Development of Repair Mechanism and Life Estimation of IN-939 Based Powerplant Components: Part Two

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Abstract

To help meet the ever increasing demands of fossil fuel powerplants and the associated stringent efficiency requirements, all aspects of material selection are critical to ensuring an overall increased material lifecycle within the plant. The increasing use of high performance materials such as nickel or cobalt-based super-alloys in powerplant components such as blades, nozzles, combustion cans and gas turbine transition pieces has certainly helped to improve these aspects but due to operation under extreme temperature and pressure conditions, it is inevitable that wear, tear, and metallurgical degradation will not be avoided.

Statement of undertaken research work:

Through a technical tenure at Ashuganj Power Station Co. Ltd, the performance of hot gas path inspections and combustion inspections on gas turbine-based power-plants with local manpower and resources were necessary. While carrying out these inspections a repair mechanism for gas turbine nozzles made of cobalt-based super-alloy FSX-414 was developed.

This mechanism comprised of TIG welding followed by stress relieving heat-treatment. Here IN-625 was used as filler metal. In this study, it was observed that by adjusting welding parameters the formation of cracks during welding can be minimized.[8] Which otherwise can cut the benefit of such a repair mechanism. Based upon the findings of the a repair mechanism for IN-939 based powerplant components will be developed. IN- 939 is nickel-based precipitation hardened super-alloy. These alloys process superior mechanical properties at high temperatures compared to their cobalt-based counterparts. [2] In this work both solid solutions strengthened, and precipitation hardened filler metal will be used. When precipitation hardened will be used both pre-weld and post-weld heat-treatment will be used, whereas when solid solution strengthened filler metal will be used only pre-weld heat-treatment will be applied.

As mentioned earlier residual stress is the main reason behind crack formation during the repair welding process. So, this parameter needs to be controlled. In this study, this parameter will be controlled by varying heat input. And after welding fractured toughness of the repaired parts will be evaluated. Through this, the remaining life of the component will be measured. And the success of the project depends on this value.

The electricity generation cost depends on the total lifecycle cost of the power plant. And maintenance cost is one of the important factors of the total life cycle cost. During operation under extreme temperature and thermal stress, these components get fractured, and they are required to be replaced or repaired periodically to achieve reliable operation of the plant. For these replacements, extra costs are incurred and it is added to the total lifecycle cost of the power plant every time these replacements occur.

So, it is quite clear that the total lifecycle cost of the power plant depends heavily upon these high- temperature material components. So, if we can reduce the maintenance cost by repairing such components the electricity generation cost will be reduced. To that end repair of these high- temperature components play a very important role.

 

OBJECTIVE OF THE RESEARCH

 

The objective of the research work is to develop a cost-effective and crack-free repair mechanism of IN-939 based powerplant components operated for a specific period under extreme temperature and pressure which can be reused. For a prolonged period of operation components made of such materials are subjected to wear and tear. If these are not addressed timely, they can lead to catastrophic failure of the machine.

To obtain crack-free and strong weld a three-step pre-weld heat treatment will be applied before repair welding and it is 1160 °C/4 h + FC/ to 920 °C(rate 2 °C min−1 ) + AC for both fillers.[19] But when precipitation hardened filler metal will be used then a post-weld heat-treatment in conjunction with the above pre-weld heat-treatment will be used. Which is: 11600 C/4 h/AC + 1000 0C/6 h/FAC + 900 0C/24 h/AC + 700 0C/16 h/AC.[17]

This pre-weld heat-treatment in which cooling is somewhat controlled makes the base metal soft and ductile which is required for repair welding.[19] And the Post weld Heat Treatment will act as stress-relieving when it will be performed. These properties of the base material are reduced because of the prolonged period of operation under extreme temperature and pressure. By varying heat input both filler metals will be used in this work. The precipitation-hardened filler metal will be used for the component which operates under higher stress whereas solid solution strengthened filler metal will be used to the components which operate under lower stress. Solid solution strengthened filler metal selection is somewhat innovative. Because solid solution strengthened filler are softer than precipitation hardened base metal. By selecting these kinds of filler and varying heat input, the main difficulty of repair welding which is residual stress formation can be overcome.

Furthermore, an ANSYS based finite element model will be developed with a moving heat source to determine the optimum value of heat input. After the repair work, fracture toughness tests will be performed. With the aid of this experimental result remaining life of the part will be calculated. And if the results are found satisfactory a repair welding process specification will be prepared for components made of IN-939 under operation for a specific period at severe temperature and pressure.