A metal for all seasons
By George Genevro
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.
The American Iron and Steel Institute (AISI) then developed the three-digit numbering system that was used for many years. It is still used, along with the recently developed Unified Numbering System, in the stock lists of suppliers and in technical literature. For stainless steels, the UNS system consists of five numbers preceded by the letter S. In cases where a letter suffix is used in the American Iron and Steel Institute system, in the UNS system it is replaced by a number, usually the fifth digit. For example, two common alloys such as 316 and 316L, (a low carbon version of 316 that contains a maximum of .03 percent carbon) are designated as UNS S31600 and UNS S31603 respectively. The UNS system is now used to designate all non-ferrous and ferrous metals.
CHART 1
17-4CuMo, 17-10P,
and others
Iron-chromium-nickel alloys were assigned 300 series numbers and the iron-chromium alloys were given 400 series numbers. These two series account for all but a few of the stainless steel alloys available. The relatively few alloys in the 200 series are usually modifications of 300 series materials. For example, 203S (UNS S20300) is a free-machining version of 303 with less nickel, more manganese, and more copper. In the three-digit AISI system minor alloy changes are indicated by a letter following the number. For example, AISI-304L is a low carbon version of 304. When selenium is added to improve machinability the chemical symbol Se is added as a suffix. Another variation in the AISI system is that carbon content in the hardenable 440 stainless steels is indicated by the letters A, B, and C.
The two major stainless-steel categories - the iron-chromium nickel 300 series and the iron-chromium 400 series - can be further subdivided by a study of their microstructures since this is the key to their various properties. As shown in the chart on page 18, the terms martensitic, ferritic, austenitic, and precipitation hardening are used to show which alloys fall into a particular microstructure category.
Martensitic - About 10 of the 400 series stainless steels are hardenable by heat treatment and are therefore useful in applications where high strength, hardness, and abrasion resistance are necessary. The chromium content varies between 11.5 and 18 percent and the carbon content is about .18 percent in all of the 400 series stainless steels except the 440A, 440B, and 440C alloys. The 440A, B, and C stainless steels have carbon contents varying from .60 to 1.2 percent, with the highest carbon content yielding the greatest hardness and least ductility when fully hardened.
Ferritic - The six stainless steels in the 400 series classed as ferritic cannot be hardened by heat treatment. How-
ever, they can be hardened slightly by cold-working processes such as rolling and stamping. The chromium content ranges from about 14 to 27 percent, giving better corrosion resistance than martensitic steels. Other features include relative ease of fabrication, good mechanical properties, and relatively low cost.
Austenitic - About 65 percent of all stainless steels fall into the austenitic category. These steels are very corrosion resistant, do not become brittle at high temperatures, and are highly ductile, and therefore relatively easy to form. The chromium content varies from 16 to 28 percent and the nickel content ranges from 6 to 22 percent, providing a wide range of metallurgical characteristics. One feature is the ability to resist temperatures up to 2,000 F when the chromium content is at or above 25 percent. Austenitic stainless steels cannot be hardened by heat treatment but can be strengthened by cold-working. They are non-magnetic.
Precipitation hardening - Several of the chromium-nickel steels are either semi-austenitic or martensitic. They are also known as precipitation-hardening types and are denoted by a number/letter system (17-4 PH, for example). These materials are solution-annealed at high temperatures and after cooling remain relatively soft at ambient temperatures. They may be fabricated or machined in this condition and then hardened by means of a relatively low temperature heat treatment (about 900 to 1,100 F) that precipitates out elements that had been held in solid solution by the annealing process. The precipitation hardening process reduces both ductility and machinability and increases hardness and tensile strength. New PH type alloys, including some produced by the Vacuum Arc Remelt process, are being introduced as the family of stainless steels expands.
This chart indicates the relative corrosion resistance of a group of commonly used metals and alloys. When two metals are in contact, the metal that is higher on the chart acts as a sacrificial anode to the metal below it. Metals that are far apart on the chart will corrode more severely when they are in contact, particularly in a wet environment. For example, an unprotected magnesium part in close proximity to a stainless-steel part would deteriorate severely in the presence of moisture.
Applications
What does all this mean for those who design and also maintain and repair aircraft? Are stainless steels the answer to all of our fabrication and corrosion problems? Not really. While stainless steels have many admirable characteristics, we must remember that in a structure as complex as an airplane many different metals are used, almost always in close contact with each other. A quick look at the listing of common metals in the galvanic series chart tells us that only three of the noble metals - silver, gold, and platinum - are more corrosion resistant than stainless steels. The metals at the top of the chart corrode more readily than those at the lower end and we should note that aluminum, in third position on the chart, corrodes readily when not coated with pure aluminum by the Alclad process or protected by other means. Two important factors to keep in mind: when two metals come in contact, the one above it on the chart serves as a sacrificial anode to the one below; and, the farther apart the metals are on the chart the more severe the corrosion will be on the sacrificial metal when they are in direct contact.
We all enjoy the clean, shiny appearance of stainless-steel fasteners for cowlings, inspection hole covers, and other parts on our aircraft, but we should be aware that aluminum and stainless steel are quite far apart on the galvanic series chart and that when these two materials are in unprotected contact corrosion of the aluminum can be quite severe. On unpainted aircraft one solution is the use of non-metallic washers while on painted aircraft a well-applied and properly maintained paint film will act as an effective barrier to corrosion. Another factor to keep in mind is that the typical stainless-steel machine screw that has a completely threaded shank is not a structural fastener and should never be substituted for regular aircraft hardware that is designed to carry shear as well as tension loads.
In the past, several attempts have been made to build entire airplanes out of stainless steel but none have been produced in large numbers. The Budd Oonestoga, a high-wing freighter type airplane, of which 11 were built in the 1940s, has faded into oblivion and only two examples of the Fleetwings Seabird amphibian were still flying in recent years. On these aircraft spot welding was used extensively in fabricating the airframe. Will someone ever tackle the demanding task of designing and building another all-stainless steel airplane for the civilian market? Probably not.
Conclusion
In the time that stainless steels have been available, their contributions
to the aircraft industry have been both valuable and widespread. Aircraft parts made from these materials are used whenever their unique properties can make a product more reliable, safer, more efficient, longer lasting, as well as more attractive. Since safety is a matter of paramount importance in aircraft, it is comforting to know that the work of many metallurgists has provided us with many critical items, such as exhaust systems, turbosuperchargers, and gas turbine parts that are durable and cost-effective.
Stainless steels are used effectively in a wide variety of aircraft. And it is comforting to know that various types of stainless steels are quietly doing their diverse tasks well and safely.
George Genevro, a retired college professor at Cal State Univ. at Long Beach, CA, is an A&P mechanic, pilot, and aircraft owner.
