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.

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According to AWS 5.18, the all weld metals produced with filler metals through GTAW should present the same minimum values in tensile tests, as follows, minimum yield strength 400 MPa, minimum tensile strength 480 MPa and minimum elongation 22%. The impact test requirements to the all weld metals produced with filler metals ER70S-2, ER70S-3 and ER70S-6 should present 27 J as the minimum average impact strength at -29 ᵒC to ER70S-2 and ER70S-6, and at -18 ᵒC to ER70S-3. Although the mechanical properties of the all weld metals produced using these three welding electrodes are quite similar, their chemical compositions are different specially in the contents of deoxidizing elements like Zr, Al, Ti, Si and Mn. The contents of deoxidizing elements are lower in ER70S-3, intermediate in ER70S-6 and higher in ER70S-2. Electrodes and rods of the ER70S-2 classification are primarily used for single-pass welding of killed, semi-killed, and rimmed steels, but may be used for some multipass applications. Due to the added deoxidants Zr, Al and Ti in the chemical composition of ER70S-2, this filler metal can be used for welding steels that have a rusty or dirty surface,with a possible sacrifice of weld quality depending on the condition of the surface. A detailed study to compare the performance of filler metals ER70S-2, ER70S-3 and ER70S-6 was conducted through GTAW bead on plate coupons using the three different electrodes on bright polished and rusty plates. The performance of the electrodes was studied through GTAW groove test weld assembly with and without purging gas. The results showed that, although ER70S-2 presented the best performance among the three welding wires tested in bead on plate in rusty plates, welding groove joints without purging gas using this welding electrode is practically unviable though GTAW. Fluid flow in the weld pool determines GTAW weld pool shape. The force driving the fluid flow is surface tension gradient. This is called Marangoni convection. Due to the higher contents of deoxidants in the chemical composition of ER70S-2, the weld pool produced using this filler wire is strongly deoxidized and the result is that when the area under the arc is heated up, the surface tension drops in the center of the weld pool and the direction of the fluid flow is from the center towards the edges of the pool. The results suggest that the change in the fluid movement due to deoxidation of the weld pool, causes slag to be trapped between base metal and weld pool.

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