Optimization of mechanical properties of an HSLA-100 steel through control of heat treatment variables

HSLA-100 steel is widely used in Navy vessels, where the requirement is a steel with good weldability possessing a high strength together with a high degree of low-temperature toughness property. However, an increase in strength is usually accompanied by a decrease in toughness. Hence there is a need for optimization of the properties. The present work attempts to optimize the mechanical properties of an HSLA-100 steel through control of heat treatment process parameters. Initially single factor experiments are carried out over a wide range of tempering time–temperature combinations to identify the heat treatment zone where the optimum combination of properties is likely to be obtained. Further experiments are done in this zone through statistical design of experiments. The present work involves quantification of properties by: (i) classical curve fitting technique with data obtained from single factor experiments; and (ii) forming regression equations from 22 factorial design of experiments. Subsequently maximization of low-temperature impact property has been done within the experimental region by Steepest Ascent method, and finally optimum combination of the properties of the present steel has been obtained by Grid Search technique with a constraint on yield strength. Transmission Electron Microscopy studies are made with a view to understanding and correlating the mechanical properties with the microstructures.