TOUGHENING HIGH STRENGTH STEEL BY WARM WORKING.
Technical rept. no. 32,
CASE INST OF TECH CLEVELAND OH
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Warm-worked steel exhibited a rheotropic recovery at subzero testing temperatures much like that found in steels which were deformed at room temperature and subsequently aged. Transition curves obtained with Charpy V-notched specimens of SAE 1340 steel which were extruded at 500 degress and 700 degrees F are presented. Extruding at 400 or 700 F lowered the transition temperatures to slightly below ambient. With the as-heat treated condition as a basis, extruding at 700 F caused a hardness drop of about 2 points Rockwell C while extruding at 500 F increased the hardness by about 2 points. The 500 and 700 F extrusions elevated the room-temperature toughnesses by factors of about 2 and 7, respectively. For the 700 F extrusion, the yield strength of the steel was raised from 216,000 to 234,000 psi, and the tensile strength and ductility were increased, respectively, from 237,000 to 238,000 psi and from 47.5 to 49.5 contraction in area. The strain-hardening exponent of the extruded material was lowered so that the fracture stress was decreased from 392,000 to 34,000 psi. Charpy impact tests were made on SAE 4340 steel both in the as-heat treated tempered at 700 F condition and in the heat-treated and extruded condition 700 F. The transition temperature point was not lowered. The toughness level at all testing temperatures was elevated to about twice that of the unworked metal while the hardness decreased about 1 point.