Delayed Failure Characteristics of Ultra High Strength Steels

Abstract

The effects of heat treatment on the characteristics of delayed failure crack propagation were investigated in 0.02N-HCI aqueous solution at room temperature using centre-notched specimens of SCM3 and 18Ni-maraging steels. Crack growth rate was directly measured by optical observation and was interpreted in terms of crack tip stress intensity factor K_I.
When tempered at 300 and 400°C, SCM3 showed marked increase in crack growth rate in addition to the deterioration in Charpy impact toughness, although tensile strengthdecreased gradually with increasing tempering temperature. Transmission electron microscopy and fractography by SEM suggested that this increase of susceptibility to delayed failure is due to theprecipitation of film-like carbide along prior-austenite grain boundaries at above-mentioned tempering temperatures, which resulted in the formation of typical intergranular fracture surface at intermediate K_I value in contrast with the duplex intergranular-transgranular fracture surface observed in the specimen tempered at other temperatures.
In the case of 18Ni-maraging steel, aging at temperatures above 500°C gave the best combination of strength and resistance to delayed failure and crack initiation was suppressed when aged at 600°C. But as the aging temperature decreased below 450°C, crack growth rate became comparable with that of SCM3 with identical tensile strength.
Thus, crack growth rate of delayed failure is not necessarily related to the tensile strength, toughness or ductility of steel, but can be affected by the variation of microstructures depending on the condition of heat treatment.