Study on the effect of ferrite number on impact toughness of austenitic stainless steels at low temperatures

Abstract

All ferrous materials, except the austenitic grades, exhibit a transition from ductile to brittle when tested above and below a certain temperature, called as ductile to brittle transition temperature (DBTT). In order to better understanding the effect of the volume fraction of δ ferrite in the impact toughness of austenitic stainless steels at low temperatures, the microstructures of welded joints of austenitic stainless steel produced through SAW, were studied. The groove welds were produced using welding electrodes and flux of the same specification, ER 316L 3.2 mm and a fluoride basic flux (CaF2-Al2O3-SiO2). The filler metals used are from different heats with different chemical compositions resulting in different ferrite numbers. The base metal used is AISI 316L TYPE plates of 25 mm thickness. The chemical compositions and the variation of the volume fractions of δ ferrite in the deposits were measured.The welded coupons were tested at -196 ᵒC to measure the impact toughness. The results confirm that the volume fraction of δ ferrite is of paramount importance in the impact toughness of austenitic stainless steels at cryogenic temperatures. Complementary techniques of microstructural analysis were used, such as optical emission spectrometry, optical microscopy and quantitative image analysis.