Fatigue Crack Propagation in Austenitic Stainless SteelWeldments J.T. AL-HAIDARY, A.A. WAHAB,and E.H.ABDUL SALAM The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two weldin techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Mag- netic d-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of d-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions ofhigh and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. Th FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propa- gated perpendicular to the weld zone, particularly in the case of SAW in which higher d-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures. I. INTRODUCTION THE austenitic stainless steel (ASS) represents the larg- estgroup of stainless steels (SS) in use, making up 60 to 70 pct of the total of SS, because it has excellent mechan- ical properties, in addition to a high level of fabricability and corrosion resistance. [1] Most of the ASS applications use the weldment construc- tions, which require a better understanding of the mechan- ical behavior of the welded components. It is wellknown thatthe majority of fractures that occurin welded steel components are of fatigue type. To minimize the susceptibility of ASS weldments to fis- sure,a smallpercentage of d-ferrite, which is generally found in thesemicrostructures 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 Crcontents 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 char- acteristics. [3] The residual stresses within a weldment result from the restrained contraction of the weld metal as it sol- idifiesand coolsto room temperature. Normally,weld metal exhibits high residual tensile stresses, while balanc- ing compression residual stresses are established in the base metal (BM). [4] The existence of residualstresses within a weldment subjected to a fluctuating load will not affectthe stress intensity range but will affectthe mean stress and hence the stress ratio (R 5 s min /s max ). Stress ratio effects can be incorporated into the Paris equation (da/dN 5 CDK m ) [5] (where C and m are the material constants) to give the Forman equation: [6] da dN 5 CDK n ð1 RÞK c DK where DK is the stress intensity factor, da/dN is the fatig crack propagation rate (FCPR), C and n are the material constants of the same type as those in Paris law, and K c is the fracture toughness of the material. From the last equa- tion,it can be seen that if, for example, the fatigue crack encountered a region of residual tensile stresses, its rate propagation would increase. [4] The fatigue crack growth rate tests for nearly all metal structural materials show that the da/dN vs DK curves ha the following characteristic: a region at low values of da/ and DK in which fatigue cracks grow extremely slowly or notat all below a lower limit of DK called the threshold value,DK th ; an intermediate region of power-law behavior is described by the Paris equation, and an upper region of rapid unstable crack growth with an upper limit of DK co responds either to the fracture toughness (K ic ) or to gross plastic deformation of the specimen. [7,8] The stress intensity factor (DK) for the adopted specim geometry could be calculated using the same formula us by Loureiri and Fernande and Langoy and Stock. [9,10] The quantitative methods of crack propagation and fin fracture require the use of fracture mechanics concepts. fundamental principle of the fracture mechanics is that t stress field ahead of the sharp crack in a structural mem can be characterized by a single parameter, DK (MNm 3/2 ). This parameter, DK, is related to both the normal stress l (s) and the size of the crack present (a). The understand J.T. AL-HAIDARY, Tutor,is with the Department of Materialand Metallurgical Engineering, Al-Balqa Applied University, Al-Salt 19117, Jordan.Contacte-mail:jalhaidary@yahoo.com A.A. WAHAB, Assistant Lecturer, and E.H.ABDUL SALAM, Lecturer, are with the Department of Production Engineering and Metallurgy, University ofTechnology, Baghdad 13050, Iraq. Manuscript submitted November 7, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A,NOVEMBER 2006—3205