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E austenitic stainless steel (ASS) represents the largest group of stainless steels (SS) in use, making up 60 to 70 pct of the total of SS, because it has excellent mechanical properties, in addition to a high level of fabricability and corrosion resistance.[1] Most of the ASS applications use the weldment constructions, which require a better understanding of the mechanical behavior of the welded components. It is well known that the majority of fractures that occur in welded steel components are of fatigue type. To minimize the susceptibility of ASS weldments to fissure, a small percentage of d-ferrite, which is generally found in these microstructures depending mainly upon nickel and chromium equivalents present in the ASS, when they are cooled to room temperature, will help. The d-ferrite content in weld deposit can be predicted by calculating the equivalents of Ni and Cr contents in the ASS weld zone using the Schaeffler diagram, as reported by Olson.[2] Dixon,[3] in his study, presented the role of d-ferrite in the control of solidification cracking. The welding process introduces residual stresses into the material and also results in the diffusion of the weld metal, both of which will alter the fatigue crack propagation characteristics.[3] The residual stresses within a weldment result from the restrained contraction of the weld metal as it solidifies and cools to room temperature. Normally, weld metal exhibits high residual tensile stresses, while balancing compression residual stresses are established in the base metal (BM).[4] J.T. AL-HAIDARY, Tutor, is with the Department of Material and Metallurgical Engineering, Al-Balqa Applied University, Al-Salt 19117, Jordan. Contact e-mail: [email protected] A.A. WAHAB, Assistant Lecturer, and E.H. ABDUL SALAM, Lecturer, are with the Department of Production Engineering and Metallurgy, University of Technology, Baghdad 13050, Iraq. Manuscript submitted November 7, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

The existence of residual stresses within a weldment subjected to a fluctuating load will not affect the stress intensity range but will affect the mean stress and hence the stress ratio (R 5 smin/smax). Stress ratio effects can be incorporated into the Paris equation (da/dN 5 CDKm)[5] (where C and m are the material constants) to give the Forman equation:[6] da CDK n 5 ð1
RÞK c
DK dN where DK is the stress intensity factor, da/dN is the fatigue crack propagation rate (FCPR), C and n are the material constants of the same type as those in Paris law, and Kc is the fracture toughness of the material. From the last equation, it can be seen that if, for example, the fatigue crack encountered a region of residual tensile stresses, its rate of propagation would increase.[4] The fatigue crack growth rate tests for nearly all metallic structural materials show that the da/dN vs DK curves have the following characteristic: a region at low values of da/dN and DK in which fatigue cracks grow extremely slowly or not at all below a lower limit of DK called the thresh