Formation condition of lacy ferrite during solidification and subsequent transformation in austenitic stainless steels solidified with primary ferrite
Kota Kadoi, M Kogure, Hiroshige Inoue
Abstract
• Lacy ferrite could form with K-S relationship between ferrite and austenite during solidification. • Misorientation for lacy ferrite was ≤ 7° (close-packed plane) and ≤ 9° (close-packed direction). • Unidirectional solidification of austenite and ferrite accelerate lacy ferrite formation. • Two pass laser welding could control directions of preferential growth of ferrite and austenite. • Formation ratio of lacy ferrite significantly increased approximately 40 % (generally 5–10 %). The formation conditions of lacy ferrite in austenitic stainless steels solidified with primary ferrite was clarified through crystallography in order to increase the amount of lacy ferrite which contributed to the improvement of properties. Lacy ferrite was formed when the misorientation between ferrite and austenite from the Kurdjumov–Sachs (K-S) relationship was ≤ 7° on the close-packed plane and ≤ 9° in the close-packed direction during solidification. In contrast, vermicular ferrite was formed if the misorientation exceeded these values. The temperature gradient during solidification was controlled unidirectionally by two-pass laser welding. Following the performance of two-pass welding with a highly unidirectional laser, it was confirmed that the formation ratio of lacy ferrite to total ferrite significantly increased in the second-pass weld metal by remelting the first-pass weld metal, in which the heat flow direction and preferential growth direction of ferrite and austenite were controlled to be almost parallel. The growth ratio while maintaining the K-S relationship established between ferrite and austenite in the first-pass weld metal, and/or the ferrite formation ratio that satisfies the K-S relationship, increased in the second-pass weld metal because the growth of both the primary ferrite and secondary austenite was crystallographically independent of each other during solidification.