Supermartensitic stainless steels have high strengths - around 550MPa (80ksi) proof strength and above - consequently, high strength filler metals are required to give matching weld metal strength. Therefore, superduplex stainless steel consumables have been adopted and employed in practice on pipeline applications. However, these may only be borderline with respect to strength, due to the high parent pipe strength and they are not ideal for applications where postweld heat treatment is desirable, e.g. to increase resistance to hot fluids with CO2 or H2S.
Matching 12/13%Cr consumables have been developed in solid wire and metal cored wire forms but have not been successfully used for industrial applications. For lower strength grades, nickel alloy fillers or 22%Cr duplex stainless steel fillers may also be considered.
At present, there is little information regarding required heat input limits, except those for the superduplex filler material; consequently, most applications have used fairly low heat inputs, e.g. <1.5kJ/mm. Interpass temperature restrictions likewise have typically been set for the superduplex filler. To date, it has generally been found that no preheat is required to avoid hydrogen cracking.
The preferred welding processes to date for pipelines have been automatic pulsed gas metal arc (PGMA) and gas tungsten arc welding (GTAW) welding. There is no metallurgical reason why other conventional arc welding processes such as manual metal arc (MMA) and flux cored arc welding (FCAW) may not be used for girth welds. Seam welding of longitudinally welded pipe has been undertaken using submerged arc (SAW) and laser welding processes.
It is becoming increasingly common to use a brief PWHT, e.g. at 650°C for five minutes, to prevent intergranular corrosion of weld HAZs that are exposed to hot acidic fluids in service.
FAQ: What is a weldable 13%Cr steel and why is it an attractive alternative to duplex stainless steel?