The Influence of an Alternating Current Field on Pack Boriding for Medium Carbon Steel at Moderate Temperature

Abstract

The influence of alternating current (AC) field on the pack boriding process for medium carbon steel was investigated through characterization of microstructure, phase composition, microhardness, and corrosion resistance of the boride layer and its mechanism was revealed. Results showed that the boride layer obtained by AC field boriding was composed of the outer FeB and the inner Fe₂B phase, which was similar to that of conventional boriding. Meanwhile, the effective thickness of the boride layer and proportion of Fe₂B increased gradually with increasing current during AC field boriding. The introduction of an AC field during the boriding process served dual purposes. First, it facilitated the decomposition of the boriding medium, leading to an elevation in the concentration of active boron atoms. Second, it reduced the activation energy required for atomic diffusion, thereby accelerating the diffusion of both boron and iron atoms. These combined effects significantly enhanced the hardness distribution and corrosion resistance of the steel. Further insights into the process were gained by fitting the parabolic kinetics curves, which confirmed that the boriding process in an AC field was exclusively controlled by diffusion. This study also clarified the growth mechanism of the boride layer within an AC field.

1. Introduction

Boriding is a sophisticated chemical heat treatment method designed to enhance the surface properties of metallic materials. By exposing the material to a boron-rich medium at a precise temperature through heating or electrolysis, highly reactive boron atoms penetrate deeply into the surface, forming a robust boride layer that comprises one or several intermetallic compound phases [1,2,3]. This boride layer dramatically increases the hardness and wear resistance of the substrate, thereby extending its operational lifespan. Consequently, boriding is widely utilized for surface treatment of various components requiring wear and corrosion resistance, including carbon steel, aluminum alloys, and titanium alloys [4,5,6,7,8]. In recent years, medium carbon steel has gradually expanded its application in emerging fields due to its excellent comprehensive mechanical properties, particularly in the energy sector. Medium carbon steel can be used to manufacture equipment for oil and gas extraction, as well as nuclear energy facilities. Taking 4145H steel as an example, this steel is primarily used for downhole drilling tools such as submersibles and drill collars, which are often exposed to high wear, high pressure, and corrosive environments. These pieces of equipment need to endure, thus placing higher demands on the surface properties of medium carbon steel [9]. Several studies [10,11,12,13,14] have demonstrated that pack boriding can produce a dense boride layer with excellent mechanical properties on the steel surface, thereby substantially enhancing its hardness and wear resistance. Therefore, it is necessary to investigate the boriding treatment of medium carbon steel, ascertain the effects of boriding on the surface properties of the steel, and further optimize the boriding process.

Pack boriding has been widely used owing to its numerous advantages, including minimal equipment requirements, a straightforward process, and exceptional surface quality [15,16]. However, the conventional pack boriding technique was not devoid of challenges, particularly its slow penetration rate, prolonged processing time, and elevated processing temperature [17]. To overcome these issues, researchers have undertaken numerous innovative attempts. For instance, rare earth elements have been incorporated into the boriding medium to bolster the reaction during the process [18,19]. Advancements in fluidized bed, plasma, and microwave technologies have been leveraged to enhance the activity of boron atoms [20,21]. Plastic deformation has been introduced as a means of increasing the surface activity and creating more diffusion channels within the materials [22,23]. Furthermore, studies have revealed that the application of direct current (DC) or alternating current (AC) fields can markedly improve the efficiency of the boriding process [24,25,26].

By applying either direct current (DC) or alternating current (AC) fields to traditional pack boriding equipment, electric field-assisted boriding can significantly lower the boriding temperature while increasing the thickness of the boride layer [27]. Typically, conventional boriding necessitates high temperatures ranging from 850 °C to 950 °C to produce a wear-resistant boride layer with superior hardness and substantial thickness. While higher temperatures during boriding can allow the boriding reaction to proceed fully, they also increase the likelihood of changes in the mechanical properties of the substrate [28]. Ding et al. [29] conducted AC field-assisted boriding on 40CrNiMo steel and discovered that a dense, single-phase Fe₂B boride layer with a thickness exceeding 13 μm could be formed on the surface of steel at 700 °C. The steel surface typically forms a boride layer consisting of FeB and Fe₂B phases. While the FeB phase boasts a hardness superior to Fe₂B, it also demonstrates considerable brittleness. Furthermore, the existence of a brittle FeB layer, combined with surface porosity and other defects, can significantly degrade the overall properties of the boride layer, thereby restricting the practical applications in engineering [30]. Consequently, the formation of a boride layer predominantly composed of Fe₂B on the surface can enhance the mechanical properties of steel, which can be achieved by AC field boriding. Cheng et al. [31] conducted pack boriding on the surface of 45 steel with the assistance of AC field and found that the AC field had a significant promoting effect on the boriding process. It enabled rapid boriding at medium to low temperatures around 800 °C and facilitated the formation of a single Fe₂B phase borided layer. Despite the clear benefits of an external AC field for pack boriding, there has been limited research on the competitive formation mechanism and the proportion of FeB and Fe₂B in the boride layer under the influence of an AC field.

In this study, medium carbon steel was treated using pack boriding enhanced by an AC field, and this method was compared with conventional boriding to investigate the impact of the AC field. By examining the microstructure and properties of the boride layer formed under the influence of the AC field, the mechanism behind the enhancement of pack boriding with AC field for medium carbon steel was elucidated. This research serves as an experimental foundation for further exploring the mechanisms of AC field on the boriding process and promotes the practical application of AC field enhanced pack boriding in engineering practice.

2. Experimental Procedure

The material selected for this experiment is medium carbon steel 4145H. The chemical composition of 4145H steel was determined using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES), as shown in

Table 1. The chemical composition of 4145H steel (wt.%).

C	Si	Mn	P	S	Cr	Mo	Fe
0.42~0.49	0.15~0.35	0.65~1.1	≤0.035	0 ≤ 0.04	0.75~1.2	0.15~0.25	Bal

. The steel was cut into samples with dimensions of 1 cm × 1 cm × 0.3 cm, and then progressively polished using sandpapers of different roughness levels, ranging from 200# to 2000#. This process removed the oxide layer formed on the substrate due to exposure to air, ensuring a uniform and smooth surface prior to the boriding process.

The components and their respective manufacturers of the solid boronizing medium used in the experiment are shown in

Table 2. The components and manufacturers of boriding medium.

Components	Percentage	Manufacturers
B4C	5%	Hebei Tengshuang Metal Materials Co., Ltd. (Xingtai, China)
C	2%	Guangdong Fangxin Biotechnology Co., Ltd. (Shaoguan, China)
KBF4	5%	Suichang Shenlonggu Charcoal Industry Co., Ltd. (Lishui, China)
SiC	88%	Henan Kesheng Abrasive Materials Co., Ltd. (Zhengzhou, China)

. The experimental setup for pack boriding enhanced by an AC field is depicted in Figure 1. The boriding medium was poured into the boriding box, with the sample positioned centrally between two stainless steel electrodes. The sealed boriding box was then placed inside a resistance furnace and heated as the furnace temperature increased. Once the temperature reached the preset value, an AC field was applied through the electrodes. After maintaining the boriding process for a specified duration, the sample was removed after cooling down with the furnace.

In this experiment, a metallographic microscope (BH200M, China) was employed to observe the longitudinal profile morphology of boriding samples and measure the thickness of the boride layer. An X-ray diffractometer (D/max2500PC, Japan) was used to analyze the types of precipitates present within the boride layers. Additionally, a microhardness tester (HV-1000Z, China) was utilized to assess the hardness of the boriding samples, using a 10 g weight and selecting 3–4 points at an equal distance from the surface to ensure accuracy. The corrosion behavior of the boriding samples in a 3.5 wt.% NaCl solution was investigated using a CHI600E electrochemical workstation. The samples, both before and after boriding, served as the working electrodes, while a platinum sheet electrode of 1 cm² was used as the counter electrode and a saturated calomel electrode served as the reference electrode, forming a three-electrode system. Polarization curves for the samples before and after boriding were obtained using the electrochemical workstation, and detailed values for corrosion potential (E_corr), corrosion current density (i_corr), and corrosion rate (V_corr) were derived by fitting the polarization curves based on the Tafel linear fitting model.

3. Results

3.1. The Current Intensity

To investigate the impact of an AC field on the boriding of 4145H steel surfaces, an experimental scheme was established as outlined in

Table 3. Process parameters of boriding for 4145H steel.

	Boriding Medium	Boriding Temperature /K	Boriding Time /h	The Current Intensity /A
CPB	5%B4C + 2%C + 5%KBF4 + 88%SiC	1073	4	/
ACFPB-1				1
ACFPB-2				2
ACFPB-3				3
ACFPB-4				4
ACFPB-5				5
ACFPB-6				6

. For ease of description, conventional pack boriding was referred to as CPB, while pack boriding with an AC field was designated as ACFPB.

3.1.1. Microstructure of Boride Layer

As depicted in Figure 2, the microstructure of the boride layer obtained through various boriding processes for 4145H steel exhibited similar characteristics. The boride layer comprised an outer FeB phase and an inner Fe₂B phase. Notably, the thickness of the boride layer achieved through boriding with an AC field was considerably greater than that obtained through conventional boriding. Furthermore, an increase in current density resulted in a thicker boride layer.

Figure 3 presents the XRD analysis of samples subjected to conventional pack boriding (CPB) and pack boriding with an alternating current field (ACFPB) at various intensities. As evident from Figure 3, the boride layer obtained through AC field enhanced boriding consisted of FeB and Fe₂B phases, which was comparable to that obtained through conventional boriding. However, based on the relative intensity of the diffraction peaks, the content of both FeB and Fe₂B in the boride layer formed under an applied AC field was higher compared to that in the conventional boride layer.

The thicknesses of the boriding layers for 4145H steel subjected to various boriding treatments were measured and are presented in Figure 4. The conventional boriding process resulted in a boride layer thickness of 24 μm. As the current intensity was increased from 1 A to 6 A, the thickness of the boride layer grew from 49 μm to 116 μm. This indicated that the application of an AC field facilitated the growth of the boride layer, and the promotional effect became more pronounced with higher current intensities.

3.1.2. Microhardness of Boride Layer

Figure 5 illustrates the microhardness distribution along the depth of the boride layer after AC field boriding treatment for 4145H steel. The microstructure of the boride layer is composed of an outer layer of FeB and an inner layer of Fe₂B. Since the hardness of the FeB phase is greater than that of the Fe₂B phase, correspondingly, the microhardness curve of the boride sample gradually decreases. As the current intensity increases, the hardness curve of 4145H steel decreases more gradually, which is attributed to the increase in the thickness of the boride layer. The smooth hardness distribution contributes to alleviating the brittleness of the boride layer, decreasing the potential for spalling and thereby improving the overall performance of the boride layer [32]. It is well-known that the hardness distribution of the boride layer significantly impacts the material’s wear resistance [33]. The hardness curve of the boride layer ultimately depends on the proportion of constituent phases and the thickness of the boride layer. The microhardness of FeB is greater than that of Fe₂B, but FeB is more brittle. Therefore, forming a boride layer dominated by Fe₂B can achieve better overall performance. Previous studies [31] have shown that applying AC field boriding on the surface of medium carbon steel can promote the formation of a borided layer composed solely of the Fe₂B phase. It is speculated that the smoother microhardness curve of the borided samples under high current intensity is due to the AC electric field not only increasing the thickness of the borided layer but also enhancing the proportion of the Fe₂B phase within the layer.

3.1.3. Corrosion Resistance of Boride Layer

The potentiodynamic polarization curves for 4145H steel samples, both in their original state and after undergoing AC field-assisted boriding, were measured in a 3.5 wt.% NaCl solution, as depicted in Figure 6.

Table 4. The electrochemical parameters obtained by fitting polarization curves of matrix and boride layer.

	The Current Intensity/A	Ecorr/V	icorr/A·cm−2	Vcorr/mm·a−1
Bare 4145H	/	−1.13	1.21 × 10−3	14.20
ACFPB-1	1	−1.09	4.16 × 10−5	0.49
ACFPB-2	2	−1.02	6.02 × 10−5	0.71
ACFPB-3	3	−1.14	1.09 × 10−4	1.28
ACFPB-4	4	−1.11	8.10 × 10−5	0.95
ACFPB-5	5	−1.12	5.63 × 10−5	0.66
ACFPB-6	6	−1.04	4.84 × 10−5	0.57

lists the corrosion parameters obtained by fitting the corresponding polarization curves using the Tafel linear fitting model, including the corrosion potential (E_corr), corrosion current density (i_corr), and corrosion rate (V_corr). The E_corr of the untreated 4145H steel matrix in a 3.5 wt.% NaCl solution was −1.13 V, suggesting a high propensity for self-corrosion. The i_corr for the untreated steel was 1.21 × 10⁻³ A/cm², with a corresponding V_corr of 14.2 mm/a, indicating severe corrosion in the 3.5 wt.% NaCl solution. However, according to Table 4, the corrosion potential of the 4145H steel after AC field boriding was significantly higher than that of the untreated matrix, and the corresponding corrosion current density and corrosion rate were substantially lower. This demonstrates that the boride layer effectively inhibits corrosion and enhances the corrosion resistance of the 4145H steel matrix.

3.2. Growth Kinetics of Boride Layer

To investigate the growth process of the boride layer under an AC field, boriding tests were conducted at temperatures of 1023 K, 1073 K, and 1123 K with a current application of 4 A. The detailed experimental parameters are presented in

Table 5. Experimental parameters of boriding for 4145H steel.

Boriding Medium	The Current Intensity/A	Boriding Temperature /K	Boriding Time /h
5%B4C + 2%C + 5%KBF4 + 88%SiC	4	1023, 1073, 1123	0
			1
			2
			3
			4
			5
			6

.

To visually illustrate the correlation between the thickness of the boride layer on 4145H steel and the boriding duration at various temperatures, a curve graph has been plotted and presented in Figure 7. Regardless of whether the AC field boriding was conducted at 1023 K, 1073 K, or 1123 K, it was observed that the thickness of the boride layer progressively increased with extended boriding time. Notably, during the initial stages of boriding at different temperatures, the thickness of the boride layer grew relatively rapidly, leading to an approximate parabolic relationship between the thickness of the boride layer and the boriding time. Consequently, the thickness of the boride layer, boriding time, and diffusion rate adhere to Fick’s second diffusion law [34], which can be expressed as:

$$d^2=Kt$$

(1)

Here, d represents the thickness of the boride layer in meters, K is the diffusion coefficient in m²/s, and t is the boriding time in seconds.

Based on the experimental data presented in Figure 7, the relationship between the square of the boride layer thickness and the boriding time was plotted in Figure 8. The results demonstrated a linear correlation between the square of the boride layer thickness, achieved through AC field application, and the boriding time, aligning with Formula (1). Consequently, the diffusion coefficient K, which represents the slope of the fitted line in Figure 8, was determined to be: K₁₀₂₃ = 3.95 × 10⁻¹⁴ m²/s, K₁₀₇₃ = 2.15 × 10⁻¹³ m²/s, K₁₁₂₃ = 3.70 × 10⁻¹³ m²/s. These values indicated that the diffusion coefficient of boron atoms in 4145H steel positively correlates with the temperature during AC field boriding. In simpler terms, as the boriding temperature increased, the diffusion rate, or diffusivity, of boron atoms also accelerated.

The Arrhenius equation establishes a relationship between the diffusion coefficient, activation energy, and temperature [35]:

$$K=K_0{e}^{-Q/RT}$$

(2)

In this equation, K represents the diffusion coefficient measured in m²/s; K₀ is the pre-exponential factor; R, which equals 8.314 J/(mol·K), is the molar gas constant; T is the thermodynamic temperature measured in Kelvin; and Q is the diffusion activation energy measured in J/mol. Taking the natural logarithm of both sides of Equation (2) yields:

lnK = lnK₀ − (Q/RT)

(3)

From Equation (3), it can be observed that the average diffusion activation energy Q and the pre-exponential factor K₀ depend on the linear relationship between lnK and 1/T. To visualize this relationship, a plot between lnK and 1/T was created, as shown in Figure 9. By fitting a line to the data points in Figure 9, the slope of the line, which corresponds to −Q/R, was found to be −25,888.37, and the intercept of the line, which corresponds to lnK₀, was −5.39. Through calculation, the average diffusion activation energy Q of boron atoms in 4145H steel at temperatures ranging from 1023 K to 1123 K was determined to be 215.13 kJ/mol, and the pre-exponential factor K₀ was found to be 4.56 × 10⁻³. The value of Q represents the energy required for a boron atom to jump over a barrier and occupy an adjacent vacancy. Consequently, a smaller value of Q indicated that boron atoms were more mobile and can diffuse more rapidly into the matrix to form compounds.

From Formulas (1) and (2), the following equation can be derived:

$$d^2=K_0{e}^{-Q/\mathrm{R}T}t$$

(4)

By substituting the previously calculated values of Q and K₀ into Equation (4), we can determine the relationship between the boride layer thickness d (in meters), the boriding temperature T (in Kelvin), and the boriding time t (in seconds) for 4145H steel:

$$d^2=4.56\times {10}^{-3}{e}^{-25888.37/T}t$$

(5)

This equation is valid for boriding temperatures within the range of 1023 K to 1123 K.

3.3. Discussion

Based on the aforementioned experimental results, it can be deduced that the microstructure and phase composition of the boride layer on 4145H steel, when subjected to an AC field, resemble those achieved through conventional boriding methods. However, the thickness, microhardness, and corrosion resistance of the boride layer obtained via AC field boriding exhibited notable enhancements, with these improvements being intricately linked to the mechanism through which the AC field exerts its influence on the boriding process.

During the pack boriding, the boriding medium undergoes reactions at high temperatures to generate active boron atoms. These atoms are initially adsorbed onto the sample surface through diffusion, with boride nucleation preferentially occurring at the grain boundaries on the surface [36]. At the commencement of the boriding process, the active boron atoms first react with iron to form Fe₂B. Subsequently, Fe₂B reacts further with boron atoms to produce FeB, resulting in a dual-phase boride layer that consists of inner Fe₂B and outer FeB phases [37]. The chemical reactions involved can be summarized as follows:

[B] + 2Fe → Fe₂B

(6)

Fe₂B + [B] → 2FeB

(7)

After the formation of the dual-phase boride layer, the growth of this layer primarily proceeds through a series of reactions. Figure 10 illustrates a schematic diagram of boride layer growth, detailing the potential reactions at each interface. The reactions involved can be described as follows:

Fe₂B + [B]^I→2FeB

(8)

FeB + [Fe]^I→Fe₂B

(9)

2Fe + [B]^II→Fe₂B

(10)

In these reactions, [B]^I and [B]^II denote the boron atoms that have diffused to the FeB/Fe₂B and Fe₂B/Fe interfaces, respectively. Additionally, [Fe]^I represents the iron atoms that have diffused to the FeB/Fe₂B interface.

The conventional pack boriding method relies on the absorption of heat from an external source to complete the reaction of the boriding medium, the diffusion of boron, and the reaction of phase interface. However, achieving full decomposition of the boriding medium and rapid diffusion of boron and iron atoms becomes challenging when boriding at temperatures below 1123 K. Due to its larger radius, the diffusion of iron atoms is slower than that of boron atoms, causing the reaction rate of (9) to be slower than that of (8). Furthermore, the diffusion of boron atoms from the surface to the FeB/Fe₂B interface is notably easier than to the Fe₂B/Fe interface, resulting in the reaction (8) proceeding faster than (10). Consequently, in conventional boriding, the growth of the boride layer is primarily driven by reaction (8), leading to a higher content of FeB in the final boride layer.

From the aforementioned reaction equations, it can be intuitively deduced that the formation of the boride layer is intimately linked to the diffusion of boron and iron atoms. One of the key factors influencing diffusion is the vacancy concentration within the matrix. Similar to DC fields, it is speculated that AC fields also possess the capability to elevate the vacancy concentration and migration ability within the sample [38]. Furthermore, the thermal and electromagnetic effects of AC fields can intensify the vibrational motion of internal atoms, augment the point defects in the matrix, and provide channels for the diffusion of boron atoms. The electric current flowing through the boriding medium under the influence of an AC field generates heat, which facilitates the chemical reaction between the boriding medium components. Consequently, AC field boriding enhances the quantity of active boron atoms and accelerates the diffusion of both boron and iron atoms. By ensuring that reaction (8) proceeds fully, AC fields also expedite reactions (9) and (10). Notably, the increase in the quantity and diffusion rate of active boron atoms prompts a significant influx of boron atoms into the Fe₂B/Fe interface, leading to the formation of Fe₂B through reaction (10). Ultimately, the application of an AC field results in an increased total thickness of the boride layer, with a higher proportion of Fe₂B compared to FeB within the boride layer.

The steel surface underwent boriding to form a boride layer composed of FeB and Fe₂B, significantly enhancing its hardness and corrosion resistance. While FeB exhibits a higher microhardness range (1890–2340 HV) compared to Fe₂B (1290–1680 HV), Fe₂B boasts a fracture toughness that is more than four times greater than FeB. Therefore, it is desirable to achieve a boride layer on the steel surface that is predominantly composed of Fe₂B [39,40]. The preceding analyses revealed that conventional boriding methods often result in a thin boride layer with a high proportion of FeB due to incomplete reaction of the boriding medium and slow diffusion of boron atoms. In contrast, AC field boriding can effectively increase the thickness of the boride layer and elevate the proportion of Fe₂B within it. As the brittle FeB phase decreases in the boride layer, so does its brittleness and propensity for spalling, leading to a more gradual hardness distribution curve. This aligns with the hardness distribution curve results depicted in Figure 5. Notably, FeB and Fe₂B phases possess different thermal expansion coefficients, often leading to crack formation at their interface, which can adversely affect the properties of the boride layer [41]. The Fe₂B phase adheres more closely to the steel matrix, resulting in a more compact boride layer when using AC field boriding. This improved compactness enhances the corrosion resistance of the steel.

4. Conclusions

In this work, the influence of AC fields on pack boriding for medium carbon steel was researched. By comparing it with conventional pack boriding, the mechanism of AC fields was discussed. The major findings obtained are as follows:

• The microstructure of the boride layer obtained through AC field boriding resembled that of conventional boriding, comprised of an outer FeB and an inner Fe₂B on the 4145H steel. In addition, as the current density increased, the thickness of the boride layer gradually increased, and the thickness obtained at a current density of 6A was five times that of conventional boriding.
• The AC field boriding process was primarily controlled by diffusion. Based on the parabolic growth law of the boride layer, this study established a growth kinetic model within the temperature range of 1023–1123 K.
• The AC field boriding samples exhibit a relatively flat microhardness curve and exhibit significantly reduced corrosion current density and corrosion rate in a 3.5 wt.% NaCl solution. The application of AC field boriding has enhanced the microhardness and corrosion resistance of 4145H steel. This is primarily attributed to the AC field’s ability to increase the number of active boron atoms and accelerate the diffusion of iron and boron atoms, thereby effectively increasing the thickness of the boride layer and the proportion of Fe₂B phase within it.