Effect of Heat Treatment 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 processing routes of Micro-alloyed steels. However, experimental information on their suitability to fracture mechanics based approaches towards design and manufacture is found to be relatively scarce in two distinct areas: Alteration of fundamental fracture mechanics properties of Micro-alloyed steels in presence of structural restraints in the form of pre-stress and pre-strain. Comparative study of the effect of heat treatment practice on fracture mechanics properties of Micro-alloyed steels relative to their as-rolled conditions. It is in this context that the present work explores experimental determination of quasi-static initiation fracture toughness (J(1c)) (as per ASTM E-1820) of low carbon (0.19%) micro-alloyed steel in as-rolled condition without any heat treatment. The study also explores the effect of normalizing, shot-peening, and cyaniding followed by shot-peening on fracture toughness parameter so obtained. The normalizing heat treatment, shot-peening and cyaniding followed by shot-peening indicate a positive influence on initiation fracture toughness of this Micro-alloyed steel. Results, when compared with as-rolled condition, show that cyaniding followed by shot-peening have led to a 2.7 times increase in J(1c) of the Micro-alloyed Steel under study. Thus the surface hardening followed by shot-peening of (CT) specimens appears to have most positive influence in terms of enhancement of qualified J(1c) values. Although fracture initiation toughness is in general known to decrease with increase in yield strength, the Micro-alloyed steel under study when normalized displayed simultaneous improvement in yield strength and J(1c). An attempt was made to explain this inter-relationship between yield strength and J(1c). Furthermore, the observed effects, in elastic-plastic regime, of heat treatment, mechanical pre-stress and chemical surface hardening on the initiation fracture toughness were also explained.)