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 0 D istortion can generally be divided into two catego- ries: size change and shape change. Solid-state phase changes occurring in steel alloys during thermal pro- cessing can result in permanent size change, due to the dif- ference between the starting microstructure and the micro- structure after heat treatment. Size change is unavoidable but can easily be predicted and accounted for in part design. Permanent shape change is a result of nonuniform plastic strain, caused by nonuniform phase transformations, ther- mal strains, or creep while at high temperature, and is more difficult to predict and control. The nonuniformities can be a result of alloy segregation, uneven heating or quenching, poor support while at high temperature, thermal expansion or contraction restrictions, or residual stresses from prior manufacturing operations. Traditionally, liquid quenchants were used to quench most steel parts to obtain a martensitic microstructure. Liquid quenchants undergo a unique phenomenon, com- prised of three stages, when a red-hot part is immersed into the liquid [1] . First, a thin vapor film is formed around the red-hot part, with extremely slow heat transfer rates. Nucleate boiling commences as the vapor blanket breaks down. Nucleate boiling results in the fastest heat transfer due to a combination of the latent heat of vaporization and aggressive convection. Convective cooling, the final stage, begins as the nucleate boiling subsides [2] . The continually changing heat transfer rates associ- ated with liquid quenching can severely affect the cooling uniformity of a given part. First, the breakdown of the va- por blanket rarely occurs evenly on all part surfaces, being dependent on the part surface temperature, local flow be- havior, and the liquid properties, creating brief periods of nonuniform heat transfer. The chaotic nature of this phe- nomenon is difficult to predict and can lead to inconsistent distortion within a single load of parts. Part geometry and immersion orientation also play a significant role in non- uniform cooling when quenching in liquids [3-5] . High-pressure gas quenching (HPGQ) does not in- volve a phase change of the quenching media, and there- fore, has a more stable heat transfer rate. However, due to its low density and specific heat, gas is unable to absorb MINIMIZING DISTORTION DURING HIGH-PRESSURE GAS QUENCHING, PART I A new method to control distortion in difficult-to-quench geometries addresses the nonuniform cooling inherent in most gas quenching processes. Justin Sims,* Zhichao (Charlie) Li,* and B. Lynn Ferguson, FASM* DANTE Solutions Inc., Cleveland *Member of ASM International energy as well as liquids, and will suffer a temperature change as heat is removed from the part. The gas’s low density also makes it more susceptible to local flow varia- tions. HPGQ equipment can also significantly contribute to local flow variations [6] . In response to large distortion during HPGQ of com- plex geometries, DANTE Solutions devised a novel pro- cess, termed DANTE Controlled Gas Quenching (DCGQ), by which the martensitic phase transformation is controlled during gas quenching [7] . Because the transformation from austenite to martensite is driven by a reduction in tem- perature, and is not time dependent like the diffusive phase transformations, the simplest way to control the martensite transformation is to control the rate of tem- perature change within the component. By controlling the uniformity of martensitic transformation throughout the part, distortion can be significantly reduced, easily predict- ed, and consistently reproduced. This article describes the DCGQ prototype unit design and operation. EQUIPMENT DESIGN AND CONSTRUCTION The DCGQ process was developed after hundreds of hours were spent evaluating DANTE quenching models and determining temperature gradients which allowed for minimal distortion of difficult-to-quench geometries, gen- erally encountered in power transmission applications. It was determined that by maintaining a set temperature dif- ference between the fastest cooling point and the slowest cooling point on a part, distortion could be significantly reduced. If the temperature difference is kept sufficiently small, shape change can be completely eliminated, and only the resulting size change from the phase transforma- tions is realized. Atmosphere Engineering (now part of United Pro- cess Controls), in Milwaukee was contracted to design and construct the DCGQ prototype unit. The system includes a 1 m 3 working zone within the quench chamber, separate hot and cold chambers for temperature manipulation of the quench gas, a human machine interface (HMI) for sys- tem manipulation and process monitoring, and custom program logic developed by Atmosphere Engineering to 5 6