Effect of Tempering Treatment on Microstructure and Mechanical properties of the HSLA-100 steel

The high strength low alloy (HSLA) 100 steel has been widely used in various industries such as shipbuilding, military, oil and gas. An increase in yield strength with a reasonable toughness is highly demanding in this steel. In the present study, the quench-tempering heat treatment was investigated to improve mechanical properties of this steel. The samples were austenitized at 930 °C for 1 h followed by water quenching. They were then tempered in the temperature range of 410 and 710 °C for 1 to 4 h. Microstructures of the heat treated specimens were evaluated by optical and scanning electron microscopy, while mechanical properties measured by hardness, tensile, and toughness tests. The results showed that microstructural changes during tempering took place mainly after 650 °C, where the martensite laths and acicular ferrite transformed to polygonal ferrite. The yield and ultimate strengths were increased by increasing tempering temperature from 410 to 525 °C, beyond which both was dropped up to 710 °C. The maximum yield and ultimate strengths were obtained by tempering at either 525 °C for 1 h or 475 °C for 2 h. Increasing tempering temperature and duration both resulted in an increase in the absorbed impact energy.