Welding of Martensitic-Austenitic Duplex Stainless Steel
Duplex stainless steels (DSS) are those stainless steels which contain appreciable amount of both ferrite and austenite in their micro structure. These low alloyed steels are widely used for their interesting combination of high strength and excellent corrosion resistance when compared to conventional austenitic or ferritic steels. DSS are suitable for applications in environments widely varying from mild to very severe and for large temperature ranges. DSS are widely used in oil and gas industry, desalination plants, chemical and pharmaceutical industry, paper industry, food processing and nuclear industry. Martensitic-austenitic duplex stainless steel (MA-DSS) is a type of steel having a composition of ferrite and austenite. It has better mechanical properties than the conventional austenitic and ferritic stainless steels such as higher yield strength and hardness, better corrosion resistance and excellent weldability. The most widely used MA-DSS is the 25Cr-5Ni-3Mo-0.2N steel which has increased mechanical strength due to the addition of molybdenum while maintaining the corrosion resistance.
The weldability of MA-DSS is very important in order to produce components with sound welds and consistent reinforcing structures. The mechanical and microstructural properties of welded MA-DSS would be largely controlled by the welding parameters and the resulting microstructure. As MA-DSS are of two-phase structure, the welding should be done with caution. The welding parameters should be suitable enough to produce sound welds with adequate mechanical properties.
The most commonly used welding procedure for MA-DSS is the shielded metal arc welding (SMAW) technique due to its simplicity and ready availability of equipments for this technique. Other commonly used welding procedures for MA-DSS are tungsten inert gas (TIG) welding and metal inert gas (MIG) welding. These welding procedures are used to varying degrees depending on the nature of the component to be manufactured. For example, SMAW is more preferred in the oil and gas industry due to its cost effectiveness and wide range of consumables available. On the other hand, MIG/TIG welding is usually used for the production of thin components due to its lower heat input.
The most important factor to keep in mind while welding MA-DSS is the welding parameters. The welding parameters for various welding procedures should be selected in such a way that the resulting weld is of sound quality and the corresponding mechanical and microstructural behavior is retained. Appropriate pre-heat and inter-pass temperature should be applied to provide sufficient time for the formation of desired microstructure. Too high pre-heats and inter-pass temperature should be avoided as they could lead to loss of quality in the weld. Appropriate welding current and voltage needed to form sound welds should be selected. The welding of MA-DSS requires more attention with respect to welding speed, interpass temperature and post-weld heat treatment.
Apart from the welding parameters the choice of filler material is also important in the welding of MA-DSS. Generally, ferritic stainless steel filler materials are used for SMAW though austenitic filler materials are also used to a small extent. In the case of MIG/TIG welding, usually austenitic stainless steel and nickel base filler materials are used. The selection of appropriate filler material is important in order to ensure adequate toughness and corrosion resistance despite the presence of ferrite or austenite in the filler material.
It can be seen that the welding of MA-DSS needs considerable care with respect to the welding parameters, choice of filler material and proper post weld heat treatment to produce sound welds with superior mechanical properties.
