FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 32 fraction, residual stress in the longitudinal direction, and bow distortion after carburizing and double normalizing process steps. The temperature is at room temperature of 20°C. The retained austenite in the base carbon material is negligible, and the retained austenite in the carburized case is about 25%. The longitudinal residual stress at the case surface is ~250 MPa in compression, the residual stress at the mid-thickness is ~260 MPa in tension, and the residual stress in the opposite surface is ~360 MPa in compression. The compressive residual stress at the carburized section is caused by the delayed martensitic transformation. The tension in the core is to balance the surface compression. The residual compression at the opposite surface is caused by the bending of the sample. SUMMARY Using a selectively carburized Almen strip sample, the bow distortion caused by a series of heat treatment steps were measured and modeled to investigate its mechanisms. The factors contributing to distortion are summarized: 1) residual stress relaxation during heating of the part, 2) nonuniform austenitizing transformation, 3) thermal stress, especially concentrated stress during heating and quenching, and 4) phase transformations during cooling or quenching. In this case study, the normalizing step with a mild cooling rate causes a similar amount of distortion as the fast-quenching step. For most heat treatment processes, each heating and cooling cycle will contribute to distortion or increase the distortion inconsistency. The common quench hardening process steps of aerospace parts include carburizing, cooling down to room temperature, then reheating and quench hardening. Reheating is used to refine the microstructure. However, from the distortion control aspect, the part should be quenched directly after carburizing. ~HTPro For more information: Charlie Li, DANTE Solutions, 7261 Engle Road, Suite 105, Cleveland, OH 44130, 440.876.7578, charlie.li@dante-solutions.com, dante-solutions.com.
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