Lorenz Schlier

EVIDENCE AND ANALYSIS OF THERMAL STATIC STRAIN AGING IN THE DEFORMED SURFACE ZONE OF FINISH-MACHINED HARDENED STEEL

After heat treating, finish machining of the hardened steel represents the last manufacturing step of machine elements. The practically most important operation of grinding is applied to achieve edge zone compressive residual stresses, best surface quality and dimensional accuracy. Metal removal involves high plastic deformation work. Glide and intersection processes raise the density and produce lower energy substructures of dislocations. The temperature and time behavior of post-machining thermal treatment is analyzed on ground and honed martensitic SAE 52100 rolling bearing steel. Microstructure stabilization is reflected in a large XRD peak width decrease on the surface. The kinetics are modeled by rate-controlling carbide dissolution as the carbon source for Cottrell-type segregation at dislocations. This thermal static strain aging is verified by the formation of a slight white etching surface layer. The model is extended to also consider superimposed thermal dislocation recovery. Both effects are separable. In rolling contact fatigue tests under mixed friction conditions, air reheating below the tempering temperature, which avoids hardness loss, leads to a significant lifetime increase. The effect also occurs after cold working.