Laser Additive Manufacturing and Post-Heat Treatment Optimization on Microstructure and Mechanical Properties of 9CR Steel

Abstract

Using powders of different particle sizes, 9Cr steel samples are additively prepared by laser coaxial powder feeding technology. 9Cr steel is a material that has been widely used in pressure vessel parts in thermal power plants and nuclear power plants, and has good high-temperature creep properties. Laser Melting Deposition (LMD) is a promising method for preparing complex 9Cr steel components. It provides a rare opportunity to improve existing designs and produce fine features and complex geometries with higher efficiency. The constructed LMD-9Cr sample has high density, the maximum tensile strength of the sample is 1057.75Mpa, which is much higher than the standard cast 9Cr steel of 650MPa. And along the construction direction, we found that there are Unaffected area by heat (UAH) and heat-affected zone (HAZ) areas in the single layer, and there are early austenite and lath martensite in the UAH area, there is only tempered lath martensite exists in the HAZ area. We use 760°C tempering heat treatment, after heat treatment, the average grain size of the material is reduced, the Charpy impact performance is improved, and the tensile strength and microhardness are slightly reduced. The main reason is that tempering heat treatment greatly reduces the high dislocation density of lath martensite, and the supersaturation behavior of Cr, Mo and C elements weakens the effect of solid solution strengthening. In addition, through the nanoindentation test, we found that although the M₂₃C ₆ precipitated phase can harden the material, at the micro level, the elastic modulus and nanohardness of the precipitated phase are lower than that of the uniform phase.

Keywords: Laser Melting Deposition (LMD), 9Cr Steel, Different Powder Size, Post-Heat Treatment, Mechanical Properties, Microstructure