FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 31 with the transformation to austenite. During cooling, the material volume decreases due to thermal shrinkage. When the temperature drops below the martensite start temperature (Ms), martensitic transformation starts with material volume expansion. With increased carbon content, the Ms transformation rate decreases. It is well known that thermal stress contributes to distortion during heat treatment. In this study, the thermal stress is negligible due to the thin part thickness and the mild HPGQ quench. The other major factors affecting distortion are shown by experiments and demonstrated by modeling. They include 1) material volume expansion caused by carburizing, 2) stress relaxation during heating, carburizing and soaking at the austenitizing temperature, 3) gradient of martensitic transformation during cooling or quenching due to the carbon gradient, and 4) austenitizing difference in the thickness direction due to retained austenite in the carburized case. The cooling rate in the carburizing step is the same as that of the double normalizing steps. However, the bow distortion caused by normalizing is much higher than that of the carburizing step, which indicates that a large portion of the bow distortion is from the heating step. The distortion from each normalizing step is close to that of the 10-bar quenching step, which proves the negligible effect of thermal stress on distortion. The mechanisms of the above-mentioned factors causing distortions are demonstrated by modeling. Figure 8 shows the simulated temperature, retained austenite All types of High Temperature Ceramics Processing Vacuum Furnaces PRODUCTION AND LABORATORY All non-oxides: SiC, AlN, BN, TiB2, B4C & Si3N4 Hot Presses from 0.5 to 1500 tons Over 6,500 lab and production furnaces built since 1954 •Max Possible Temperature: 3,500°C (6,332°F) •Hot Zones: 10 cc to 28 cu meters (0.6 cu in to 990 cu ft) •Debind, Sinter, Anneal, Hot Press, Diffusion Bond, CVD, CVI, MIM, AM •CVI testing in our lab to 2,800°C (5,072°F) •Worldwide Field Service, rebuilds and parts for all makes Centorr Vacuum Industries 55 Northeastern Blvd., Nashua NH 03062 USA •603-595-7233 sales@centorr.com • www.centorr.com Batch Hot Press Continuous Fig. 8 — Temperature, retained austenite, residual stress, and bow distortion at the beginning of reheating for 10-bar HPGQ hardening (a er carburizing, double normalizing, and cool down to room).
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