The austenitic stainless steels (ASS) are generally preferable for critical parts of power plants as they can stand the properties at elevated temperatures. The joining of critical components such as reheater and superheater sections are by Tungsten Inert Gas (TIG) welding process.

The wider softening zone encountered during welding of thick sections results in deteriorated properties of welded joints and lowers the service life. Implementation of a higher heat input than GTAW can overcome the existing challenges.

The keyhole mode of Plasma arc welding (K-PAW) can be useful to take over the existing problem. The higher arc constriction of KPAW adds an advantage to minimise the weldment region compared to the conventional ones. 304HCu, tailored austenitic stainless-steel tubes were fabricated using KPAW and GTAW welding processes.

The weld zone has dendritic structures formed due to the cooling rate. The size of the dendrite and the space between them are quantified using optical microscopy (OM). The strength of the KPAW joints was 12 % higher than the GTAW joints. The microstructure of the fusion zone has control over the strength of the joints.

The KPAW joints had finer equiaxed grain with minimum dendritic spacing than the GTAW joints due to the difference in their cooling rate.

This article is shared by P.G. Sam Infant Jones, S. Rajakumar and V. Balasubramanian.