Consider replication to verify metallurgy of buckets/blades after restoration heat treatment

To achieve the creep rupture life expected from a gas-turbine bucket/blade, and to ensure reasonable mechanical properties, the airfoil’s microstructure must have the ideal shape and distribution of its constituent phases, reminded John P Molloy, PE, of M&M Engineering Associates Inc, Leander, Tex, in a telephone call to CCJ’s offices.

The primary beneficial phases of interest are the gamma-prime and carbide constituents, he said.  The first probably is the most scrutinized phase during microstructural evaluation, and often is used to determine if a blade requires refurbishment. Carbide shape and distribution is similarly important, but often takes a back seat to the gamma prime in condition assessment.

Ideally, the gamma-prime morphology should be cuboidal, which essentially implies that the microstructure, in cross section, should look like an array of squares (Fig 1). As the square shape of the gamma prime becomes more rounded, or “coarsened” the mechanical properties of the blades are compromised, particularly with regard to remaining creep life, Molloy continued. This is the reason most GT buckets/blades are limited to approximately 24,000 fired hours between refurbishment cycles (Fig 2).

1. Gamma prime after typical ageing heat treatment

1. Gamma prime after typical ageing heat treatment

2. Gamma prime after a complete hot-gas-path cycle

2. Gamma prime after a complete hot-gas-path cycle

Carbide morphology (shape) also influences mechanical behavior, he said. Carbides, like gamma prime, also will change shape and coarsen as operating hours accumulate. They may dissolve in the matrix and coalesce at the grain boundaries as well. If the carbide distribution in the grain boundaries becomes a continuous film over time, the mechanical properties can become severely degraded. 

Other phases are also of concern—such as sigma, laves, and mu.  Several of these are known as topologically close-packed (TCP), and have a detrimental effect on the mechanical properties if present in any form. By contrast, carbides can be beneficial or detrimental depending on their shape and distribution. TCP phases are often needle-like and are readily discernible in a microstructural analysis. 

Having established the need for microstructural evaluation of turbine buckets/blades, Molloy turned to the how. The most common method for assessing the microstructure of a turbine airfoil is to perform a destructive analysis, he noted, where a bucket or blade is sacrificed. This normally entails sectioning the airfoil in three locations (at 10%, 50% and 90% of span) as well as the root. These cross sectional areas are metallographically evaluated for many of the features discussed above. 

However, today’s turbine buckets and blades are engineered to ensure they will retain enough of their properties to survive the typical service cycle. It is during the hot gas path (HGP) refurbishment that the blades receive critical solution annealing and age-hardening heat treatments to ensure that the appropriate microstructure has been restored. Some service centers also utilize hot isostatic pressing (HIP) to reduce void content, but this is normally an optional process.  Depending on the service center performing the refurbishment, the bucket/blade set may or may not have a destructive analysis to verify the proper restoration of microstructure. Not everyone is willing to destroy a $30,000 turbine airfoil. 

If your goal is simply to ensure that the heat treatment has been effective, the bucket or blade need not be destroyed. Metallographic replication can be performed near the tip of the airfoil, where stress is low and weld repairs often are performed. Such replications can be evaluated at 5000X to establish the gamma-prime and carbide morphologies (Fig 3). Ideally, such a replication would be performed after the ageing heat treatment, and before any other processing—such as coating. 

3. Gamma prime morphology is interpretable after a field replication

3. Gamma prime morphology is interpretable after a field replication

The bottom line: Bucket or blade microstructure can be confirmed without sacrificing an airfoil.  Moreover, once a bucket or blade is received by a qualified laboratory, a typical replication and evaluation can be done in a day’s time. Alternatively, a fully equipped metallurgist can visit to the location of the blades to perform the replications onsite.

Once the replications have been obtained, the samples must be coated for electrical conduction and evaluated using a scanning electron microscope (SEM) at high magnification (5000X) to establish the gamma-prime and carbide conditions.  The replications can be archived indefinitely for future reference as needed.

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