November_December_2021_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 1 4 2 as the part cools relatively quickly and uniformly. For thin geometries with a slightly nonuniform cross-sectional thickness, such as ring gears, temperature reductions should be kept small, due to the mass differences in the part, but the ramp and hold times can be relatively short, as the part will cool quickly. For parts with significant un- balanced mass distributions, such as crankshafts or eccen- tric bores, temperature reductions should be kept small, and ramp and hold times should be long. To reduce recipe design time, and ensure an optimal recipe is achieved, with respect to distortion minimization and processing time, the equipment must be thermally characterized so heat treatment simulation and design software can be used for DCGQ recipe design. The follow- ing section describes the characterization of the DCGQ pro- totype unit. EQUIPMENT CHARACTERIZATION To properly characterize the DCGQ prototype unit for modeling and process design, or any other type of thermal equipment, it is necessary to determine the heat transfer coefficients (HTCs) and ambient temperatures acting on the component being treated. For liquid quenching oper- ations, the convective HTC should be described in terms of part surface temperature, due to the significant differ- ence in heat transfer between the vaporization, nucleate boiling, and convective stages of liquid quenching. The ambient temperature is assumed to remain constant, due to the liquid’s large specific heat and overall volume in the quench vessel. However, for gas quenching operations, it is assumed that the convective HTC remains constant because there is no phase change associated with cooling in gas, and the ambient temperature is a function of process time, due to the gas’s low specific heat (gen- erally a magnitude less than liquids) and volume in the quench vessel. The ambient temperature as a function of time will vary for any single piece of gas quenching equip- ment, operating at the same conditions, and is dependent on the total mass being quenched, surface area of the load, and the initial tempera- ture of the load. Figure 5a shows a cylindrical quench probe, made of AISI 304 stainless steel, used to characterize the DCGQ equipment. The cylinder has a 100 mm diame- ter and 100 mm height. There are five holes drilled to mid-height, four approximately 3mm from the outer diameter and one in the center, which are fitted with K-type thermocouples. Thermocouples are also located in the DCGQ chamber at various dis- 7 Fig. 4 — DCGQ prototype unit temperature comparison between quench gas entering the quench chamber and the recipe setpoint temperature, with an empty chamber. 8