Weld Heat-Affected Zone Subjected to Multi-Thermal Cycles and Properties of Multi-Quenched Steel Similar to Heat-Affected Zone

Abstract

A atudy is made on structural features and mechanical properties, especially on notch toughness of duplicated weld heat-affected zone and multi-quenched steel similar to refined weld heat-affected zone for Si-Mn and Si-Mn-Cr-V steels by induction heating and tempering in furnace, based on the results of notch toughness surveys of welded joint for quenched and tempered steel.
Refined microstructure in weld heat-affected zone is produced by rapid heating and cooling thermal cycles above Ac₁ temperature, whose maximum heating temperature are presumed to be lower in later ones. Shift of Ac₁ and Ac₃ transformation temperature was little observed, as compared with that of slower rate of heating, however, tranformation products in cooling were much influenced by heating temperature of thermal cycles, being martensitic in higher heating temperature, whereas rather ferritic in lower temperature.
Hardness in duplicated weld heat-affected zone initially heated to 1350°C was lower after subjecting to the thermal cycles of lower heating temperature. Carbide grains in those steels were rather globular than ordinarily quenched and tempered steels.
Subjecting to singly quenched thermal cycle, steels had the best notch toughness, when heated to just above Ac₃ temperature followed by tempering, and toughness of singly quenched steel was improved further by repeated heating between Ac₃-Ac₁ temperature range followed by tempering.
Microstructure of those steels consists of extreamly refined ferrite and globular carbides in both Si-Mn and Si-Mn-Cr-V steels, thus suggesting superior impact properties of those steels and special heat treatment for such steels.
Generally steels have the poor impact properties, when repeatedly heated between Ac₃-Ac₁ temperature, however, tempering treatment gives the favourable effect on the impact properties.