Crystal plasticity analysis of instrumented indentation on a service-aged Cr–Mo steel

Samples extracted from pressure vessels that had been in service for 40 years at 480 °C revealed a 24% increase in tensile strength and a 30% decrease in elongation compared with new material. However, artificially aging at 700 °C for 2 h revealed a decrease in tensile strength and a slight decrease in elongation despite having a similar Holloman–Jaffe (HJ) tempering parameter to the service-exposed samples. Mesoscale crystal plasticity simulations were conducted to understand the aging microstructure on microhardness indentation, incorporating a dislocation density-based constitutive law. Experimental results in dislocation density, grain orientations, and particle sizes were used as inputs for the crystal plasticity model. Atomic force microscopy (AFM) was used to measure the depth profile of the nanoindentation marks, validating the model. Both experimental and simulation results indicated that precipitation hardening by nanoscale semi-coherent M o 2 C significantly strengthened the ferrite matrix during aging. The dissolution of F e 3 C in pearlite had decreased the tensile strength, while the increased carbide precipitation and dislocation densities had increased the tensile strength of the ferrite matrix in the service-exposed samples. An indentation model for microhardness of the pearlite regions confirmed that the spheroidization of carbides in pearlite during aging had contributed to the decrease in hardness and strength.