Microstructural response to heat affected zone cracking of prewelding heat-treated Inconel 939 superalloy

The microstructural response to cracking in the heat-affected zone (HAZ) of a nickel-based IN 939 superalloy after prewelding heat treatments (PWHT) was investigated. The PWHT specimens showed two different microstructures: 1) spherical ordered γ′ precipitates (357–442 nm), with blocky MC and discreet M23C6 carbides dispersed within the coarse dendrites and in the interdendritic regions; and 2) ordered γ′ precipitates in “ogdoadically” diced cube shapes and coarse MC carbides within the dendrites and in the interdendritic regions. After being tungsten inert gas welded (TIG) applying low heat input, welding speed and using a more ductile filler alloy, specimens with microstructures consisting of spherical γ′ precipitate particles and dispersed discreet MC carbides along the grain boundaries, displayed a considerably improved weldability due to a strong reduction of the intergranular HAZ cracking associated with the liquation microfissuring phenomena.

► Homogeneous microstructures of γ′ spheroids and discreet MC carbides of Ni base superalloys through preweld heat treatments.
► γ′ spheroids and discreet MC carbides reduce the intergranular HAZ liquation and microfissuring of Nickel base superalloys.
► Microstructure γ′ spheroids and discreet blocky type MC carbides, capable to relax the stress generated during weld cooling.
► Low welding heat input welding speeds and ductile filler alloys reduce the HAZ cracking susceptibility.