Study on Improvement in the Hot Workability of High-Alloy Steels

Abstract

Stainless steels which acquire γ+a mixed structure at high temperature tend to crack during hot working, because the deformation by working concentrates upon the ferrite parts that have a small flow stress. To improve the hot workability of stainless steels of (γ+a) structure at high temperature, we have selected typical stainless steels of the duplex structure 20Cr-12Ni-2Mo, 20Cr-9Ni-0·9Nb and 24Gr-13Ni steel, and investigated the effect of high temperature homogenizing treatment, and addition of rare earth and nitrogen upon the hot workability of the ingot of these stainless steels by means of high temperature torsion test.
The results obtained are as follows:
(1) The high temperature homogenizing treatment is effective to improve the hot workability for all these steels tested, and above all, the effect upon 20Cr-12Ni-2Mo steel is remarkable. Homogenizing treatment at 1200°C for 50h is the best to improve the hot workability of these steels.
(2) The effect of rare earth (misch metal and lantan alloy) addition on the hot workability of these steels is recognized and an addition of 0·25% is the most effective.
The difference of hot workability among the tested steels is almost negligible, and lantan alloy addition is a little less effective than misch metal addition.
(3) The effect of nitrogen addition upon the hot workability for these steels is remarkable except 20Cr-12Ni-2Mo steel that naturally has little δ ferrite. However the weldability of the steel containing nitrogen is not good, therefore it is not advisable to make nitrogen addition to improve the hot workability of these steels, if good weldability is required.