THE EFFECT OF COBALT ON THE STRENGTH AND TOUGHNESS OF NI-CR-MO HIGH-YIELD-STRENGTH STEELS.

Quenched and tempered steels also hold promise as base metals for an HY-180210 weldment. Therefore, a program was initiated to evaluate quenched and tempered steels for this application. The effect of varying cobalt content from 0 to 8 percent on the strength and toughness of 5Ni-1.5Cr-0.5Mo steels containing either 0.18 or 0.25 percent carbon was determined. Studies on 12-inch-thick water-quenched plates showed that all the steels exhibited optimum toughness at high yield strengths when tempered at 400 F. Steels without cobalt and a steel containing 0.23 percent carbon and 8.04 percent cobalt had the best notch toughness at yield strengths in the range 170 to 210 ksi. The toughness of these steels approached that obtained in 12-inchthick plates from production heats of 12Ni-5Cr3Mo maraging steel. Except for the steels containing 8 percent cobalt, increasing cobalt content generally lowered toughness without markedly increasing strength. Raising the carbon content of the base steel from 0.19 to 0.26 percent resulted in an increase in yield strength from 170 to 189 ksi when the steel was tempered at 400 F. Author