Thermal fatigue testing of Stellite 6-coated hot work tool steel

The performance of Stellite 6 coating deposited on X32CrMoV33 hot work tool steel via Plasma Transfer Arc (PTA) process was investigated under steel thixoforming conditions. The Stellite 6 coating made a very favorable impact on the thermal fatigue performance of the hot work tool steel. The coated hot work tool steel survived much longer, for nearly 5000 cycles before the first thermal fatigue crack was detected on the coating. This marked improvement is attributed to the higher oxidation resistance of Stellite 6 alloy and its ability to retain its mechanical strength at elevated temperatures. The Cr content of the Stellite 6 alloy facilitated the formation of stable Cr-rich oxides which sustained the thermal stresses generated at the surface without spalling and thereby retarded crack initiation. The peak compressive and tensile stresses acting on the coating were estimated to be 500 MPa and 170 MPa, respectively, and eventually led to thermal fatigue cracking. Once the crack initiated, the impact of microstructural features was only minor.