Microstructure and fatigue strength in a low-carbon martensitic stainless steel remelted by plasma torch

Abstract

This study aimed to evaluate the effect of surface remelting by a plasma torch on fatigue resistance of a soft martensitic stainless steel used in the manufacture of hydraulic turbine rotors. The remelting was performed on the surface of this type of steel using direct and pulsed current with the fatigue tests performed by bending at four points at 750, 850 and 1000 MPa stresses. It was found better performance of the remelted condition in relation to cast material without this treatment and no significant differences in fatigue performance between the two remelted processes. The presence of compressive residual stresses resulting from the martensitic transformation partly explains the better performance of the remelted condition. Microscopic analysis further revealed that the lower the fatigue performance of the base material was also associated with the presence of defects (microshrinkage) from the casting process, which promoted the nucleation of fatigue cracks next to them. The presence of delta ferrite together with martensite laths oriented at 45° to the loading direction promoted fatigue crack nucleation in specimens with surface remelted treatment.