Defect-dominated fatigue behavior in type 630 stainless steel fabricated by selective laser melting

Fatigue behavior of type 630 (17-4 pH) stainless steel specimens fabricated by selective laser melting (SLM) was studied. The results obtained were compared with those of the conventionally-melted (CM) type 630. In the microstructure of SLM type 630, martensite-poor and ferrite-rich areas were recognized, while in the CM type 630, acicular martensite uniformly distributed in a matrix. Relatively high temperature was held due to the additive lamination of melted powder during SLM process, which prevented full-martensite structure formation. In the SLM-quenched type 630, the microstructure consisted of acicular martensite due to the post heat treatment at 1050 °C followed by water quenching. The hardness was the highest in the CM type 630, and followed by SLM-quenched and SLM type 630 in decreasing order. The microstructural observation revealed that some defects (<50 µm) distributed in the SLM and SLM-quenched type 630, which were formed during additive lamination of melted powder. Rotating bending fatigue tests were performed at a frequency of 57 Hz in laboratory air. The SLM and SLM-quenched type 630 exhibited much lower fatigue strengths compared to the CM one. One of the reasons of lower fatigue strengths in the SLM and SLM-quenched type 630 could be attributed to their lower hardness. Furthermore, fatigue cracks initiated from the defects in both SLM type 630. Consequently, fatigue crack initiated at an early stage of fatigue life in the SLM and SLM-quenched type 630, resulting in the lower fatigue strengths. However, fatigue crack growth resistances of the SLM and SLM-quenched type 630 were higher than that of the CM one due to frequent crack branching and deflection.