In the early 1980s, stainless steel flux cored arc welding wires became very popular. These wires contained small additions of bismuth (Bi), often in the form of bismuth oxide (Bi2O3) to improve slag removal and help the welder. This is still common practice today, including for TETRA wires.
In the early 1990s, premature creep failures were reported in high service temperature applications in power plants. Since then, research has shown that segregation of bismuth to the grain boundaries at high temperatures results in reduced ductility at temperatures above 650ºC and inter-granular cracks at temperatures above 700ºC.
As a result, special FCAW wires, free of bismuth, are required for applications involving high service temperatures or where a post weld heat treatment (PWHT) above 600ºC is specified. To resist high-temperature degradation under these conditions, weld metal must contain less than 20 ppm (0.002 %) bismuth.
High service temperature applications are mostly found in the power generation and petrochemical industries. If we consider cladding, our focus is on the petrochemical industry where the majority of stainless steel cladding applications are, and where PWHT cycles above 600ºC are often required.
The first bismuth-free wire for welding heat resisting stainless steel 253 MA was designed by Welding Alloys in 1998. The third generation of Welding Alloys’ bismuth-free TETRA wires contains an exhaustive range of compositions. It is available now and will be officially launched at the Schweissen und Schneiden exhibition in September 2017 in Düsseldorf.
Jean-Marie Bonnel, WAG Technical Director