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Article

Effect of B Content on Microstructure and Wear Resistance of Fe-3Ti-4C Hardfacing Alloys Produced by Plasma-Transferred Arc Welding

by
Lin Zong
1,
Ning Guo
2,*,
Rongguang Li
1 and
Hongbing Yu
3
1
School of Mechanical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
2
School of Materials and Energy, Southwest University, Chongqing 400715, China
3
Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
*
Author to whom correspondence should be addressed.
Coatings 2019, 9(4), 265; https://doi.org/10.3390/coatings9040265
Submission received: 3 April 2019 / Revised: 15 April 2019 / Accepted: 16 April 2019 / Published: 19 April 2019
Browse Figures
Versions Notes

Abstract

:
The Fe-3Ti-xB-4C (x = 1.71, 3.42, 5.10, 6.85 wt. %) hardfacing alloys are deposited on the surface of a low-carbon steel by plasma transferred arc (PTA) weld-surfacing process. Microstructure, hardness and wear resistance have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), Rockwell hardness tester and abrasive wear testing machine, respectively. The results show that the microstructure in all alloys is composed of austenite, martensite, Fe23(C,B)6, Ti(C,B) and Fe2B. The volume fraction of eutectic borides and Ti(C,B) carbides increases with increasing B content. Many brittle bulk Fe2B phase arises when the boron content increases to 6.85%, which causes the formation of microcracks in the hardfacing layer. The microhardness of the hardfacing alloys is significantly improved with the B addition, however, the wear resistance of hardfacing alloys increases firstly and then decreases with increasing of B content. The hardfacing alloy with the 5.10% B content has the best wear resistance, which is attributed to high volume fraction of eutectic borides and fine Ti(C,B) particles distributed in the austenite and lath martensite matrix with high hardness and toughness. The formation of brittle bulk Fe2B particles in the hardfacing alloy with the 6.85% B leads to the fracture and spalling of hard phases during wear, thus, reducing the wear resistance.

1. Introduction

Low-carbon steels are widely used infrastructure materials due to their low costs and excellent combinations of strength and toughness. However, low surface hardness usually causes surface abrasion and accelerates corrosion when used in harsh conditions of high speed, heavy load for long time [1,2,3]. Generally, surface modification technology is applied to improve the surface property of the low-carbon steels [4,5,6,7,8,9,10]. Plasma transferred arc (PTA) welding which uses high-energy plasma arc as heat source for coating of a hardfacing layer on the material surface is considered as an effective method to increase surface hardness and improve wear resistance of the steels. Under the irradiation of high-energy plasma, the hardfacing layer formed on the steel surface obtains the required performance, but the substrate performance is almost not affected by the heat source due to the fast cooling speed in the process of irradiation. The boundary between the substrate and the hardfacing layer has a good metallurgical combination. PTA has a broad prospective application in the manufacturing and the repairing of the wear resistant engineering material due to high efficiency, low cost, and easy operability [11,12,13,14,15].
In order to further improve the wear resistance of hardfacing alloys under severe working conditions, researchers have explored the properties of Fe-based hardfacing alloys reinforced by the precipitation of micrometric or nanometric secondary hard phases [16,17,18,19,20,21,22,23,24]. Besides, boron was also added into the iron base hardfacing alloy to improve their hardness and wear resistance due to the formation of harder carbides and borides. Liu et al. [25] investigated the effect of boron content on microstructure and wear properties of hardfacing alloys and discovered that boron could increase the carbide volume fraction (CVF) and improve the wear resistance. Although some progresses on the study about the influence of B on the hardfacing layer have been achieved [26,27], the effect of different boron contents on the microstructure and wear properties of hardfacing layer remains lack of in-depth research. It is found that increasing boron content (above 1 wt. %) promotes the formation of hard phases (Fe23(C,B)6 and B0.7Fe3C0.3), increasing the wear resistance [27]. In this study, we aim to investigate the effect of different B contents (up to 6.85 wt. %) on the microstructure and properties of the Fe-3Ti-xB-4C hardfacing layer using PTA.

2. Experimental

2.1. Materials and Methods

A commercial low-carbon steel (ASTM A283-C) was selected as the substrate materials, its chemical composition is listed in Table 1. Sheet samples with a dimension of 100 mm × 80 mm × 12 mm were cut from the steels. Before hardfacing, the steel substrates were ground and cleaned with acetone and it was not pre-heated. Commercially available gas atomized powders of Fe-16%B, Fe-30%Ti and Fe (99.0% in purity) and graphite (and 99.5% in purity) were used as initial materials. All the powders have a spherical shape with a diameter of 75–180 μm. The blended powders were mixed with a small amount of sodium silicate to keep the powders on the surface. Then, the blended powders with sodium silicate were dried in hot air. Finally, the blended powders were pre-placed on the surface of the substrate. The thickness of pre-placed layer was about 3 mm. Four samples with different boron contents were prepared in this study via changing the proportion of the Fe-16%B in the packed powders. The chemical compositions are listed in Table 2.
PTA experiments were performed using a plasma powder hardfacing apparatus (DML-V02BD, Shanghai Duomo Industry Co., LTD. Shanghai, China). The PTA processing parameters are listed as follows: the welding current and voltage were 160 A and 30 V, the arc longitudinal movement speed was 0.05–0.25 cm/s, arc lateral oscillation frequency was 0.35 s−1, arc lateral oscillation width was 2.0 cm, and arc-workpiece distance was 3 mm, the flow rate of Ar gas was 15 L/min.

2.2. Characterization

The samples cut by spark-erosion wire cutting were ground and polished, and then etched with 10% nitric acid alcohol solution for microstructural observation. The microstructures were characterized by a field emission gun scanning electron microscope (FEG-SEM, Nova400, FEI, Hillsboro, OR, USA) equipped with an energy dispersive spectroscopy (EDS, Aztec, Oxford Instrument, High Wycombe, UK) detector operating at 20 kV voltage. X-ray diffraction (XRD, Cu Kα, Bruker D8, Bremen, Germany) experiments with a measuring step of 4°/min were conducted to determine the phase components of the coatings.
Hardness tests were performed on an HRD-150 electric Rockwell hardness tester (Henwall Tech Co., LTD., Laizhou, China). A rubber wheel abrasion machine (MLS-23, Yihua Tribology Testing Technology Co., TTD., Jinan, China) was employed to determine the abrasive wear properties. The roller diameter is 150 mm with a speed of 4 r/s, and the surface pressure of the roller is 1.5 MPa. The size of quartz sands with a total weight of 1.5 kg is about 300–500 μm. Samples with a size of 56 mm × 27 mm × 11 mm were machined for 3 min of wear testing. The TG328A model analysis balance with an accuracy of 0.1 mg was used to give the weight of the sample before and after the wear test. The loss of weight of the sample during wear tests was denoted as ΔG:
ΔG = G0 − G1
where G0 and G1 represented weights of samples before and after wear tests, respectively. Three tests were conducted for each wear test, and the average value was taken as the final result. The worn surfaces were characterized by SEM. In this study, two samples were investigated for Rockwell measurement.

3. Results and Discussion

3.1. Phase Analysis of the HardFacing Layer

Figure 1 shows the XRD patterns of the hardfacing layers of the A, C and D samples. From Figure 1a, it can be seen that when the B content is 1.71%, the matrix microstructure of the hardfacing layers mainly contains austenite and martensite. The volume fraction of martensite is larger than that of austenite from the higher diffraction peak intensity of martensite shown in Figure 1a. When the B content increases up to 5.10%, though the volume fraction of austenite decreases by comparative analysis of the intensity value of diffraction peak between the A sample and the C sample, there is no change of the phase component in the hardfacing layer of sample C as shown in Figure 1b. With the B content added to 6.85%, the diffraction peak intensity of austenite and martensite markedly decreases and the peak intensity of Fe2B and Ti(C,B) phases obviously increases shown in Figure 1c, which suggests the volume fraction of austenite and martensite decreases and the volume fraction of Fe2B and Ti(C,B) phases increases with increasing B content. Moreover, the diffraction peaks of the Fe2B phases are higher than that of Ti(C,B) and Fe23(C,B)6, indicating that the volume fraction of the Fe2B phases are larger than that of the Ti(C,B) and Fe23(C,B)6 phases.

3.2. Microstructure of the Hardfacing Layer

The microstructure of the hardfacing layers of various samples with different B contents is shown in Figure 2. Figure 2a–c shows that when the B content is 1.71%, 3.42% and 5.10%, a typical hypoeutectic structure is observed in the hardfacing layers, which contains the primary cellular morphology surrounded by eutectic structure (austenite + carbo-borides). During solidification, the austenite precipitates firstly from the liquid with the decreasing of the temperature, resulting in the formation of the coarse cellular morphology. A part of the austenite is changed into martensite in the hardfacing layer due to the high cooling rate during PTA process. So the austenite and the martensite co-exist in the A, B and C samples, identical with the X-ray data in Figure 1. Also, it can be clearly seen that the cellular morphology becomes finer and the volume fraction of cellular morphology decreases with increasing of B content, which suggests that the nucleation and growth rates of austenite are markedly influenced by boron. The refinement of the cellular morphology is related to the left shift of the eutectic point in the Fe-C binary phase diagram influenced by the addition of B element in the hardfacing layer. When the containing of the boron element in austenite is very low, and most boron atoms in the austenite phase diffuse and segregate into the grain boundaries as austenite grows during solidification [28]. Thus, as the temperature drops to the eutectic temperature, the eutectic transformation starts and a eutectic microstructure is formed. However, when the B content increases to 6.85%, a hypereutectic microstructure containing the gray bulk primary phase with quadrangular shape is observed in the D sample, which precipitates directly from the liquid phase shown in Figure 2d. Such primary phases with a diameter of 20–30 µm distribute uniformly within the matrix. Along that, finer dark hard phases with the quadrangular or irregular shape are also observed. According to the XRD analysis shown in Figure 1c, the gray bulk primary phase is identified as Fe2B and a finer dark hard phase is identified as Ti(C,B).
In order to observe clearly the morphology of the carbo-borides in the hardfacing layer, SEM is employed for observing the microstructure of the A and D samples, as shown in Figure 3. From Figure 3a, three different kinds of carbo-borides in the hardfacing layer of sample A are mainly observed: the eutectic Fe23(C,B)6 carbo-borides with pearlite morphology, the eutectic Fe2B phase with the honeycomb shape, and the dark Ti(C,B) particle with quadrangular or irregular shape. The Fe23(C,B)6 can significantly improve the wear resistance of hardfacing layer due to its high fracture toughness [29,30] and high microhardness (HV0.2 = 1400–1700). The eutectic boride Fe2B phase precipitates at the grain boundary when the B content is relatively low, attributed to the low solubility of boron in the austenite.
As shown in Figure 3b, the gray boride (Fe2B) with a quadrangular block shape and a microhardness value of 1600 HV [31] in the hardfacing layer of sample D precipitates directly from the liquid phase. Figure 4 shows the Fe2B morphology in the surface and the profile of the hardfacing layer, respectively. In the surface of the hardfacing layer, the Fe2B phase with cylindrical shape distributes in the matrix, as shown in Figure 4a, while in the profile of the hardfacing layer, the lengthwise direction of the strip-shaped Fe2B phase tends to be perpendicular to the surface of the hardfacing layer, as shown in Figure 4b. The temperature during the PTA process gradually decreases with increasing distance from the fusion zone and forms a series of isotherm lines in the surface of hardfacing layer [32]. The crystal growth of the Fe2B phase has the fastest rate along the direction perpendicular to the isotherm line since it is the direction of the maximum temperature gradient along the heat dissipation direction. However, the bulk Fe2B phase is too brittle to improve the wear resistance of the hardfacing layer. The volume fraction of the Fe3(C,B)6 phase with a low hardness value decreases gradually, while the volume fraction of Fe23(C,B)6 and Fe2B phases with high hardness increase with increasing B content according to the Fe-B binary phase diagram.
Figure 5 shows the EDS map analyses of the hardfacing layer of sample D. The elemental distribution of Fe, C, Ti are shown here to analyze the chemical composition of the dark quadrangular block in Figure 3b. As can be seen, the dark particle is rich in Ti and C elements, but poor in Fe element. Combined with the XRD results, the dark particle in Figure 5 is considered to be the Ti(C,B). The amount of Ti(C,B) is increased and the cellular morphology is refined with increasing of B content, as shown in Figure 2. It is inferred that the refinement of the cellular morphology is related to the Ti(C,B) precipitation. Ti is a very strong carbide-forming element and can react with carbon to form Ti(C,B) in the early stage of the solidification process. The activity of carbon can be increased in the liquid by the addition of B element, resulting in the formation of Ti(C,B) particles. The more solidification latent heat is released due to the formation of Ti(C,B), the higher nucleation rate of austenite grains due to the reduction of the undercooling of solid-liquid interface. On the other hand, it is reported that the Ti(C,B) particles can promote heterogeneous nucleation of austenite, which further enhances the nucleation rate of austenite grains during solidification [25].
Figure 6 shows the crack morphology arising in the hardfacing layer of sample D. Both the wide micro-crack marked by the solid arrow, as well as the narrow transgranular micro-crack pointed by the dotted arrow are observed. The PTA treatment is the process of the localized heating and cooling, and different levels of thermal stress and phase transformation stress exist in both the hardfacing layer and the heat affected zone. The hardfacing layer and the heat affected zone tend to deform under the thermal stress and phase transformation stress, and the cracks can be readily induced in the large-sized Fe2B phase in the hardfacing layer due to the low ductility of the Fe2B phase under the thermal stress during hardfacing, shown in Figure 6.

3.3. Hardness and Wear Resistance

The effects of B on the hardness and wear resistance of samples are shown in Figure 7. It can be seen that the hardness of hardfacing layer increases gradually with increasing B content, and the maximum value of hardness is about 66 HRC for the higher B content sample. However, the wear resistance of the hardfacing layer increases firstly and then decreases with increasing B content. With 1.71% B, the hardfacing layer exhibits a poor wear resistance due to the small amount of eutectic carbo-borides and Ti(C,B), which causes a bigger mass loss during the wear test. Since the amount of eutectic carbo-boride and Ti(C,B) increases with increasing B content, the wear resistance increases. Increased amount of eutectic borides combined with Ti(C,B) with the increasing of B content from 1.71% to 5.10% makes the wear resistance rapidly increase, and the mass loss of the C sample nearly two times less than that of the sample A. When the B content is more than 5.10%, many primary Fe2B phases with a high hardness arise, and the largest amount of the Ti(C,B) phase can also be obtained, which causes the highest hardness value in hardfacing layer. However, the cracks are observed in the bulk Fe2B phase (Figure 6), which causes the wear resistance drop rapidly due to the brittleness increase of hardfacing layer.
Figure 8 shows the wear morphology of hardfacing layers. Lots of the deep, long grooves and some spalling pits appear on the worn surface of the hardfacing layer of sample A, as shown in Figure 8a. With increasing B content, the number of grooves gradually decreases and the wear scratches get narrower and shallower, as shown in Figure 8b,c. When the B content is 5.10%, the wear scratches are even much narrower and shallower in Figure 8c, which suggests the wear mechanism is dominated by micro-cutting. When the B content further increases to 6.85%, as seen from Figure 8d, many deep and long grooves arise, and a lot of primary carbo-borides fall off on the worn surface which suggests that the wear mechanism is the combination of abrasion and spallation. In general, the two constituents of the wear-resistant materials serve different functions: the hard phases can impede wear by grooving or by indenting mineral particles [33], while the matrix with a good toughness can effectively protect the hard phases [34], resulting in improvement of the wear resistance. The carbon-borides possess high hardness, which means that they can blunt the quartz sands and enhance the wear resistance. The austenite matrix with the cellular morphology gets finer when the B content increases, making the austenite matrix get the better strength and toughness, as shown in Figure 1, which can effectively protect the hard phases from cracking during wear. Therefore, the best wear resistance is obtained in the sample C, and the wear morphology shows a small quantity of the shallow grooves. With 6.85% B content, networks of micro-cracks formed in the large bulk primary Fe2B phase significantly harm the wear resistance. In addition, some cracks at the Fe2B/matrix interface propagates along the boundary of the Fe2B phases, and it is easy to propagate into the matrix to form big cracks, which causes the hard phases to crack and spall off [35]. The broken hard phases become the new abrasive particles, which generate more wear for the surface of the hardfacing layer, and a lot of deep, long grooves and the trace of the spalling hard phases can be observed on the worn surface. Therefore, the wear resistance of hardfacing layer depends not only on the amount of B and the morphology of the hard phases but also depends on the compatibility of hardness and toughness of the matrix microstructure, as well as the interfacial bond strength between the hard phases and the matrix.

4. Conclusions

  • In the Fe-3Ti-xB-4C hardfacing layers, when the B content is lower than 5.10%, the hypoeutectic structure Fe-3Ti-xB-4C hardfacing layers is composed of the cellular austenite matrix and the eutectic structure (cellular + carbo-borides) surrounding the cellular austenite, and the austenite matrix is refined markedly with increasing B content. However, when the B content is 6.85%, the hypereutectic containing the primary bulk Fe2B borides and the eutectic phase is obtained.
  • The hard phases in the hardfacing layers are mainly composed of Fe23(C,B)6, Ti(C,B) and Fe2B phase. The Fe23(C,B)6 eutectic boride has a pearlite morphology and the Fe2B eutectic boride has a honeycomb shape, and amount of both eutectic boride and Ti(C,B) carbide gradually increase with increasing B content. When 6.85% B content is added in the hardfacing layer, quadrangular bulk Fe2B primary phase forms in the hard-acing layer.
  • The hardness value of the hardfacing layer increases with increasing B. The wear resistance of the hardfacing layer increases firstly and then decreases with increasing B content. The hardfacing layer with the 5.10% B content shows the best wear resistance, which is attributed to a large amount of eutectic carbo-borides combined with fine Ti(C,B) particles distributing in the fine grain austenite and the lath martensite matrix with high hardness and toughness. The wear resistance of the hardfacing layer with 6.85% B content decreases due to the formation of brittle Fe2B in the microstructure, which leads to the fracture and spalling of hard phases.

Author Contributions

L.Z. and N.G. conceived the project; L.Z. did the experiments; L.Z. and N.G. wrote the manuscript; R.L. and H.Y. reviewed the manuscript; all authors were involved in data analysis and commented on the manuscript.

Funding

This research was funded by Natural Science Foundation of Liaoning Province of China (Grant No. 20180551117), and Liaoning Provincial Science Research Project (Grant No. LQ2017014).

Acknowledgments

Ning Guo thanks the 2019 Experimental Technology Research Project at Southwest University (SYJ2019012).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD spectra of various hardfacing layers with different of B additions: (a) 1.71%; (b) 5.10%; (c) 6.85%.
Figure 1. XRD spectra of various hardfacing layers with different of B additions: (a) 1.71%; (b) 5.10%; (c) 6.85%.
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Figure 2. SEM images showing microstructure of the hardfacing layers with different of B additions: (a) 1.71%, (b) 3.42%, (c) 5.10%, and (d) 6.85%.
Figure 2. SEM images showing microstructure of the hardfacing layers with different of B additions: (a) 1.71%, (b) 3.42%, (c) 5.10%, and (d) 6.85%.
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Figure 3. SEM images of the surface of the hardfacing layers with (a) 1.71% B and (b) 6.85% B.
Figure 3. SEM images of the surface of the hardfacing layers with (a) 1.71% B and (b) 6.85% B.
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Figure 4. SEM images of borides of the hardfacing layer with 6.85% B contents: (a) surface; (b) the top of the cross-sectional view.
Figure 4. SEM images of borides of the hardfacing layer with 6.85% B contents: (a) surface; (b) the top of the cross-sectional view.
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Figure 5. SEM image and EDS element distribution maps of the hardfacing layers with 6.85% B addition.
Figure 5. SEM image and EDS element distribution maps of the hardfacing layers with 6.85% B addition.
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Figure 6. Microcracks in the hardfacing layer with 6.85% B addition.
Figure 6. Microcracks in the hardfacing layer with 6.85% B addition.
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Figure 7. Rockwell hardness and mass loss (during wear testing) plotted as a function of the B contents of the hardfacing layers.
Figure 7. Rockwell hardness and mass loss (during wear testing) plotted as a function of the B contents of the hardfacing layers.
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Figure 8. Worn morphology of various hardfacing layers: (a) 1.71% B; (b) 3.42% B; (c) 5.10% B; (d) 6.85% B.
Figure 8. Worn morphology of various hardfacing layers: (a) 1.71% B; (b) 3.42% B; (c) 5.10% B; (d) 6.85% B.
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Table
Table 1. Chemical composition (in wt. %) of ASTM A283-C steel.
Table 1. Chemical composition (in wt. %) of ASTM A283-C steel.
CMnSiSPFe
<0.180.35–0.80<0.3<0.04<0.04Bal.
Table
Table 2. Chemical composition (in wt. %) of various hardfacing layers.
Table 2. Chemical composition (in wt. %) of various hardfacing layers.
SamplesBTiCSiMnFe
A1.712.953.910.450.9390.5
B3.422.933.450.420.9788.81
C5.103.093.980.391.0286.42
D6.853.133.760.431.0184.82

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MDPI and ACS Style

Zong, L.; Guo, N.; Li, R.; Yu, H. Effect of B Content on Microstructure and Wear Resistance of Fe-3Ti-4C Hardfacing Alloys Produced by Plasma-Transferred Arc Welding. Coatings 2019, 9, 265. https://doi.org/10.3390/coatings9040265

AMA Style

Zong L, Guo N, Li R, Yu H. Effect of B Content on Microstructure and Wear Resistance of Fe-3Ti-4C Hardfacing Alloys Produced by Plasma-Transferred Arc Welding. Coatings. 2019; 9(4):265. https://doi.org/10.3390/coatings9040265

Chicago/Turabian Style

Zong, Lin, Ning Guo, Rongguang Li, and Hongbing Yu. 2019. "Effect of B Content on Microstructure and Wear Resistance of Fe-3Ti-4C Hardfacing Alloys Produced by Plasma-Transferred Arc Welding" Coatings 9, no. 4: 265. https://doi.org/10.3390/coatings9040265

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