The Effects of Nitrogen Gas on Microstructural and Mechanical Properties of TIG Welded S32205 Duplex Stainless Steel

Abstract

Duplex stainless steels are gaining greater interest due to their increasing amounts of application fields. Accordingly, there is a need for awareness of problems associated with improper microstructural distributions such as δ-ferrite (delta-ferrite), austenite and other important intermetallic phases that may form in these steel weldments. Since δ-ferrite versus austenite ratio profoundly influences corrosion and mechanical properties, optimum δ-ferrite ratios must be kept approximately within 35-65 vol % and balance austenite to maintain satisfactory corrosion and mechanical properties on welding of these steels. Cooling rates of welds and alloying elements in base metal are the major factors that determine the final microstructure of these steels. In this work, 3 mm thickness of 2205 duplex stainless-steel plates were TIG (Tungsten Inert Gas) welded with various amounts of nitrogen gas added to argon shielding gas. Specimens were joined within the same welding parameters and cooling conditions. As nitrogen is a potential austenite stabilizer and an interstitial solid solution hardener, the effects of nitrogen on mechanical properties such as hardness profiles, grain sizes and microstructural modifications are investigated thoroughly by changing the welding shielding gas compositions. Increasing the nitrogen content in argon shielding gas also increases the amount of austenitic phase while δ-ferrite ratios decreases. Nitrogen spherodized the grains of austenitic structure much more than observed in δ-ferrite. The strength values of specimens that welded with the addition of nitrogen gas into the argon shielding gas are increased more in both austenitic and delta-ferritic structure as compared to specimens that welded with plain argon shielding gas. The addition of 1 vol % of nitrogen gas into argon shielding gas provided the optimum phase balance of austenite and δ-ferrite in S32205 duplex stainless-steel TIG-welded specimens.

Physical and Theoretical Modeling of the Nitrogen Content of Duplex Stainless Steel Weld Metal: Shielding Gas Composition and Heat Input Effects

“…This observation has practical importance, because this is the N2 content range, where shielding gases are used for DSS TIG welding. Moreover, many researchers [23,24,54] observed sometimes contradictory results of γ content in the WM, using this shielding gas range for TIG welding, which could originate from this effect of the arc voltage changes. Table 3.…”

“…During arc welding of DSSs, the nitrogen loss from the weld metal (WM) leads toward more ferritic microstructures and to the loss of the abovementioned properties [21,22]. In order to balance this nitrogen loss, nitrogen (N 2 )-argon (Ar) (or helium) mixed shielding gases are used in industrial applications for DSS tungsten inert gas (TIG) welding [23][24][25]. The N 2 dissociates at the arc plasma temperature and the atomic nitrogen can dissolve in the molten pool [26].…”

Effect of Nitrogen Addition to Shielding Gas on Cooling Rates and in the Microstructure of Thin Sheets of Duplex Stainless Steel Welded by Pulsed Gas Tungsten Arc Welding Process

“…Several authors have been mentioning the important role of N 2 in the microstructure evolution by being the strongest austenite forming alloying element 6,9,10,[13][14][15][16][17] . Moreover, N 2 is an economical and efficient substitute for nickel 10,17 .…”

“…Moreno et al 17 determined using electron probe microanalysis (EPMA) technique an average content of N 2 in the range 0.045% -0.070%wt in ferrite and 0.520% -0.770%wt in austenite for five different DSS alloys. The addition of C and N 2 strengthens both ferrite and austenite by dissolving at interstitial sites in the solid solution 2,16,17,18 .…”

“…Nitrogen is added to argon shielding gas in duplex stainless-steel welds to preserve weld metal nitrogen levels 15,16 . According to Zhang et al 15 , there are two ways to compensate for the nitrogen loss during welding: one is directly adding the nitrogen to the BM and filler wires in the originally producing supplies 13,17,23 or a proper proportion of nitrogen gas is added into shielding gas and backing gas during welding 11,14,16,[24][25][26][27][28] .…”