Enhancing the Wear Resistance of 100Cr6 Bearing Steel Using Cryogenic Treatment

The purpose of this study is to examine the effect of cryogenic treatment on the enhancement of the wear resistance of 100Cr6 bearing steel. The study also aims to reveal the underlying mechanisms responsible for the enhancement of wear resistance by deep cryogenic treatment (DCT) at −185°C. The wear behavior was assessed by a reciprocatory friction and wear monitor under varying normal loads (ASTM International, Annual Book of Standards, 1996). It was found that the wear resistance was increased by 37% due to DCT when compared with that of conventional heat treatment (CHT). A scanning electron microscopy (SEM) study was also conducted to identify the possible mechanism that augments the improvement in the wear resistance of cryogenically treated 100Cr6 bearing steels. The microstructural study suggests that the improvement in the wear resistance is attributed to the conversion of the retained austenite into martensite, along with the precipitation and distribution of the carbides brought in by the cryogenic treatment. The differential scanning calorimeter (DSC) analysis conducted on the bearing steel samples showed that the DCT mainly enhances the destabilization of martensite by activating carbon clustering and transition carbide precipitation. Deep cryogenic treatment demonstrated more improvement in the wear resistance and hardness compared with conventional heat treatment.

Keywords: Cryogenic; 100Cr6; Microstructure; Carbides; Wear

The fuel injection pump plunger is one of the elements limiting the life of diesel engine fuel systems. The plunger is precisely fitted with a very small clearance to facilitate a perfect motion. The failure that occurs in the fuel injection pump is mainly due to the failure of the plunger (Sergeev, et al. ([1])). Traditional diesel fuel injector plungers and bore material are made of 100Cr6 bearing steels. The plunger failure modes include wear, corrosion, and fracture (Sergeev, et al. ([1])). Plunger wear has been a problem for designers and manufacturers for many years. Due to plunger wear, the flow of the fuel increases, and it reduces the fuel pressure delivered inside the combustion chamber, which results in incomplete combustion, more smoke, etc. Even though new plunger materials and production techniques are constantly being developed, these changes face difficulties in keeping pace with the high demands placed upon the plunger, due to the continual increase in the requirements of fuel injection pump performance in a worldwide competitive environment. Plunger manufacturers are continuously working with fuel pump manufacturers in an effort to improve the plunger quality and life. Hence, definite ways and measures are needed to improve the performance and life of a plunger. Cryogenic treatment has been attempted as one of the ways to improve the properties of materials by reducing wear, which, in turn, will increase the performance and life of the components (Cajner, et al. ([2]); Podgornik, et al. ([3]); Arockia Jaswin, et al. ([4])). The present work is an experimental study to compare the wear resistance improvement in 100Cr6 bearing steel samples.

Table 1 Chemical Composition of the 100Cr6 Bearing Steel.

Cryogenic treatment is a process attempted by researchers to supplement the conventional heat treatment (CHT) to improve a material's mechanical properties. Molinari, et al. ([5]) mentioned that cryogenic treatment improves the mechanical properties of a material by allowing the molecules of the material to compress and expand in a uniform, homogeneous manner and then to be realigned in a more coherent fashion, thus reducing internal stress and thereby increasing the life of the components. Huang, et al. ([6]) carried out cryogenic treatment by soaking the samples in liquid nitrogen for 1 week and studied the microstructural changes of M2 tool steel before and after cryogenic treatment. The results showed that cryogenic treatment facilitates the formation of carbon clusters and increases the carbide density in the subsequent heat treatment, thus improving the wear resistance of the steels. Vimal, et al. ([7]) studied the effect of cryogenic treatment on the mechanical properties and microstructure of En 31 materials. The refrigeration of metals to improve their performance is generally classified as either shallow cryogenic treatment, sometimes referred to as subzero treatment, or deep cryogenic treatment (DCT) based on the treatment temperature.

Collins ([8]) studied the effect of deep cryogenic treatment of D2 tool steel on its hardness, toughness, wear resistance, and microstructure. The study concluded that those samples treated at lower austenitizing temperatures, with more martensite in the initial as-quenched condition, underwent more conditioning at the deep cryogenic temperature with a consequent increase in both toughness and wear resistance. Dong, et al. ([9]) studied the effect of DCT with respect to the microstructure of high-speed steels (T1 and M2). The results showed that DCT not only transforms austenite into martensite but influences carbide precipitation as well. Kalin, et al. ([10]) investigated the effect of four different tempering temperatures on ESR AISI M2 high-speed steel subjected to vacuum and cryogenic treatment. Das, et al. ([11]) studied the effect of DCT on the carbide precipitation and tribological behavior of D2 steel. Deep cryogenic treatment of D2 steel markedly reduces the amount of retained austenite and results in significant changes in the precipitation behavior of secondary carbides. Mohan Lal, et al. ([12]) conducted a study on the improvement in the wear resistance and significance of the treatment parameters in D3, M2, and T1 tool and die steel in various treatment conditions. It was found that the cryogenic treatment imparted nearly 110% improvement in tool life. Samples cooled to 93 K in 3 h, soaked for 24 h, and ramped up to room temperature in 6 h yielded 20% extra life compared to the maximum life achieved through cold treatment. Das, et al. ([13]) studied the microstructure, hardness, and wear behavior of AISI D2 steel subjected to varied subzero treatments have been examined with reference to CHT. Gill, et al. ([14]) reported that the prominent reasons found to be responsible for improving the mechanical properties of tool steels are transformation of retained austenite to martensite and precipitation of fine carbides. The wear on bearing steel is an important factor that affects the life of the plunger in the fuel injection pump, and cryogenic treatment has the potential to improve the wear resistance of bearing steel, which has not been attempted. Plunger wear is very complex, involving many mechanisms, and cryogenic treatment is expected to address at least a few mechanisms, due to the enhanced hardness and precipitation of fine carbides, which lead to better wear resistance. An attempt was made to study the effect of cryogenic treatment on plunger wear characteristics and characterize the structure of the CHT and DCT specimens using scanning electron microscopy (SEM) analysis. In this work, DCT was applied to 100Cr6 bearing steel, and the effect on the wear resistance was investigated.

100Cr6 Bearing steel rod samples of 6 mm diameter and 6 mm length were prepared for the wear study. Initially, the composition of the samples was confirmed by the optical emission spectroscope analysis, as shown in Table 1. The samples were heat-treated as per the procedure prescribed in the ASM standards ([15]). The machined specimens were segregated into two groups, viz. Group A (CHT) and Group B (DCT), and subjected to two different treatment processes, as explained in the flowchart shown in Fig. 1. The following heat treatment was given to the 100Cr6 bearing steel specimens: The Group A samples were subjected to hardening (austenitizing) at 850°C for 1 h, followed by an oil quench, and tempered immediately after quenching at 200°C for 2 h. In the deep cryogenic treatment (Group B) the hardened 100Cr6 samples were cooled from room temperature to −185°C in 3.5 h at 1°C/min, soaked at −185°C for 24 h, and allowed to warm up to room temperature for another 6 h at 0.6°C/min. Finally, the samples were tempered immediately at 200°C temperature for 2 h. The deep cryogenic processing was performed in a laboratory by a computer–controlled cryogenic processor (A.C.I. CP-200vi, Applied Cryogenics Inc., Burlington, MA, USA) using liquid nitrogen as the medium. These samples were kept inside the well-insulated treatment chamber to prevent the steels from being subjected to liquid nitrogen and to eliminate the risk of thermal shock.

Graph: Fig. 1 Thermal treatment processes for Group A and B samples.

The test samples for the Vickers hardness test were machined as per the ASTM standard designation E 92–82 (ASTM International ([16])) and the hardness test was carried out. Five samples were taken from each group and subjected to the hardness test. The Vickers hardness test was carried out in such a way that four indentations were made in each test sample. The hardness number was determined based on the formation of the indentation due to the applied force. The load applied was 30 kgf for a dwell time of 10 s.

The enhanced wear resistance of the cryo-treated bearing steel samples was experimentally measured using a reciprocatory friction and wear monitor (DUCOM TR-281M-M4, Ducom Instruments Pvt. Ltd, Bangalore, India) by the weight loss method, as per ASTM standard G-133 (ASTM International ([17])). The weight loss due to wear of the bearing steel samples during the reciprocating motion was measured to find the wear resistance of the materials (Wei, et al. ([18])). A photographic view of the reciprocating wear tester is shown in Fig. 2. The reciprocatory motion required for the sample holder was obtained through a Scotch yoke mechanism, which was driven by an AC motor through a toothed pulley. The stroke length of the reciprocating motion was 10 mm.

Graph: Fig. 2 Reciprocatory friction and wear monitor.

The Scotch yoke mechanism was attached with a reciprocating arm, which carried the bearing steel samples to be tested, with a specially designed holder. The reciprocatory sliding motion was made to occur against a stationary plate (counterpart) held in a special fixture. The fixture for the counterpart was supported on a plunger and lever arrangement, in order to apply a normal load, ranging from 5 to 150 N, through a counterweight at the other end of the lever. The frequency of the reciprocating arm, which was achieved by a variable-frequency drive, can be varied from 1 to 30 Hz. The plunger and the counterpart assembly were heated in a suitable chamber so that the sliding surfaces could be heated to the required level (max. 300°C) during the test, which was monitored and regulated through a feedback controller. A data acquisition software was used for logging the test data and setting the test parameters.

The material samples were cleaned with acetone to remove any contamination and further cleaned using an ultrasonic cleaner for 10 min. After drying with an electric dryer, the sample was weighed using a semi-micro-balance (10−5 g accuracy). The initial weight of the sample was noted. Then the sample was attached to the reciprocating arm of the wear tester through a specially designed holder so that the cylindrical face made contact with the counterpart. The specimen was then allowed to reciprocate over the hardened 100Cr6 flat plate with different normal loads and reciprocating frequency. The test was conducted for 5 h. Finally, the specimen was taken out of the holder, cleaned and dried, and the weight loss due to wear was estimated using the semi-micro-balance. The weight loss (wear loss) is a measure of the wear resistance; the lower the wear loss, the better the wear resistance.

The samples were first polished using emery paper of grits 80, 120, 200, 600, 800, 1,000, and up to 1 μm, followed by polishing using diamond paste on a rotating linen disc, and finished with polishing on a velvet cloth using white kerosene as a coolant. These samples were etched with 2% nital and dried in air. The etched samples were studied using SEM (Hitachi [5]× to 300,000×). The sizes of the carbides were also measured in the SEM. High-resolution digital micrographs were taken randomly at different regions of the specimens.

Differential scanning calorimetry (DSC) was carried out using a Q10 DSC apparatus (TA Instruments, New Castle, DE, USA) in an argon atmosphere to prevent oxidation. Basically, the DSC technique measures the net heat evolved as a result of exothermic and endothermic reactions occurring in the sample as it is heated under controlled environmental conditions. The DSC curves indicate the phase changes as any transformation is accompanied by endothermic or exothermic reactions reflecting in peaks. Sixty milligrams of the bearing steel samples was heated from room temperature to 900°C. The ASTM E1269 standard test method for determining the specific heat capacity by differential scanning calorimetry was followed to determine the specific heat capacity of the bearing steel materials (ASTM International ([19])). This test method covers the determination of the specific heat capacity by DSC.

A dilatometer is a thermoanalytical instrument used to measure the thermal expansion or contraction of a given specimen for a specific controlled temperature–time program. The thermal expansion and contraction are measured digitally with a high-resolution (1 μm) linear variable differential transformer (LVDT) measuring system. This instrument can also be connected to a computer system, and the samecan be obtained graphically by using the inbuilt software. The dimensional stability of the 100Cr6 bearing steels after CHT and DCT was studied as per ASTM standard E831 (ASTM International ([20])). The specimen to be tested was machined to 45 mm length and 10 mm diameter and subjected to CHT and DCT. The treated specimen was held in an enclosure and contacted by a probe leading to a displacement sensor, with the temperature sensor in contact with the specimen. The specimen was heated at a constant heating rate of 5°C/min from room temperature to 900°C. The expansion of the specimen was measured using an LVDT over the temperature range, and the data obtained were used to compare and describe the dimensional stability of 100Cr6 bearing steels during service.

The results obtained from the Vickers hardness test of the bearing steel showed an equivalent hardness of 700 HV for the CHT samples and 850 HV for the DCT samples. The cryogenically treated samples had an increase in the hardness level compared to conventional heat-treated samples. The hardness of the DCT samples showed an improvement of 18% over the CHT samples. This observation is unlike that of Bensely, et al. ([21]), who indicated no or negligible variation in hardness due to cryotreatment. However, the observed increase in hardness is in agreement with some other observations (Mohan Lal, et al. ([12]); Arockia Jaswin and Mohan Lal ([22])). The increment in hardness may be attributed to the complete transformation of the austenite to martensite.

Cryogenic treatment is more effective in reducing the amount of austenite and can make a larger number of fine secondary carbides precipitate, which can increase the dispersion strengthening effect; both are beneficial for increased hardness. Additionally, as is well known, the decrease in grain size can improve the hardness. The grain size of martensite is smaller than that of austenite; cryogenic treatment can produce more martensite, which implies a fine grain strengthening effect. So, this can also be ascribed to the fact that the hardness of the alloy in cryogenic treatment is higher than that in the non-cryo-treated samples.

Graph: Fig. 3 Wear loss of 100Cr6 at 5 Hz frequency for the two different treatments.

A reciprocatory wear test was conducted on the CHT and DCT samples to study the enhancement in the wear resistance by the weight loss method. The improvement in the wear resistance of cryogenically treated samples compared to conventionally treated samples was evaluated. The improvements in the cryogenically treated samples were studied with respect to varying loads. It was observed from the wear test data that for a maximum load of 50 N and 5 Hz frequency the wear resistance of the 100Cr6 improved by 37% due to DCT compared to CHT. The test results revealed that the DCT samples had a high wear resistance (low wear loss) in all test conditions. In addition, while assessing the wear behavior under the most severe test conditions (i.e., maximum load 50 N and frequency 5 Hz), it was revealed that the CHT and DCT samples experienced wear rates of 2.751 × 10−3 and 1.740 × 10−3 mg/m, respectively. This study confirmed that DCT can be used to improve wear resistance.

Figure 3 presents the wear loss for various loads (10, 20, 30, 40, and 50 N) at a frequency of 5 Hz for the 100Cr6 specimens. From the figure it can be seen that the wear weight loss for the DCT samples was less than that of the CHT specimens at all loads. Figure 4 shows the wear rate at different loads of 10, 20, 30, 40, and 50 N for the treated 100Cr6 specimens. The figure indicates that at any normal load, the wear rates of the CHT samples were greater than those of the cryogenically treated specimens. This trend is in agreement with earlier reports of Mohan Lal, et al. ([12]).

Graph: Fig. 4 Wear rate of 100Cr6 bearing steel at 5 Hz frequency for the two different treatments.

Graph: Fig. 5 Microstructure of 100Cr6 bearing steel for the CHT sample.

Graph: Fig. 6 Microstructure of 100Cr6 bearing steel for the DCT sample.

The wear resistance of the cryogenically treated samples was always better than that of the non-cryogenically treated samples in the whole range of test conditions. To account for the observed improvement of the wear resistance in the cryogenic treatment, three important factors should be considered. First is the transformation of abundant retained austenite into martensite during cryogenic treatment, which can offer stronger support for carbides to inhibit their spalling and prevent large grooves from forming during abrasion. Secondly, the precipitation of finer carbides and their more homogeneous distribution as a result of the cryogenic treatment is responsible for the improved wear resistance. The distribution of the carbides in the cryogenically treated samples is more homogeneous than that in the non-cryogenically treated samples, and the carbide volume fraction in the cryogenically treated samples is also more than that in the non-cryogenically treated samples. Thirdly, cryogenic treatment can result in greater martensite transformation and make the alloy present a refiner matrix,

Graph: Fig. 7 DSC graph for 100Cr6 bearing steel.

and a refiner matrix implies a fine grain strengthening effect, which would contribute to the wear resistance improvement. The presence of numerous ultrafine carbides in cryo-treated steels assists in attaining the micro-stress distribution of the material, in a way that results in favorable crack growth resistance of the material. This, in turn, leads to higher wear resistance.

The microstructural examination was carried out to study the changes that influence the wear resistance; Figs. 5 and 6 depict the representative SEM micrographs of CHT and DCT. The microstructure of the 100Cr6 DCT samples shown in Fig. 6 reveals that most of the austenite retained in the CHT sample was converted into martensite, thereby increasing the hardness, which resulted in an improvement in its wear resistance compared to the CHT sample. In addition to the conversion of the retained austenite into martensite, the microstructure of the DCT samples showed a marked increase in carbide precipitation. These were also responsible for the improvement in the wear resistance and hardness. Finer carbides were precipitated in the martensitic matrix of the cryo-treated specimens. This was also responsible for the improvement in the properties of the cryo-treated 100Cr6 bearing steel.

Furthermore, after cryogenic treatment, there was less retained austenite in the CHT samples because the retained austenite is more unstable at lower temperatures and likely to transform into martensite. It can be seen from the micrograph of the DCT sample in Figure 6 that a large amount of fine carbides of micrometer size ware precipitated throughout the structure. The fine carbides precipitated through the cryogenic treatment tie up with certain elements and restrict the promotion of instabilityduring service. It is noted that holding martensite at a lower temperature increases its lattice distortion and thermodynamic instability, both of which drive carbon and alloying atoms to segregate at the nearby crystal defects. These segregated regions act as sites for nucleation of fine carbides, which explains the increase in the population density of fine carbides in DCT specimens compared to CHT specimens.

Graph: Fig. 8 Linear expansion coefficient of the 100Cr6 bearing steels for the CHT and DCT specimens.

Graph: Fig. 9 SEM image of the worn surface: (a) CHT and (b) DCT specimens.

Figure 7 shows the DSC graph of 100Cr6 bearing steel for the temperature range 30–900°C. It displays three partially overlapped peaks. Two of them (I and II) are quite small, whereas the third one (III) is well pronounced. The peak at the highest temperature can be attributed to the alloy carbide precipitation from martensite, which is responsible for secondary hardening.

The coefficients of thermal expansion (CTE) estimated for the 100Cr6 bearing steel subjected to the CHT and DCT are shown in Fig. 8. The CTE was low for the 100Cr6 DCT bearing steel specimens, which indicates that the dimensional stability was high. The CTE increased for the CHT and DCT samples until 450°C and then remained fairly constant for the CHT samples, whereas it decreased for the DCT samples due to precipitation of the transition carbides. Again, a reduction in the CTE was noted at around 750°C for the DCT sample, which was attributed to precipitation of the alloy carbides and grain coarsening.

The 100Cr6 bearing steel samples of CHT and DCT specimens subjected to identical wear tests were examined under SEM. The wear characteristics of CHT and DCT specimens were carried out along with an examination of worn surfaces. The estimated values of wear loss for the specimens tested under varying normal loads with a frequency of 5 Hz at room temperature were discussed earlier. The improvement in wear loss was significant by DCT but less significant by CHT. Figures 9a and 9b show representative SEM micrographs of worn surfaces of CHT and DCT specimens tested at 30 N and 5 Hz at room temperature. The worn surface of CHT specimens was rough but metallic in nature and also exhibited fracture ridges and deformation lips stretched parallel to the sliding direction along with occasional subsurface cracking (Fig. 9a). The presence of a deformation lip implies that the CHT specimen underwent heavy plastic deformation during the wear test. In contrast, the worn surface of the DCT (Fig. 9b) specimen was smoother and exhibited less presenceof a deformation lip. The microscope of CHT depleted region of worn surfaces shows cracking and pull-out of carbides in addition to the presence of surface grooves, as illustrated in Fig. 9a. These observations revealed that the wear mechanism of the CHT specimen was plastic deformation, which causes a deformation lip and subsurface cracking, whereas that for the cryogenic-treated specimen was predominantly low deformation lip and subsurface cracking. The wear losses of the DCT specimen at all applied loads were lower than those of the CHT specimen.

The reciprocatory wear test showed that the wear resistance of bearing steel was improved by cryogenic treatment compared to conventional heat treatment.

• • The hardness of the DCT samples showed an improvement of 18% compared to the CHT samples.
• • The wear resistance of the 100Cr6 was improved by 37% due to DCT compared to CHT.
• • Cryogenic treatment produce more martensite transformation and facilitated carbide formation, which made the alloy present a refiner matrix and produced a fine grain strengthening effect, thereby increasing the wear resistance.
• • The CTE was low for the 100Cr6 DCT bearing steel specimens, which indicates that the dimensional stability is high.

This study confirmed that cryogenic treatment can be used to improve the wear resistance of bearing steel.

Review led by Richard Neu