Relationship between composition, impurity content, cooling rate, and solidification in austenitic stainless steels

Abstract

The solidification mode at slow cooling rates has been accurately predicted from previous work, and the amount of primary δ-ferrite in mode B alloys related well to the equilibrium data and the (Cr/Ni)_equiv values. The correlation between these two parameters is fortuitous and appears to hold only over the range of commercial compositions. Similarly, the residual δ–ferrite values predicted using the Schaeffler–DeLong method do not relate directly to the δ–ferrite contents at the solidification temperature. For anyone residual ferrite content a number of values of the high-temperature phase can be obtained. Sulphide and phosphide distribution was found to be a function of the amount of primary ferrite in mode B alloys. With a relatively high content of primary ferrite, the growth of austenite into both liquid and solid created an increased number of interfaces and more dispersed impurities. At high cooling rates a mode B to mode A change occurred for low (Cr/Ni)_equiv values, and when sulphur and phosphorus were present a mode B to mode C change was produced in the region of the welds with the highest cooling rates. The former change is attributed to undercooling and the latter to the influence of impurities on the relative stability of austenite and ferrite.

MST/322