Toughness of Warm Worked Martensitic Steels

The Charpy “V” notch transition temperature of quenched and tempered armor steel was lowered by warm stretching at a temperature just below the initial tempering temperature. The transition temperature was lowered almost linearly with prestrain; and a 30 percent deformation suppressed it approximately 150 deg F. This toughness improvement occurred with no change in hardness, although there was a loss in super-transition temperature shelf height. The initial tensile strength of the steel was only slightly changed, while the yield strength was increased and ductility reduced. The enhanced toughness persisted through retempering after warm working. The added heating did not change the hardness while the supertransition shelf was brought back to its “as-received” level. In addition, the tensile strength, yield strength, elongation, and reduction in area of the warm stretched plus tempered and “as-received” steel were essentially identical, resulting in a net increase in toughness. A severe room temperature toughness loss was produced by compressive prestrains in excess of about 10 percent. Retempering after straining not only delayed this precipitous loss until the strain exceeded 25 percent, but also raised the Charpy energy after small compressions to about 140 percent of its “as-received” values. Step-wise prestraining was found to be as effective as a single straining step in lowering transition temperatures. In a single test series warm working by rolling was compared with stretching. The suppression of the transition temperature was found to be almost identical for the two deformation processes.