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

摘要

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 aquenched and tempered condition and is highly susceptible to hydrogen assisted cracking associated withconventional fusion welding. FSW/P (400 RPM/ 2 IPM) was conducted using a polycrystalline cubic boron nitridetool having a pin length of 6.35 mm. Two sets of processing runs were completed on HY-80 plates, 6.35 mm inthickness, one dry and the other under seawater. Analysis of the residual hydrogen content revealed little change inhydrogen content associated with FSW/P. Detailed microstructural characterization (optical and scanning electronmicroscopy) with emphasis on the differences in the distribution of constituent phases in the rolling, transverse andnormal planes was carried out. Microstructural analysis of the “plan-view” section at the tool extraction sites ofFSW/P samples was executed. Mechanical properties of these samples were evaluated by tensile, microhardness, andCharpy V-notch impact resistance tests. The results indicated that austenitization occurred within the stir zones andsubsequent cooling influenced the final microstructure and properties. The thermomechanically affected zonedisplayed a gradual change in near-equiaxed austenitic grain size towards both the tool axis as well as the base platewith little grain shape change throughout. These results have been rationalized on the basis of expected temperatureand deformation fields generated setup in these steel samples and the nature of phase transformations occurring in lowcarbon steels such as HY-80.