Influence of microstructural evolution and localized delta ferrite number on high-cycle fatigue crack opening and propagation rate

Abstract

In this research, percentages of nickel as austenizer and chromium as ferritizer, in the molten metal pool of SS304L weldments are varied to analyze the effect on the produced delta ferrites morphology and volume. The resulting localized precipitation of delta ferrite and its effect on the fatigue crack propagation rate of the material are examined in this work. The estimation of localized delta ferrite number for a particular weld zone is assessed through the development of a MATLAB code for image processing. It is observed that a reduction in the percentage of nickel in filler alloy yields randomly scattered granular ferrites in the fusion zone and equiaxed grain growing in the heat-affected zone with the development of blocky ferrites on the grain boundaries, whereas increased percentage of chromium in the filler causes the precipitation of dendritic clusters and vermicular ferrites in fusion zone and heat-affected zone of weldments, respectively. The effect of ferrite numbers and their morphology on the crack opening displacements and the corresponding fatigue crack propagation rate in different weld zones are analyzed. It is observed that the ferrite number ranging between 15 and 19 provides optimum fatigue strength for SS 304L. Paris curves are plotted from the test data, which shows that the weldment attained with filler alloy having Cr 26.60% and Ni 10.92% depicts the highest fatigue life compared to other studied weldments.