Effect of base metal solution annealing on mechanical and metallurgical properties of GMA welded nitronic steel

Abstract

In the present experimental investigation, an attempt has been made to study the weldability aspects of 21-4-N nitrogen-containing austenitic stainless steel (nitronic steel). The welding was performed on as-received and solution treated conditions of the base metal by gas metal arc welding (GMAW) process employing ER2209 filler metal. Metallurgical and mechanical properties of the base metal and the weld joints were studied in light of optical microscope, (scanning electron microscope) SEM micrographs, XRD (X-ray diffraction), Microhardness, tensile and Charpy impact toughness test results. The weld metal initially solidified as austenite + δ-ferrite, a dual-phase microstructure near interface regions without showing any solidification cracking. The maximum δ-ferrite (approximately 20% measured with ferritescope) was found at the weld center which decreases towards the weld-HAZ (Heat affected zone) interface. The solution annealing improved the tensile strength, ductility and impact toughness of the base metal due to the dissolution of carbides. The carbide phases like Cr23C6 and Cr7C3 confirmed in the X-ray diffraction pattern of the base metal. The average hardness of the base metal decreased after solution annealing (950 °C and 1150 °C) and found minimal variations in hardness for 1150 °C treated base metal. The tensile strength of the weld joint was found maximum (for weld metal 897 MPa and HAZ 937 MPa) for 1150 °C treated base metal. Also, the impact toughness was found maximum for weld metal followed by HAZ and base metal. The tensile and impact samples for as-received and 950 °C solution annealed base metal were fractured in brittle mode while the samples with 1150 °C treated base metal were fractured in mix mode (quasi-cleavage). The weld failure was purely ductile fracture.