Enhanced cyclic stress response and low-cycle fatigue life of modified 9Cr-1Mo steel by wire-arc additive manufacturing and post-heat treatment

Wire-arc additive manufacturing (WAAM) is a promising technique to fabricate large-size components with high efficiency. However, the changes in microstructure induced by WAAM and its impact on the fatigue behavior remain unclear. In this study, the microstructure and low-cycle fatigue mechanisms of modified 9Cr-1Mo ferritic/martensitic steel fabricated by WAAM at 350 °C are investigated, and different post-heat treatment methods are explored to improve the fatigue property. It is found that compared to conventional hot-rolled steel, the tensile strength and elongation of WAAM steel are 23 % and 21 % higher, respectively. In addition, both higher cyclic stress response and longer low-cycle fatigue life are obtained by the synergy of WAAM and optimized post-heat treatment. After HT2, the fatigue life increases by 70 % at the strain amplitude of 0.5 % compared to WAAM-HT1 steel. Such an enhancement in fatigue property can be ascribed to the finer martensite laths and higher dislocation density due to continuous rapid heating and cooling cycles of WAAM, as well as the smaller prior austenite grain size induced by optimized post-heat treatment through increasing normalizing cycles. The optimized post-heat treatment decreases the prior austenite grain size without changing the width of martensite lath.