Effect of tempering time on mechanical and fracture behavior of SA 387 Gr.11 pressure vessel steel

The effect of tempering time on J-integral fracture toughness, J-integral fracture resistance and mechanical properties for SA 387 Gr.11 pressure vessel steel is investigated. For this investigation tensile test, fracture test, hardness test, scanning electron microscopy and optical microscopy were performed. With incremental tempering time, fracture toughness and fracture resistance are dramatically increased due to significant rise in plastic component of J-integral, while material strength is decreased. The changes in mechanical and fracture behavior with tempering time are attributed towards the decomposition of martensite/austenite phase into ferrite and uniform distribution of coarse carbides at grain boundaries which is confirmed with microstructural analysis. Fractographical investigations show cleavage facets along with small dimples up to tempering time of 2.5 h. However, with further incremental tempering time large and deep dimples are observed.