Synergetic Effects of Macro- and Microscopic Residual Stresses Induced by High-Frequency Mechanical Impact Post-weld Treatment on Fatigue Strength Enhancement of S335 Steel T-Weld

The effectiveness of high-frequency mechanical impact (HFMI) treatment is studied in this paper considering the reasons of the fatigue life improvement of S335 steel T-welded joints with non-load transverse stiffeners. The main attention is paid to the synergic effects of macroscopic and microscopic residual stresses induced by HFMI post-weld processing as the key factors for superior fatigue life. Macroscopic residual stresses and their distribution nearby (at a distance of 3 mm) and away from (13 mm) the weld toe or the HFMI-produced groove were studied by electronic speckle interferometry and x-ray diffraction (sin 2 ψ -based method). It was established that HFMI treatment resulted in the transformation of the welding-induced tensile residual stresses (of about yield stress (~ 0.9 σ Y )) into compressive ones (of ~ 0.9 σ Y ) near the weld toe, while the stresses did not relax away from the HFMI-formed groove. Microscopic residual stresses induced by HFMI were studied as a factor affecting the physical broadening (full width on half maximum—FWHM) of the x-ray diffraction maxima. The correlations of microscopic stresses (lattice micro-strains) with dislocation density, strain-induced dissolution of cementite, and carbon content in the ferrite lattice observed by TEM and the HFMI-induced crystallite refinement with surface hardness are discussed. For various HFMI treatment working speeds employed (1, 5, and 10 mm/s), the hardened layer depths of the S335 steel specimens were in the range of 0.6–1.0 mm. After HFMI, the fatigue strength of the T-welded joint of the S335 steel was observed to be increased by 54% (from 163 to 251 MPa) at 2 × 10 6 cycles, and fatigue life prolongs by 10 times.