Austenitic stainless steels are used in diverse fields such as chemical and petrochemical industries, marine structures, nuclear and agri-food sectors, etc. This is due to their excellent ductility, high notch toughness, corrosion resistance, and good formability. The fusion zone in their welded joints has a duplex structure consisting of austenite and ferrite phases. The ferrite phase has the beneficial role of preventing the occurrence of hot cracking.
Solidification in the fusion zone by epitaxial growth has been studied for many years. The microstructure within this zone is dendritic whose chemical composition is not homogeneous due to the fast cooling rate. The high thermal gradient generated by the solidification gives rise to columnar grains. This phenomenon occurs by organising atoms from the liquid metal on the grains of the substrate. The crystalline structure in the welded zone may differ from that of the base metal because the chemical composition of the welded zone consists of a mixture between the filler metal and the base metal.
During the welding operation, current, voltage, welding speed, welding energy and temperature between the welding passes are the main parameters affecting the microstructural evolution in the fusion zone. The morphology and the amount of this ferrite phase are different from one pass to another. In the zone corresponding to the last welding pass, the ferrite forms a continuous network in the austenite matrix. Its amount, size, and distribution in the duplex structure are important parameters in controlling the charpy impact properties of austenitic stainless steel welded joints.
Charpy impact test
The charpy impact and toughness properties of the tow-welded joint prepared with type ER316LN and ER308LN austenitic stainless steel filler metals have been compared. The results obtained have shown that the charpy impact energy of the weld is sensitive to the filler metal. The highest energy (91J) absorbed was noted in the case of the welded joint (316L/ER308LN).
The metallographic examination of a longitudinal section of Charpy test specimens has indicated that the failure occurs by plastic deformation in the form of bands. In the case of the (316L/ER308LN) welded joint, these bands appear to be much more diffused than that of the (316L/ER316LN) welded joint. This suggests that the plastic deformation needs more energy in the case of (316L/ER308LN) than (316L/ER316LN) welded joint. The crack starts near the notch, and propagates in the surface along different secondary directions. Both types of welds fractured in a ductile mode. This consists of nucleation of void, growth and coalescence associated to a plastic deformation in the matrix. Furthermore, the presence of inclusions in the material are an important factor for nucleation of micro-voids. Generally, large inclusions cause crack initiation sites in the early stages of plastic deformation. In addition, grouped inclusions considerably reduce the cohesion of steel regardless of their location in the matrix, and thus require a low plastic energy for the formation and growth of micro-voids (Amina Sriba et al. 2018).
The use of ER308LN instead of ER316LN as a ﬁller metal is beneficial to improve the Charpy impact toughness of the welded joint. The authors in the work of Amina Sriba et al. 2018 have suggested many reasons for this behaviour. According to them, the improvement in toughness properties of the welded joint (316L/ER308LN) was probably due to the high amount of chromium and the lowest copper content in its fusion zone compared to (316L/ER316LN) welded joint. On the other hand, they have also explained the improvement in ductile properties by the fact that the (316L/ER308LN) welded joint had the good combination in terms of low phosphorus concentration (Table 1) and high δ-ferrite content (Figure 1).
Phosphorus has a strong tendency to segregate at grain boundaries during the solidification of the weld and leads therefore to a severe intergranular embrittlement phenomenon that attenuates toughness and ductility. The role of δ-ferrite in the fusion zone remains important for controlling solidiﬁcation during welding and inhibiting the formation of low melting compounds such as phosphorus, which promotes hot cracking. This δ-ferrite phase behaves in a ductile manner at ambient temperatures with sufﬁcient ability to accommodate plastic deformation.
I would like to thank Jean-bernard VOGT, Professor at ENSCL (Lille-France) for his assistance and support during the course of this research.