Microstructure and Mechanical Properties of Friction Stir Welded 17-4 Martensitic Stainless Steels

17-4 steel, also known as UNS S17400 is a chromium-nickel-copper precipitation hardening (PH) martensitic stainless steel. This material is widely used as a structural material because of its excellent combination of mechanical properties and corrosion resistance. Along with high strength, it offers a good combination of corrosion resistance, fabricability and toughness and hence finds many applications in the aerospace, petrochemical, nuclear and food processing industries. Fusion welding of these materials is problematic and requires post weld heat treatment (PWHT) before putting into service. In this study, friction stir welding (FSW) which is a solid state joining process was investigated with the objective of understanding the microstructure and mechanical properties of the welded joints. Successful welds were produced with a W-Re-4%HfC pin tool at a rotational speed of 200 rev●min^-1 and travel speed of 50 mm●min^-1.  FSW produced recrystallized and extremely fine grain size (~2 µm) in the weld nugget region. Detailed microstructural analysis was performed for parent material, nugget region, heat affected zone using optical and scanning electron microscopy. The welds were subjected to various post weld aging treatments and the microstructure, grain size and mechanical properties were evaluated and compared with the as-welded specimens.