A research review on deep cryogenic treatment of steels

A research review on deep cryogenic treatment of steels
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     Int. J. Materials and Structural Integrity, Vol. 8, Nos. 1/2/3, 2014 169  Copyright © 2014 Inderscience Enterprises Ltd. A research review on deep cryogenic treatment of steels D. Senthilkumar Department of Mechanical Engineering, P.A. College of Engineering and Technology, Pollachi, 642 002, Tamil Nadu, India Fax: 04259-221386 E-mail: E-mail: I. Rajendran* Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi, 642 003, Tamil Nadu, India E-mail: *Corresponding author Abstract:  The effect of deep cryogenic treatment on properties of different types of steels was extensively studied. Cryogenic treatment can convert the retained austenite into martensite along with the carbide precipitation and hence enhances the wear resistance when compared to the conventional heat treatment of steels. It also increases the compressive residual stresses in the component, which leads to better fatigue life. However, this study is carried out to reveal the mechanism of deep cryogenic treatment with respect to different materials,  pretreatment conditions and properties of steels for the practical application. Keywords:  conventional heat treatment; deep cryogenic treatment; DCT;  properties; steels. Reference  to this paper should be made as follows: Senthilkumar, D. and Rajendran, I. (2014) ‘A research review on deep cryogenic treatment of steels’,  Int. J. Materials and Structural Integrity , Vol. 8, Nos. 1/2/3, pp.169–184. Biographical notes:  D. Senthilkumar is currently working as Professor in the Department of Mechanical Engineering, P.A. College of Engineering and Technology, Pollachi, Tamilnadu, India. He obtained his Bachelor degree in Mechanical Engineering from Bharathidasan University, India in 1999. He obtained his Masters in 2005 and PhD in 2012 from Anna University, Chennai, India. He has 13 years of teaching experience. He has published seven papers in international journals, two papers in international magazines and three  papers in international conferences. His research interest is on influence of cryogenic treatment on steels and heat transfer. He is very much interested to  build research collaborations with the international scientists and researchers for further innovations. He is a life member of Indian Society for Technical Education.   170  D. Senthilkumar and I. Rajendran I. Rajendran is Professor and Head of the Department of Mechanical Engineering in Dr. Mahalingam College of Engineering and Technology, Pollachi, India. He obtained his BE in Mechanical Engineering and ME in Engineering Design from the College of Engineering, Guindy, Anna University, Chennai, India, in 1991 and 1994, respectively, and his PhD in Composite Leaf Spring from PSG College of Technology, Coimbatore, India, in 2001. His research interests include composite materials, design optimisation, finite element analysis, machining and vibration analysis. He has  published 61 papers in national and international journals and conferences. He organised international and national conferences in the area of materials and mechanical engineering. 1 Introduction Cryogenic treatments of steels are quite often studied and they plays an important role in developing the tribological properties of steels. One of the most common claims in cryogenic treatment is an increase in wear resistance of steels as investigated by Barron (1973). Supplementing cryogenic treatment to conventional heat treatment process will aid the manufacturers to attain better wear resistance of steel components (Senthilkumar and Rajendran, 2011). Mohan Lal et al. (2001) pointed out that the cryogenic treatment is an affordable one-time permanent treatment process that affects the entire cross section of the steel component. Collins and Dormer (1997) found out that the cryogenic treatment  promotes the transformation of the retained austenite into martensite at cryogenic temperatures and facilitates the formation of fine carbides in the martensite, thus improving the wear resistance. It has many benefits and it gives dimensional stability to the material by improving wear resistance, strength and hardness of the materials as stated by Molinari et al. (2001). However, scientific research on cryogenic treatment has  been very limited and only a few research papers have been published on medium carbon steels as indicated by Zhirafar et al. (2007). The capability to attain high strength and toughness by heat treatment is a main advantage of steel components. In conventional heat treatment of steels, the phase transformations in steel are of great technological importance. The heat treatment of steels to achieve the optimum properties depends on the mechanism and kinetics of phase transformation. The problem of retained austenite after conventional heat treatment is still prevailing (Senthilkumar and Rajendran, 2008). In conventional quench-hardening  process, the steels were heated and soaked for certain period of time and suddenly quenched in oil at room temperature. This rapid cooling from the hardening temperature causes the transformation of austenite. This austenite is a soft allotropic form of iron that on cooling, transforms to other phases in which martensite is the desirable harder phase. This transformation is diffusion-less and time independent and also there is no change in chemical composition. It also suppresses the conversion of austenite into ferrite and cementite. The hardening of steel depends entirely on the formation of martensite. During martenistic transformation, the shear process converts the face centred cubic (FCC) structure of austenite to the body centred tetragonal (BCT) structure of martensite (Raghavan, 1999). If martensite is not formed, other phases may be formed and even austenite itself may be retained. This retained austenite is unstable at lower temperatures     A research review on deep cryogenic treatment of steels 171   and is likely to transform into martensite under service. This undesirable amount of austenite in the hardened steel has a detrimental effect on its mechanical properties. It reduces hardness and dimensional stability due to austenite transformation during use and storage. Hence, the structure of hardened steel consists mainly of tetragonal martensite and some amount of retained austenite that depends on chemical composition of steel. 2 Cryogenic treatment In this literature review, the aim, scope, main arguments, prominent theories, practical application and the knowledge gaps of the cryogenic treatment pertaining to the various steels are discussed in relation to the wear behaviour and hardness, transformation of retained austenite, precipitation of carbides, tensile and fatigue behaviour, residual stress, toughness and optimisation of cryogenic treatment. Kalia (2010) pointed out that cryogenics is an exciting, important and inexpensive method that has already led to main discoveries and holds much future assurance. It improves wear abrasion, erosion and corrosion resistivity, durability and stabilises strength characteristics of various materials. Cryogenic processing is the treatment of the materials at very low temperature around –196°C. This technique has been proved to be efficient in improving the physical and mechanical properties of materials such as metals, alloys, plastics and composites. During the last decade, cryogenic treatment techniques have been developed and are now broadly used by industry to improve the mechanical  properties of steel components. Charles and Arunachalam (2006) pointed out that the cryogenic treatment of materials are gaining importance in recent days because of their potential to produce steel components that find enormous application in industries, nuclear power plants, fertiliser  plants, medical, aerospace and avionics. This is due to the fact that materials treated under cryogenic environments attain superior properties that call for operation under severe environments. It is a one time homogenous process that provides significant extension in the performance and productive life of steel components namely brake rotors, gears, engines, machine parts, transmission to machine tools and gun barrels. The benefits of deep cryogenic treatment (DCT) of steels are increased strength, greater dimensional stability or micro structural stability, improved wear resistance, hardness, toughness, fatigue life and compressive residual stress. The improvement of the mechanical properties depends on the chemical composition of steels and the prior hardening cycles. Darwin et al. (2007) studied the key parameters involved in DCT as: cooling rate, soaking temperature, soaking time, heating rate, tempering temperature and tempering  period. The cooling rate is the rate at which the sample is cooled to the soaking temperature. Soaking temperature is the temperature at which the sample is held, while the soaking period is the time for which the sample is held at the soaking temperature. The heating rate is defined as the rate at which the sample is heated back to room temperature. Their results show that the significance of the parameters for improving mechanical properties of steel prevails in the following order of importance: 1 soaking temperature (72%) 2 soaking period (24%)   172  D. Senthilkumar and I. Rajendran 3 rate of cooling (10%) 4 tempering temperature (2%); and the parameter called tempering period is insignificant. Research shows that the deep cryogenic cycle should start with a slow cooling, continue with fairly long soak (24 to 72 hours or more hours at temperature), and finally end with a slow warming to room temperature (Barron, 1974a; Collins and Dormer, 1997). In DCT, after conventional hardening process the machined samples were slowly cooled from room temperature to 77 K at 1.24 K/minute, which is soaked at 77 K for 24 hours and finally heated back to room temperature at 0.62 K/minute as pointed out by Bensley et al. (2011). The typical cycle of DCT is shown in Figure 1. These very low temperatures are achieved using computer controls in a well-insulated treatment chamber with liquid nitrogen LN2 as working fluid. Since 1965, when commercial DCTs first  became available, a number of reports have been published showing the improved  performance of some tools steels treated in this way. This treatment enhances the desired metallurgical and structural properties by completing the transformation of austenite (a softer metal phase) to martensite (a tougher, more durable metal phase). Then the samples are finally subjected to tempering. By shallow cryogenic treatment (SCT), the samples are directly kept in a freezer at 193 K for five hours. Then the samples are subjected to tempering process. Figure 1  Typical DCT cycle (see online version for colours) 0501001502002503003501 3 5 7 9 11 13 15 17 19 21 23 25 Time, Hours      T    e    m    p    e    r    a     t    u    r    e ,     K 1.24 K/minute 0.62 K/minute Soak    Jaswin et al. (2010) investigated the wear resistance improvement in En 52 and 21-4N valve steels through shallow and DCT using a reciprocatory friction and wear monitor as per the ASTM standard G-133. It has been observed that the wear resistance of En 52 and 21-4N has improved by 81.15% and 13.49% respectively, due to SCT, 86.54% and 22.08% respectively, due to DCT, when compared to the conventional heat treatment. The microstructural study suggests that the improvement in wear resistance and hardness is attributed to the conversion of retained austenite into martensite, along with precipitation and distribution of the carbides brought in by the cryogenic treatment.     A research review on deep cryogenic treatment of steels 173   Barron (1974b) and Harish et al. (2009) found that DCT of SAE 52100 bearing steel enhances wear resistance. Dong et al. (1998) studied the effect of DCT with respect to the microstructure of T1 high speed steels. It was proved that DCT can enhance wear resistance by the precipitation of nano-sized eta-carbides in the primary martensite. Koneshlou et al. (2011) studied the effects of low temperature (subzero) treatments on microstructure and mechanical properties of H13 hot work tool steel. Cryogenic treatment at –72°C and DCT at –196°C were applied and it was found that by applying the subzero treatments, the retained austenite was transformed to martensite. As the temperature was decreased more retained austenite was transformed to martensite and it also led to smaller and more uniform martensite laths distributed in the microstructure. The cryogenic treatment at a very low temperature and long sample holding times also appear to result in the precipitation of more uniform and very fine carbide particles. However, the most important effect of tempering the deep cryogenically treated samples was the improving of the wear properties of the H13 tool steel. Stratton (2007) put forward the following processing route for the cryogenic treatment of steels to achieve maximum wear resistance. 1 heat to austenitising temperature that will reduce retained austenite for the steel  being treated 2 hold for the recommended time for the steel 3 quench at a rate sufficient to give fully martensitic structure 4 condition at 60°C for a maximum of one hour and immediately go to step 5 5 cool to liquid nitrogen temperature (–196°C) at a rate slow enough to prevent cracking, preferably in a nitrogen atmosphere to avoid condensation 6 hold at liquid nitrogen temperature for a minimum of 24 hours, preferably in a nitrogen atmosphere to avoid condensation 7 reheat to room temperature at a rate slow enough to prevent cracking, preferably in nitrogen atmosphere to avoid condensation 8 temper at the temperature recommended for the steel being treated. 3 Mechanism of cryogenic treatment Most researchers believe that the cryogenic treatment promotes the complete transformation of retained austenite into martensite at cryogenic temperatures, which is attributed to improve the wear resistance. This cryogenic treatment is an extension of quench cycle, which continues the process of martensite formation. This treatment modifies the material microstructure and develops the strength and wear properties. 3.1 Transformation of retained austenite Vanvlack (1998) explained that the percentage of tempered martensite increases as the initial quenching temperature drops toward –73°C when for all practical purposes it reaches 100%. It is also concluded that the transformation of austenite to martensite is dependent only on temperature and the time required for transformation is so short as to
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