Influence of Cold Rolling Threads Before or After Heat Treatment on the Fatigue Resistance of High Strength Fine 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 at five different preload values. Bolts used were 3/8 UNRF-24 (fine) 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 % of proof stress (to keep load the same). The tests produced a range of R ratios (R=S_min/S_max) between 0.03 and 0.92. Maximum near surface residual compressive stresses, obtained via x-ray diffraction, ranged from −500 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. Multiple ratchet marks (separate crack nucleation sites) occurred for roll before heat treatment bolts, while fewer or no ratchet marks, were evident for the roll after heat treatment. Scanning electron microscopy evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions contained ductile dimpling. Cyclic creep/ratcheting was monitored and little, or none, was observed for preload tests equal to or less than 75 %. Replication data indicated a log-normal distribution on life was very reasonable. The fatigue resistance for fine threads rolled after heat treatment with preload stresses of 1 % (R ratio less than 0.05) caused very large increases (158 %) in 10⁷ cycles fatigue strength compared to roll before heat treatment. This is in agreement with other roll before/after low R-ratio results. The roll after heat treatment bolts when tested at the higher proof loads had 10⁷ fatigue strengths of 69 to 30 % increase. These increases are much less than the 158 % at 1 % preload, but still significant. Constant life Haigh diagrams at 10⁵ and 10⁷ cycles were in qualitative agreement with VDI 2230 bolt preload guidelines.