Optimization of composition for thick NM450 steel plates and elimination of delayed cracking phenomena

Abstract To address the phenomenon of delayed cracking during the flame cutting of traditional thick NM450 steel, a new variant, 0Ni-NM450 steel, was developed. This steel design reduces the manganese (Mn) content to improve segregation while increasing the chromium (Cr) content to maintain hardenability, without adding nickel (Ni) to lower alloy costs. The absence of Ni also contributes to a reduction in internal stress within the thick plate. The microstructure was analyzed following phase transformation, quenching, and tempering, leading to the determination of the optimal tempering temperature and other relevant parameters. Additionally, it was observed that the uniformity of the martensitic structure in 0Ni-NM450 steel was enhanced compared to that of cracked NM450. The original austenite grain boundaries, martensitic bundles, and martensitic blocks were more refined, resulting in a more uniform martensitic structure. Finer original austenite grains could decrease the diffusible hydrogen content, thereby improving resistance to hydrogen embrittlement. High angle grain boundaries exhibit higher binding energy than low angle grain boundaries, which reduces the diffusible hydrogen content and consequently lowers the sensitivity of 0Ni-NM450 steel to hydrogen embrittlement, effectively mitigating the occurrence of delayed cracking. This study provides a comparative analysis of the microstructure and properties of traditional NM450 steel and the optimized 0Ni-NM450 steel, examines the factors influencing delayed cracking, and investigates the role of hydrogen embrittlement. The findings offer valuable insights into the behavior of this steel alloy under specific conditions and propose potential strategies for enhancing steel performance in industrial applications while preventing delayed cracking.