“Stainless steel is a material that we wish never was labeled “stainless” because it can do so much more than simply resist rust. Technically, Stainless Steel is strictly a trade name applied to what are known as corrosion-resistant steels. It is a fabulous material that outperforms mild and alloy steels in so many different applications in racing that no other material can match it, and I think all racers should consider it as a vital element in their fabricating efforts.” - Rick Popovits
However, stainless steel does have some unique properties that the fabricator needs to know about before launching into a project. An interesting characteristic of many types of stainless steel is that they are non-magnetic, a quality that makes them very important in the aerospace industry. Compared to mild steel, stainless steel has superior high temperature characteristics. It is an excellent material for headers and exhaust systems, or any application where high heat is encountered.
Stainless steel is an iron alloy that contains at least 12% chromium and low carbon content, usually less than .15%. This high chromium content retards corrosion giving the steel its “stainless” quality. There are many stainless steel alloys that fall into two basic categories:
- High-nickel grades (austentic)
- Low-Nickel grades (martensitic, ferritic and duplex)
The high nickel, austentic grades (300 series) are the most commonly used in race car fabrication due to the nickel content which provides excellent weldability and corrosion resistance. Also, nickel improves mechanical properties such as fatigue strength, toughness and ductility. Austentic stainless steels are normally non-magnetic. Stainless steels are often referred to by their chromium and nickel content: for instance, 18-8 stainless has 18% chromium and 8% nickel. 400-series stainless steels contain little or no nickel and are often used in production automobile exhausts. These are also magnetic.
As mentioned, stainless steels have a rather low carbon content. Some carbon is needed for hardness, but it also causes the stainless to become susceptible to corrosion at high temperatures. When heated to a temperature range of 800° to 1590°F, the carbon in the steel combines with chromium to form chromium carbides. This transformation is called carbide precipitation and reduces the corrosion resistance of the steel by allowing intergranular corrosion to occur. Some stainless steels are known as low carbon grades to minimize this carbide precipitation; others, such as 321, are special alloys that reduce carbide precipitation by combining and stabilizing the chromium at elevated temperatures.
Another great property of stainless steel is low thermal conductivity, 219% less than 1010 steel. You may recall Smokey Yunick talk about increasing the scavenging effect of headers by covering headers with a thermal wrap. Also, thermal ceramic coatings for headers have become popular for this purpose. The low coefficient of thermal conductivity inherent to stainless steel provides the same effect. More importantly, it protects the car and the driver from excessive, fatiguing high temperatures and lower air induction temperatures. It should be noted that wrapping mild steel headers can lead to premature failure due to high temperatures.
These many characteristics, such as superior heat retention properties, high temperature fatigue resistance, and to a lesser extent, the cosmetic value of a non-rusting finish, combine to make stainless steel an ideal choice for headers and exhaust systems. Such a system will produce more horsepower and last “’til the cows come home.” It is an excellent solution.
Now that you are sold on the merits of stainless steel, there are a number of things you need to know about the different types available before you launch into a header and exhaust system project.
A three-digit numerical classification system is used throughout the industry. The racer needs to be familiar with only one of these three-digit series within the system - the 300 series. They offer the fabricator a wide array of choices, from ornamental quality up through the highest-temperature and closest-tolerance aircraft quality. Within the 300 series of stainless steels, there are four types that are suitable, available and cost effective for the racer. These are 304, 316L, 321, and 347.
Stainless steels come in both tubing and pipe sizes. Since certain pipe sizes are almost identical in dimension to tubing sizes, pipe may sometimes be substituted for tubing, and vice versa. Numerous wall thicknesses are available, but for headers, normally .049” (18 gage) to .065” (16 gage) is used.
304 is the most inexpensive and available stainless in the 300 series. It is suitable for normallyaspirated header applications, and has been successfully used by many racing teams. It does not have the high temperature fatigue resistance that 321 does, but is easier on the budget.
316L is an extra low carbon (ELC) grade of stainless that has only .03% carbon, making less carbon available to precipitate with the chromium. It is recommended for marine exhausts where salt water corrosion mixed with diesel exhaust particulates and electrolysis create such a hostile environment that even other grades of stainless have difficulty coping with it.
321 and 347 are known as stabilized grades of stainless and are alloyed with either titanium (321) or columbium (347), both of which have a much stronger affinity for carbon than does chromium at elevated temperatures. This minimizes intergranular corrosion due to carbide precipitation. Both 321 and 347 are top choices for exhaust headers, especially turbocharger systems and rotary engines.
Different specifications are used to meet particular requirements for the military (MIL), the American Society of Testing Materials (ASTM), and the Society of Automotive Engineers (SAE). Burns Stainless offers these grades:
ASTM A-269 304 stainless is a general service commercial specification that is higher quality and is fully annealed for better ductility. It is available in both welded seam and seamless, and is a good spec for the racer to use. In our experience, both, seamless and welded seam tubing are very durable in header applications, but seamless tubing is considerably more expensive.
MIL-T-8808/8606\MIL-T-6737 321 stainless are military specifications for aircraft tubing. Suffice it to say that some MIL-specs are not necessarily better or even as good as some ASTM standards. There is no particular magic here.