Effects of Cooling Rate Fluctuation on Cooling and Transformation Behavior of Steel upon Direct Quenching

TTT-diagrams and CCT-diagrams, which are derived from isothermal Newtonian cooling conditions, have been traditionally used for developing and examining heat treating processes. However, actual cooling and transformation behavior of a steel part upon direct quenching is controlled by a number of factors. When examining actual quenching practice, it is important to correlate cooling performance of the quenchant and the cooling and transformational behavior of steel, especially if there is a variation of cooling rate during the quenching process. This behavior is very difficult to detect by a cooling curve method only. Theoretically, dynamic measurement may be used to detect any transient, even a very small variance in thermoelectric potential. Thus, measurement of thermoelectric potential may provide more detail and insight into the quenching process than a simple temperature-time response as reflected by conventional cooling curve analysis. Therefore, to detect dynamic cooling and steel transformation behavior upon direct quenching, an experiment was conducted using plain carbon steel rods of ⊘5–8 mm diameter with a thermocouple located at the geometric center of the bar. Cooling rate variation upon direct quenching was characteristically demonstrated by dynamic cooling curves. The smaller the diameter of the rods, the more drastic the quench severity and the more sensitive the cooling curve. By comparison to an appropriate temperature-time-transformation (TTT)-diagram or continuous cooling transformation (CCT)-diagram, the cooling process and the transformation behavior of steel upon direct quenching was analyzed with respect to the cooling rate fluctuation obtained in this experiment. The presence of lamellar pearlite of 0.70 % carbon steel wire substantiated the effects of cooling methods and quenchant on the cooling and transformation behavior of the steel wire upon direct quenching. Dynamic measurements revealed new and helpful details related to the direct quenching process.