This study describes the mechanisms of emergence and propagation of fatigue cracks caused by mechanical tension stress fluctuations in dissimilar steels butt and overlap welded joints under axial tension fatigue loads. A structural (ASTM A537, class I) and a stainless (ASTM A240, 304L) were soldiers through GMAW, Argon as protecting gas and a stainless (ASTM A240, 308L) as a supplier material, not being submitted to pre and post welding thermal treatment. Microstructures (Scanning Electron Microscopy, SEM) were contrasted in different zones of each joint, focus on Heat Affected Zone (HAZ) and fusion lines. Samples were inspected by not destructive test (penetrating liquids and ultrasound), to discard surface and internal defects. The following mechanical tests were compared between both welding joint (WJ): Vickers hardness profile, tension, bending, impact, axial fatigue, and speed of propagation of fatigue cracks. Vickers show high values of micro hardness in the HAZ, near the fusion line between weld and stainless. Tension and axial fatigue tests indicated similar behavior between WJ and structural (butt joint); and similar behavior between WJ and stainless (overlap joint). Pre-cracked test evidence a faster growth of crack in the fusion line between structural steel and stainless. Dissimilar unions (butt and overlap) have mechanical and microstructure properties, which can be considered adequate to withstand the mechanical requirements in service conditions, despite relatively high values of hardness in the HAZ, particularly in the fusion line between the welding cord and the stainless 304L, as well as inclusions between the structural and the stainless one.
Random non-proportional fatigue tests with planar tri-axial fatigue testing machine