Behavior of High-Temperature Embrittlement and Its Mechanism in Heat-Affected Zone of Low-Carbon Alloy Steels

Abstract

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