RESEARCH ARTICLE Effect of Cryogenic Treatment on the Mechanical Behavior of En8D Alloy Steel

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1 International Journal of Advances in Engineering, 2015, 1(3), ISSN: (printed version); ISSN: (online version); url: RESEARCH ARTICLE Effect of Cryogenic Treatment on the Mechanical Behavior of En8D Alloy Steel M. Arockia Jaswin and M.D. Antony Arul Prakash Department of Mechanical Engineering, Aksheyaa College of Engineering, Chennai , India Received 25 February 2015 / Accepted 21 March 2015 Abstract - This study investigates the effect of cryogenic treatment on the mechanical behaviour of En8D alloy steel. The material subjected to cryogenic treatment at 85K and the tensile strength, wear resistance & hardness of the material is compared with that of conventional heat treatment (CHT). Cryogenic treatment is the process of cooling a material to very low temperatures to enhance the mechanical and physical properties. The study also aims to reveal the mechanism behind the enhancement in material properties. The test result shows an enhancement in the cryogenically treated En8D material when compared with the CHT samples. The ultimate tensile strength, yield strength, hardness and wear resistance of cryo-treated En8D shows an improvement of 7.5%, 11%, 19% and 39% over the CHT samples respectively. The microstructure of cryo-treated samples showed the alloy carbides are evenly precipitated and distributed. Scanning Electron Microscopy analysis showed that the bonding in cryo-treated specimen improved drastically. The grains are unevenly distributed in CHT samples compared to more even distribution in DCT samples. The cryo-treated samples showed reduction in percentage of elongation when compared to CHT samples. It may be concluded that the precipitation of secondary carbide is the reason behind the improvement in mechanical properties of En8D when it is cryo-treated. 1. INTRODUCTION There are many of the developed processes apply treatments in a range of temperature higher than room temperature. But, lately focus of researchers shifted towards the concept of sub-zero treatments (at liquid nitrogen temperature). Cryogenic treatment also known as cold or sub-zero treatment is a very old process and is widely used for high precision parts. In general, unlike surface treatments, the cryogenic treatments influence the core properties of the materials. Cryo-treatment (CT) is a supplementary process to conventional heat treatment, that involves deep freezing of materials at cryogenic temperatures (-190 C) to enhance the mechanical and physical properties. The basic cryogenic treatment consists of a gradual cooling of the component till the defined temperature is obtained, then holding it for a particular time (soaking time) and then gradually leading it back to room temperature, and then low temperature tempering is done to decrease the brittleness of the martensite. Fig.1 shows the Deep Cryogenic Treatment process. Barron, (1982) [1] showed that Cryogenic treatment has been successfully applied to die and high speed steel (HSS) ferrous alloys. The cryogenic process enhances the conversion from austenitic phase to martensite phase, the cryogenic treatment increases hardness and wear resistance of ferrous alloys. Stewart, (2004) [2] applied cryogenic treatment to tungsten carbide and compared with untreated carbide, the results indicated that tool wear was reduced with cryogenic treatment. Gallagher et al., (2005) [3] studied the effect of cryogenic treatments on tungsten carbide tool life. Cryogenically treating tungsten carbide tooling has the capability of extending tooling life. As the number and size of the Eta phase increases, the tool life is reduced and vice-versa. The treatments also cause the gamma phase to become more evenly dispersed through reductions in vein and pocket size. The ability of the gamma phase to inhibit the alpha grain growth influences the tool hardness during cutting, thereby affecting the tool life. S. Zhirafar et al., (2007) [4] studied the effect of cryogenic treatment on the mechanical properties of 4340 steel. This study investigated the effect of cryo-treatment on mechanical and micro structural behaviour of AISI It was shown that in general, hardness and fatigue strength of the cryogenically treated specimens were a little higher whereas the toughness of the cryogenically treated specimens was lower when compared to that of the conventionally treated steel. Neutron diffraction showed that the transformation of retained austenite to martensite occurred which resulted in carbide formation during tempering, is a crucial factor in enhancing hardness and fatigue resistance of the cryogenically treated specimens. M.Arockia Jaswin, (2011) [5] studied the effect of Cryogenic Treatment on the Microstructure and Wear Resistance of X45Cr9Si3 and X53Cr22Mn9Ni4N Valve Steels. The materials are subjected to shallow(193 K) and deep cryogenic treatment (85 K), and the microstructure and wear resistance are compared with those of conventional heat treatment. The results showed significant improvement in wear resistance. It was concluded that the formation of fine carbides dispersed in the tempered martensite structure was the main reason for the enhancement of wear resistance along with the retained austenite transformation. ZHU Yuan-zhi, (2008) [6] studied the effects of cryogenic treatment on mechanical properties and microstructure of Fe-Cr-Mo-Ni-C-Co alloy. The alloy was quenched in liquid nitrogen and held for 24 h. The results show that the hardness increases by 1 2 (HRC) and the compressive strength decreases slightly after cryogenic treatment. The increase in hardness is attributed to the transformation from austenite to martensite and the precipitation of the very tiny carbide. Harish et al. [7] studied the effect of shallow cryogenic treatment (SCT) at 193 K and deep cryogenic treatment (DCT) at 77 K on the microstructure of En 31 bearing steel. This study concluded that tempering should be performed after cryogenic treatment to increase secondary carbide

2 223 Int. J. Adv. Eng., 2015, 1(3), precipitation, which results in improvement of hardness and wear resistance. The material En8D is highly applied as gear material, which are highly subjected to wear. From the literature it can be inferred that the cryogenic treatment has the potential to enhance various mechanical and physical properties of the material. The aim of this study is to investigate the effect of cryogenic treatment on the mechanical behaviour of En8D alloy steel and conducted various mechanical testings. The paper aims to reveal the difference in the properties of the gear material, when it is subjected to Deep Cryogenic Treatment (DCT) and Conventional heat treatment (CHT). Figure.1 Deep Cryogenic treatment process II.EXPERIMENTAL INVESTIGATION The specimens required for the study are machined as per the ASTM standards. The machined specimens are grouped into two and are subjected to two different treatment processes as explained below. The process of Conventional Heat Treatment (CHT) and Deep Cryogenic Treatment (DCT) are shown in fig 2. Figure.2 CHT and DCT process Conventional heat treatment: The conventional heat treatment for the En 8D Alloy steel is conducted based on the alloy heat treatment.the following heat treatment is given to one group of the En 8D specimens. Hardening (austenitizing) at 1123 K (850 º C) for two and half hour, is followed by oil quenching and tempering at 723 K (450 ºC) for one hour. The conventional heat treatment (CHT) for the En 8D Alloy steels are shown in Fig. 3. Figure.3 Conventional Heat Treatment Cycle

3 224 Int. J. Adv. Eng., 2015, 1(3), Deep cryogenic treatment: The present study investigated the En 8D Alloy steel material which has undergone the conventional hardening and the En 8D Alloy steel are slowly cooled from room temperature to 85 K (-196ºc), soaked at 85 K for 24 h, and finally heated back to room temperature. This processor used here is A.C.I. CP-200vi (Massachusetts, USA) cryogenic treatment processor. The processor is a well-insulated chamber with liquid nitrogen as the working fluid. The cryogenic processor consists of a treatment chamber, which is connected to a liquid nitrogen tank (MVE DURA-CYL 160MP) through a vacuum insulated hose pipe. The temperature sensors inside the chamber sense the temperature and accordingly the proportional integral derivative (PID) temperature controller operates the solenoid valve to regulate the flow of liquid nitrogen. The liquid nitrogen passes through the spiral tubes of heat exchanger and enters the duct leading to the bottom of the chamber as nitrogen gas (evaporated state). The blower at the top of the chamber sucks the gas coming out at the bottom and makes it to circulate inside the chamber and reduces the chamber temperature drastically. Through the data acquisition system, the deep cryogenic treatment processes are recorded and stored. The En 8D samples taken out from the processor are tempered at 473 K (200ºC) for an hour. The process of Deep Cryogenic treatment is as shown in figure.4below Figure.4 Deep Cryogenic Treatment Cycle Tensile test : The tensile test is to determine the ultimate tensile strength, yield strength, percentage elongation, reduction in area, etc. As per the ASTM standard the tensile test is carried out for the round bar test specimens. The nominal diameter of a specimen is 25 mm and the gauge length is 62.5 mm; as shown in Figure.5. Figure.5 Tensile test specimen The tensile specimens are grouped into two batches, namely, CHT and DCT. Total of 3 specimens in each batches (i.e. CHT and DCT) were tested. The specimen is gripped in the tensile testing machine (i.e. universal testing machine). After clamping the specimen on the holder the machine is switched on. The load is applied on the specimen along the uniaxial direction. The specimen is continually subjected to increasing tensile force till it breaks down. As machine is connected with computerized device, a graph between stress & strain is drawn and it shows the ultimate tensile and yield stress values. The percentage elongation of the specimen after fracture is obtained by putting the specimen back together on the elongation gauge. Hardness Test: The hardness test carried out for conventionally treated material and for the deep cryogenic treatment was carried out on Vickers hardness testing machine as per the ASTM standards. The testing machine consists of indenting the test material with a diamond indenter, which is in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces. Totally 6 specimen were taken for the test (3 specimens were tested conventionally and other 3 specimens were tested cryogenically). A load of 10 kg was applied on each specimen for 10 to 15 sec (dwell time). The results were compared and calculated. It was noted that the hardness of the cryogenically treated specimen was much improved when compared to the conventionally treated specimen.

4 225 Int. J. Adv. Eng., 2015, 1(3), Scanning Electron Microscopy (SEM): SEM is used to examine surface features, textures and particles that are too small to see with standard optical microscopes. The fully automatic low vacuum system allows observation of specimens which cannot be viewed at high vacuum due to excessive water content or due to a non-conductive surface. Its asynchronous five-axis stage rotation and tilt can accommodate a specimen of up to 8-inches in diameter. Standard automated features include auto focus, auto gun and automatic contrast and brightness, which provide fast and unattended data acquisition. III. RESULTS and DISCUSSION Tensile test : The results of the tensile test for the material En8D is shown in table 1&2 below for CHT and DCT specimen respectively. The tensile strength of En8D for CHT specimen has an average value of MPa compared to MPa of DCT specimen. When we compare the results, we can see significant improvement in tensile strength of DCT specimen over CHT specimen. The tensile strength of En8D alloy steel improved by 7.48% when it is cryogenically. The yield strength of En8D alloy steel is shown in table 1&2 for CHT and DCT specimen respectively. The yield strength of En8D for CHT specimen has an average value of MPa whereas yield strength of DCT specimen is MPa. The yield strength has improved by 11.24% when it is cryo-treated. The transformation from retained austenite to martensite and precipitation of secondary carbide as shown in SEM images below has resulted in enhancement of ultimate tensile and yield strength. The percentage elongation of the DCT samples is less compared to that of the CHT samples. When comparing the results, it can be inferred that conversion of martensite from retained austenite; and precipitation of secondary carbide are the primary reasons behind the enhancement of mechanical properties of En8D alloy steel. The results suggest that Deep Cryogenic Treatment (DCT) show better enhancement of the metal when compared with Conventional Heat Treatment (CHT). Table 3&4 show improvement in tensile and yield strength for CHT and DCT specimen. Table.1 Tensile strength of CHT specimen PARAMETERS SAMPLE-1 SAMPLE-2 SAMPLE-3 AVG. Tensile Strength (in MPa) Yield Stress (in MPa) Elongation in gauge length of % % % 22.53% 50 mm Reduction Area (in %) % % % 64.07% Table.2 Tensile strength of DCT specimen PARAMETERS SAMPLE-1 SAMPLE-2 SAMPLE-3 AVG. Tensil strength (in MPa) Yield stess (in MPa) Elongation in GL of 50mm Reduction area (in %) % % % 22.6% % % % 61.89% Mechanical Property Table.3 Percentage of improvement in Tensile Strength CHT DCT (Conventional Heat (Deep Cryogenic %improvement Tensile strength (in MPa) % Mechanical Property Table.4 Percentage improvement in Yield Strength CHT DCT (Conventional Heat (Deep Cryogenic treatment) % improvement Yield Stress (in MPa) %

5 226 Int. J. Adv. Eng., 2015, 1(3), Hardness Test : The hardness test results for En8D alloy steel are as shown in table 5 & 6 for CHT and DCT specimen respectively. The hardness tests performed here is Vicker s hardness test. The hardness of En8d alloy steel of CHT specimen has an average value of HV whereas for DCT specimen the hardness value is 264 HV. The results clearly show drastic improvement in the hardness of the material. The hardness of the material has improved by 19.19%. The improvement in hardness of the material is attributed to precipitation of secondary carbide which improves carbon bonding and reduces the grain size. The table 7 below shows the percentage of improvement in hardness of the material. Table.5 Hardness of CHT specimen Property LOAD (in Kg) SAMPLE 1 SAMPLE 2 SAMPLE 3 AVG. Vickers Hardness (HV) Property LOAD (in Kg) Table.6 Hardness of DCT specimen SAMPLE 1 SAMPLE 2 SAMPLE 3 AVG. Vickers Hardness (HV) Property Table.7 Percentage of improvement in Hardness CHT (Conventional Heat DCT (Deep Cryogenic % improvement Vickers Hardness (HV) % Scanning Electron Microscopy (SEM) analysis: The surface of En8D alloy steel was analysed for both CHT and DCT specimens. The fig. 6 & 7 below shows the SEM image of CHT & DCT specimen respectively. The image clearly shows that the bonding in cryo-treated specimen improved drastically. The grains are unevenly distributed in CHT samples compared to more even distribution in DCT samples. The microstructure of deep cryogenically treated specimen are observed to be finer and more uniformly distributed than the conventionally heat treated specimen. It can be clearly seen in fig. 7 which shows image of DCT specimen having more white spots which are precipitation of secondary carbides. The presence of more carbides and elimination of retained austenite exhibits the variations in hardness and wear resistance of conventionally heat treated and deep cryogenically treated bearing steel. This suggests that the variation in mechanical properties in deep cryogenically treated specimen must be due to the consequences of microstructural modifications, such as variation in amount of retained austenite, martensite and carbides. Figure.6 SEM image of DCT specimen

6 227 Int. J. Adv. Eng., 2015, 1(3), Figure.7 SEM image of DCT specimen CONCLUSION The material En8D alloy steel is highly used as a gear material. One of the main mechanical failures faced by gear is wear. The aim of the research was to analyse the effect of Cryogenic Treatment on the gear material EN8D alloy steel. The papers show the different in properties of the material when it was subjected to Conventional Heat Treatment (CHT) and Deep Cryogenic Treatment. Various tests were performed to analyse the mechanical properties of the material. The Tensile test performed here showed improvement in various properties such as tensile strength; yield stress; %elongation & reduction area. The reports showed that there was a significant difference in the properties in Cryo-treated sample when compared with conventionally heat treated sample. The hardness test performed showed that cryo-treated (DCT) samples possessed higher order of hardness when compared to conventionally heat treated samples. The result of this work indicates 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 also improve the hardness. The grain size of the martensite is smaller than that of austenite; cryogenic treatment can produce more martensite, which implies a fine grain strengthening effect. So, this is also one of the reason for the hardness of the alloy in cryogenic treatment being higher than that in the non-cryo-treated samples. REFERENCES [1]. R.F. Barron, Effects of cryogenic treatment on lathe tool wear. In: progress in refrigeration science and technology proceedings of the 13th international congress of refrigeration, vol. 1. AVI Publishing Company; p , [2]. H.A Stewart, Cryogenic treatment of tungsten carbide reduces tool wear when machining medium density fibreboard, Forest Products Journal, 54(2): 53 56, [3]. A.H. Gallagher, C.D. Agosti, J.T. Roth, Effect of cryogenic treatments on tungsten carbide tool life: Micro-structural analysis, Transactions of the North American Manufacturing Research Institute of the Society of Manufacturing Engineers, NAMRI/SME, 33: , [4]. S. Zhirafar, A. Rezaeian, M. Pugh Effect of cryogenic treatment on the mechanical properties of 4340 steel, Journal of Materials Processing Technology 186 (2007) [5]. M. Arockia Jaswin, D. Mohan Lala, A. Rajaduraib(2011) Effect of Cryogenic Treatment on the Microstructure and Wear Resistance of X45Cr9Si3 and X53Cr22Mn9Ni4N Valve Steels, Tribology Transactions, 54: 3, [6]. ZHU Yuan-zhi Effects of cryogenic treatment on mechanical properties and microstructure of Fe-Cr-Mo-Ni-C-Co alloy, J. Cent. South Univ. Technol. (2008) 15: [7]. Harish S, Bensely A, Mohan Lal D, Rajadurai A, Lenkey Gyongyver B.Microstructural study of cryogenically treated En 31 bearing steel. J Mater Process Technol 2009;209: