Effect of Al alloying on microstructures and mechanical properties of Co-free maraging steel

Abstract

<p indent="0mm">To improve the mechanical properties of maraging steel, it is important to control the precipitation and morphology of high-density nano-scale precipitates. In this work, Co-free maraging steel with a dual-phase structure was prepared by composition design and an optimized heat treatment process. The results showed that semi-coherent η-Ni₃Ti and coherent B2-NiAl ordered phases were precipitated in maraging steel with a content of 1.5 wt% Al. The steel had good mechanical properties with a high ultimate tensile strength of <sc>2065 MPa</sc> and an elongation of 9.5%. With an increase in the Al content, the morphology of the precipitated phase changed from a needle-like structure (η-Ni₃Ti) to a spherical structure (B2-NiAl). The addition of Al was also beneficial for the competitive growth of Ni₃Ti and NiAl, and it accelerated the precipitation process and promoted the size refinement of the nano-sized precipitates. Nano-scale austenite existed in the maraging steel and was distributed at the boundary of lath-like martensite. However, with an increase in the Al content, the sizes of both the nano-scale austenite and the lath martensitic decreased. The good mechanical properties of maraging steel were mainly attributed to the dual-phase structure, high-density precipitation, and fine grain strengthening.