Development of Repair Mechanism and Life Estimation of IN-939 Based Powerplant Components: Part 1

Background and Literature Review

Welding nickel-base superalloys is always a challenging job. To obtain crack-free and high strength welding the minimization of residual stresses are crucial. This variable of welding can be controlled by appropriate heat-treatment, decreasing heat input, and choosing filler metals with low modulus of elasticity. Several researchers have performed studies on different aspects of welding nickel-based superalloys which are reviewed in the following summaries:

Dae-Young Kim et al. [12] in their research work found that the rate of cracking of autogenous GTA welds in IN 738 alloy decreased with lowering heat input. The properties of IN 738 were very similar to IN 939 which was precipitation hardened superalloy. This was because high heat input caused high residual stress which in turn produced cracks. Ainsworth [13] in his study found that for a low level of external load, the initial crack driving force was induced by residual stresses. This hypothesis was very much applicable to gas turbine nozzles because they were subjected to mainly thermal stresses. Furthermore, the adverse effect of residual stress was also investigated by Park et al. [15] They performed their study on IN 617 and found that in presence of residual stresses the crack growth rate was increased by 20%. So, it was obvious that by minimizing residual stresses the crack growth could be decreased. Whereas heat treatment played a very vital role in this scenario.

Shaw [9] in his study suggested that repairing nozzles made of IN 939 pre-weld heat treatment was helpful. His prescribed heat treatment was 4h/11600C+ 6h/10000C+500C cooling per minute. He mentioned that by adopting this technique an elongation of over 9% could be achieved compared to 4.5% for material treated with the same procedure except control cooling. But this controlled cooling is not practically viable. Mottari et al. [11] in his study found that the average fracture toughness of weld specimens of IN 939 was reduced by about 40% by residual stress. In their experiment, they used pre-heat treatment without control cooling and IN 625 as filler material. Their results suggested the hypothesis mentioned above. As solution strengthened filler material used for welding precipitation hardened filler material, in this study the strength of the final weld was already compromised. So, if residual stress of such a high magnitude was permitted to develop during repair welding the circumstances would be catastrophic and the main objective of repair welding might be completely missed.

Although several studies have been performed on welding the nickel-based precipitation hardened superalloy IN 939, which is widely used for many structural components of a modern gas turbine-based power plant, but no study has been performed to minimize residual stress during repair welding of such components. Especially using controlled cooling during pre-weld heat treatment, minimizing heat input and using softer solid solution strengthened filler material than IN-625 to produce crack-free and strong weld which can be reused in gas turbines. These issues are addressed in the present study. Furthermore, precipitation hardened filler material will be used for high-stress components with post-weld heat treatment, and fracture toughness values for both groups of base and filler metal selection will be investigated experimentally.