Effect of Magnetic Field Coupled Deep Cryogenic Treatment on Wear Resistance of AISI 4140 Steel

In this study, a new magnetic field coupled deep cryogenic treatment (MDCT) is developed and its effect on wear resistance of AISI 4140 steel is investigated. Compared with wear resistance of untreatment (UT), wear resistance of MDCT increases by 29%. Wear resistance is inversely proportional to the friction coefficient. .e treatment promotes the phase transformation and dislocation movement to generate more martensite in multidirectional distribution and optimized carbide. It enhances material property and repairs surface defect. Moreover, the wear mechanism of MDCT is only abrasive wear in the form of microscopic cutting, while other process groups are oxidative wear and abrasive wear in the form of microscopic cutting and microscopic fracture.


Introduction
Conical pick is a cutting tool that is widely used in mining coal. It is composed of cemented carbide tip embedded in alloy steel body. Due to excellent mechanical property and low cost, the AISI 4140 steel is often selected as the pick body material. However, wear resistance of this material is still lower than cemented carbide tip. Under the complicated working condition, the pick body is most likely to cause wear failure [1]. It leads to the pick tip falloff and directly affects efficiency of the coal mining machine ( Figure 1). erefore, it is especially necessary to improve the wear resistance of material. Wear resistance is often related to structure design, material choice, and process treatment. Among them, process treatment is a convenient and effective measure.
Cryogenic treatment is an efficient method to enhance the materials property and the service life, which has been widely recognized by scholars [2][3][4][5].
e principle of cryogenic treatment is to lower the temperature of the material to the ultralow temperature state. e low temperature environment strengthens the material to improve performance [6,7]. Cryogenic treatment has a very early research history. In the 1930s and 1940s, many researchers begin to explore the effect of low temperature treatment on the property of high-speed steel T1 [8][9][10]. e process is then gradually extended to other materials. e grinding wear performance of cryotreated Cr-Mn-Cu alloy iron is better than that of conventionally destabilized [11]. For AISI 4140 steel, the paper obtains optimal level of the cryogenic treatment process parameters and improves the wear resistance of the material [12].
In addition to cryogenic treatment, scholars magnetize material with ferromagnetism to enhance the material property [13][14][15][16]. ere are two ways to apply magnetic field. e first way is to test performance after magnetic field treatment. Ma [17] investigates mechanical property changing mechanism of high-speed steel by pulsed magnetic treatment.
is proposed method can be effective in improving the high-speed steel tool life. e other way is to test performance under magnetic field treatment. Ayhan [18] studies fatigue life of AISI 4140 steel under magnetic field intensities. e fatigue life approximately improved by 20%, when magnetic field intensity is increased. e above two treatments have common advantages such as green and controllable cost and easy implementation. However, few scholars study the combination of magnetic field and cryogenic treatment and comprehensive analysis of wear is also lacking. erefore, based on the failure mode of pick body, this paper applies the magnetic field coupled cryogenic treatment to study the wear resistance of AISI 4140 steel. Table 1. It has high strength, good hardness, and wear resistance, but no obvious temper brittleness. It has been widely used with high fatigue reliability and good load carrying capacity after heat treatment. e test specimen is shown in Figure 2.

Process Formulation.
Conventional heat treatment generally involves quenching and tempering. e researcher has found that the sequence of cryogenic treatment is best between quenching and tempering [19]. is paper was to integrate static magnetic field on the basis of cryogenic treatment. e process was set to four stages: quenching, magnetic field coupled deep cryogenic treatment, low temperature tempering, and medium temperature tempering. Low temperature tempering can effectively resist coarsening of carbide [20]. e specific process setting is shown in Figure 3. As shown in Figure 3, cryogenic treatment was carried out in cryogenic treatment system by gasification endothermic of liquid nitrogen. e best cryogenic temperature set as 113 K [21]. It belongs to deep cryogenic treatment (113-77 K). e realization of the magnetic field was based on two NdFeB permanent magnets [22]. e magnetic field should be placed in the center of cryogenic treatment system.
After the process setting was determined, it was necessary to design a scientific test program. According to the process setting, there were four process groups in test: untreatment group (UT), heat treatment group (HT), deep cryogenic treatment group (DCT) and magnetic field coupled deep cryogenic treatment group (MDCT). e specific process program is shown in Table 2.

Hardness and Wear Test.
Hardness is the ability of a material to resist the pressing of hard objects into its surface. e hardness was measured as a HRC-150A Rockwell hardness tester with a range of 20-70 HRC. Five hardness points were selected near the wear scar ( Figure 2).
After the hardness test above, the same specimen can been used for wear resistance test. e wear equipment was the CFT-I material surface wear tester. It also records the dynamic friction coefficient in wear. Based on the tested to better simulate the actual working conditions, the specimen selected the rotary friction. e grinding block material was selected from Si 3 N 4 ceramic ball of 5 mm in diameter. e other parameter setting is shown in Table 3 [21]. Before and after wear, the average was m 1 and m 2 , respectively. So, the wear ratio was (m 1 -m 2 )/m 1 .

Microscopic Observation.
e main instruments used in microscopic observation were scanning electron microscope   (SEM) and X-ray diffraction (XRD). Microscopic specimens were taken at the wear scar location ( Figure 2). e manufacturer of SEM is VEGA3 SBH from the Czech Republic. e manufacturer of XRD is MiniFlex600 Rigaku from Japan.

Mechanical Property.
Friction coefficient is an important parameter in tribology, which can reflect the friction surface condition to a certain extent. As shown in Figure 4(a), friction coefficient of process groups tend to be consistent in line spectrum shape. e numerical fluctuation range is small. is means that friction and wear stages are relatively stable. But friction coefficient values of each group are different. It can also be seen that the average friction coefficient decreases from UT about 0.63, HT, DCT to MDCT about 0.52. e maximum reduction rate is 17%. ese indicate that MDCT reduces the degree of friction. Figure 4(b) shows the wear rate, average friction coefficient, and hardness of different process groups. It is found that wear rate is reduced from UT, HT, DCT to MDCT. It indicates that the wear resistance of each group increases in order. Compared with wear resistance of UT, wear resistance of HT, DCT, and MDCT increases by about 3.2%, 8.8%, and 29%, respectively. Furthermore, the wear rate has the same reduction tendency as the friction coefficient. Hardness has no trend as above. Although the trend is to decrease gradually, the maximum reduction rate is only 4.7%. e values are mainly distributed around 45HRC. So, we can conclude that wear resistance is inversely proportional to friction coefficient but is irrelevant to hardness. Friction coefficient reduction may enhance wear resistance. However, Bowden's [23] friction theory indicates that the coefficient of friction cannot satisfactorily explain the reason for enhancement of wear resistance. We need to obtain a more sufficient reason through microscopic analysis. Figure 5 shows the tempered structure of the specimen by SEM observation. It can be judged in the figure: irregularly shaped grain boundary, white grainy carbide, gray-black ferrite, and white strip tempered troostite. Carbide and ferrite are conventional structure under heat treatment. ere are three reasons why the white strip is troostite: the temperature of medium temperature tempering is 633 K; the carbon content is about 0.42%; and the hardness is about 45HRC [24]. In the quenching stage,

Microstructure.
"Y" means yes and "N" means no.      Figure 6(a), the white troostite has sparse and brittle acicular distribution with many microgaps. White acicular troostite is the least and the distribution is poor. ere are many large particle carbide and black honeycomb pit. is characteristic causes uneven force in the wear test. e pit is easily broken by the ceramic ball. ese lead to the worst performance on wear resistance. Figure 6(b) shows a little distribution of small granular white carbide and uneven white strip troostite. e troostite alternately distributes with thin strip ferrite. e quenching promotes the transformation of austenite to martensite, which in turn transforms into troostite in tempering. e pit is also significantly reduced. ey prompt a slight increase on wear resistance. Figure 6(c) shows a quantity of thickness strip troostite and ferrite. A small amount of gaps were distributed on the surface of the material. DCT transforms more troostite and carbide. e reason is that DCT can lower the temperature below M s (start temperature of martensite transformation) to generate more martensite [26]. e transformed troostite also increased in the later tempering. Carbon accumulates at the martensite boundary to form cementite (FeC 3 ) or carbide (Cr 7 C 3 ) [27]. e granular carbide is mostly distributed in discontinuous areas. e result is that the hardness can be

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increased. In addition, the matrix martensite is also optimized during the precipitation of carbon. erefore, the wear resistance of the DCT is greatly enhanced.

Effect of Magnetic Field Coupled Deep Cryogenic
Treatment. Figure 6(d) reveals best troostite distribution. e strip troostite is thicker and denser in multidirectional distribution. is indicates that more optimized troostite is produced than in cryogenic treatment. On the basis of the deep cryogenic treatment to lower the temperature, the magnetic field treatment changes Gibbs free energy. is will increase M s and more easily induce martensite transformation [28,29]. Besides, the magnetostrictive effect activates flexible dislocation movement in all directions [30]. Multidirectional troostite distribution optimizes surface roughness. So, the treatment eliminates unfavorable gaps and repairs microscopic defect. Dislocation movement can also break down carbide [31]. e refined carbide strengthens the matrix martensite. ese factors make a large amount of troostite to fill material surface. erefore, the group has the highest wear resistance. e content of troostite and carbide can also be obtained by XRD analysis . Figures 7(a) and 7(b) show the results of ferrite and cementite, respectively. ey are the necessary phases of the composition of the troostite. It can be seen that the peaks of DCT and MDCT are matched, while other processes have large deviations.
is shows that a large amount of troostite is produced. In addition, it can be seen from Figure 7(c) that MDCT has the highest carbide content. According to the above analysis, MCDT is the most ideal distribution of troostite and carbide.

Wear Mechanism.
e analysis of wear mechanism is necessary to wear. SEM can observe the surface wear scar of different groups. Figure 8 shows a clear elongated cutting groove with curvature in four images. In the wear test, the rotary friction motion is performed between the ceramic ball and the specimen. e higher hardness ceramic ball can cause plastic deformation on the surface of specimen. e huge cutting stress pushes the material to both sides. e surface forms many small width and depth cutting grooves with small chips. e wear mechanism is abrasive wear in the form of microscopic cutting [32]. Figures 8(a) and 8(b) have similar wear characteristics. In addition to cutting groove, the surface of specimen presents large delaminated areas and patchy dark areas. When cut depth reaches a critical depth, a large amount of brittle structure is cracked in the wear test. e expansion of the crack and its own unfavorable pit lead to a large delaminated area. e wear mechanism is abrasive wear in the form of microscopic fracture [33]. e dark area made oxidation reaction with friction heat. e surface is subjected to oxidative wear [34]. erefore, the main wear mechanisms of UT and HT are oxidative wear and abrasive wear in the form of microscopic cutting and microscopic fracture. e result is a higher wear rate in both groups. However, the delaminated area and the black area are significantly reduced in Figure 8(c). e surface has many evenly spaced cutting grooves with few black areas. e wear mechanism is microscopic cutting but little oxidative wear. e reason is that the surface has tempered troostite with high strength and toughness. In Figure 8(d), there are only grooves on the surface. As material property is further enhanced, the cutting stress only pushes the material to both sides without greater damage. erefore, MDCT is the most satisfactory wear resistance performance. In summary, type and degree of wear are successively reduced from UT, HT, DCT to MDCT.

Conclusion
In this study, the effect of magnetic field coupled deep cryogenic treatment on wear resistance of AISI 4140 steel has been investigated. From the mechanical performance testing and microanalysis, the following conclusions have been drawn.
(1) For AISI 4140 steel, compared with wear resistance of UT, wear resistance of HT, DCT, and MDCT increased by about 3.2%, 8.8%, and 29%, respectively. Besides, wear resistance is inversely proportional to friction coefficient but irrelevant to hardness. (2) DCT lowers temperature to generate more martensite. e carbon in martensite precipitates as cementite or carbide. e matrix martensite is also optimized during the precipitation of carbon. e wear resistance of the DCT is greatly enhanced.
(3) MDCT can raise Ms to generate more martensite. e magnetostrictive effect activates flexible dislocation movement in all directions.
is leads to multidirectional distribution and optimized carbide. e result strengthens material property and repairs surface defects. Based on the cryogenic treatment, the best wear resistance is obtained. (4) e mechanism of MDCT is abrasive wear in the form of microscopic cutting, while other process groups are oxidative wear and abrasive wear in the form of microscopic cutting and microscopic fracture.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.