Rejuvenation Heat Treatments and Their Role in the Repair of IN738 Turbine Components

Abstract

The thermal processing of turbine engine components is a critical step in the repair and rejuvenation of turbine section hardware to ensure optimal performance and reliability. In the repair process, the thermal process regime must meet the following requirements; improving the weldability of the alloy prior to the repair process (if necessary), returning the microstructure of the alloy to a solutioned state prior to precipitation hardening the alloy, and an aging cycle in order to achieve optimal mechanical properties for the alloy. This paper will focus on the criticality of each step and discuss the typical mechanical properties seen after engine service and the repair process. We will show the importance of these steps and how they will ultimately effect the repair of the hot section component. For almost three decades, gas turbine original equipment manufacturers (OEM’s) have cast high-pressure turbine blades/buckets from In738 Ni-base superalloys. Although significant turbine experience has been gained in the use of this material, little or no standardization of repair heat treatments has been established in the industry. Currently OEM’s and component repair shops utilize a variety of refurbishment heat treatments, all targeted at achieving maximum restoration of mechanical properties and base metal microstructure. This paper also summarizes the results of stress rupture testing of service-run material both before and after six different rejuvenation heat treatments. Microstructures in the service-run and heat-treated conditions are also characterized.

Mechanical Properties of New Ni-Mn-Based Braze Alloys for the Fast Epitaxial Braze Repair of Single-Crystalline Ni-Base Superalloys

“…A well-known technology which is used to improve the filling is the hot isostatic pressing (HIP, also hipping). The influence of hipping on the strength of brazed samples was investigated by Miglietti and Du Toit [12,13]. In this work braze alloys with Hf and Zr as MPDs are applied to wide gap samples, fabricated from IN 738 with a gap width of 1.5 mm.…”

Microstructure and high temperature tensile properties of wide gap brazed cobalt based superalloy X-40

“…17 From the results reported in the earlier section, it is evident that brazed joints containing borides can suffer from reduced ductility and ultimate tensile strength although the yield strength may increase. With extended post brazing or diffusion heat treatment, the tensile strength could reach that of the baseline 4,16,19 but the ductility can not be recovered by heat treatment. Low cycle fatigue (LCF) and thermal fatigue (TF) properties have been found to be strongly influenced by ductility; a material with higher ductility exhibits superior LCF and TF properties.…”

“…Low cycle fatigue (LCF) and thermal fatigue (TF) properties have been found to be strongly influenced by ductility; a material with higher ductility exhibits superior LCF and TF properties. 19,20 In addition to the formation of brittle boride which contributes to crack initiation and propagation, the occurrence of porosity was also found to adversely affect the mechanical properties of the wide gap brazed joint. 15 The effect of porosity on the tensile strength was further illustrated by the reduction in tensile strength with the increase in the additive alloy to filler alloy ratio since increased additive alloy contributes to the occurrence and the size of porosity 15,21 In summary, the mechanical properties of WGB joints are controlled by the occurrence of both hard phases and porosity.…”

Practical Experience With the Development of Superalloy Rejuvenation

“…Several factors may be contributing to the discrepant conclusions reached by different authors. Firstly, as found by both Koul et al [8] and Miglietti et al [14], the specific parameters used for rejuvenation affect the resulting properties. It may be that, in those instances where rejuvenation was found to be of no benefit, the parameters had not been adequately optimized.…”

“…Koul et al studied the effect of a rejuvenation cycle on service exposed IN 738 material and showed that different cycles had varying effectiveness at restoring creep properties [8]. Subsequent studies, have continued to reach mixed conclusions regarding the effectiveness of rejuvenation in general and HIP rejuvenation specifically, with some authors concluding that rejuvenation is effective [9 -11], others concluding that it is not [12,13] and yet others identifying that the effectiveness was a function of the specific processing parameters [1,14].…”