The microstructure of the weld can be described by studying the Creq/Nieq ratio in relation to the solidification type. The liquidus and solidus projections of the Fe–Cr–Ni system along with the related binaries are shown in Fig. 4(a)[22]. The Fe–Cr system is isomorphous down to temperatures well below the solidification range. Also, a eutectic reaction can be observed in the Cr–Ni system at 1345 °C and 49 wt.% nickel. In the Fe–Ni system, there is a short peritectic loop containing δ-ferrite and after that the system is completely soluble. Thus, in the Fe–Cr–Ni ternary system, the liquidus projection starts with a peritectic reaction on the Fe–Ni system (δ + L → γ) and moves down to the eutectic reaction (L → δ + γ) on the Cr–Ni system. The initial solidifying phase is determined by the position of the alloy with respect to the liquidus surface. The 70 wt.% Fe isopleth of the ternary Fe–Ni–Cr system shown in Fig. 4(b) is commonly used to identify the primary solidifying phases or solidification modes for AISI 304L stainless steel [22]. The amount of Creq/Nieq is the most effective parameter in determining the solidification mode. By increasing the ratio of Cr equivalent to Ni equivalent, the primary solidification phase changes from austenite to δ-ferrite. In the usual cooling rate range during GTAW, this change normally occurs at Creq/Nieq = 1.5. It should be noted that this change in ratio is different in welding processes with more rapid cooling such as laser welding. During welding without filler metal, the weld pool consists completely of the molten AISI 304L with Creq/Nieq = 1.8 and so according to Fig. 4(b), the solidification mode is FA type.