Modeling heat treatment of steel parts

The ability to achieve high strength and toughness by heat treatment is a primary advantage of steel alloys. However, the development of internal stress and geometric distortion accompanies these hardening processes. Simulation of heat treatment processes must include the evolution of microstructural phases in order to calculate the mechanical behavior of the composite microstructure as the alloy changes phase. This paper discusses an optimization method to derive the phase transformation kinetics parameters from dilatometry experiments. A discussion of a method based on the lattice parameters of individual phases and a method based on a lever rule for building the bridge between phase transformations and dilatometry strains is offered. The determination of kinetics parameters using an optimization algorithm was implemented into a commercial heat treatment simulation software package, DANTE®. Using Pyrowear 53 steel as an example, the kinetics parameters were fit for various carbon contents. The heat treatment process steps for a 3-D test bar model with a notch on the top surface were simulated, with steps including furnace heat up, carburization, air transfer from the furnace to quench tank, quenching in heated oil, cryogenic treatment, and tempering. Because of a high amount of retained austenite after oil quenching, a cryogenic treatment is used to complete the martensite transformation in the high carbon case of the test bar. After the deep freeze, the test bar was tempered. The predicted distortion and residual stresses were verified by the experimental testing.