Effects of Deep Cryogenic Treatment on Wear Resistance and Structure of GB 35 CrMoV Steel

Wear resistance of metallic materials can be effectively improved by the deep cryogenic treatment. In this study, different deep cryogenic treatment conditions were considered, with different soaking durations between quenching and tempering. The main objective is investigating the effects of deep cryogenic treatment and exploring the relationship between the mechanical properties and the microstructure of GB 35CrMoV steel. Hardness and relative wear ratios of samples were evaluated by the Vickers-hardness test and the pin-on-disk wear test, respectively. Worn surface was characterized by a non-contact optical surface profiler. Microstructures were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD). Significant improvements in hardness and wear resistance are observed for higher cryogenic soaking times; the root mean square deviation (RMS) parameter (Sq) was employed to evaluate the effect of deep cryogenic treatment on the worn surface roughness; the improvements were ascribed to the precipitated carbides. The mechanism can be interpreted not only as the promoted effect of deep cryogenic treatment in the decomposition kinetics of martensite, but also as the acceleration on the Ostwald ripening process.


Introduction
Over the last several decades, deep cryogenic treatment has been recognized as an effective method to improve hardness, fatigue, toughness and wear resistance of metallic materials [1][2][3][4].With the advantages of low cost, low energy consumption and free pollution, it has increasingly attracted the attention of researchers all over the world.
The significant effects of appropriate deep cryogenic treatment on the mechanical properties, wear resistance of high-speed steels [5][6][7] and tool steels [8][9][10][11][12][13][14] have been reported in many studies.Podgornik et al. [15] investigated the effect of deep cryogenic treatment on the wear resistance of different tool steels, and suggested that the improvement in properties can be related to the formation of finer needle-like martensite and the martensitic transformation accompanied by plastic deformation of primary retained austenite.Das et al. [16] demonstrated that the improvement of wear resistance by deep cryogenic treatment of AISI D2 steel samples was unambiguously related to both the substantial modification in the precipitation behavior of secondary carbides and the reduction in retained austenite content.Existing in steels, carbides have been recognized as an important hard phase.Yan et al. [17] studied the improvement of wear resistance on W9Mo3Cr4V high-speed steel through different deep cryogenic treatments; the improvement was attributed to the strengthening effect on the matrix caused by the precipitation of fine secondary carbides, the formation of fine twinning and the more transformation from retained austenite to martensite.Li et al. [18] pointed out that the wear resistance of M2 steel under deep cryogenic treatment related to the cryogenic temperature, the holding time and the cooling rate, and that the governing mechanism was ascribed to the fine carbide precipitation that enhanced the strength and hardness of the martensite matrix.
Although numerous benefits, especially the wear resistance of the deep cryogenic treatment on high-speed steels and tool steels, were reported in many literatures, there is little research focused on the effect of deep cryogenic treatment on the wear resistance of 35CrMoV steel.Therefore, the purpose of this paper was to investigate the effect of deep cryogenic treatment on the wear resistance of 35CrMoV.Samples were treated at different cryogenic soaking durations (0 h, 1 h, 3 h and 6 h, respectively) between quenching and tempering; wear resistance was investigated by the pin-on-disk wear test.The mechanism was explored by means of scanning electron microscope (SEM) and X-ray diffraction (XRD) to understand the relationship between properties and microstructures, and the wear behaviors accordingly.

Materials
Reference material used in this investigation was commercial GB 35CrMoV steel, which is a mid-carbon steel commonly used in the industry due to its high fatigue limit, high static strength and good creep strength.Thus, it is widely used in the production parts, such as main shafts, crankshafts, gears and turbine impellers [19][20][21][22].Excellent wear resistance is necessary for smooth operation and long service life of parts.The material composition of the GB 35CrMoV steel is shown in Table 1 and the experiment profile is shown in Figure 1.

Heat Treatments
After the specimens (cubes 20 mm × 20 mm × 20 mm) were machined from soft annealed blocks, they were first austenitized at 920 • C, maintained for 0.5 h in a high-temperature intelligent box furnace (SX2-4-10, Yuandong, Changsha, China) and then quenched in oil.In order to evaluate the effects of deep cryogenic treatment, specimens were then kept in liquid nitrogen (LN2) in a commercial LN2 tank for 1 h (designated as 1HCT), 3 h (3HCT) and 6 h (6HCT), respectively, left in the air to reach room temperature after taken out from LN2 and finalized by a subsequent single tempering for 0.5 h.The same tempering temperature was used in the case of conventional heat treatment.The traditional heat-treated sample was labeled as 0HCT to be distinguished from the deep cryogenic treated samples.Heat treatment procedures and detailed conditions are presented in Figure 2.

Hardness Test and Wear Test
Room temperature measurement of hardness was conducted by a Vickers-hardness tester (HV-1000A, Huayin experimental instrument Co. Changsha, China), with a load of 300 gf for 10 s.Samples were wet ground with abrasive papers to get rid of the oxidation layer and were ultrasonic cleaned before the hardness test.Each sample was measured with 20 points of Vickers-hardness and the mean value was calculated.
The effect of deep cryogenic treatment on wear resistance was determined under dry sliding conditions by using a pin-on-disk configuration on a microcomputer controlled wear tester (MRS-10w, Crown precision measuring instrument Co. Jinan, China).The schematic diagram of the pin-on-disk is shown in Figure 3. Four ∅8 × 20 mm cylinders were cut from the heat treated cubic samples by a computer numerical controlled (CNC) wire cutting machine (DK7735, Jiangzhou CNC Machine Tool Manufacturing Co. LTD, Taizhou, China) and acted as the pins, when the disc was a commercial abrasive disc with the particle size of 80.The end face of columnar samples were wet ground with abrasive papers to remove the influence of the Electrical Discharge Machining (EDM) layer and polished with 0.5 µm diamond suspensions to make the surface smooth.Wear tests under dry sliding conditions were performed on room conditions, with an average sliding speed of 0.367 m/s, a load of 10 N and a total sliding distance of 329.4 m.Wear mass loss was obtained by measuring the pin mass before and after the wear test.The relative wear ratio (W i ) was obtained using the following Equation (1) [10]: where W A is the weight loss of a target sample and W B is the weight loss of a standard (or reference) sample.In this study, the traditional heat-treated sample (0HCT) was used as the reference sample.
The worn morphology of the wear scar was characterized by a non-contact optical surface profiler (Wyko NT9100, Veeco Instruments Inc., Plainview, NY, USA) with a magnification of 20× on an area of 313.1 × 234.7 µm 2 after the wear test.

Microstructural Examinations
An SEM (Phenom ProX, PhenomWorld, Eindhoven, The Netherlands) was used to observe the microstructure.Samples were wet ground with abrasive papers and polished with diamond suspensions, and then etched with a nital etchant (4 mL of 70% nitric acid and 96 mL of anhydrous ethyl alcohol).XRD (D8 Advance, Bruker, Billerica, MA, USA) with Cu Kα radiation (the wavelength was 0.15406 nm) was used for the study of phase composition, with the scanning range of the diffraction angle 2θ from 10 • to 90 • in steps of 0.02 • .

Hardness
The result of the hardness test (HV 0.3 ) on samples experiencing different cryogenic treatments is illustrated in Figure 4.The hardness values for the 0HCT, 1HCT, 3HCT and 6HCT samples were 371.8 HV 0.3 , 385.6 HV 0.3 , 395.9 HV 0.3 , 410.8 HV 0.3 , respectively.The HV 0.3 of 1HCT sample was improved about 3.7%, and 6HCT sample was improved about 10.5%, compared with the 0HCT sample.In general, the hardness of samples increased consistently with the prolonged deep cryogenic treatment time and reached maximum when the soaking time was 6 h.

Wear Resistance
According to Equation (1), a relative wear ratio of the reference sample is 100%, and a lower W i of target sample means better wear resistance.Data of the weight change are listed in Table 2.The effect of different cryogenic soaking time on the relative wear ratio of 35CrMoV samples is shown in Figure 5.As shown in Figure 5, the relative wear ratio changed when samples experienced the deep cryogenic treatment.Compared with the sample that experienced 0HCT, the relative wear ratio of the sample that experienced 1HCT was decreased by 40.7%, the relative wear ratio of the sample experienced 6HCT was significantly reduced 56.5%, when the holding time was 6 h.Thus, the deep cryogenic treatment can dramatically enhance the wear resistance of GB 35CrMoV steel, and the effect can be markedly increased along with the extension of cryogenic holding time.

Worn Surface Observation
The data obtained by the non-contact optical surface profiler was further treated using the Vision Software (Vision.exe,Veeco Instruments Inc., Tucson, AZ, USA).Three surface profile lines of samples (h, j and k) were obtained correspondingly from the selected scanning areas (H, J and K) via smoothing treatment, and the cursor widths of H, J and K were all 50 pixels.3D (Three-dimensional) surface morphology and smoothing treated surface profile lines of samples are shown in Figure 6.
As shown in Figure 6a, wide and deep grooves can be clearly observed on the surface of the sample experiencing 0HCT, indicating the effects of typical microplowing and microcutting during the pin-on-disk wear test.The surface of the sample experiencing 1HCT revealed similar features of grooves after the wear test (Figure 6b).By contrast, the surface topography of the sample experiencing 3HCT was covered with shallow, thin grooves (Figure 6c), and the 6HCT sample possessed a much smoother appearance with fewer grooves (Figure 6d) in comparison to the 3HCT sample.A parameter of 3D surface characteristic microtopography was provided to present more visualized information by using the function of 3D surface roughness analysis in the Vision Software.The root mean square deviation (RMS) parameter (Sq) was employed to evaluate the effect of the deep cryogenic treatment on the worn surface roughness [23,24], which is defined as the following Equation (2) [25]: The result is shown in Table 3.Values of Sq were decreased by 24.7%, 44.0% and 81.0% for 1HCT, 3HCT and 6HCT, respectively, indicating that the degree of smoothness of the worn surface improved with the prolonging of deep cryogenic treatment time.The deep cryogenic treatment reduced the relative wear ratio, while improving the wear surface morphology of 35CrMoV steel.

Mechanisms Analysis and Discussion
In order to reveal the mechanisms in different wear performances between 35CrMoV samples, microstructural features were observed with SEM.Typical microstructures of samples were illustrated in Figure 7. Structural characterization was tested by means of XRD of the samples, which experienced 0HCT and 6HCT (Figure 8).The precipitation of carbide was clearly affected by deep cryogenic treatment, as is shown in Figure 7. Using the "analyze particles" function in Image J software, the quantity, average size and area fraction of carbides in Figure 7 were measured, and particles larger than 0.1 µm were taken into account.The effect of the deep cryogenic treatment time on the distribution condition is shown in Table 4. Compared with the sample experiencing 0HCT (Figure 7a), the sample experiencing 1HCT (Figure 7b) clearly revealed the small-size carbides; the average size was about 2.2 times larger and the area fraction was about 1.7 times higher than the 0HCT sample.As the deep cryogenic holding time was increased to 3 h, the precipitation of carbides was observably promoted.The average size and area fraction of carbides continued to increase; the medium-size carbides appeared, but the amount of carbides decreased.When the holding time was prolonged to 6 h, the size of part carbides continued growing up; the average size of carbides was 0.838 µm and the area fraction was 0.773%.Based on the above observations and measurements, the precipitated carbide may have considerable contributions to the improvement in hardness and wear resistance of 35CrMoV steel.
In order to explore the mechanism of martensite decomposing and carbide precipitation, researchers explored the effects of different sub-zero treatments.Compared with cold treatment (193 K), deep cryogenic treatment (77 K) had a more significant effect accelerating the decomposition of martensite and the precipitation behavior of carbides during tempering [16].With the decrease in temperature of the deep cryogenic treatment, martensite became more supersaturated than on the indoor temperature condition and developed high internal stresses in the process of transformation from austenite to martensite, thus increasing lattice distortion and thermodynamic instability of itself, resulting in the segregation of carbon atoms to nearby defects and forming clusters [26].Therefore, the mechanism for the observed difference in microstructures between the 0HCT sample and the 1HCT sample could be attributed to the promotion of deep cryogenic temperature on the martensite decomposition kinetics, leading to the promoted precipitation behavior of carbides.However, the samples of 35CrMoV were soaked at the same deep cryogenic temperature, so it was apparent that the acceleration of martensite decomposition was not enough to explain the observed difference between the 1HCT, 3HCT and 6HCT samples.
Normally, carbide nuclei are generated along with the decomposition of martensite at the initial stage of tempering heat treatment.Particles are formatted from the agglomeration of carbide nuclei and ar coarsened with the expense of other particles.This selective growth for carbides is referred to as Ostwald ripening and is well demonstrated in many papers [27][28][29][30].By combining the experiment results, the possible mechanism related to the observed phenomena in this study could be attributed to not only the acceleration of the decomposition of martensite and carbide nucleation during deep cryogenic treatment, but also the promotion of the Ostwald ripening process.
During the deep cryogenic treatment, nanoclusters of carbon atom were generated along with the decomposition of martensite, but did not grow up because of the deep cryogenic temperature.When temperature began rising, nanoclusters gathered and agglomerated, acting as nuclei of carbides [26].These nanoclusters resulted in a promoting effect on the selective growth of the Ostwald ripening process.When the cryogenic soaking time was prolonged, the amount of nanoclusters, or the amount of carbide nuclei, increased gradually, accelerated the growth of carbides effectively; the samples experiencing the deep cryogenic treatment showed the increased amount and size of carbides.When the soaking time reached the longest time (i.e., 6 h), the size and amount of carbides were significantly increased.
SEM pictures and XRD spectrograms indicated that there was mostly tempered sorbite [31] in the heat-treated 35CrMoV steel, and there was no significant difference of matrix composition between samples experiencing 0HCT and 6HCT, respectively (Figure 8a,b).It can be inferred that the transformation from the retained austenite to martensite during deep cryogenic treatments was not the major concerning reason to be responsible for the improvements of 35CrMoV samples.From the above discussion, the size and amounts of carbides increased along with the prolonging of cryogenic soaking time, and the improvements in hardness and wear resistance of 35CrMoV showed a trend similar to what was observed during mechanisms analysis of SEM.Thus, these improvements were attributed to the precipitated carbides.Carbides that were generated at dislocations and grain boundaries could effectively enhance the strength and hardness of matrix and impede the movement of dislocations, leading to increased hardness and improved resistance to plastic deformation of 35CrMoV steel.Carbides with good fracture toughness can effectively resist the crack initiation and prevent further extension of the fracture, thus making a significant contribution to the wear resistance [32].

Conclusions
Based on the results obtained in the present investigation, the following conclusions were drawn: (1) Deep cryogenic treatment could effectively enhance the hardness and wear resistance of GB 35CrMoV steel.The hardness (HV 0.3 ) was increased by 3.7%, 6.1% and 10.5% for 1HCT, 3HCT and 6HCT, respectively, with respect to the hardness with no cryogenic treatment.For the wear ratio, decreases of 40.7%, 53.3% and 56.5% were observed for 1HCT, 3HCT and 6HCT, respectively.(2) Worn surface of deep cryogenic treated samples after the pin-on-disk wear test was obviously different from the conventional heat-treated sample, which showed scars suffering from typical microplowing and microcutting.Such scars were much less and lighter on the worn surfaces of samples that experienced the longer-time cryogenic treatment, especially the sample experiencing 6 h cryogenic treatment.The RMS parameter (Sq) indicated that the degree of smoothness of worn surface was improved with the prolonged deep cryogenic treatment time.(3) Improvements in hardness and wear resistance of 35CrMoV were ascribed to the precipitated carbides.Deep cryogenic treatment between the quenching and the tempering process promoted the precipitation of carbides.The mechanism could be interpreted not only as the promoted effect of deep cryogenic treatment in the decomposition kinetics of martensite, but also as the acceleration on the Ostwald ripening process by the prior precipitated nanoclusters of carbon atoms during deep cryogenic treatment.Precipitated carbides could effectively enhance the strength and hardness of matrix and resist the crack initiation and propagation.

Figure 3 .
Figure 3. Schematic diagram of a pin-on-disk wear test set-up.

Figure 5 .
Figure 5.The relative wear ratios of different 35CrMoV steel samples.

Table 2 .
The weight change of samples after different cryogenic treatments.

Table 3 .
The root mean square deviation parameter (Sq) of worn surface.

Table 4 .
The distribution of carbides.