8. Uluslar Arasõ Kõtõlma Konferansõ Bildiriler Kitabõ 7 – 9 Kasõm 2007 Prooceedings of 8th International Fracture Conference 7 – 9 November 2007 Istanbul/TURKEY 518 INVESTIGATION OF THE EFFECTS OF AGING ON THE ROOM AND HIGH TEMPERATURE TENSILE PROPERTIES AND FRACTURE OF STAINLESS STEEL 316L WELD METAL Asadollah KARIMIYAN, Hassan FARHANGI and Ali Amari ALLAHYARI School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran. ABSTRACT In this research, variations of room and high temperature tensile properties and fracture behavior of 316L austenitic stainless steel weld metal as a function of aging temperature and time have been investigated. Stainless steel plates were welded using gas tungsten arc welding technique. Tensile test specimens of weld metal were subjected to various aging heat treatments at temperatures of 750 and 850°C for periods of 1 to 100 hours. Microstructural analyses were performed using techniques including metallography, EDS analysis, and measurement of ferrite content with ferritoscope. Fracture behavior of the weld metals was characterized by conducting tensile tests at 25 and 500°C, and performing fractographic and microstructural observations using SEM. Transformation of delta-ferrite to the intermetallic sigma phase during aging resulted in a mild increase in tensile strength and significant reduction in ductility. Slant and flat macroscopic fracture modes were observed in the aged weld metals, with the slant mode being dominant at 500°C. This mode was associated with deformation localization along arrays of primary voids, nucleated at cracked sigma phase particles, oriented at about 45° to loading direction. The transition in the fracture mode is further discussed based on variations in the dimpled fracture morphologies and strain hardening exponents. Keywords: 316L weld metal, aging, dimpled fracture, flat fracture, slant fracture. 1. INTRODUCTION Austenitic stainless steel alloys are used extensively in heat resistant structural components in power generating and chemical industries due to their metallurgical stability, excellent corrosion resistance, and good creep strength and ductility properties at elevated temperatures [1-3]. It has been established that successful welding of this type of stainless steel requires the presence of at least 5%, by volume, of h-ferrite in the microstructure [2-4]. The ferrite is beneficial because it restricts grain growth and prevent liquid cracking by limiting impurity element diffusion and inhibiting wetting of liquid films [1,2]. During post weld heat treatment or high temperature exposure as encountered in service, the ferrite transforms to a variety of secondary phases such as M23C6 carbides as well as the intermetallic phases such as j and ぬ [4-7]. These phases are known to degrade the mechanical and corrosion properties of the weld metal [4]. However, the formation of these phases is influenced by a number of factors such