Duplex stainless steel castings of 25Cr-5Ni-2Mo-3Cu added with 0.12% and 0.23% N were made, and solution annealing was carried out on the specimens under the heat treatment conditions of 1273, 1323, 1373, and 1453K for 3.6ks, followed by aging between 623K and 823K for 18Ms (5000h) at maximum. The effects of nitrogen content and solution annealing temperature on microstructure, volume fraction of austenite, and behavior of aging were clarified.
Ferrite phase precipitated as a primary phase in the nitrogen free specimens. On the other side, ferrite and austenite phases precipitated in the specimens with nitrogen because solidification started in the (L ＋ α ＋ γ) region due to large undercooling. The austenite content increased with increasing nitrogen content and with decreasing solution annealing temperature.
The specimens were hardened by aging between 623K and 748K regardless of the nitrogen content and solution annealing temperature. The highest hardness was obtained in the specimen aged at 748K for 3.6Ms (1000h). TEM observation suggested that the reason of hardening was not the precipitation of the α' phase but the formation of the Cu-rich phase. Since age hardening occurred only in the ferrite phase, increased nitrogen content and decreased solution annealing temperature reduced the ferrite content, resulting in the reduction of the age hardening and embrittlement.
