The aircraft industry is one of the most demanding industries. Materials used to manufacture gas turbine components must meet high requirements. Rotating elements work under difficult condition, and their damage often have catastrophic effects. One of the cause’s compressor damage is foreign objects that are sucked by the inflow. It could happen while taxiing or as a result of reverse thrust. It is important to follow the procedures to keep the runways in good condition. But is erosion the only one problem for the gas turbines?

Have you ever wondered what the maximum temperature in a gas turbine is? Which gas turbine components work at the highest temperatures, compressor, turbine blades or elements of combustion chamber? Maybe, you know what kinds of construction materials are widely used to gas turbine components? 

trent 1

Figure1. Trent 800, Rolls-Royce.

Let's have a look at the air and exhaust flow (and temperature) in gas turbine. Air with a temperature between -50-40°C is sucked in through the three-meter diameter fan at the front of the engine. The compressor compresses the air to high pressure and the temperature grows up to 300–650°C. Then, the fuel is mixed with the air within the combustion chamber and is ignited. The temperature of this mix is 2000°C. The hot combustion products push the turbine blades which then rotate the turbine shaft. The temperature of this combustion falls down to 1500°C. Because compressor and turbine are attached to the same shaft, the rotation of the turbine rotates also the compressor, keeping the system operating. In the last stage, a gas is blown out of the rear of the engine.

As you can see, the work conditions of gas turbine components are very difficult. The gas turbine components are required to work under conditions of high temperature (max2000°C) and mechanical loading. These components are loaded with longitudinal and centrifugal forces, vibrations and erosive environment. The most important material features include creep and fatigue resistance, high yield strength, lightness and erosion resistance. The next material requirement is good machinability. Many companies deal with modern cutting tools manufacturing, for instance, Ceratizit Group and WalterTools. Those companies offer special solutions for aircraft industry such the advanced milling heads and turning tools with carbide inserts. The inserts have a special coating and geometry to make the machining easier. 

Material groups that meet above requirements contain aluminum, steel and titanium alloys. However, those alloys do not retain strength and hardness at temperatures above 400°C. When temperature goes above about 400°C, those alloys are no longer strong enough for the gas components. To work under these conditions, engineers created the superalloys.

The superalloys fall into three groups, nickel-based, cobalt-based and iron-based alloys. The most popular group is nickel-based superalloys. Popular commercial designations of these alloys are Inconel 718 content 50.0-55.0% Ni with hardness 425 HB (aged) and Hastelloy R-235, 310HB, content 61.0% Ni. Other popular alloys are Waspaloy and Nimonic PK33. Nickel-based superalloys are the most used for blades, impellers, rings and discs in gas turbines. A next group is iron-based alloys, that are suited for shafts, rings, and casings. However, these alloys do not retain strength and hardness at high temperatures as good as the other two groups. The last group is cobalt-based alloys. That group has got the best hot corrosion resistance at high temperatures, but at the same time, alloys from that group are more expensive than the others. 

One of the most important material properties used in aircraft engine industry is strength as a function of temperature.  Regarding to these properties the superalloys have no equal. While titanium, steel and aluminum alloys lose their high temperature properties (above 400°C), superalloy components can safely work at twice as high temperature. As you can see, the superalloys are, indeed, super.

Author: Joanna Kołakowska

REFERENCE

[1] Matthew J. Donachie, Stephen J. Donachie, Superalloys: A Technical Guide, 2nd Edition, 2002 [2] ROGER C. REED, The Superalloys: Fundamentals and Applications, Cambridge University Press, 2009

[3] Heat Resistant Super Alloys, application guide, Sandvik Coromant, 2010

[4] http://users.encs.concordia.ca/~kadem/Rolls%20Royce.pdf

[5] http://www.rolls-royce.com/products-and-services/civil-aerospace.aspx