Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy
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INTRODUCTION
INCONEL 738* superalloy is one of the most widely *INCONEL is a trademark of Inco Alloys International, Huntington, WV.
used hot-corrosion-resistant superalloys in both land-based and aero gas turbine engines. In addition to its remarkable hot corrosion resistance, it also exhibits excellent hightemperature strength primarily due to precipitation hardening by ordered intermetallic Ni3(Al,Ti)- phase. The weldability of heat-resistant nickel-base superalloys has gained importance because of the wide use of welding to fabricate and repair hot section components of aero and industrial gas turbine engines that are made from these materials. Microstructural development in superalloy welds, which invariably controls the properties and reliability of the weldments, can be significantly influenced due to microsegregation and nonequilibrium phase transformation that occur during solidification. Dendritic microsegregation is known to often result in formation of intermetallic secondary solidification constituents along the dendrite interstices, as well as inhomogeneous distribution of secondphase precipitate particles.[1–4] In some cases, important strengthening elements may be tied up in the resultant nonequilibrium intermetallic particles, which could also exhibit other damaging effects on high-temperature performance.[1]
O.A. OJO, Postdoctoral Fellow, N.L. RICHARDS, 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] Manuscript submitted November 10, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Postweld heat treatment (PWHT) is usually applied to most superalloy weldments to modify the as-weld microstructure and relax residual welding stresses in order to impart optimum reliable properties. INCONEL 738, like other precipitation-hardened nickelbase superalloys that contain a substantial amount of Al and Ti (6 wt pct), is generally considered very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during welding and PWHT by strain age cracking.[5] Cracking during welding of this alloy has been attributed mostly to large shrinkage stresses that occur as a result of rapid precipitation of particles during cooling from the welding temperature.[6] However, it is generally known that weld cracking results from competition between the mechanical driving force for cracking (stress/strain generation) and the material’s intrinsic resistance to cracking. It has been found that liquation, which could occur by different mechanisms, is the primary cause of low HAZ crack resistance in most austenitic alloys including precipitation-hardened Ni-base superalloys.[7] The combination of thermally induced welding strain and very low ductility in the alloy, due to localized melting at grain boundaries, results in HAZ liquation cracking. The HAZ liquation is known to occur either by nonequilibrium i
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