Stainless steel is a cost effective, low-maintenance metal used in civil and mechanical engineering projects Australia wide. Stainless steel is particularly popular for pipe fittings because it is resistant to oxidation and has no effect on other metals. Stainless steel has applications everywhere from shipping, oil and gas, water treatment facilities, energy industries, to the textile machinery.
Stainless steel is an umbrella term for a family of corrosion resistant alloy steels containing 10.5% or more chromium. Stainless steel has a chromium-rich oxide film on the surface, which makes it resistant to corrosive attack, and although stainless steel is relatively thin and lightweight, it is highly protective and strong, with a tensile strength ranging from about 621 MPa to 1300 MPa.
As a result, stainless steel is the material of choice for durable long-life span pipes and pipe fittings. There are five basic grades of stainless steel that you will frequently come across.
Stainless steel can include other elements besides chromium, such as nickel, molybdenum, titanium and niobium, giving rise to a wide range of different steel types and grades. The grade of stainless steel refers to its quality, durability and temperature resistance. The most common grades are 304 and 316, characterised by their different levels of corrosion resistance in saline and chloride exposed environments, as well as differing levels of heat tolerance. Stainless steel can be further divided into five basic categories:
18% chromium content and 8% nickel. Excellent corrosion resistance, excellent weldability, excellent in high and low temperatures, hardened by cold.
Varying chromium content between 12 and 18% and a low carbon content. Moderate to good corrosion resistance, hardened by heat. Poor weldability.
High chromium (18 to 28%), moderate nickel (between 4.5 and 8%) and in many cases molybdenum (2.5 to 4%). High resistance to stress and corrosion, increased resistance to chloride iron, high tensile and yield strength, good weldability.
Relatively high carbon of 0.1 to 1.2% and chromium of 12 to 18%. Moderate corrosion resistance can be hardened for high strength. Poor weldability.
Formulated so they can be supplied in a solution treated condition. Characterised by corrosion resistant properties, high strength and weldability.
Following the above, selecting stainless steel piping and fittings by grade and category is the next step. Pipes and fittings are chosen through the careful review of their purpose and environment, including any chemicals they may come into contact with.
Austenitic and martensitic stainless steels are frequently employed when corrosion is the biggest hurdle. Austenitic steel, with its high quotient of chromium and nickel, is especially resilient.
High pressure pipes require alloy-elements to resist stress and high temperatures. Piping made from solid chromium or chromium and nickel alloys is common, particularly in seamless, welded and electric fusion combinations.. Use of press fitting connections has revolutionised stainless steel installation, saving time, specialised labour, welding gas, hot work permits etc.
Used for fuel lines and hydraulic systems, hydraulic line tubing is typically smaller in diameter than other tube types. Austenitic steel types are ideal for their strength and corrosion resistance.
As the name suggests, sanitary tubing is designed to maintain sanitary standards. Depending on your requirements, sanitary tubing can be austenitic or ferritic, and is defined by being non-porous and easy to clean, suitable for pharmaceuticals, food and beverage, and drinking water applications.
Mechanical tubing is typically martensitic, composed of high carbon and chromium ratios that can be hardened for strength.
Air Energy supplies a comprehensive range of stainless steel pipes and fittings. We have 20 years of experience across a wide variety of technical industries across Australia. We promise fast, dependable, cost-effective solutions to suit your specifications. Browse online our innovative range of stainless steel EUROPRESS Press piping and fittings or contact us today for assistance.
Stainless steel is a material that we wish never was labeled "stainless" because it can do so much more than simply resist rust. The origins of stainless steel date back to the early 1900s when an English metallurgist developed a type of steel for making knives that would not 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 all racers should consider it as a vital element in their fabricating efforts. 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 similar to mild and alloy steels; it is an alloy of iron that contains at least 12% chromium. This high chromium content retards corrosion giving the steel its "stainless" quality. There are many alloys of stainless steel, which are broken down into two basic categories:
Chromium-nickel grades
Straight chromium grades
The chromium-nickel grades are the more common stainless steels used in race car fabrication compared to the straight chromium types, due to the nickel content which provides excellent weldability and corrosion resistance. Also, this nickel improves some mechanical properties such as fatigue strength, toughness and ductility. People sometimes refer to stainless steels based on their chromium and nickel content: for instance, 18-8 stainless has 18% chromium and 8% nickel in it.
Stainless steel typically has a rather low carbon content, in the range of .08% to .15%, and sometimes as low as .03%. The carbon is needed for hardness, but it also can cause the stainless to become susceptible to corrosion at high temperatures. What happens is this: when chromium-nickel steel is 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. The chromium is reduced in this heat-affected area and makes the steel subject to what is known as intergranular corrosion. 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.
You may have heard Smokey Yunick talk about maintaining high exhaust velocity and increase scavenging by covering headers with a thermal wrap. In addition, there are companies that coat headers with a thermal barrier, typically some type of ceramic formula, in order to keep the heat inside the exhaust system. Stainless steel performs this function without the need for add-ons because it has a much lower coefficient of thermal conductivity, thereby keeping more heat inside and transmitting it to the header outlet. Radiated heat is perhaps the most important reason to wrap or ceramic coat the headers to protect the car and the driver from excessive, fatiguing high temperatures.
Typical 1010 carbon (mild) steel conducts 219% more heat per foot than do the types of stainless steel we use in header fabrication. By contrast, quite a bit more heat stays inside the stainless header tubes and does not get passed into the surrounding air. By not allowing the contraction of the cooling gases as they flow down the tubes, more exhaust velocity is retained which promotes better scavenging at the collector. This retention of velocity increases the overall header efficiency.
You've probably seen Indy cars with their enclosed engine compartments and thermal clam-shell enclosures around their turbocharger headers. They must thermally wrap their exhaust pipes just so the radiant heat off the tubes won't cause fires or melt any critical systems. In this case headers made out of mild steel would completely fail and break apart due to the severe heat retention, let alone scale and send death particles into the turbocharger, ruining the turbine blades. 321 stainless steel tubing has excellent high temperature fatigue resistance in this enclosed application and does a darn good job of living in this hostile environment better than any other material except the ultra-high nickel content steels ( such an Inconel ), which are hard to find, very difficult to work with and extremely expensive.
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.
321 and 347 are known as stabilized grades of stainless. These 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 eliminates carbide precipitation leaving the chromium where it belongs for corrosion protection...remember our discussion of intergranular corrosion? Both 321 and 347 are top choices for exhaust headers, especially turbocharger systems and rotary engines. Since 321 is much more available than 347, that leaves 321 as the first choice, with no sacrifice in needed qualities.
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 used extensively in marine exhausts where salt water corrosion mixed with diesel exhaust particulates and electrolysis create such a horrible environment that even other grades of stainless cower and run away!
304 is the most inexpensive and available stainless in the 300 series. It is suitable for normally-aspirated 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 considerably less costly and much more available. Most 304 tubing these days has the dual designation of 304/304L.
Practically speaking, there are overlapping applications of 304 and 321 stainless in header construction, but knowing you've got the insurance of the aircraft-grade 321 for the job is definitely worth consideration of the extra cost... if your application requires it.
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-gauge) to .065" (16-gauge) is used.
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). Examples of what to look for when you order stainless tubing are as follows:
ASTM A-554 304 stainless is a welded mechanical tubing used primarily for ornamental purposes. It is not fully annealed and is work-hardened slightly in manufacturing. It has good column strength and good bendability.
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. We have not seen any difference in longevity between welded seam and seamless stainless tubing in header use, but there is a substantial cost difference. A249/269 has better dimensional tolerances, higher pressure rating and excellent bendability. Some header manufacturers successfully use A554, but A249/269 is the better choice.
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.
There are as many uses for stainless steel as there are projects in the shop. There is nothing else that transmits an image of quality and skill to the majority of fabricators than a cleanly constructed stainless steel project. Whether it is a set of headers, intake stacks, or even a stand for one's dyno engine cooling fan, stainless steel has such great mechanical properties that its use should be considered for many projects beyond exhaust systems.
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