低炭素合金添加鋼の溶接熱影響部における高温脆化挙動とその機構

Abstract

For the purpose of understanding the mechanism of high temperature embrittlement, especially in the heat affected zone of B bearing low-carbon alloy steel, the role of B addition is studied in terms of grain boundary segregation and nitride precipitation of B. BN precipitates at prior austenite grain boundary are supposed to be a dominant cause to the embrittlement of the steels when tensile stress is applied at 600℃ followed by heat cycle of welding, where the environment of fire is simulated. Nitride formation is changed from intragranular TiN after hot rolling followed by reheating to 600℃ for tensile test to intergranular BN at prior austenite grain boundary after reheating to 600℃ following heat cycle of welding. Consequently, the grain boundary fracture takes place for the specimens subjected to the heat cycle of welding when tensile stress is applied after reheating to 600℃, because the intergranular BN leads to the formation of cavity along prior austenite grain boundary. This mechanism is experimentally verified by the fact that high temperature embrittlement is able to be prevented by either the addition of Zr or the more addition of Ti which may fix nitrogen as more stable nitride and inhibit the dissolution of nitride during welding heat cycle.