Phase transformation mechanism of ferritic stainless steel by nitrogen absorption

Fe-Cr binary alloys with 12mass%Cr or more are originally fully ferritic in a high temperature range, but the crystal structure can be changed from bcc (ferrite) to fcc (austenite) by nitrogen absorption in 1atm N₂ gas atmosphere at 1473K. In this paper, the mechanism of the phase transformation with nitrogen absorption was investigated in association with crystallographical characteristic by means of optical microscopy and scanning electron microscopy. The alloy used is a 27mass% Cr ferritic stainless steel. This steel can absorb a great amount of nitrogen (about 1.4mass% at the maximum) in 1atm N₂ gas atmosphere at 1473K, so that the austenite formed at the temperature is kept to be stable even at room temperature. Results obtained are as follows: (1) The austenite formed by nitrogen absorption has the Kurdjumov-Sachs orientation relationship with ferritic matrix: (111) γ // (011) α, [101] γ // [111] α (2) The austenite grows in rod-like shape toward one of four 〈111〉 α directions, and the side faces of rod-like austenite is confirmed to be constructed with two kinds of γ/α interfaces which have a good coherency in the Kurdjumov-Sachs orientation relationship.