Determination of surface heat transfer coefficient during quenching of medium carbon steel

Abstract

Quenching is an important heat treatment processes during manufacturing of mechanical component. The purpose of quenching is intentional introduction of desired mechanical properties in the components, which will improve their structural as well as functional appearance. Heat treatment is generally followed by transfer of heated component to the environment which uses the quenchants such as forced gas, oil or water flow. During the quenching process, heat is transferred from hot metal component to the quenching medium. This heat transfer in quenching process results in faster cooling of surface than core of the component. This variation in cooling rates may give introduce residual stress and cause distortion and cracking. This variation is due to difference in conduction and convection heat transfer rates. It needs to optimize the quenching process for both part geometry and quenching process design to minimize such problems while improving mechanical properties. In this work the attempt is made to conduct experimental trial of quenching of medium carbon steel with the intention of determination of surface heat transfer coefficient of cylindrical component of material EN 9 steel. The temperature variation during the quenching process is recorded by using K type thermocouple and temperature indicator. Data obtained by experimental work is used to plot temperature variation to observe the variation of cooling rate. From the obtained data HTC is calculated and plotted against time. Keywords— Quenching, HTC, heat flux, cracking, and temperature gradient. I. INTRODUCTION Heat is transfer either by conduction, convection or radiation. The quenching process can be realized by considering the modes of heat transfer that take place during conventional quenching. Boiling of the quenchant on the heated part surface starts immediately when the part is immersed into the quenching tank. The high heat flux from the part surface results in very high rate of water Evaporation that a vapor blanket (a vapor film) covers all or part of the heated surface. This mode of heat transfer is called film boiling. The film boiling is a non-controllable process because the vapour blanket appears and disappears throughout the part surface. Film boiling is the least uniform phase during quenching causing the most part distortion. During film boiling stage of cooling a heat flux from the part surface decreases since the vapour blanket acts as an insulator and results in a high thermal resistance for moving the heat from the heated component to the quenchant. The decrease in the heat flow rate from the heated component surface results in the reduction of the quenchant evaporation rate. As the time goes, the vapour blanket collapses and the film-boiling mode of heat transfer disappears and this leads to the nucleate boiling stage. During the nucleate boiling mode of heat transfer, small bubbles are formed at the surface. As the time goes there is no more vapour formation around the heated part surface, and then the heat flow rate decreases, due to the reduction of the temperature gradient. The heat transfer throughout the nucleate boiling stage of cooling is the highest value of heat flow rate during the quenching process. After nucleate boiling , further heat transfer takes place by convection. Convection heat transfer is the slowest (least intensive) mode of the heat extraction during the conventional quenching processes.