Turbine system having more failure resistant rotors and repair welding of low alloy ferrous turbine components by controlled weld build-up

Abstract

System for repairing worn surfaces of steam turbine components and especially high pressure turbine rotors, are disclosed. These systems include depositing a first layer of weld metal on a worn surface of the component, whereby a heat-affected zone is created. A second layer of weld metal is then deposited over the first layer using a greater amount of heat to temper at least a portion of the heat-affected zone produced by the first layer. The preferred embodiments include the use of gas tungsten arc welding for providing fine-grain size and more creep resistance, especially in the weld and heat-affected zone. The resulting build-up can be machined, for example into a blade fastening to produce a component having properties equal to or better than the base-metal alloy. The invention also provides a longer lasting turbine system, including rotors which have serrated steeples that are more resistant to failure.

Description

CROSS REFERENCE TO RELATED APPLICATIONSThis application is related to co-pending application Ser. No. 168,097, filed Mar. 14, 1988, in the names of E. Clark and D. R. Amos, entitled "Repair Welding Low Alloy Turbine Components", which is assigned to the assignee of this application and which is hereby incorporated by reference.This application is also related to co-pending application Ser. No. 092,851, filed Aug. 24, 1987, in the names of R. T. Ward and J. M. Butler, entitled "Repair of High-Pressure Turbine Rotors By Ring Welding", which is assigned to the assignee of this application and which is herein incorporated by reference.This application is also related to application Ser. No. 763,744, filed Aug. 8, 1985, in the names of R. E. Clark and D. R. Amos, entitled "Method for Repairing A Steam Turbine Or Generator Rotor", now U.S. Pat. No. 4,633,544, issued Jan. 6, 1987, which is assigned to the assignee of this application and which is herein incorporated by reference.This appli...

AI Technical Summary

Benefits of technology

Improved turbine systems including more failure resistant rotors and novel methods for repairing worn surfaces of steam turbines, especially high pressure turbine rotors are disclosed. The methods include welding procedures and heat treatments that minimize weld stresses and cracking. The procedures employed substantially reduce the risk of failure in ferrous Cr-Mo-V base metals of high-pressure, high temperature rotors and discs commonly found in steam turbines. This invention presents an improvement over welding forged fastenings to rotors, since welder accessibility and weldment integrity are improved. These features are particularly important with respect to high pressure, (HP), turbine components, such as rotors, which have been known to operate at pressures over 2400 psi and temperatures over 1000.degree. F.

Accordingly, improved welding methods are disclosed for overcoming the occurrence of metallurgical structural problems within the heat-affected zone. The additional heat generated by the deposition of the second layer of weld metal produces an immediate heat treatment of the heat-affected zone, whereby coarse grains of the base metal are recrystallized and tempered. It is understood that when these course grains are reformulated into a finer grain structure, stress-relief cracking in the vicinity of the weld repair can be minimized.

The methods employed by this invention also avoid the over-tempering, or softening, of the base metal created by the heat of welding the first layer of weld metal. This loss in strength occurs, to a greater extent, when a stress transverse to the weld is applied, for example, high and low cycle fatigue, tensile, or creep-to-rupture. The proper control of the initial layers of weldment can significantly reduce the failure in the heat-affected zone and prevent the loss of strength in this zone below the levels of the unaffected base metal.

Further improvements disclosed by this invention include the use of bead sequencing for minimizing heat input into the base metal. Run-off tabs are also taught for minimizing weld defects created by starting and stopping the arc. In addition, a weld trail-shield is disclosed for minimizing carbon losses in the base metal which could result in lower tensile properties. Finally, parameters such as preheat-interpass temperatures, shield gas-type and flow rates, current, tungsten size and weld speed are also disclosed for achieving a higher quality weld. Procedures for single "steeple" repairs and for 360.degree. rotor repairs are also separately disclosed.

It is another object of this invention to provide welding procedures, alloys, and heat treatments which provide improve notch sensitivity and increased creep ductility to repaired or new turbine components.

It is still another object of this invention to provide a repaired turbine rotor for use in high pressure service which is relatively free of weld porosity, lack of fusion and cracking resulting from the welding process.

Problems solved by technology

This loss in strength occurs, to a greater extent, when a stress transverse to the weld is applied, for example, high and low cycle fatigue, tensile, or creep-to-rupture.

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