Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel

This comprehensive study gives evidence of reversible and irreversible embrittlement in ultra-high-strength steel through destructive testing. Several tempering temperatures ranging from 350 °C to 700 °C were designated to observe temper embrittlement in the steel. Charpy impact, hardness, and tensile tests were performed for assessing the mechanical behavior of the steel. Stereo microscopy was utilized to examine the fractured surfaces and microstructural analysis was performed with the help of an optical microscope. During tempering, reversible temper embrittlement occurs due to the formation of alloy carbides between martensite plates while irreversible temper embrittlement is preferably due to the segregation of impurity elements (i.e. P and S) on grain boundaries as well as due to the coarsening of alloy carbides. It was found that irreversible temper embrittlement transpired when tempering was done in the range of 400–450 °C and reversible temper embrittlement occurred in 650–700 °C temperature range. Results of the notched-bar impact test clearly showed a reduction in impact toughness in susceptible temperature ranges; moreover, the revealed fracture surface and micrograph also validate these findings. Experimental results also validate that both types of embrittlement are detrimental to the mechanical properties owing to the decrease in hardness and tensile strength.