Technical ReportEffect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints
Highlights
► Welding procedure is established for welding 6 mm thick AISI 304 using GTAW process. ► Mechanical properties of the weld joints are influenced strongly by the heat input. ► Highest tensile strength of 657.32 MPa is achieved by joints using low heat input. ► Welding parameters affect heat input and hence microstructure of weld joints. ► Extent of grain coarsening in the HAZ increases with increase in the heat input.
Introduction
Austenitic stainless steels have been used widely by the fabrication industry owing to their excellent high temperature and corrosion resistance properties. Some of the typical applications of these steel include their use as nuclear structural material for reactor coolant piping, valve bodies, vessel internals, chemical and process industries, dairy industries, petrochemical industries etc. Out of 300 series grade of these steels type 304 SS is extensively used in industries due to its superior low temperature toughness and corrosion resistance. One of the typical applications of type 304 SS include storing and transportation of liquefied natural gas (LNG), whose boiling point is −162 °C under 1 atmosphere.
A study on fatigue crack growth rate for type 304 SS over a temperature range from room to −162 °C has shown that base metal possesses superior resistance to crack growth relative to weld metals over the entire temperature range [1]. Another typical application of this material includes its use as bellows used as conduit for liquid fuel and oxidizer in propellant tank of satellite launch vehicle [2].
Chen et al. [3] found that when Cu–Si enriched type 304 SS (containing 2–2.5 wt.% copper and 1–1.5 wt.% silicon) and a conventional type 304 SS was welded using gas metal arc welding (GMAW), process ductility decreased and ferrite levels increased in both weldments, as the heat input was increased. A comparative study by Yan et al. [4] on the microstructure and mechanical properties of 304 SS joints by tungsten inert gas (TIG) welding, laser welding and laser-TIG hybrid welding showed that laser welding could give highest tensile strength and smallest dendrite size in all joints whereas TIG welding gave lowest tensile strength and biggest dendrite size. Work reported by Muthupandi et al. [5] on the effect of weld chemistry and heat input on the structure and properties of duplex stainless steel welds using autogenous-TIG and electron beam welding process shows that chemical composition exerts a greater influence on the ferrite–austenite ratio than the cooling rate. Jana [6] has reported the effect of varying heat inputs on the properties of the HAZ of two different duplex steels and found that as arc energy increased hardness of both weld metal and the HAZ decreased, whereas width of the HAZ increased with increased arc energies. Study on the influence of welding heat input on submerged arc welding (SAW) welded duplex steel joints imperfections has been reported by Nowacki et al. [7] where heat input from 2.5 to 4.0 kJ/mm was used for plate thickness of 10–23 mm and it was concluded that usage of larger welding heat input provided the best joints quality.
Zumelzu et al. [8] studied the mechanical behaviour of AISI 316L welded joints using shielded metal arc welding (SMAW) and GMAW process with different electrodes types. Their work concludes that a direct correlation exists between the thermal contribution and tensile strength for the materials studied. The effects of minor elements and shielding gas on the penetration of TIG welding in type 304 SS have been studied using bead on plate experimentation technique and it is concluded that minor elements such as oxygen, aluminium and sulfur have a significant effect on the weld depth to width ratio [9]. Experimental investigations on the effect of hydrogen in argon as a shielding gas in TIG welding of austenitic stainless steel show that mean grain size in the weld metal increases with increasing hydrogen content besides increasing the weld metal penetration depth and its width [10]. Lu et al. [11] have reported in their experimental results that small addition of oxygen content to the He–Ar mixed shielding can significantly change the weld shape from a wide shallow type to a narrow deep one.
Lee et al. [12] have reported in their studies on effects of strain rate and failure behaviour of 304L SS SMAW weldments and find that as the strain rate increases, the flow stress increases and the fracture strain decreases. Korino et al. [13] have reviewed the considerations for weldability of 304L SS and recommend Creq to Nieq ratio of 1.52–1.9 to control the primary mode of solidification. Lee et al. [14] while investigating the pitting corrosion behaviour of welded joints of AISI 304L using flux cored arc welding (FCAW) process found that tensile and yield strengths were increased with increasing equivalent ratio of Creq/Nieq. Milad et al. [15] found that yield and tensile strengths of 304 SS increased gradually at the same rate with increasing degree of cold work. Shyu et al. [16] have investigated the effect of oxide fluxes on weld morphology, arc voltage, mechanical properties, angular distortion and hot cracking susceptibility of autogenous TIG bead on plate welds. Their results indicate that penetration is significantly increased which in turn increases depth to bead-width ratio and tends to reduce angular distortion.
Other studies which show that 304 SS and 304L SS grade has been the topic of research of many researchers include various studies like experimental determination of grain boundary composition of 304 SS in low temperature sensitization condition using a scanning Auger microprobe [17], measuring chromium depletion after various thermal heat treatments [18], modelling of low temperature sensitization of austenitic stainless steel [19], studying sensitization behaviour of grain boundary engineered austenitic stainless steel [20], arresting weld decay in 304 SS by twin-induced grain boundary engineering [21] etc.
From the literature reviewed on the material processing of 304 SS it is observed that no systematic work on the effect of heat input on microstructure and tensile properties of gas tungsten arc (GTA) welded has been reported. In view of the fact that arc welding processes like GTAW offer a wide spectrum of thermal energy for joining different thicknesses of steels it was considered important that undertaking the present study would be beneficial in gaining an understanding about the metallurgical aspects that affect the service performance of these welded joints made using different heat input combinations.
Section snippets
Base and filler material combination
The base material used in the present investigation was in the form of AISI 304 SS plates of sizes 200 mm × 100 mm × 6 mm which were cut from a rolled sheet and the filler was 308 SS solid electrode of 3.15 mm diameter. Table 1 shows the chemical composition of the base and the filler used.
Welding procedure
In the present work double V-groove design was used so that welding could be accomplished in two numbers of passes ensuring full penetration. Before welding all the edges were thoroughly cleaned mechanically and
Metallographic studies
Full penetration welds were obtained in all the three combinations of heat input as shown in Fig. 4. Measured areas of fusion zone and HAZ of different weldments are shown in Table 3. As indicated by these values it is found that as heat input increases the fusion areas of the joints also increase proportionately. The same trend is followed for the HAZ area associated with each of these joints. Yan [4] and Jana [6] have reported similar trends while studying TIG welded 304 SS and SMAW welded
Conclusions
The following conclusions can be drawn from the present work:-
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Good joint strength is exhibited by all the joints which show that for welding 6 mm thick AISI 304 SS the operating envelope of GTAW process offers a wide range of parameters to the fabricator.
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As the dendrite size in the fusion zone is smaller in low heat input joints than the dendrites in medium and high heat input joints, it is found that maximum tensile strength and ductility is possessed by the weld joints made using low heat
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