Welding and weldability of nickel-base superalloys

News från Development of weldability assessment and understanding of hot cracking in boiler and gas turbine materials, KME-719.

The most important results so far are related to the hot cracking study on cast alloy 718. Hot cracking is a welding related issue that concerns highly alloyed materials such as nickel-iron based superalloys. The cracking occurs during the actual welding as combination of complex metallurgical reactions and stresses/strains in the weld metal and adjacent heat affect zone. Often, components undergo to heat treatments prior to any welding process in order to homogenize the material and for stress relief.

In this work it was found that increasing heat treatment temperature improves the homogenization of the material, however the cracking susceptibility in the base metal does not necessarily improves after high temperature homogenization heat treatments. A careful control of grain size and the percentage of the segregating elements in the microstructure are contributing factors when it comes to weldability of cast alloy 718.

The results can be utilised as recommendation of process parameters to be followed when welding of gas turbine components in order to reduce cracking related issues.  This also can be used as a base for further weldability studies involving other type of alloys.

During the last year studies involving the relatively new cast alloy ATI®718TM are expected to shed some light regarding its weldability.

The new alloy is candidate for gas turbine component applications which could operate at a maximum temperature of ~700°C. This would give an advantage of 50°C over the standard grade alloy 718, thus having an important impact on the gas turbine efficiency. Within the following months there will be substantial amount of Varestraint weldability testing on high temperature stainless steels apart from the dedicated weldability studies on superalloys that now have been completed.

It was unexpected that the high temperature homogenization provided worst hot cracking susceptibility.

PhD students, project participants and project leader of project:
Lars Nyborg (Project leader and assistant supervisor, Chalmers University of Technology)
Joel Andersson (Main supervisor, University West)
Sukhdeep Singh (Phd Student, Chalmers University of Technology)
Johan Ockborn (Welding Expert, GKN)
Jan Häggander (Welding Expert, GKN)
Søren Aakjær Jensen (Dong Energy)
Jouni Mahanen (Amec Foster Wheeler-Sumitomo)

Figure 1. Average total crack length (TCL) measurements for the As Cast and heat
treated conditions (1120 and 1190)

Figure 2. Optical microscopy images showing the microstructure in the base metal of a)
as cast, b) 1120 and c) 1190

Participating companies:
GKN Aerospace Sweden AB
Amec Foster Wheeler-Sumitomo
Dong Energy

To the project-site.