Microstructural Evolution and Mechanical Response of AISI 304 Stainless Steel Under Multiaxial Fatigue Loading: Relevance to Pressure Vessel Design in Nigeria

This study investigates the microstructural evolution and mechanical response of AISI 304 austenitic stainless steel under multiaxial fatigue loading conditions representative of pressure vessel service in Nigerian chemical processing facilities. Pressure vessels in Nigerian process industries experience complex multiaxial stress states at nozzle attachments and discontinuities, yet fatigue design guidance available to Nigerian engineers is predominantly based on uniaxial data, potentially underestimating multiaxial damage accumulation. Solid cylindrical specimens machined from Type 304 plate are subjected to proportional and non-proportional in-phase and out-of-phase combined bending-torsion fatigue cycles using a biaxial fatigue testing machine. Test conditions cover von Mises equivalent strain amplitudes of 0.2, 0.3, and 0.5 percent. Interrupted fatigue tests are conducted for microstructural examination at 10, 30, and 70 percent of fatigue life. Microstructural characterisation includes EBSD texture analysis, deformation-induced martensite fraction by XRD and magnetic permeability, slip band density by SEM, and dislocation density by TEM. Findings reveal that non-proportional loading produces approximately 35 percent shorter fatigue lives than proportional loading at equivalent strain ranges, associated with higher martensite transformation rates and greater slip band accumulation in non-proportional specimens. A modified fatigue life prediction model incorporating deformation-induced martensite fraction is proposed. Keywords: multiaxial fatigue, AISI 304, stainless steel, pressure vessel, microstructural evolution.