焼戻マルテンサイト鋼に対する多重焼入熱サイクルの効果

Abstract

Study is continued on the effect of multi-quenching thermal cycles similar to refined weld heat-affected zone on the mechanical properties of Ni-Cr-Mo and Ni-Cr-Mo-V-B high tensile steel.Results indicate that those steels have as good mechanical properties as ferritic steels when subjected to multi-quenching thermal cycles with incresaingly lower heating temperatures and tempering in furnace.Most suitable initial heating temperature locates around Ac3 temperature; however, quenching from Ac3 below makes the strength lower. Subsequent most suitable quenching temperature is between Ac3-Ac1 temperature range. In those cases a slight decrease in tensile strength appears, but notch toughness is much improved, when tempered in furnace.Multi-quenching from just above Ac3 slightly lowered vTr15 transition temperature; however, it didn't give such superior impact properties as given by quenching from Ac3 above, then Ac3-Ac1 temperature range.Multi-quenching from just above Ac1 retained large ferrite grains and impact properties of those steel were not so good as suitably treated steel.From the results of microstructural studies it was presumed that those superior mechanical properties were developed as a result of refining of γ grains, agglomeration and spheroidization of carbides and sink effect with locally produced r when steel was reheated in later cycles.