Influence of process chains with thermal, mechanical and thermo-mechanical impact on the surface modifications of a grind-strengthened 42CrMo4 steel

Abstract

Abstract Since the term surface integrity was introduced in 1964 by Field and Kahles, the final processing step and its influence on the surface microstructure was heavily investigated. In particular, a lot of research on the surface microstructure and its connection to the functional properties of the workpiece like life time and wear resistance has been performed. However, the influence of the process chain on the final surface microstructure of the workpiece was in industrial applications most of the time not taken into account, although the former manufacturing steps do affect the final result. In this work, samples of ferrite-perlite 42CrMo4 steel (AISI 4140) were first processed by grinding with mechanical main impact (grind-strengthening) and then mechanically, thermally and thermo-mechanically treated to investigate the change in the microstructure depending on the chosen process chain. Electron backscatter diffraction (EBSD) measurements as well as backscatter electron images (BSE), secondary electron images (SE), energy dispersive spectroscopy (EDX) and transmission electron microscopy (TEM) were used to characterize the change in the surface modifications. The first applied grind-strengthening process caused a heavily deformed surface zone of ~4 µm with nano-crystalline grains and high dislocation densities in the following ~10 µm. Thermal treatment resulted in grain growth and dislocation annihilation due to static recrystallization whereas mechanical treatment either increased the dislocation density further or decreased the dislocation density by annihilation of dislocations and dislocation rearrangement. Thermo-mechanical treatment resulted in small equiaxed grains in the surface zone. Cementite lamellae of perlite grains were deformed, partly dissolved and formed spherical carbides at grain boundaries in the surface zone for all process chains including thermal treatment.