Study of structural changes in S700MC steel thermomechanically treated under the influence of simulated welding thermal cycles

This paper describes the structural changes of S700MC steel under the influence of simulated welding thermal cycles that have been completed to develop a continuous cooling transformation (CCT) chart. This includes controlled analysis of the chemical composition of S700MC steel with nitrogen content, simulation studies, performance chart of phase transformation of austenite for CCT welding conditions and metallographic microscopy and hardness measurements of samples of S700MC steel as a function of the cooling time t_8/5. Methodology of structural changes is based on the simultaneous use of three measurement methods: dilatometric, magnetometer and thermal analysis. In this study, three different techniques are carried out: dilation of the sample material, the magnetic flux changes and temperature. Three types of thermal cycles implemented are fast, natural and free. The obtained results show that the processes associated with the allotropic transformation of γ-α, occurring in S700MC steel during cooling cannot be the basis for the assessment of its welding properties. Carbon in solution has a small extent effect on strengthening the steel and it is not the dominant factor in the transformation phase. The S700MC steel has a low hardenability due to the low concentration of unbound carbon (about 0.03%), and other alloying elements. With a short cooling time, martensite is formed, but it is a low carbon martensite, which does not reduce the yield strength of the steel.