Carburizing, Case Depth, Phase transformation, Austenite Grain Growth, CCT, TTT

Carburized quenching is a surface hardening process applied to low and medium carbon steel parts to improve fatigue and wear resistance. Distortion, cracking, inadequate case depth and surface/core hardness are the most frequently encountered problems during this process. During the last two decades, computer simulations became popular to predict and avoid those problems instead of conventional analytical and trial-and-error approaches. Aside from troubleshooting, heat treatment simulations enable determination of optimal process parameters yielding the desired microstructure and residual stress distribution to improve the performance of the part.Primary aim of this study is to complement computational materials engineering methods to develop a material data set for the carburized quenching simulation of DIN 22NiCrMo2-2 (SAE 8620H) steel. For that purpose, first, the raw material is characterized chemically and microstructurally to provide necessary input to computational techniques. Then, the kinetics of austenite growth, which has strong impact on phase transformations during succeeding quenching step, is investigated. Finally, critical temperatures and transformation kinetics are determined and presented in the form of TTT and CCT diagrams.

Raw material characterization results indicate that the billets are qualified for the process and the validation study as the billets are free of macro-segregation and exhibit a homogeneous and mildly banded equiaxed ferritic/pearlitic grain structure. Moreover, austenite grain growth study revealed that the grain growth in 22NiCrMo2-2 can be expressed by an ideal grain growth law. Finally, computationally and experimentally determined CCT diagrams are in a good agreement. The largest differences between computationally and experimentally determined TTT diagrams are observed in the bainitic transformation range which is more sensitive to hard to control factors such as local chemical composition and local prior austenite grain size.Secondary aim of this study is to conduct microstructural investigations for the quality assessment and validation of computer simulations conducted in complementary studies within the scope of the same project. In one of those complementary studies, an experimental Design of Experiments (DoE) using the Taguchi method is conducted on steel shafts made of DIN 22NiCrMo2-2 and 16MnCr5 steels to minimize the variability of the industrial process. In the other study, the same DoE is investigated using computer simulations. This thesis complements those studies with determination of microstructure and hardness distributions. The experimental results indicate an agreement with the simulations results and the agreement can be improved with better characterization of bainite transformation kinetics including its dependence on stress, local grain size and chemical composition.