The combustion gases expelled from a gas turbine will quickly erode and can even melt most steels. For internal components to survive in such an environment, they are investment cast in nickel-based superalloys, precision machined and carefully inspected. Here’s an introduction to their function and the production process.

Axial Flow Compression

A gas turbine produces force and rotary motion from continuous combustion. Fan blades at the front rotate to draw air in. This air moves through the turbine along the axis of the fan until it reaches the combustion chamber where a continuous stream of fuel is injected. The mixture burns and the hot gases expand rapidly, pushing out through turbine blades at the rear of the engine. As they turn, the shaft on which the blades are mounted drives the compressor fan blades at the front of the engine and any other connected rotating machinery.

Gas Turbine Applications

Aircraft use a form of gas turbine optimized for aerospace, but most of these engines go into power generation. These “heavy frame” gas turbines are bigger and more powerful than the aircraft versions. Heavy frame gas turbines for producing electricity have a generator attached to the output shaft, letting them produce megawatts of power.

Gas Turbine Nozzle Assemblies

The nozzle in a gas turbine guides the exhaust gases out of the back of the engine. The multiple rings of turbine blades surrounding it are what produce the rotary motion. These blade assemblies must maintain their strength and dimensions despite exposure to fast-moving combustion gases and temperatures as high as 2,500 °F.

Alloys

Most steel alloys suffer from a phenomenon known as “creep” before reaching their melting point. This is where the metal loses strength and begins to deform, which makes them unsuitable for gas turbine applications.

The solution is to use nickel-based superalloys, particularly those with a nickel-chromium-cobalt composition like Inconel 617®. These have a lower melting point than many steels but don’t suffer from creep. They also have excellent resistance to the corrosive gases in gas turbine exhaust.

The melting point limitation is addressed by incorporating cooling channels into the individual blades. However, as superalloys are hard to machine, these channels are incorporated during the casting process.

Production Processes

Gas turbine blades are produced by investment casting, followed by heat treatment and limited precision machining. Building these blades into nozzle assemblies requires machining of other components, plus welding and testing.

Investment Casting

Wax patterns replicating the part being cast are used to form ceramic molds. Holes and cavities are created by inserting cores into the pattern. These remain in the ceramic mold after the wax is melted out and are removed once the ceramic shell is broken away.

Investment casting yields parts very close to the final dimensions, with thin walls where needed and excellent surface finish. This minimizes the amount of machining required.

After casting, superalloy parts usually undergo heat treatment to reverse metallurgical changes caused by alloy constituents solidifying at different temperatures. Particularly tight tolerances are then achieved by precision machining.

Machining Processes

Very hard parts, and those with tightly toleranced features, are ground to final size. EDM is also employed to produce small features and achieve tight tolerances. Elsewhere, CNC turning and 3 and 5-axis milling are used extensively.

Welding

Where needed, components are joined by brazing, gas tungsten arc welding, (GTAW) or electron beam welding (EBW). Stress relieving prevents cracking or distortion and various NDT techniques are used to verify weld integrity.

Assembly and Test

Once a gas turbine nozzle assembly is complete it undergoes thorough inspection to verify fitness for purpose. This includes flow testing and inspection by borescope and ultrasound.

The Casting and Machining Competencies You Need

It takes a lot of expertise to manufacture nozzle assemblies for gas turbines. They feature complex geometries to optimize gas flow and cooling, and are made from nickel-based superalloys. Investment casting creates near-net shape forms which are precision machined to tight tolerances before welding and assembly.

Impro is one of relatively few companies with the resources and skills needed to make these highly engineered products. If we can satisfy the casting and machining requirements for gas turbine engine components, we can almost certainly produce the parts you need. Contact us to find out.