Radial-shrinkage mechanism of 18Ni Co-free maraging steel thin-wall pipes

The 0. 20% -0. 38% radial-shrinkage of the thin-wall pipes made of 18Ni Co-free maraging steel after heat treatment was studied with hardness test, micrographs observation, and X-ray diffraction analysis. The results show that the microstructure of the material after hardness spinning presents texture patterns, the grains are stretched and arranged in certain orientation, and corresponding hardness values are increased. After solution treatment, grains are refined and resume to normal state, most of the residual stresses are eliminated. During the aging process, certain amounts of reversed austenite are formed. At the same time, dispersed strengthening phase is precipitated. The analysis indicates that the main reasons for the radial shrinkage are caused by elimination of the residual stress resulted from deformation and the reversed-austenite and other microstructure changes accompanying with precipitation are significantly contributing to the radial shrinkage.