Stainless Steel: A metal for all seasons

A metal for all seasons By George Genevro The exhaust manifold, waste gate, hot side of the turbo, and the exhaust elbow are made of various grades of stainless steel. Exhaust system and turbosupercharger for the Orenda engine. The...


Stainless Steel

A metal for all seasons

By George Genevro

exhaust manifold The exhaust manifold, waste gate, hot side of the turbo, and the exhaust elbow are made of various grades of stainless steel.
exhaust system
Exhaust system and turbosupercharger for the Orenda engine. The turbo runs at red heat at full power.

Stainless steel is present in many areas of an aircraft. It's resistance to corrosion and ability to withstand high temperatures makes it the metal of choice in many different applications. In this article, we will look at different stainless steel types as well as considerations to keep in mind when working with these metals.

Stainless steels came into general commercial use around 1930 and were regarded by some as "wonder metals." Only a few types and grades were available then but their ability to resist corrosion and heat made them very valuable. While stainless steels were much more expensive than plain carbon and alloy steels, they were far more effective in applications such as aircraft exhaust systems, turbosuperchargers, equipment for the chemical and food industries, and hospital equipment.

What was different about these steels? How could they resist heat and corrosion so well? Were they really "wonder" metals or did they have limitations? The word "stainless" is not an absolute term since stainless steels are not all completely resistant to heat, corrosion, or attack by certain chemicals. They are, however, more corrosion resistant than other steels and all but a few of the non-ferrous metals or alloys. When the proper type of stainless steel is chosen for a specific application - and this is a critical consideration for those who work on aircraft - these steels are extremely effective.

While all stainless steels share certain basic characteristics, some can be very different from others. This makes selection for a particular application a very critical process, especially when cost, component reliability and durability, compliance with original equipment specifications, and in-flight safety are major considerations. The problem becomes more critical when fabrication or repair processes involve welding and close contact with other metals in service.

General characteristics
All stainless steels are basically alloys of iron and chromium; iron, chromium, and nickel; or iron, chromium, nickel, and manganese. Other elements, such as selenium, are often added in trace amounts to alter characteristics such as machinability and weldability. Carbon content can also be varied to alter a given alloy's reaction to heat treatment, welding, and contact with certain chemicals. Along with iron, chromium is the alloying element common to all stainless steels. The chromium content varies from a low of 4.6 percent in some of the 500 series alloys to a high of almost 30 percent in some of the 300 series steels.

In addition to corrosion and heat resistance, stainless steels offer the aircraft designer and builder a number of other valuable characteristics. As the percentage of primary alloying elements is varied and secondary alloying materials are added or deleted, other characteristics will vary. Tensile strength, fatigue resistance, weldability, response to heat treatment, forgeability, machinability, resistance to corrosion in certain atmospheres, and other properties can all be changed. These are truly versatile metals.

Classification
Stainless steels were originally designated by noting the percentages of the major alloying elements (other than iron and carbon). For example, one of the first commonly used alloys was known as "18-8" because, along with iron, it contained 18 percent chromium and 8 percent nickel. As more stainless-steel alloys were developed to meet specific needs, it became evident that this designation system was no longer adequate.

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