Stainless Steel and Superalloys in Turbine Engines

For those of us who are fortunate enough to travel internationally, we may take for granted the components that make up the incredibly robust body of the air carrier we are in – or maybe not. Let’s face it, at 35,000 feet above the ground there is little margin for error which means the materials chosen for the internal and external parts of these amazing flying machines are critical.

Stainless steel is a strong, durable and corrosion resistant material which makes it perfect for some turbine blades which need to work at 100 percent at high temperatures and low-pressure environments.

However, when we take a closer look at the heart of these power generating beasts, we see that there is a lot more to consider.

The Role of Stainless Steel and Other Materials in Turbine Engines

One of the issues standing in the way of aviation advancement at the turn of the century was the lack of materials strong enough to withstand the forces that were required for bigger, faster aircraft. The aluminium block engine used by the Wright brothers was light and agile but could only withstand temperatures up to 660°C before melting.

Why do we say, “only 660°C”?

Consider this:

“Turbine blades are subjected to stress from centrifugal force (turbine stages can rotate at tens of thousands of revolutions per minute (RPM)) and fluid forces that can cause fractureyielding, or creep[nb 1] failures. Additionally, the first stage (the stage directly following the combustor) of a modern turbine faces temperatures around 2,500 °F (1,370 °C),[7] up from temperatures around 1,500 °F (820 °C) in early gas turbines.[8] Modern military jet engines, like the Snecma M88, can see turbine temperatures of 2,900 °F (1,590 °C)” (Source)

It's with this in mind that engineers are making use of superalloys with refractory metals such as tungsten, molybdenum, as well as ceramics and ceramic-metal mixes in combustion chambers, turbines and exhaust nozzles.

The 2007 Nasa Guide to Engines offer some fascinating insights as noted below:

“Temperatures can exceed 1800 °C and again superalloys are used, but without the titanium or aluminum for strength because there are no moving parts. Instead, refractory metals are often added to a superalloy.

“These are metals of unusually high resistance to heat, corrosion, and wear such as tungsten, molybdenum, niobium, tantalum, and rhenium. They are used in alloys and not as pure metals because they are among the densest of all the elements, a negative property when it comes to aircraft that need to keep weight to a minimum.

“Ceramics and ceramic-metal mixes are also used here because of their high heat resistance. We are familiar with pottery, tile, crucibles, and fire bricks as types of ceramics. They have very high melting points and don’t require the cooling systems like those needed to keep metals from melting, so they make for lighter, less complicated engine parts. The down side is that they tend to fracture under stress, so engineers seek to create new ceramics composites that incorporate other materials to improve properties”

Whew.

So, while stainless steel remains a strong contender and a useful addition to turbine engines, the incredible technical leaps that this industry has enjoyed has put increasing pressure on engineers to find stronger, lighter and more heat-resistant mixes.

We’ll leave these complex searches to the engineering brains for the moment while we hop on the plane and enjoy our hard-earned year-end break.

 

Comments are closed.