Study On Microstructure And Properties Of T250 Martensitic Aging Steel

T250 maraging ultrahigh-strength steel is modelled by China that has characteristics of high strength, good toughness and simple heat treatment process simple, etc. Therefore it is widely applyied to the field of aviation and aerospace fields, such as rocket engine case, missle shell, landing gear, and so on. As a new processing technology, laser rapid prototyping technology is developing rapidly in recent years. This technique can achieve high performance complicated structure forming directly, needing less maching distortion. Laser forming of T250 maraging steel can provide a new approach to manufacturing solid rocket engine case.This article explores the influence of different process parameters, including spot diameter, laser power, scanning speed, powder feeding rate, overlap rate and Z axis single increment, on the characteristics of laser forming, obtains the opimized process parameters combination of laser rapid forming of T250 maraging steel, manufactures original state sample with compact texture and less macro detect, also observes the micro stuctuer of the original state sample. In addition, this article explors the influence of heat treatment system on the microstructure and mechanical properties of laser rapid forming of T250 maraging steel, and obtains the opimized heat treatment process.T250 maraging steel with fine microstructure and high-performance can be manufactured by laser rapidfoming. First, the single factor experiment explors the influence of main process parameters on the characteristics of lase forming. Then, the orthogonal test and range analysis serve the opmized process parameters combination of single-pass cladding. Finally, by adjusting parameters of overlap rate and Z axis single increment, the opitimized precess parameters combination of sample couponwas determined: laser power 1500 W, spot diameter 4.5mm, scanning speed 7.0 mm/s,powder feed rate 3.5 rpm, overlap rate 35%~40%, Z axis single increment1.0mm~1.1mm.The microstructure of the original sample is composed of the cellular crystal and dendrite with a certain growth direction, which has serious micro segregation.High-temprature treatment of 1050℃ can improve the micro segregation efficiently,but that leads to coarse grain as a result, which can be sovled by variable temperature cycle phase ransition process. Based on this regulagation, the solid solution process was determind. With the aging process was obtained by changing aging temperature,the opitimized heat treatment process was finally determined. After heat treatment, the microstructure consists of fine and homogeneous martensitic structure, a small amount of reverse austenite and large number of fine dispersed second phase precipitated in the matrix. The tensile strength, elongation and impact thougness achieve 1875 MPa, 5.84% and 36 J respectively.The microstructure on the joint with the matrix is similar with that of wedment,which can be classified as fusion zone, overheated zone, dark etching zone, matrix and so on. After heat treatment, the microstructure on the joint is transformed into indiscriminating, fine and homogeneous martensitic structure. Although declining,The tensile strength of this zone still achieves 1800 MPa.