Influence of microstructure on near-threshold fatigue crack growth behavior of 18Ni maraging steel weldment

The influence of microstructure of weld metal on near-threshold crack growth behavior under the condition of low and high stress ratio has not been deeply understood. Therefore, experiments were carried out to study the influence of microstructure on the near-threshold fatigue crack growth behavior of 18Ni maraging steel and weld. Microstructure characteristics of base metal and weld metal were analyzed by optical microscope and scanning electron microscope. Results show that the segregation of elements such as Mo, Ti, and Ni leads to the formation of massive reversed austenite phases in the weld interdendritic region and correspondingly reduces the content of these elements in martensite matrix of weld metal, thereby reducing the amount of N3 (Mo, Ti) in the weld matrix, leading to decrease of microhardness and tensile strength of weld metal compared to that of base metal. Fatigue tests demonstrate that when the stress ratio is 0.1 and fatigue crack growth rate is less than 10-5 mm/cycle, roughness-induced crack closure effect has a great influence on crack growth behavior of base metal. The near-threshold crack growth rate of weld metal is lower than that of base metal under the stress ratio of 0.9, which can be attributed to the larger average size of martensite blocks and lower density of high-angle boundaries in weld metal than in base metal.