Application of Response Surface Method in Optimization of Impact Toughness of EN24Steel

Impact testing methodology is findings the applications for determining the impact strength of the different materials. Oneof the most important characteristics of structural steels, toughness is assessed by the Charpy V-notch impact test. The objective of the research was to maximize the impact toughness by selecting various combinations of Charpy Impact test parameters. In this paper, experiments were carried out to Study the Effect of Thermal Treatments (annealing, cryogenic treatment and tempering) on Impact Toughness. Cryogenic treatment (CT) is the supplementary process to conventional heat treatment process in steels, by deep– freezing materials at cryogenic temperatures to enhance the mechanical and physical properties of materials being treated. For this purpose, the temperature was used -196C as deep cryogenic temperature. The effects of cryogenic temperature (deep), cryogenic time (kept at cryogenic temperature for 36 hr) on the wear behavior of EN24 steel were studied. The findings showed that the cryogenic treatment decreases the retained austenite and hence improves the wear resistance and hardness. The process has various advantages like increase in hardness, increase in wear resistance, reduced residual stresses, fatigue resistance, increased dimensional stability, increased thermal conductivity, toughness, by transformation of retained austenite to martensite, the metallurgical aspects of etacarbide formation, precipitation of ultrafine carbides, and homogeneous crystal structure. The findings show that the cryogenic treatment improves the wear resistance and hardness of EN24 steel. En24 steel is generally used in the hardened and tempered condition to achieve an optimum combination of hardness and ductility. In the present study, heat treatment process of En24 steel was done which includes Annealing and Tempering at high temperature. The specimens tempered at different temperatures (in the range 473–823 K) exhibited decreasing hardness with increase in tempering temperature. Response surface methodology was adopted in designing the experiments for three factors with 27 levels. Keywords—EN24 Steel, Impact Toughness, Thermal Treatment, Cryogenic Treatments, Hardness, Austenite, Martensite, Carbide formation, Tempering, Wear Resistance. INTRODUCTION Engineering materials, mostly steel and their alloys are heat treated to alter their mechanical and physical properties so as to meet the engineering applications. Impact testing methodology is finding its applications for determining the impact toughness of any materials. The absorbed impact energy and the transition temperature defined at a given Charpy energy level are regarded as the common criteria for toughness assessment[1]. The Charpy impact testing process consists of hammering the steel specimens with a reasonable height with certain velocity on the reverse side of the notch, so that the amount of energy required for the failure of steel specimen can be determined [2]. Here, EN24 steel specimen is used for the impact testing experiment& the effect of different process parameters on the value of impact toughness of EN24 steel is being determined in this paper [2]. During the World War II, the US Army used many ̳Liberty Ships‘ but a lot of them got damaged due to brittle fracture. The term brittle fracture is used to describe rapid propagation of cracks without any excessive plastic deformation at a stress level below the yield stress of the material. The brittle fracture that occurred in the ―Liberty Ships‖ was caused by low notch toughness at low temperature of steelspecimens [3]. CryogenicSteel experiences ductile fracture at high temperature and brittle fracture at low temperatures; therefore steel shows the characteristic of ductile-to-brittle transition. Brittle fracture usually occurs under the conditions International Journal of Engineering Research and General Science Volume 2, Issue 4, June-July, 2014 ISSN 2091-2730 488 www.ijergs.org of low temperature, high loading rate. Cryogenic treatments are useful in these types of cases [3].As recent researches were going on, the scientists showed a lot of confidence and interest on the Deep Cryogenic Treatment of steels.According to the experimental results, martensitic transformation occurred after the deep cryogenic treatment[4]. Grain shape and size gets refined and is made uniform. Defect elimination takes placeand inter atomic distance is reduced [5].Cryogenic treatment is an extension of conventional heat treatment process which converts austenite to martensite.The findings showed that the cryogenic treatment decreases the retained austenite and hence improves the wear resistance and hardness [6].A thermal treatment of steel specimens of EN24 grade steel is done. Heat treatment of the steel specimens consists of Austenitizing (Annealing) and Tempering.The mechanical properties such as ductility, toughness, strength, hardness and tensile strength can easily be modified by heat treating the steel specimens to suit a particular design purpose [7]. And secondly the Cryogenic treatment is done at low temperature in a jar known as Cryocan filled with liquified nitrogen. The Cryogenic treatment involves cooling of steel specimens of EN24 at very low temperature (-1960 C) for about 36 hours. Due to more homogenized carbide distribution as well as the elimination of the retained austenite, the deep cryogenic treatment demonstrated more improvement in wear resistance and hardness compared with the conventional heat-treatment [8].Cryogenic treatment improves hardness, microstructure of metal (retained austenite to marten site), dimensional stability and decreases residual stresses [17].A Comparative study on conventionally heat treated and cryogenic treated EN24 steel specimens has been presented in this paper. Specimens initially subjected to conventional heat treatment at austenitizing temperature of 810 C̊ and goes under deep cryogenic treatment at –195 C̊ for 24 hours [10].