Abstract
The effect of Mn on transformation behaviour in synthetic weld heat-affected zone of steel was investigated using simplified steels made of pure metals and graphite.
Mn element displaced the SH-CCT diagram for each transformation region to a longer time and to a lower reaction temperature. Each critical cooling time Cf', Cp' or Cc' obtained from the SH-CCT diagram for welding increased with Mn content. Especially, the effects of Mn above 1 % on each critical coloing time were remarkable.
Morphologies of ferrite, pearlite and Zw (Zwischenstufengefüge) were strongly influenced by Mn. When Mn content increased, the morphology of ferrite formation was more liable to develop sideplate and rodlike or needlelike ferrite than massive ferrite. Pearlite morphology was affected by Mn. The fine colony pearlite formed between both ferrite sideplates or both rodlike ferrites precipitated more easily than lamellar or degenerate pcarlite. Zw precipitated easily in the region of longer cooling time (cooling time from A3 or Ac3 to 500°C; above 60 see). In this case, Zw seemed to have the morphology in which austenite between both ferrite sideplates or both thick rodelike ferrites decomposed to ferrite and a relative large cementite. Martensite transformation was promoted by Mn content in steel.
The effect of Mn on the hardness of martensite was a little, while Mn element (above 0.5 %) raised the hardenability curves obtained from the diagram that expresses the relation between cooling time and hardness in synthetic weld heat-affected zone of steel. It seems that the volumes of Zw and martensite increase with Mn content.
