Fracture resistance of low-carbon alloy irons

The temperature dependence of the critical stress intensity factor and of the fracture energy were measured on six low-carbon iron alloys, one containing 0.002 wt pct C and five containing 0.02 wt pct C. Either Ni, P, Si, or Si and Mn were added to four of the five 0.02C irons in quantities typically found in ferritic steels. The fracture tests were conducted at rapid (but less than impact) speed of 1 ips on fatigue cracked, three-point bend beam specimens. Each alloy was tested over a temperature range of —195° to 24°C in both furnace-cooled and quench-aged states. Both alloying and heat treatment produced wide differences in the fracture resistance of these alloys. The quench-aged 0.002C iron and furnace-cooled phosphorus alloy failed by intergranular separation, whereas the remaining alloys exhibited cleavage fractures. With the exception of 0.002C iron, an alloy in the quench-aged condition had higher fracture toughness than the same alloy in the furnace-cooled state. The transition temperature, however, was influenced by heat treatment only in the plain carbon irons. In this case the transition temperature was independent of carbon content but the furnace-cooled specimen had a lower transition temperature than the quench-aged specimens. D. C. A. R. COX, formerly Exchange Scientist at the Naval Research Laboratory