March_2022_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 | M A R C H 2 0 2 2 3 6 I n the November/December 2021 issue of HTPro magazine, part I of this article series described a process, termed DANTE Controlled Gas Quenching (DCGQ), by which the martensitic phase transformation is controlled during gas quenching. The process was devised in response to large distortion during high-pressure gas quenching of complex geometries and is a method to control distortion. Part I de- scribed the equipment design, construction, operation, and characterization. This article discusses material property testing, microstructural evaluation, andmechanical testing. MATERIALS TESTING RESULTS The previous article showed that the temperature of quench gas, at atmospheric pressure, could be controlled within the range of martensitic formation for high harden- ability steels. However, doubt still existed as to whether or not a martensitic structure formed over such a long time period could perform as well as a standard high-pressure gas-quenched structure. Therefore, a testing program was launched to compare DCGQ to a standard high pressure gas quenching process. Ferrium C64 was chosen as the candidate alloy, due to its high hardenability, use in high-stress powertrain ap- plications, and high-tempering temperature. And because the martensite transformation occurs slowly using DCGQ, self-tempering may occur during quench if the tempering temperature is less than the martensite start tempera- ture (MS). Further testing is needed to evaluate this ef- fect. All comparative testing was performed on identical coupons machined from one, 100 mm diameter bar. For tests which required car- burization, a single LPC pro- cess was executed by Solar Atmospheres in Souderton, Pa., on all coupons as one batch using a predefined LPC MINIMIZING DISTORTION DURING HIGH-PRESSURE GAS QUENCHING, PART II The second article in this series looks at materials testing, microstructural evaluation, mechanical testing, and residual stress and distortion using the DANTE Controlled Gas Quenching process. Justin Sims, * Zhichao (Charlie) Li, * and B. Lynn Ferguson, FASM* DANTE Solutions Inc., Cleveland recipe. The HPGQ coupons were processed first, at Solar Atmospheres, and the austenitizing step was analyzed such that the time the coupons spent in the austenite phase was noted and duplicated with the DCGQ coupons. This was done to ensure any additional carbon diffusion occurring during austenitizing was similar between the two sets of coupons. All processed coupons, DCGQ and HPGQ, were also subjected to cryogenic and tempering treatments. The testing included, on carburized coupons, microstructural evaluation, hardness, and residual stress and on noncarburized coupons, tensile, impact strength, and distortion. MICROSTRUCTURAL EVALUATION Previous experiments had been conducted on base carbon Ferrium C64 prior to the design and construction of the DCGQ prototype unit to evaluate a slow transforma- tion rate on C64’s microstructure. Those preliminary re- sults showed no noticeable difference between slowly and rapidly transformed martensitic microstructures. Figure 1 shows the microstructure, magnified 1000x, of a carbu- rized (a) DCGQ processed coupon and (b) HPGQ processed coupon. As with previous experiments, there is no discern- able difference between the two microstructures. The thin gray film on the DCGQ coupon is oxidized copper, which was removed prior to testing. The microhardness profiles *Member of ASM International 7 Fig. 1 — Microstructure of a carburized coupon processed using (a) DCGQ and (b) HPGQ, magnified 1000x. 4 (a) (b)