Does the pressure‐induced a‐e phase transition occur for all low–alloy steels?

When designing many products, especially vehicles, it is important to take into consideration impact properties. This paper presents fundamental dynamic data for two examples of bainitic steels. Hugoniots and shear–strength variation with longitudinal stress is reported. These were similar for both materials and in agreement with values for mild steel. The ferrite in the lower–temperature bainite was found to undergo a shock–induced phase transition at 13 GPa, whereas no phase transition was observed in the upper bainite. Further research is being conducted to account for this difference by studying the microstructure and the effect of impact on it. The lower–temperature bainite was found to behave in a brittle fashion, fragmenting extremely, whereas upper bainitic samples were usually recovered intact indicating ductile behaviour. The dynamic tensile or spall strength was also measured in these materials over a range of impact conditions. For the upper bainite, the spall strength dropped slightly with increasing longitudinal stress. However, for the lower–temperature bainite, there was a significant drop in spall strength above the longitudinal stress at which the phase transition occurs. Microstructural studies were also undertaken.