The effect of microstructure on the fracture toughness of a high strength steel

Abstract

The relationship between heat treatment, microstructure, fracture topography and room temperature fracture toughness has been determined for a high strength low-alloy steel, En 25. 0ptical, transmission and scanning electron microscopy were used to characterize the structure and morphology while fracture toughness, notched impact and tensile tests were used to determine the mechanical properties. The as-quenched microstructure was predominantly autotempered lath martensite. A little retained austenite was observed in all as-quenched martensite. Austenitizing at 1200°C instead of the conventional 850°C resulted in a 30 percent increase in as-quenched fracture toughness with no loss in tensile strength. Step quenching from 1200 to 850°C for 15 minutes before oil quenching resulted in embrittlement of the austenite grain boundaries and consequently a loss of fracture toughness. The superior fracture toughness of the coarse grain l200°C material was maintained for tempers up to 200°C. Severe intergranular embrittlement and a considerable reduction in toughness were observed when the coarse grain 1200°C material was tempered in the 300-400°C tempered martensite embrittlement range. The fine grain 850°C material also exhibited a drop in toughness in the 300-400°C tempering range but in this case the fracture mode was mainly transgranular cleavage. Transgranular cleavage fracture was tentatively associated with the precipitation of lath boundary cementite, Fe3C. Tempering the fine grain material above 400°C eliminated cleavage and brittle intergranular fracture and resulted in a significant increase in toughness. A little brittle intergranular fracture was observed in the coarse grain material tempered at 500°C. The transverse fracture toughness for material of yield strength less than 1200 MPa was controlled by the major MnS stringer population. The fracture profile was zig-zag, each peak or trough terminating at a MnS stringer. A shear fracture mechanism has been proposed to account for the observed zig-zag fracture mode. The longitudinal fracture toughness was significantly greater than the transverse fracture toughness for tempers of 400°C and above. Mixed microstructures containing lower bainite and martensite did not have inferior relative toughness to wholly martensitic structures. Relationships were found between relative toughness and yield strength and relative toughness and shear lip thickness. There was no meaningful relationship between notched bar impact and fracture toughness data.