Not many alloys can survive in the environment downstream of the combustion chamber in a gas turbine engine. It’s too hot, forces are high, the gases are extremely corrosive and they’re flowing quickly. Together, these factors make material selection one of the most challenging aspects of gas turbine design.

As a leader in the production of gas turbine components, Impro combines materials knowledge with extensive manufacturing expertise and resources. Here are the main factors involved in selecting materials for nozzle assemblies.

A Uniquely Challenging Environment

The basic principle of a gas turbine is that air is drawn in continuously through a compressor section and mixed with fuel before being ignited. The combustion gas expands rapidly and flows out through stationary nozzle guide vanes. These direct the gas onto rotating turbine blades that operate the compression section of the engine and power a generator or provide motive power.

At peak output combustion gases can be as hot as 3,090°F (1,700°C) and are extremely corrosive. Both stationary vanes and rotating blades experience high loads, although centrifugal forces on the blades create additional challenges for the design team, the materials and the assembly methods.

Engineered Cooling

A few alloys have melting points above 3,090°F, (tungsten and molybdenum are two), but their cost, and in some cases the difficulty of working them, render them impractical for gas turbine applications. Accordingly, to enable use of lower melting point alloys, cooling features are incorporated into nozzle assembly components. The main methods are to make internal regions hollow, and to add channels that flow cooler air over the external surfaces.

Suitable Alloys

Depending on composition, stainless steels melt between 2,550 and 2,790°F (1,400 to 1,530°C). With the help of cooling features, this is enough to ensure survivability, but stainless has two problems:

• Strength diminishes rapidly at temperatures above 1,472°F (800°C)
• Stainless is prone to oxidizing in corrosive atmospheres

These limitations are addressed by superalloys. This term refers to a broad family of metals that retain their strength at temperature. Most also have excellent corrosion resistance.

The three families of superalloys are based on either nickel, cobalt, or iron. For nozzle assemblies, nickel-based superalloys are the most widely used. Sold under trade names like Inconel, Hastelloy and Nimonic, these are composed primarily of nickel, supplemented with chromium and iron. Iron-based superalloys have lower melting points than those based on nickel and are rarely used in gas turbine applications.

In a limited number of situations, titanium is an alternative to superalloy materials. However, cost, properties, and machinability challenges generally make it a less attractive choice.

Manufacturing Gas Turbine Components From Superalloys

Superalloys like the widely used Inconel 718 are difficult to machine. This stems from high hardness and toughness, combined with a tendency to work harden. Consequently, near-net shape forming processes are preferred in order to minimize material removal.

Impro routinely investment casts nickel and cobalt-based superalloys. This permits formation of tightly-toleranced complex features with excellent surface finishes and thin walls. The process is ideal for the internal cavities and small cooling channels needed in nozzle assembly components. Crack detection and X-ray inspection technologies ensure the quality of parts delivered to customers.

Where precision machining is required, Impro uses 4 and 5 axis milling, in addition to CNC turning. Heat treatment is often needed, both before and after machining, to help maintain alloy properties and tight tolerances. Joining methods include vacuum brazing and electron beam welding, which eliminate or minimize heat affected zones.

Your Source for Highly Engineered Components

Conditions inside a gas turbine engine necessitate use of superalloy materials such as Inconel 718. Casting, machining, and joining metals like these requires a combination of materials and manufacturing process expertise.

Impro is one of very few vertically integrated manufacturers capable of producing such parts complete, from casting to inspection and assembly. To learn more about our ability to manufacture complex, highly engineered components like those used in gas turbine engines, we invite you to contact us.