Residual stress relaxation and fatigue behavior of an induction hardened microalloyed steel

The thermal and mechanical relaxation of compressive residual stresses during tempering and cyclic loading of an induction hardened vanadium microalloyed steel has been evaluated. A microstructural analysis was also carried out on the microalloyed steel to correlate the residual stress relaxation behavior with microstructural characteristics of the material. Vanadium carbide particle size and distribution were analyzed as well as how these characteristics are affected by the application of normalizing and induction hardening heat treatments. To determine the effect of vanadium carbide particles on the residual stress relaxation response of the microalloyed steel a parallel study was conducted on a 1530 steel which is similar in chemistry to the microalloyed steel, but without the vanadium.; The thermal relaxation of compressive residual stresses due to tempering for 2 hours after induction hardening was evaluated for a range of tempering temperatures from {dollar}rm 177spcirc C (350spcirc F){dollar} to {dollar}rm 579spcirc C (1075spcirc F).{dollar} Mechanical residual stress relaxation was evaluated by subjecting specimens to cyclic loading conditions. For this part of the work a special type of specimen was designed. The specimen had an overall C-shape with a T-shaped cross section. This specimen geometry generates higher levels of stresses in the induction hardened outer layer than in the soft core material along the inner layer of the C-shaped geometry. The compressive residual stresses were generated by the phase transformation that occurs during hardening heat treatments and also by mechanical means. Additional compressive stresses were put into the outer surface region of the hardened C-shaped specimens by pre-straining them plastically through the application of compressive loads.; Fine vanadium carbide precipitates were observed in the microalloyed steel in the as-forged condition. The application of a normalizing and induction hardening heat treatments caused coarsening of the vanadium carbide particles. {dollar}varepsilon{dollar}-carbide was identified in samples tempered at {dollar}rm 177spcirc C{dollar} and cementite at higher tempering temperatures. High compressive residual stress levels were generated at the hardened outer surface layer by the phase transformation and pre-straining. The compressive residual stresses were reduced during tempering. At low tempering temperatures the residual stress relaxation occurred basically by the contraction experienced by the steel due to the precipitation of carbides. At higher tempering temperatures the relaxation of compressive residual stresses occurred mainly by creep. The relaxation of residual stresses by creep is assisted by the dislocation pipe diffusion of iron atoms in ferrite and can be represented by an Avrami type of function. During fatigue, a significant reduction of compressive residual stresses occurred in the first few loading cycles due to the Bausingher effect. After this transient phenomenon, the residual stresses monotonically relaxed with the logarithm of the number of applied cycles. The vanadium microalloyed steel showed greater resistance to mechanical stress relaxation than the 1530 steel.