Evaluation of Friction Stir Processing of HY-80 Steel Under Wet and Dry Conditions

Abstract

This thesis describes the microstructural and mechanical property changes associated with Friction Stir Processing (FSP) of HY-80 steel under dry and underwater conditions. HY-80 is a low-carbon alloy steel that is used in a quenched and tempered condition and is highly susceptible to hydrogen assisted cracking associated with conventional fusion welding. FSW/P (400 RPM/ 2 IPM) was conducted using a polycrystalline cubic boron nitride tool having a pin length of 6.35 mm. Two sets of processing runs were completed on HY-80 plates, 6.35 mm in thickness, one dry and the other under seawater. Analysis of the residual hydrogen content revealed little change in hydrogen content associated with FSW/P. Detailed microstructural characterization (optical and scanning electron microscopy) with emphasis on the differences in the distribution of constituent phases in the rolling, transverse and normal planes was carried out. Microstructural analysis of the “plan-view” section at the tool extraction sites of FSW/P samples was executed. Mechanical properties of these samples were evaluated by tensile, microhardness, and Charpy V-notch impact resistance tests. The results indicated that austenitization occurred within the stir zones and subsequent cooling influenced the final microstructure and properties. The thermomechanically affected zone displayed a gradual change in near-equiaxed austenitic grain size towards both the tool axis as well as the base plate with little grain shape change throughout. These results have been rationalized on the basis of expected temperature and deformation fields generated setup in these steel samples and the nature of phase transformations occurring in low carbon steels such as HY-80.