CYCLIC STRENGTH OF PRECIPITATION HARDENED AND STRAIN HARDENED CARBON STEEL.

This work concerns the possibility of improving the cyclic strength by the combined effect of precipitation hardening (hardening due to precipitation of dispersed particles of special carbides and nitrides during high-temperature tempering) and strain hardening. The study was made on steel 40PP (0. 4% C, 0. 2% Mn, 0. 1% Si) and steel 40Pp with vanadium (0. 4% C, 0. 2% Mn, 0. 1% Si, 0. 6% V, 0. 1% Ti). The study showed that the structural differenes in steels 40PP and 40PP with 0. 6% V resulting from quenching and tempering have a substantial effect on the fatigue test results. The low-cycle fatigue limit and the fatigue limit of the steel with 0. 6% V are double those of steel 40PP (after tempering at 550 degree C, sigma // minus //1 equals 33 kg/mm**2 for steel 40PP and 66 kg/mm**2 for 40PP with 0. 6% V). The fatigue strength of carbon steel 40PP is probably due to the favorable compressive stresses and to a lesser extent to refining of the substructure. The high cyclic strength of precipitation hardened steel 40PP with 0. 6% V is evidently due to the combined influence of the stable substructure resulting from precipitation hardening and the compressive stresses resulting from shotpeening.