Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy
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S turbines are generally designed within the limits of current technology for the best possible efficiencies and output, and as such, complex shaped parts made of nickel-base superalloys are often required to withstand the stringent demands encountered in aero engines and power generation turbines. INCONEL* 738 superalloy *INCONEL is a trademark of INCO Alloys International, Huntington WV.
is one of the most widely used hot-corrosion-resistant superalloys in both land-based and aero gas turbine engines. In addition to its remarkable hot corrosion/ oxidation resistance, this superalloy also exhibits excellent high-temperature strength primarily due to precipitation hardening by the ordered intermetallic O.A. OJO, Assistant Professor and M.C. CHATURVEDI, Distinguished Professor and Canada Research Chair are with the Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. Contact e-mail: [email protected]@cc.umanitoba.ca Manuscript submitted January 21, 2005. 356—VOLUME 38A, FEBRUARY 2007
Ni3(Al,Ti)-c¢ phase. The weldability of heat-resistant nickel-base superalloys has gained importance because of the wide use of welding to fabricate and repair service-damaged hot section components of gas turbines. Unfortunately, application of fusion welding to fabrication and repair of precipitation-hardened nickelbase superalloys such as IN 738 has been severely restricted. This is because these alloys, especially those containing a substantial amount of Al and Ti (>6 wt pct), are highly susceptible to microfissuring predominantly in the heat-affected zone (HAZ) during welding and postweld heat treatment (PWHT).[1] The HAZ microfissuring susceptibility is known to depend upon the composition and microstructure of a material, both of which have already been optimized for acceptable mechanical properties. Similar to many other precipitation-hardened nickel-base superalloys, cracking during welding of the IN 738 alloy has been attributed mostly to large shrinkage stress occurring as a result of rapid precipitation of the main strengthening phase of the alloy, c¢ precipitate particles, during cooling from welding temperatures.[2] It has been suggested that in order to minimize HAZ cracking in this class of superalloys, these materials should be welded in overaged condition.[3,4,5] Even though this preweld heat METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment condition has resulted in a considerable reduction in HAZ cracking in some superalloy weldments including IN 738,[6] it has not been able to eliminate its occurrence. The suggestion of overaging heat treatment was based primarily on the premise of minimizing the effects of rapid c¢ precipitation reaction on cracking. However, very limited information is available about other important effects of overaging heat treatment on grain boundary liquation, which is known to influence HAZ microfissuring by significantly reducing hot ductility during welding. Also, Duval et al.[7] observed that ductility, which is required for sufficient
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