Characterization of failure modes for different welding processes of AISI/SAE 304 stainless steels

Weld joints manufactured with a welding electrode type 308L and by three different arc welding processes shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW) in a AISI/SAE 304 were studied in order to compare the failure mechanisms associated with their mechanical and microstructural properties. Chemical compositions were analyzed by optical emission spectroscopy and the ferrite numbers (FN) of the welds were also identified. Relevant microstructural characteristics of the different processes were analyzed by microscopy techniques. Finally, fatigue tests were performed to study the variations in the mechanical properties of each process and to analyze their most probable failure modes by means of a fractographic study, in which the characteristic morphologies of each one (nucleation, propagation, final fracture) were identified by means of optical stereoscopy and scanning electron microscopy (SEM). Three different fracture modes were found at the welding joints that showed correlations with microstructural changes produced during the welding process. The first failure mode displayed that the nucleation of the crack was at the weld root. The second failure mode was generated at the heat affected zone (HAZ), where the crack nucleated due to a variation in the grain size produced by the process and then further propagated through the edge of the weld. The third failure mode appeared due to the presence of exogenous inclusions generated by the welding process, which acted as stress concentrators in the weld and produce the initiation and further propagation of the crack. Lastly, some welding processes presented a combination of the previous failure modes and consequently multiple sites of crack nucleation.