高窒素含有Niフリーオーステナイト系ステンレス鋼のHAZ靭性に及ぼす窒化物析出の影響

抄録

This work was undertaken to examine the influence of Cr-nitride on impact property in heat affected zone of high nitrogen containing Ni-free austenitic stainless steel. Cr-nitride precipitation of HAZ was simulated to be determined using the isothermal test and the Gleeble test by varying holding time and cooling rate, respectively. Impact toughness was determined by testing Charpy V-notch type impact specimens (55×10×5 mm³) between room temperature and 77K. The time-temperature-precipitation (TTP) diagram for Cr₂N precipitation showed that the nose of each C curve (i.e., intergranular, cellular, and transgranular Cr₂N) was around 1173K. Cr₂N precipitation at holding temperature 1173K occurred sequentially at grain boundaries, by cellular precipitation, and, finally, by transgranular precipitation within the matrix. Amount of intergranular and cellular Cr₂N precipitation increased with increasing holding time or decreasing cooling rate. High nitrogen containing Ni-free austenitic stainless steel exhibited excellent impact toughness at room temperature. At 77K, impact toughness was decreased markedly, and ductile-to-brittle (DBT) transition behavior associated with brittle transgranular fracture was observed. Intergranular and cellular Cr₂N precipitation reduced impact property of HAZ at room temperature. In addition, embrittlement occurred due to grain boundary separation and fracture through cellular precipitation regions, initiated at Cr₂N. However, regardless of amount of intergranular and cellular Cr₂N precipitation, impact toughness values of HAZ were not observed at 77K.