of HHCCI before and after the metamorphic treatment of Ce and found that the microstructure of HHCCI before and after the metamorphic treatment was composed of primary M₇C₃ + eutectic M₇C₃ + austenite + martensite. Compared with before modification, the morphology of primary M₇C₃ changed from a long hexagon to round hexagon, the size decreased by 44.9%, and the volume fraction increased by 30.2%. The Rockwell hardness, impact toughness, and bending strength of cast iron are increased by 5.0%, 43.4%, and 39.2%, respectively, and the wear loss is reduced by 50.7%. The effectiveness of Ce compound Ce₂O₂S as the heterogeneous nucleation core of M₇C₃ is moderate, and Ce can spherodize and purify the inclusions. The refined primary carbide and the matrix protect each other and enhance the surface wear resistance of cast iron. The wear mechanism of cast iron before modification is mainly micro-cutting, supplemented by fracture and fall off. After modification, the wear mechanism of cast iron becomes mainly micro-cutting.

3.4.2. La Modification

Zhou et al. [77] studied the effect of La₂O₃ on the microstructure and wear resistance of the Fe-Cr-C cladding layer during arc surface welding and found that the microstructure of the Fe-Cr-C cladding layer was composed of primary (Cr, Fe)₇C₃ carbide and eutectic (γ-Fe+(Cr, Fe)₇C₃) microstructure. With the increase in the addition of La₂O₃, the primary carbide was gradually refined, and the weal loss of the cladding layer was gradually reduced. Fe-Cr-C cladding with an addition of 4 wt.% La₂O₃ had the best wear resistance. As the heterogeneous nucleation site of the primary M₇C₃, the RE inclusion of LaAlO₃ is moderately effective and can refine the M₇C₃ carbide. In addition, the addition of La₂O₃ to the cladding layer can improve its wear resistance.

Liu [78] studied the effect of a different content of La element on the microstructure and wear resistance of Cr26 HCCI and found that the as-cast microstructure was mainly composed of austenite, ferrite, martensite, M₇C₃, and other phases. With the increase in La content, the microstructure was refined, the columnar crystals were gradually transformed into equiaxed crystals, and the long strip carbides were gradually transformed into daisy-like carbides. La₂O₃ could serve as the heterogeneous nucleation site of M₇C₃ carbides, which increased the amount of primary carbide and refined it. Ploughings and carbide spalling pits appeared during the wear process of cast iron, and the number of ploughings and spalling pits decreased significantly after La was added, which improved the wear resistance of cast iron.

3.4.3. Y Modification

Zhang et al. [79,80] studied the erosion resistance of 27Cr HCCI with an Y content of 0~1.5 wt.% in different mortars, which were a mixture of tap water, HNO₃ (pH = 3) and NaOH (pH = 12) solutions, and 30 wt.% silica sand. The results showed that Y alloying could obviously improve the properties of HCCI, and the optimal Y content was 1.0 wt.%, beyond which the beneficial effect of Y would decline. Y element mainly improves the corrosive resistance of cast iron by improving the surface passivation film, rather than by enhancing the mechanical properties of cast iron. They also studied the sliding wear properties of cold-cast yttrium-alloyed 27Cr HCCI in different media such as tap water and dilute NaOH and HNO₃ solutions and found that the wear resistance of Y-containing cast iron was improved compared with 27Cr cast iron without Y. It was also found that the cold-casting 27Cr HCCI with an Y content of 1.0 wt.% had the best wear resistance, but the excessive addition of Y element deteriorated the wear performance. The beneficial effect of Y lies in that it can effectively improve the passivation ability of HCCI and thus significantly improve the mechanical properties and damage resistance of passivation film.

There are many discussions on the modification mechanism of RE elements [65,66,74,77,81]. The reasons why RE can refine carbides can be summarized as follows: (1) RE has a low melting point and is a strong constitutional supercooling element. One study [82] investigated the primary crystallization process of HCCI before and after modification by thermal analysis technology. The results showed that the initial crystallization time and solidification interval of the alloy were shortened after modification. The nucleation rate of carbides was improved by the repartitioning of solute elements in the liquid at the crystallization front of carbides during solidification. (2) RE elements have a large affinity with O and S in liquid iron, which can purify liquid iron and avoid the directional growth of the primary phase during crystallization, thus avoiding coarse grain size. (3) A large number of high-melting-point RE compounds and the inclusions formed by desulphurization and deoxygenation can act as the heterogeneous nucleus of primary carbide, which increases the number of crystal nucleuses and further refines the primary carbide. (4) RE elements will be adsorbed on the grain growth front of carbide and austenite, the selective adsorption on different crystal planes of carbide, preferentially adsorbed on the crystal plane with higher potential energy and higher growth velocity, which reduces the growth rate of carbide in the preferred growth direction. The carbides are transformed from long strips of directional growth to short strips or clumps of non-directional arrangement [81,83].

In summary, there have been many studies on the RE modification treatment of HCCI, which basically clarified the influence of RE modification on the microstructure, hardness, wear resistance, toughness, and other properties of HCCI, as well as the mechanism of RE refining carbide. It was found that the appropriate amount of RE modification can effectively refine the microstructure of HCCI and change the morphology of carbide, thereby effectively improving the hardness, toughness, wear resistance, corrosion resistance, and even high-temperature oxidation resistance of HCCI. Therefore, RE is a very effective inoculant for HCCI. Although there have been a lot of studies on the modification of HCCI with RE, there are still many problems worthy of further study, such as the effect of RE elements on the solidification behavior of HCCI; high-temperature oxidation mechanism and corrosion mechanism in different media of HCCI modified by RE; the effect of other pure RE elements or oxides on the microstructure and properties of HCCI except cerium, lanthanum, and yttrium; and the effect of a combined treatment of RE and other alloying elements on HCCI. Elucidating these problems is of great significance to further promote the application of RE in HCCI.

4. Summary and Prospect

The reference content, existing form, and function of austenite stabilizing elements and modifying elements in HCCI are shown in

Table 2. Reference content, existing form, and function of austenite stabilizing elements and modifying elements in HCCI.

Classification	Alloying Elements	Reference Content (wt.%)	Existing Form	Function
Austenite stabilizing elements	Ni	≤5.0	Almost all solids dissolve into the matrix.	Improve hardenability, reduce martensite transition temperature MS, increase austenite, improve toughness. Strengthen and repair the passivation film and improve the electrode potential of matrix, thus improving the corrosion resistance.
	Mn	≤4.0	Dissolve in both carbide and matrix.	Improve hardenability, strongly reduce martensitic transition temperature MS, increase austenite, and promote the precipitation of polygonal secondary carbides by tempering. Appropriate addition is conducive to improving strength, toughness. and wear resistance.
	Cu	≤2.0	Almost all solids dissolve into the matrix.	Improve hardenability, reduce martensite transition temperature MS, increase austenite, refine carbide. Exhibit obvious solid solution strengthening effect and promote the precipitation of secondary carbide during heat treatment, thus improving the hardness and wear resistance. Improve the corrosion potential of matrix and strengthen and repair the passivation film, thus improving the corrosion resistance.
Modifying elements	B	≤1.2	Dissolve in both matrix and carbide.	Reduce the initial solidification temperature and the solidification temperature interval; reduce the beginning temperature of eutectic transformation. Improve hardenability, increase volume fraction of carbide and refine microstructure, improve the morphology and distribution of carbide. Improve the hardness of matrix and carbide, improve wear resistance, but reduce impact toughness and corrosion resistance.
	Si	≤3.0	Solids mainly dissolve in iron matrix.	Play a deoxidation role in melting and reduce the temperature range of eutectic reaction; reduce the austenite region, reduce the stability of austenite, and promote the formation of pearlite; refine eutectic carbides and improve the morphology of eutectic carbides; appropriate addition can promote martensitic transformation of matrix. Significant solid solution strengthening effect; appropriate addition can effectively improve the hardness and wear resistance.
	N	≤0.5	Solids mainly dissolve in iron matrix and partially exist in carbide.	Significantly improve austenite stability and expand austenite phase region; refining carbide and grain size; it can be concentrated on the interface of metal and oxide film and the active surface of metal. Play the role of solid solution strengthening, improve wear resistance and impact toughness; improve the electrode potential of matrix; increase the corrosion resistance.
	RE	≤4.0	Often present as compounds or in adsorbed state.	Effective inoculant. Effectively refine the microstructure, modify the morphology of carbide, improve hardness, toughness, wear resistance, corrosion resistance, and high-temperature oxidation resistance.

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After more than 60 years of development, the research on the alloying of HCCI is relatively complete, and the types, content, and function of the main alloying elements in HCCI have basically been clarified. However, the current development situation of HCCI is still relatively extensive, and there are still many problems regarding the alloying of HCCI that need to be further studied and solved.

(1) At present, the research on the alloying of HCCI is mainly aimed at homogeneous materials or parts, and many parts only need surface wear and corrosion resistance. In this case, if these parts are made of a single HCCI, on the one hand, due to the large brittleness of HCCI, the overall mechanical properties of these parts are poor; on the other hand, the cost of these parts is high.

(2) At present, there are usually seven or more alloying elements in HCCI, and for different service conditions, the same or similar chemical composition of HCCI is often applied; that is, the role of various alloying elements is not effectively used, which undoubtedly increases the cost and is not conducive to green, sustainable development.

(3) At present, the research methods on the alloying of HCCI or its composition design methods are still conventional experimental methods. Because HCCI often contains many alloying elements, it is difficult to obtain the optimal composition by a conventional experimental method, and the workload is relatively large.

(4) The mechanisms involved in the alloying of HCCI need more in-depth research, such as solidification behavior, toughening mechanism, corrosion mechanism in different working media, oxidation resistance mechanism under high-temperature conditions, etc.

In order to reduce the cost and increase efficiency, and further realize the refined development of HCCI, in view of the above problems, what future research work on the alloying of HCCI should focus on is proposed as follows:

(1) Conduct research on HCCI coating or gradient materials. According to the actual application scenario, surface engineering technologies are used to study the alloying of HCCI coating or gradient materials so as to maintain a good metallurgical combination between the surface of HCCI and the internal substrate and give full play to the beneficial effects of wear resistance, corrosion resistance, and even oxidation resistance of the surface of HCCI. Effectively improve the overall mechanical properties of parts while reducing costs.

(2) According to different typical service conditions, minimize the unnecessary addition of alloying elements and design new special HCCI brands so as to reduce costs and promote the green and sustainable development of HCCI.

(3) Design new HCCI using artificial intelligence. Based on data from the existing literature or industrial production, in view of the practical application requirements, the method of combining artificial intelligence and experiments is used to design a novel HCCI, which greatly promotes the updating of HCCI and truly realizes the design on demand.

(4) Deepen research on the mechanisms related to the alloying of HCCI and lay the foundation for promoting the wider application of HCCI.