Volume 3, Issue 3 (March 2006)
Influence of Cold Rolling Threads Before or After Heat Treatment on the Fatigue Resistance of High Strength Coarse Thread Bolts for Multiple Preload Conditions
SI class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using five different preload values. Bolts were 3/8 UNRC-16 (coarse) and preloads were taken as 1, 50, 75, 90, and 100 % of roll before heat treatment proof stress. Since proof stress was lowered 10 % for roll after heat treatment, these preloads for roll after heat treatment bolts were then actually 1.1, 55, 83, 100, and 110 %. The tests produced a range of R-ratios between 0.03 and 0.92. Maximum near surface residual compressive stresses, obtained via X-ray diffraction, ranged from −600 to −1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface with crescent-shaped cracks dominating in most tests. SEM evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions contained ductile dimples. Cyclic creep/ratcheting was monitored and little, or none, was observed for preload tests equal to or less than 75 %. The fatigue resistance of the threads rolled after heat treatment with preload stresses of 1 % caused a 147 % increase in 107 cycles fatigue strength compared to roll before heat treatment. This is in agreement with other roll before/after low R-ratio results. However, with higher preload stresses, the roll after heat treatment fatigue strength did not increase. Constant life Haigh diagrams at 105 and 107 cycles were not always in agreement with VDI 2230 bolt preload guidelines.