EFFECTS OF ALLOYING ELEMENTS UPON AUSTENITE DECOMPOSITION IN LOW-C STEELS

The kinetics of austenite decomposition were studied in high-purity Fe -0.1C-0.4Mn-0.3Si-X (concentrations in weight percent; X represents 3N i, 1Cr, or 0.5Mo) steels at temperatures between 500-degrees-C and 675 -degrees-C. The transformation stasis phenomenon was found in the Fe-C -Mn-Si-Mo and Fe-C-Mn-Si-Ni alloys isothermally transformed at 650-deg rees-C and 675-degrees-C but not in the Fe-C-Mn-Si and Fe-C-Mn-Si-Cr a lloys at any of the temperatures investigated. The occurrence of trans formation stasis was explained by synergistic interactions among alloy ing elements. The paraequilibrium model was applied to calculate the m etastable fraction of ferrite in each alloy. This fraction was shown t o coincide with cessation of transformation in the Mo alloy transforme d at 600-degrees-C. Transformation stasis was found in both the Ni and the Mo alloys isothermally reacted at 650-degrees-C and 675-degrees-C . The interactions among Mn, Si, and Mo, as well as interactions among Mn, Si, and Ni, appear to decrease the threshold concentrations for t ransformation stasis in Fe-C-Mn-Si systems. Segregation of Mn and Mo t o the alpha/gamma boundary, assisted by the presence of Si, was sugges ted to enhance the solute draglike effect (SDLE) and lead to transform ation stasis. In the Ni alloy, a lower driving force for ferrite forma tion resulting from the Ni addition could be responsible for the occur rence of transformation stasis.