Weld Cracking in Precipitation Hardening Ni-based Superalloys

Licentiate thesis, 2018

Manufacturing of hot structural components for aero engines requires the materials being used to be
weldable. The high demands on strength and temperature resistance make nickel based superalloys the
material of choice for this application. Alloy 718 has been the standard grade for several years,
providing high strength at elevated temperatures while being weldable due to the relatively slow
precipitation kinetics of its hardening phase gamma double prime. Increasing operating temperatures
as well as intermittent cycling of land-based gas and steam turbines motivate research on highly
temperature stable alloys such as nickel based superalloys. Increased temperature stability of
precipitation hardening superalloys is generally achieved via the gamma prime phase, which in
contrast to gamma double prime causes a very rapid hardening effect in the material.
Rapid hardening of the gamma prime phase can cause strain age cracking (SAC), a cracking
phenomenon occurring during heating towards the post weld heat treatment when stress relaxation
mechanism coincide with the precipitation of hardening phases. With the general mechanism of SAC
being established, detailed knowledge about the material response is necessary to be able to predict the
welding behaviour and to prevent SAC. This is especially relevant with regard to newly developed
alloys such as Haynes® 282®, where limited weldability data is available. This work hence sets focus
on investigating the weldability of the relatively new superalloy Haynes® 282®.
It was found that the welding response of Haynes® 282® is generally good, with the heat input during
welding being identified as main effect on the cracking response under the studied conditions.
Solidification cracks were observed in the material, while neither heat affected zone liquation cracks
nor SAC could be confirmed. A simulative Gleeble test was developed to provide more data on
ductility in the SAC temperature range and its dependence on ongoing precipitation reactions during
thermal exposure, correlating the loss in ductility with hardness evolution in the material.