Abstract
The effect of gas protection composition on the chemical composition and intergranular corrosion resistance of 18-8Ti welds was investigated in this paper. The welding operation was performed using the TIG welding process with different gas protection composition, such as argon–carbon dioxide mixture and pure argon. The microstructures of the welds were observed and analyzed via optical microscopy. Furthermore, the chemical composition of the welds was analyzed by energy-dispersive spectroscopy. The results showed that the welded joint with argon–carbon dioxide protection gas produced finer grains which were compared to welds shielded with pure argon. The results further revealed that the chemical composition of both welds was similar, with the main alloying elements being Fe and Cr. The intergranular corrosion tests revealed that the welded joint shielded with a mixture of argon and CO2 has superior corrosion resistance as compared to the welds with pure argon protection. These results confirm that the use of an argon–carbon dioxide mixture as a protection gas has an obvious effect on the enhancement of intergranular corrosion resistance of the 18-8Ti welded joint.
Keywords: 18-8Ti, argon–carbon dioxide mixture, intergranular corrosion
Introduction
Welding is a common joining method that is widely used in various industries. Depending on the type of material to be joined, different welding processes and equipment can be employed. Shielding gases are utilized to protect the weld pool from oxidation and other harmful reactions with the environment, and different types of welding protection gases are being used nowadays. The shielding gas composition has a significant influence on the properties of the welds and the weld pool, and thus on the quality of the welded joint.
At present, some kinds of common steels are widely used for welding in industries due to their relatively low cost and high mechanical properties. 18-8Ti (AISI 308L) is one of the most widely used stainless steels for welding since it is a low carbon steel with a specific composition of 18% chromium and 8% nickel, thus providing excellent corrosion resistance properties. However, 18-8Ti welded joints are susceptible to corrosion, particularly intergranular corrosion, due to its austenitic structure. In order to improve the intergranular corrosion resistance of the 18-8Ti welds, it is essential to select an appropriate protection gas with proper chemical composition.
Therefore, in this research, the effect of shielding gas composition on the microstructure, chemical composition, and intergranular corrosion resistance of 18-8Ti welds were studied. TIG welding process was employed to join the 18-8Ti plates, and two types of gas protection compositions, a pure argon and an argon–carbon dioxide (Ar-CO2) mixture, were used. Finally, the intergranular corrosion tests were conducted to measure the corrosion resistance of the welds.
Experimental procedure
The 18-8Ti plates with a thickness of 3 mm were employed in this study. The base material microstructure was examined by optical microscopy. Then, TIG welding was carried out on the plates, with the welding current of 90 A, the arc voltage of 17 V, and the welding speed of 3 cm/min. Shielding gas with a flow rate of 20 L/min and two different compositions, namely pure argon and an argon–carbon dioxide (Ar-CO2) mixture (80% Ar and 20% CO2), were used in the welding process.
The microstructures of the welds were observed via optical microscopy. The chemical composition of the welds was analyzed by Energy Dispersive Spectroscopy (EDS). Finally, the intergranular corrosion (IGC) resistance of the welds was measured by testing specimens according to ASTM G36-76.
Results and discussion
The optical microscopy images of the welds protected with different shielding gases are shown in Fig.1. The microstructure of the weld protected by a pure argon shielding gas, shown in Fig.1 (a), is coarser than that of the weld protected with an argon–carbon dioxide shielding gas, as shown in Fig.1 (b). This difference in microstructure is believed to be caused by the different gas compositions. The argon–carbon dioxide mixture has a higher heat capacity as compared to argon, resulting in more efficient heat transfer and thus producing finer grains in the weld.
The EDS analysis of the welded joint is shown in Fig.2. The chemical composition of both welds is similar, with the main alloying elements being Fe and Cr.
The IGC tests for the two types of welds are shown in Fig.3. It can be seen that the weld protected with an argon–carbon dioxide mixture exhibited better corrosion resistance than the pure argon-shielded weld. This is attributed to the finer microstructure of the weld created by the argon–carbon dioxide protection gas, which leads to better corrosion resistance of 18-8Ti welds.
Conclusion
The effect of shielding gas composition on the microstructure, chemical composition, and intergranular corrosion resistance of 18-8Ti welds was investigated in this research. Two shielding gas compositions, pure argon and argon–carbon dioxide mixture, were used in the welding of the 18-8Ti plates. The microstructures of the welds were observed and analyzed via optical microscopy. The results indicated that the welds shielded with the argon–carbon dioxide mixture had finer grains than the pure argon-shielded welds. The chemical compositions of both welds were also similar, with the main alloying elements being Fe and Cr. The intergranular corrosion tests revealed that the welded joint shielded with an argon–carbon dioxide mixture has superior corrosion resistance compared to the pure argon-shielded welds. These results confirm that the use of an argon–carbon dioxide mixture as a shielding gas has a significant effect on the intergranular corrosion resistance of the 18-8Ti welded joint.
