HIGH-TEMPERATURE INTERGRANULAR CRACK-GROWTH IN MARTENSITIC 21/4CR-1MO STEEL

Micromechanisms of high-temperature crack growth in martensitic 2 one quarter Cr-1Mo steel have been studied in vacuum, under static loading at 500 degree C. Detailed metallographic and fractographic measurements have been combined with Scanning Auger Microscopy and crack growth resistance curves to characterise the micro-mechanisms of failure. At low stress intensities, less than 48-55 MPam** one-half , the mode of crack growth is high-temperature, brittle intergranular fracture (HTBIGF) and is controlled by the dynamic segregation of sulfur to crack-tip regions. Crack advance appears to occur by discrete jumps when a critical concentration of sulfur is achieved over the jump-distance. At high stress intensities, greater than 48-55 MPam** one-half , the mode of fracture changes to intergranular microvoid coalescence (IGMVC), and is controlled by the distribution of sulfides. Of crucial importance are the relatively fine sulfides that reprecipitate from solid solution during the austenitising treatment.