Dependence of the Yield and Fatigue Strength of the Thread Rolled Mild Steel on Dislocation Density

Dependence of the yield and fatigue strength of steel bolts with composition in accordance to AISI 1035 manufactured by thread rolling and machining process on dislocation density were investigated. The results indicate that the fatigue strength of the rolled bolts are 55% higher than the machined bolts and by full annealing at $850°C$⁠, it reduced to the extent of machined specimen. Partial annealing of the thread rolled bolts at $680°C$ caused a reduction of fatigue strength by approximately 61% due to reduction in the dislocation density. Fatigue strength was improved by deformation rate (i.e., rolling speed), which is also due to the increasing dislocation density. Yield stress of the studied specimens followed the same pattern as fatigue strength. Considering the obtained results from the low and high speed, partial and full annealed thread rolled specimens, yield stress of the thread rolled bolts has been modeled based on the dislocation density. The obtained results from the model are in good agreement with the experimental results. The contribution to fatigue strength by thread rolling stems from the strain hardening effect which would facilitate the formation of compressive residual stress near the surface layer. The strengthening may be attributed to increasing dislocation density in the ferrite phase (i.e., substructure formation), in addition to the formation of a fine layered structure consisting of elongated pearlite colonies and ferrite grains.