Effect of Heat Treatment, Pre-stress and Surface Hardening on Fracture Toughness of Micro-Alloyed Steel

Micro-alloyed steels are being increasingly accepted by industry in various fields of application and are available with a wide variety of microstructures. Extensive literature is available on their microstructure-property relationships. The superior mechanical properties of micro-alloyed steels are caused by fine-grained microstructures and precipitation of micro-alloying elements such as V, Ti and Nb that led to an improvement in yield strength, in the product of tensile strength and total elongation and in Charpy V-notch impact energy as well. The microstructural changes caused by heat treatment or residual stress state caused by surface hardening or mechanical means may influence the fracture toughness of these micro-alloyed steels. It is in this context that the present work begins with experimental determination of quasi-static initiation fracture toughness (J _1c) of low carbon (0.19%) micro-alloyed steel in as-rolled condition without any heat treatment. The study further explores the effect of normalizing, shot-peening and cyaniding followed by shot-peening on fracture toughness of as-rolled steel under study. The normalizing heat treatment, shot-peening and cyaniding followed by shot-peening—each indicates a positive influence on initiation fracture toughness. Results, when compared, show that cyaniding followed by shot-peening have led to a 2.7 times increase in J _1c. Cyaniding followed by shot-peening may therefore be considered as having the most positive influence on initiation fracture toughness in as-rolled condition for the type of micro-alloyed steel under study. Although initiation fracture toughness is in general known to decrease with increase in yield strength in LEFM arena, the micro-alloyed steel under study when normalized displayed simultaneous improvement in yield strength and J _1c. All these observed effects of normalizing, shot-peening and cyaniding on initiation fracture toughness (elastic-plastic fracture mechanics) were explained on the basis of microstructural study and stress depth profiles.