Effect of Austempering andMartempering on the Properties of AISI 52100 Steel

e mechanical properties of steel decide its applicability for a particular condition. Heat treatment processes are commonly used to enhance the required properties of steel. e present work aims at experimentally investigating the effect of austempering and martempering on AISI 52100 steel. Different tests like microstructure analysis, hardness test, impact test, and wear test are carried out aer heat treatment process. It was found that annealed steel was least hard and more wear prone, while martempered steel was hardest and least vulnerable to wear. Austempered steel had the highest impact strength and it is increased with soaking time up to certain level. Least wear rate is observed in martempered sample both in abrasion and dry sliding. However, least friction coefficient is shown by annealed samples.


Introduction
e knowledge of materials and their properties is of great signi�cance for a production engineer. e machine elements should be made of a material that has properties suitable to the operating conditions. For instance, AISI 52100 is a high carbon alloy steel which achieves a high degree of hardness with compressive strength and abrasion resistance used in ball and roller bearings, spinning tools, punches, and dies. In such cases, to achieve the required properties heat treatment methods are commonly used.
Martempering is a common heat treatment process that quenches the material to an intermediate temperature just above the martensite start temperature ( ) and then cools air through the martensitic transformation range to room temperature [1][2][3][4]. It is important to air-cool throughout the transformation range since rapid cooling through this range is required to produce residual stress patterns similar to those produced by a direct quench and negate any advantages of the process [5]. Modi�ed martempering (MM) is a similar technique wherein the intermediate quench temperature is below but above the martensite �nish temperature ( ) [3,4]. Tempering of martempered or modi�ed martempered steels to the desired hardness and tensile strength is performed identically to that in quench and temper operations with better impact resistance. Commonly martempered steels include AISI 1090, 4140, 4340, 6050, and SAE 52100 [3].
Austempering is a method of hardening steel by quenching from the austenitizing temperature into a heat extracting medium (usually molten salt) which is maintained at speci-�ed temperature level between 200 ∘ C and 400 ∘ C and holding the steel in this medium until austenite is transformed to bainite. is method is used to increase strength, toughness, and to reduce distortion. e two processes are heating a medium-to-high carbon ferrous metal to an austenitic condition then cooling the object rapidly enough to avoid the formation of pearlite to a temperature above temperature and isothermally holding the part for a time sufficient to produce the desired microstructure. But these two processes are generally limited to small components.
Due to their high applicability, these processes are explored by many researchers. e amount of retained austenite in Cr-Mo steels used in mill liner was studied by Shaeri et al. [6]. e effects of heat treatments including direct quenching, martempering, and austempering on the retained austenite existing in the microstructure of these steels were investigated. Specimens were austenitized at 950 ∘ C followed by direct quenching using compressed and still air. e specimens were also isothermally quenched in salt bath at 200 ∘ C and 300 ∘ C for 2, 8, 30, and 120 min. e existence of the retained austenite in the microstructure of this steel led to some drawbacks. Wear resistance of the material was reduced as a result of the presence of phase with low hardness and strength. Unfavorable dimensional variations appear in the specimens resulting from the transformation of austenite to martensite during tempering or upon severe impacts applied to the liners during milling process. Transformation of austenite to martensite during tempering gives rise to a volume change in austenite resulting in the formation of a severe compressive stress at the austenite-martensite boundary. Such a defect forms a suitable place for crack nucleation and therefore reduces durability of the specimen. e results showed that the lowest amount of retained austenite in the microstructure was obtained in the specimens quenched isothermally at 300 ∘ C for 120 min.
e effect of austempering treatment on microstructure and mechanical properties of high-Si steel was studied by Mandal et al. [7]. In this investigation, the in�uence of austempering treatment on the microstructure and mechanical properties of silicon alloyed cast steel has been evaluated. e experimental results showed that an ausferrite structure consisting of bainitic ferrite and retained austenite can be obtained by austempering the silicon alloyed cast steel at different austempering temperature. TEM (Transmission Electron Microscope) observation and �-ray analysis con�rmed the presence of retained austenite in the microstructure aer austempering at 400 ∘ C. e austempered steel has higher strength and ductility compared to as-cast steel. With increasing austempering temperature, the hardness and strength decreased but the percentage of elongation increased. A good combination of strength and ductility was obtained at an austempering temperature of 400 ∘ C.
MacIejewski and Regulski [8] studied the fracture assessment of martempered and quenched and tempered AISI 4140 low alloy steel. e reported advantages of martempering include less distortion, elimination of quench cracking, improved fatigue resistance, and improved absorbed impact energy. Data regarding improved impact energy are sparse and appear to be most widely reported for the high-carbon steels. e results of impact energy and tensile strength that are compared between quenched and tempered to that of modi�ed martempered had no much difference, and the analyst must check for the martempering process.
Wear resistance properties of Austempered Ductile Iron (ADI) were studied by Lerner and Kingsbury [9]. A detailed review of wear resistance properties of ADI was undertaken to examine the potential applications of this material for wear parts, as an alternative to steels, alloyed and white irons, bronzes, and other competitive materials. Two modes of wear were studied: adhesive (frictional) dry sliding and abrasive wear. In the rotating dry sliding tests, wear behavior of the base material (a stationary block) was considered in relationship to counter surface (steel sha) wear. In this Jetley [10] reported improvement in wear properties of aircra brake steel rotors by martempering. Martempering process using oil-and water-based quenchants at lower temperature is adopted in this work. e test samples were evaluated for hardness, distortion, and wear under accelerated simulated tests. e results show that although both hardness and wear resistances were lower compared to the austempering, they met the design intent. Also the wear rate of martempered samples was more consistent which may provide advantages for maintenance purposes.
Wear of hard-turned AISI 52100 steel was studied by Bartha et al. [11]. High precision machining such as hard turning changes the surface and the material properties of steel alloys. A sliding block-on-cylinder wear tester was used for the purpose of testing the wear performance of AISI 52100-bearing steel. e effect of microstructure on the wear performance of hard-turned steel showed that the white layer and overtempered martensite (OTM) had a higher wear resistance than martensite. e wear mechanism dependence on the surface hardness was attributed to this increase in wear performance. e near-surface residual stress of the material was shown to become more compressive as the material wore down. e applied normal loads affected the surface roughness, residual stresses, and, in turn, the wear performance of the material.

Experimentation
e chemical composition of the investigated steel is determined by optical emission spectrometer and shown in Table  1. e dimensions of the as-cast specimens were 75 mm × 25 mm × 12 mm for abrasive wear test, 75 mm × 10 mm × 8 mm for impact test, and 6 mm diameter pins of 30 mm length for pin on disc test, respectively. All specimens were annealed at 950 ∘ C for 1 hr to homogenize the as-cast microstructure of the specimens. Same set of specimens was, initially, austenitized at 950 ∘ C for 1 hr and then, held in salt bath (mixture of sodium nitrate-30%, potassium nitrate-40% and calcium nitrate-30%) at 160 ∘ C for 15 min, aer that quenched in water in martempering process. In austempering, specimens are austenitized at 950 ∘ C for 1 hr and held in salt bath (mixture of sodium nitrate-50% and potassium nitrate-50%) at 350 ∘ C for 10 min, 20 min, and 30 min and then cooled to room temperature in still air. For microstructural analysis, specimens have been prepared based on the standard ASTM E3. To etch the specimens, the sodium metabisul�te solution (15%) has been used in accordance with the ASTM E407. e optical micrographs were taken according to the standard ASTM E883 using radical microscope (RMM-77) with ×100 magni�cation. All the specimens are tested for impact strength in Izod impact test of standard ASTM D256. Hardness tests were performed and BHN is calculated as per ASTM E10. Abrasive wear of the specimens is measured using DUCOM abrasion tribometer according to ASTM G65 with application of 5 kg load at 200 rev/min. Dry sliding test is conducted on pin disc apparatus against EN32 steel disc having hardness of 470Hv to measure coefficient of friction based on standard ASTM G99.

Results and Discussions
3.1. Microstructure. e images were captured in a metallurgical microscope from prepared samples to study the microstructure changes. Figure 1(a) shows the microstructure of annealed specimen, which consists of small black dots with good distribution. ese black dots are interpreted as carbide present in the structure. e structure of the martempered sample (Figure 1(b)) is completely covered with carbides and has a very rich density of these all over the surface. Austempered specimen with 10 min soaking time shows good density of carbides (Figure 1(c)) but is lesser than the martempered sample. Compared to 10 min soaking time, specimen with 20 min soaking time has less carbide ( Figure  1(d)). Figure 1(e) shows the microstructure of austempered specimen with 30 min soaking time. e sample has good number of carbides but occurs only in some areas. e above samples have different microstructures and their hardness varies with amount of carbides.
e samples which were annealed have fewer carbides with massive pearlite compared to remaining samples and as such it is least hard as the carbides are retained in solution. Martempered sample has a microstructure rich with carbide or martensite which is the hardest steel structure. In austempered sample, the structure is �lled with the carbide but lesser than martempered sample, the reason being that it is soaked in salt bath maintained at 340 ∘ C for 10 min and then quenched it in water. is allows conversion of austenite into bainite only for 10 min with the remaining converting into martensite. Similarly, the sample which was soaked for 20 min has had bainite conversion for only 20 min, aer quenching the remaining converts in to martensite. Also as the austempered sample with soaking time 30 min gets converted to bainite structure from austenite till 30 min and aer quenching the remaining converts in to martensite. is is evident from the structures observed which shows the density of carbide.

3.2.
Hardness. e Brinell's hardness number of different heat treated samples showed (Figure 2) that the martempered sample was the hardest among the samples followed by the austempered samples with soaking time 10, 20, and 30 min, respectively, and annealed, which was least hard of all. As expected with reference from the microstructure test, the martempered sample is the hardest because of conversion of austenite into martensite structure. Austempered sample, which has been soaked in salt bath maintained at 340 ∘ C for 10 min, has conversion of austenite into bainite only for 10 min, with the remaining converting into martensite aer quenching in water. As the soaking time increases the conversion time and conversion of austenite into bainite increase and the conversion of martensite decreases as such the hardness decreases. e annealed sample exhibits the least hardness among the tested samples for the hardness.

Impact Strength.
Impact strength of all the specimens obtained from Izod impact test is shown in Figure 3. e moderate impact strength was observed for annealed sample. Martempered sample shows least impact strength due to formation of martensite. Austempered samples impact strength was improved because of the presence of bainite and it is observed that impact strength was improved with soaking time in austempering.

Abrasion Wear.
Aer the experiments were conducted in the prescribed procedure, the weight loss for every reading was noted till a steady or nearer to steady state arrived. e wear rate is given by weight loss for one min. Variation of weight loss for each minute with constant speed of rotation is measured and average weight loss is calculated. From weight loss, wear rate for each minute and average wear rate are calculated and presented in Figure 4. e result clearly indicates that the martempered sample has the least wear rate. e austempered sample with soaking time of 10 min has the next least wear rate followed by 20 min soaked and 30 min soaked. Annealed sample has the more wear rate compared to other samples. is shows that the martempered sample is having good wear resistance followed by the austempered samples. is also indirectly indicates the hardness acquired by the sample in the heat treatment process.
3.5. Dry Sliding Wear. e weight loss of heat treated samples with respect to time in dry sliding test is measured. e average wear rate of heat treated samples with respect to time in dry sliding test is presented in Figure 5. e bar graph clearly indicates that the most effected pin is annealed when compared to all the pins and the least effected is martempered. e annealed pin had a burr formation at the end which was kept on the tungsten disc. is indicates that a lot of heat was formed at the end which deformed the portion of that end plastically. Also this suggests that the material was more ductile than that of the remaining samples.
e pin on disc experiment was done till steady friction value was obtained. Aer every reading, the friction value for each sample was measured and also the average friction coefficient value (Figure 6) was calculated. It is observed that average friction value is less for annealed one and increased in martempered and austempered samples.

Conclusion
AISI 52100 steel was subjected to various heat treatments for enhancing the material properties. From the present study the following conclusions are drawn. (i) Annealed samples have less carbide in micro structure and are least hard, whereas martempered samples have dense carbide indicating highest hardness.
Hardness increased three times with martempering process.
(ii) Austempered samples have highest impact strength, the least being martempered samples. e impact strength increased with soaking time in austempered samples up to certain level. 20% improvement is observed with austempering process.
(iii) Annealed samples have the highest wear, while martempered samples have the least wear. Approximately 50-60% wear resistance is increased with martempering process.
(iv) Friction coefficient is increased with both the heat treatment processes.
Based on the functional requirement, the choice can be made among the heat-treated AISI 52100 steels.