The Effect of Impurity Elements on Transition Temperatures and 475°C Embrittlement of Ferritic Stainless Steels

Abstract

The brittle-to-ductile transition curves of a series of ferritic stainless steels containing carbon, nitrogen, silicon or manganese, respectively, were determined, and also the effect of aging at 475°C on the transition curves of high purity ferritic stainless steels was discussed.
The main results obtained were as follows:
(1) In 18% chromium steels, at the 0.002% carbon and 0.004% nitrogen levels, the indicated transition temperature is found to be 85°C However, there is a remarkable increase in transition temperature and a decrease in upper shelf energy, as carbon or nitrogen content increases. The effect on transition temperature and upper shelf energy is attributed to the exsistence of solute carbon or nitrogen atoms and the dispersed carbide or nitride precipitates.
(2) Addition of aluminium as stabilizer is effective in lowering transition temperature.
(3) Silicon and manganese additions tend to raise transition temperature.
(4) In high purity 18% chromium steel aged at 475°C the upper shelf energy decreases, but the transition temperature has little change.
(5) In 0.05% nitrogen-18% chromium steel aged at 475°C, the upper shelf energy decreases, and the transition temperature is shifted toward higher temperatures.
(6) The hardness increase with the aging time for high purity 18% chromium steel is less than for 0.05% nitrogen-18% chromium steel.
(7) These phenomena caused by aging at 475°C must be depended on the precipitation of chromium rich bcc phase and chromium nitrides.