The Influence of Residual Stresses on Fatigue Behaviour of Medium Carbon Microalloyed Steel

Licentiate thesis, 2009

Subjected to the dynamic loading, truck components are exposed to fatigue loading. Hence the methods to improve the fatigue properties of the components are widely employed by manufacturers. One of the well known methods in this field is shot peening. Shot peening introduces compressive residual stresses in the surface zones and changes the work hardening state there. Compressive residual stresses provide higher resistance to crack initiation and may postpone the crack propagation. However the surface roughness is considerably increased by shot peening, which of course is not favourable for the fatigue durability. Steering arm as a truck component is the subject of present study. This component may be made of two different materials: medium carbon microalloyed steel or quenched and tempered steel. The former is continuously cooled leading to pearlitic-ferritic microstructure and the latter has a tempered martensite microstructure. Although the pearlitic-ferritic steering arm made of microalloyed steel is the main focus of this study, the tempered martensite microstructure was also investigated to a certain extent. Strain controlled, constant amplitude push-pull fatigue tests at different strain amplitudes were performed on cylindrical specimens. The specimens were machined out of the microalloyed steel steering arms. The specimens then were tested in two groups: one group in mirror finished condition and the other group in shot peened condition. The results showed that the life time of specimens at higher strain levels (higher plastic strain amplitudes) is not effected by shot peening. However at smaller plastic strain amplitudes the peened specimens exhibit a life time at least one order of magnitude longer than unpeened specimens. The residual stress relaxation and changes of work hardening state were also monitored at the surface and in depth for different strain amplitudes. The influence of plastic strain amplitude on relaxation of residual stresses was studied in detail as a function of fatigue cycle number. The fatigue behaviour in bending, including relaxation, was also studied for the tempered martensitic steel with different degrees of decarburisation and subsequent shot peening. Also in this case a positive influence of shot peening is mainly seen at small strain amplitudes. In parallel whole components were also tested in constant and variable amplitude loading fatigue, with and without shot peening. The results here also show a longer life time for the peened components.