Media.Player

Pre-weld heat treatment of γ' precipitation strengthened nickel-based superalloys

詳細技術說明

This technology relates to a method for improving the weldability of nickel-based superalloys. The method involves pre-weld heat treating a nickel-based superalloy, such as INCONEL 738 (IN 738) at a temperature sufficient to dissolve any borides that might have formed during casting and prior to processing. The heated nickel-based superalloy is then slowly cooled at a rate that is slow enough to minimize further boron segregation. By minimizing boron segregation at grain boundaries and eliminating boride formation in the treated pre-weld nickel-based superalloy, resistance to intergranular heat-affect zone (HAZ) liquation cracking is improved and a pre-weld microstructure that exhibits the desirable hardness and ductility properties suitable for welding are achieved.

*Abstract

Technical Details
Precipitation strengthened nickel-based superalloys, such as IN 738, are very difficult to weld by fusion welding techniques due to their high susceptibility to HAZ intergranular liquation cracking, which is usually caused by a combination of metallurgical and mechanical factors. Improvement in the ease of repairing gas turbine components made of IN 738 superalloy requires improving its weldability. An improvement in weldability of IN 738 and other precipitation strengthened nickel superalloys could be realized by modifying the materials microstructural characteristics through thermal treatment. The pre-weld thermal treatment that is usually applied to IN 738 superalloy is the widely known standard solution heat treatment (SHT), which has been assessed to result in a highly crack susceptible pre-weld microstructure in the alloy. Other thermal treatment procedures designated as UMT and NUMT have been found to significantly reduce HAZ intergranular liquation cracking in IN 738 superalloy during TIG welding and CO2 laser beam welding respectively. Although UMT and NUMT pre-weld thermal treatments are more desirable than SHT for IN 738 Superalloy, these heat treatments are not industrially applicable. The non-applicability of UMT and NUMT results from their requirement for water quenching as a process step during heat treatment, which is usually difficult to achieve in the aerospace industry where most of the thermal processing are performed under vacuum conditions. Importantly, most of the existing thermal treatment procedures result in boron segregation to grain boundaries and other interfaces prior to welding which results in a crack-susceptible microstructure. Therefore, in addition to avoiding water quenching, an effective thermal treatment for IN 738 Superalloy must consider both the elimination of boride phases present in the cast alloy and the minimization of boron segregation at grain boundaries prior to welding.

In collaboration with Standard Aero Ltd, a pre-weld thermal treatment procedure was developed for IN 738 superalloy and was tested with hybrid laser-arc welding technology, which is an emerging joining technology for structural engineering materials. On the premise that the elimination of the deleterious boride phases and the minimization of boron segregation at grain boundaries can be achieved by a careful choice of temperature and cooling rates, it was demonstrated that the thermal processing of 1120°C/16 h/Furnace cooled, designated as FUMT, is practical and effective for improving the weldability of IN 738 superalloy. No boride particles were observed in the FUMT material. Utilizing the robotic 6 kW laser – 500 A MIG hybrid welding equipment at the Centre of Aerospace Technology and Training in Winnipeg, an 80% reduction in HAZ cracking susceptibility was realized in IN 738 Superalloy materials subjected to the newly developed FUMT compared to currently used SHT.

Advantages
The overall economy of operation of turbine engines is dependent upon the ability to maintain, repair and overhaul the cost intensive engine parts. However, known weld repair techniques for superalloy materials have met with only limited success, due primarily to the propensity of superalloy materials to develop cracks during such welding operations. In this respect, precipitation strengthened nickel-based superalloys are known to be very difficult to weld due to their high susceptibility to HAZ intergranular liquation cracking.

Improvement in the ease of repairing gas turbine components made of IN 738 superalloy requires improving its weldability. In order to improve the weldability of IN 738 superalloy during laser arc hybrid welding, it was necessary to develop another pre-weld thermal treatment procedure that was effective in minimizing or totally eliminating HAZ intergranular liquation cracking, while being industrially deployable at the same time.

The newly developed thermal treatment procedure is based on furnace cooling as opposed to water quenching, and minimizes non-equilibrium boron segregation during cooling from the thermal temperature treatment resulting in the elimination of boride particles from the heat treated material as it. For these reason, this welding technique is found to be both practicable and effective in drastically reducing HAZ intergranular cracking in IN 738 superalloy.

Applications
Precipitation hardened nickel-based superalloys are used in the production of hot section components of modern power generation turbine and aerospace engines due to their excellent high temperature strength and corrosion resistance. Increased performance requirements have necessitated that components manufactured from this alloy suffer in-service damage, which necessitates costly repairs or outright replacement of turbine components. The present disclosure is based on an innovative approach to treat nickel-based superalloys to improve the weldability of such materials. In particular, the methods described herein are based on the observation that most of the existing thermal treatment procedures result in boron segregation to grain boundaries and other interfaces prior to welding which results in the formation of borides in nickel-based superalloys. The decomposition of the borides contributes, significantly, to extensive intergranular liquation in the superalloy resulting in a crack-susceptible microstructure. Accordingly, the present disclosure describes a pre-weld method for heat treating nickel-based superalloys that involves a combination of heating and cooling steps that is believed to achieve both elimination of boride phases present in the cast alloy and the minimization of boron segregation at grain boundaries prior to welding.

Stage of Research
The research is complete and the resulting methodology is fully developed and ready to be commercialized.

Publications Related to this Technology

Oyedele Ola. 2013. A Study of Laser-Arc Hybrid Weldability of Nickel-Base IN 738 LC Superalloy. PhD Thesis, University of Manitoba.

Egbewande A.T., Buckson R.A., Ojo O.A. 2010. Analysis of Laser Beam Weldability of Inconel 738 Superalloy. Materials Characterization. 61(5): 569-574.

Patents
A provisional patent application was made in the USA under Serial number 61/858,042 on June 24th, 2013.

*Inquiry

Jody Dexter,Ph.DTechnology ManagerPh:+1(204)474.8966Email: jody.dexter@umanitoba.ca

國家/地區

美國