TO STUDY THE EFFECT OF CRYOGENIC HEAT TREATMENT ON HARDNESS AND THE AMOUNT OF RESIDUAL AUSTENITE IN 1/2304 STEEL

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1 TO STUDY THE EFFECT OF CRYOGENIC HEAT TREATMENT ON HARDNESS AND THE AMOUNT OF RESIDUAL AUSTENITE IN 1/2304 STEEL 1. Kamran Amini, 2. Said Nategh, 3. Ali Shafiey, 4. Mohhamad ali soltany 1. A member of Young Researchers Club Islamic Azad University Majlessi Branch 2. Islamic Azad University Science and Research Branch 3. Isfahan University of Technology 4. Isfahan alloy steel complex 1: kamran_amini1978@hotmail.com Abstract 1/2304 steel is a kind of steel which is used very much for manufacturing cold rolling. In these parts, percent of residual Austenite has considerable effects on the life of roll, because in working conditions due to exerted stresses, there is possibility for transformation of residual Austenite to martensite. In this manner, the obtained martensite can cause the roll to be broken. The results of the performed tests in this research indicate that it is possible to reduce the amount of residual Austenite in the structure with cryogenic heat treatment. In the present study the effect of cryogenic heat treatment on hardness and the amount of residual Austenite in the structure of the said steel was surveyed. Results indicated that the best austeniting temperature is 1000 centigrade degree and duration is 45 minutes. Then with exertion of these conditions and quenching the samples in each weather their hardness was 64 HRC and residual Austenite was determined between 6 7 %. Then, immediately after the stage of samples quenching, with putting them under cryogenic heat treatment ( in liquid and nitrogen) for a duration of 6 hours, hardness of samples increases to 67 HRC and the amount of their residual Austenite decreases to about 1.5 to 1.7 %. Therefore performance of cryogenic heat treatment can decrease the amount of residual Austenite in addition to increasing hardness (about a few HRC). Key Words: cryogenic heat treatment, hardness, residual Austenite 1. Introduction 1/2304 steel is alloy-steel containing elements of Cr, Mo,etc. which is applied in manufacture of cold roll. In the presence of such alloy elements temperatures of M s and M f decrease. So during quenching, some Austenite remains that causes to decrease hardness[1]. γ Presence of residual Austenite ( re ) reduces wear resistance that this matter results in reduction of roll life. On the other side, while using roll, due to stress and plastic deformation, better conditions will be provided for performing martensite. Nucleation of martensite sheets becomes easier and so temperature of M s increases; Therefore there is probability for transformation of residual Austenite to martensite during working stage of roll. This created martensite which was not tempered, is rough and can create required conditions for breaking rolls.[2] With rising temperature of transformed martensite, martensite percent reduces. The highest temperature in which martensite is created under stress is M d and this martensite is called stress induced martensite [2] So, considering presented explanations we can conclude that presence of residual Austenite can create negative effects on the life of roll. In this direction, effective factors such as Austenite temperature, quenching speed, 1

2 temperatures of quenching environment,etc. can have effect on the percentage of residual Austenite and its amount.[3] But in each case in these steels full deletion of Yre is almost impossible due to the situation of Mf temperature. Cryogenic heat treatment is a proper operation (treatment) for reducing percent of γ re. Cryogenic treatment consists of heating steel up to Austenite temperature, cooling it in quench environment and then immediately putting it in sub- zero centigrade degree and then tempering heat treatment. Increasing resistance to wear, reduction of internal stresses, consistency of dimensions and deposition of micro carbides in the field can be regarded as the most important privileges of using cryogenic heat treatment. The less the temperature of cryogenic environment, improvement in properties is performed with more rapidity. [4] In figure 1 the effects of cryogenic heat treatment have been shown in temperatures of -84 C and -190 C on a high carbon steel. As it is observed the lower the sub-zero temperature, the more obvious is improvement of properties. [5] It shall be mentioned that keeping sample for a long time in quench conditions, causes to stabilize Austenite and reduces the effect of cryogenic heat treatment. In the present study the effect of cryogenic heat treatment on the amount of residual Austenite and hardness in 1/2304 steel has been surveyed. Figure 1 Wear resistance of 5 cryogenic treated steels in -84 o c, -190 o c 2. RESEARCH METHOD: 1/2304 steel applied in this research is a production of Isfahan Alloy Steel Complex. Analysis of this steel has been presented in Table No.1. In order to study the effect of Austenite temperature on hardness of this steel and also effect of cooling speed, samples were Austenitizied in temperatures of 1000, 970, 950, 930, 900, 870, 840, 820, 800 centigrade degree for a period of 45 minutes and then they were quenched in air conditions. Then using light microscope equipped with photograph analysis system, and also using the technique of colored etch with the following composition: the percent of residual Austenite in samples were measure.[6] (3gr)K 2 S 2 O + (10gr)( Na 2 S 2 O 3 ) + (100ml)( H 2 O) (1 In the next stage samples were put under cryogenic heat treatment 2

3 In liquid nitrogen with the temperature of -80 o c for a period of 6 hours and the percent of residual Austenite and hardness resulting from these treatments were compared with the conditions prior to the exertion of cryogenic treatment. Table1 Chemical composition of steel Element C Si Mn Cr Mo V P S Ni Fe Weight Percent Bal. 3.Results and Discussion In diagrams 2 & 3 temperature of Austenitizing on steel hardness have been shown in the manner of quenched in air conditions. As it is observed, by increasing Austenitization temperature up to 1000 centigrade degree in the fixed period of 45 minutes, hardness resulting from samples quenching in water or air is increasing. Although increasing hardness concerning quenched samples is more in the air, but with increasing Austenitization temperature this difference decreases. Whereas in Austenitization temperature of 1000 centigrade degree for a period of 45 minutes, hardness resulting from samples quenching in water or air is similar. The reason of this affair is increase of steel hardenability with increasing Austenite temperature. So the range of Austenitization temperature in this type of steel centigrade degree for a period of 45 minutes and quench environment is proposed for narrow profiles of air and thicker sections of water. In the next stage samples quenching in water and air (with Austenitization temperature of centigrade degree) are etched by colored etch technique and were studied quantitatively. During colored etch each one of the phases are appeared by a special color that percent of each phase is calculated by quantitative measuring of each phase from its color, by a microscope equipped with photograph analysis. Residual Austenite during colored etch is introduced with etch solution, and appeared in a white color. After studying phases quantitatively, the samples were taken under hardnessmeasurement test. Results obtained from hardness-measurement of samples and the percent of residual Austenite have been presented in Table No.2. Also microscopic structure resulting from colored etch of samples after quenching is brought in figure 4. After all of these phases, samples were transferred to sub-zero environment and were put in liquid nitrogen for a period of 6 hours in the temperature of -80 centigrade degree. After exiting samples and performance of metallography and hardenability tests, ( in accordance with presented results in Table 3 ) it is observed that hardness is increasing and Austenite percent is decreasing. The obtained structure from sub- zero treatment has been presented in figure 5. Even though it is possible that the effect of these treatments on increasing hardness of the said samples is not significant, but due to the reason of considerable reduction of the amount of residual Austenite in their structure, it can prevent from untimely martensite transformation during roll usage that can cause reduction of roll life. 3

4 Table 2: Residual Austenite and hardness of quenched samples Heat treatment Cycle Hardness (HRC) Remain Austenite Percent Sample1: Austenited at 950 o c for 45 min and water quenched 62 9 Sample2: Austenited at 1000 o c for 45 min and water quenched 64 7 Sample3: Austenited at 1000 o c for 45 min and air quenched 64 6 Table 3: Residual Austenite percent and hardness of cryogenic samples Heat treatment Cycle Hardness (HRC) Remain Austenite Percent Sample Sample Sample Figure 2: Effect of Austenitizing temperature and water quenching on steel hardness Figure 3: Effect of Austenitizing temperature and air quenching on steel hardness 4

5 Figure 4: Structure of Quenched samples by color etching, x500 5

6 Figure 5 Structure of samples after cryogenic treatment, x Conclusion 1- Appropriate Austenitization temperature for the said steel is 1000 centigrade degree for a period of 45 minutes. 2- With increasing Austenitization temperature (up to 1000 centigrade degree) we can obtain higher hardenability. The reason for this matter can be expressed as increasing steel hardenability with increasing Austenitization temperature. 3- In the steel under study, hardness obtained after Austenitizing in the temperature of 1000 centigrade degree and quenching in water and air is similar. 4- The amount of hardness in samples depends on the amount of residual Austenite after quenching. 6

7 5- the less the percent of residual Austenite in cryogenic heat treatment the more hardness will result, so sub-zero treatment can create increasing hardness. In the present study hardness of samples increased with performance of cryogenic heat treatment from 64 HRC to 67 HRC, whereas the percent of residual Austenite decreased from 6-7 % to %. Therefore performance of martensite transformation during cryogenic heat treatment can cause reduction in probability of crack resulted from untimely transformation while using rolls. 5. References 1-. Fanju Meng, Kohsuke Tanasbira and Hideaki Sohma, Wear Resistance and Microstructure of Cryogenic Treated Fe-1.4, Cr1C Bearing Steel, Scripta Metallurgica et Material, Vol 31, 1994, pp Torsten Ericson, Principles of Heat treating of Steels, ASM Handbook Volume 4, ASM International Committee, 1991, PP. 3-5, 204, Publishing, Boston, K.M. Smith,:An Investigation in to the Influence of Cryogenic Treatment on the Wear Resistance of Tool Steels,Lanchester(England) Polytechnic, May Barron, A Study on the Effect of Cryogenic Treatment on Tool Steel Properties, Louisiana Technical University Report, Agust 30, D.Mohanlal, S. Renganarayan, A. Kalanidhi, Cryogenic Treatment of Augment Wear Resistance of Tool and Die Steel, Cryogenics 2001, pp R.F. Barron, Cryogenic Treatment of Metals to Improve Wear Resistance, cryogenics,august 1982, page