ASTM A312 is an American Standard specification which covers seamless, straight-seam welded, and heavily cold worked welded austenitic stainless steel pipe intended for high-temperature and general corrosive service. The most common grades are 304/304L Stainless Steel and 316/316L Stainless Steel .In this article, we illustrate the chemical and mechanical properties of the A312 specifications.
ASTM A312 STAINLESS STEEL PIPE
Stainless steel pipes are used for high-temperature applications where corrosion is a key issue.
Stainless steel has been developed to cope with the increasing demand, across many industries, for more durable steels that could withstand demanding service conditions. Stainless steel was conceived as an evolution of standard carbon steel and obtained by adding alloying elements to the base iron like Nickel and Chromium. The addition of such alloying elements enhances the resistance of the steel to corrosion in harsh applications.
Before learning about the different grades, let’s review the types of stainless steel available on the market and their classification.
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STAINLESS STEEL TYPES
As a general rule, any steel alloy that has at least 10.5 percent of chromium may be considered stainless steel. However, a multitude of grades is available depending on the mix of the alloying elements (Nickel. Chromium, Moly, Titanium, Copper, Nitrogen, etc.) Each alloy has a specific structure and chemical and mechanical properties.
The general attribute of stainless steels is that they show resistance to corrosion thanks to an outer chromium oxide layer. Such oxide acts like a microscopic protection layer that reacts with oxygen and blocks corrosion. Further, stainless steel alloys feature better toughness in cryogenic applications than carbon steel, better strength and hardness, improved ductility and low maintenance costs.
Stainless steels may be grouped into a few families, designated as “series”. Let’s take a closer look.
Austenitic Stainless Steel (Series 300)
These are the most common grades of stainless steel. The microstructure of austenitic stainless steels is obtained with the addition of nickel, manganese, and nitrogen that give weldability and formability properties to the alloy. The resistance to corrosion can be further improved by augmenting the percentage of chrome, moly, and nitrogen to the base alloy.
Nevertheless, the basic austenitic grades are vulnerable to stress corrosion cracking (higher percentages of nickel are necessary to enhance the stress corrosion cracking). Austenitic stainless steel cannot be hardened by heat treatment but have can work-hardened to high strength levels while retaining a reasonable level of strength and ductility.
Even if austenitic steels are generally non-magnetic, they can show some magnetic property based on the actual alloy composition and the work hardening given during production. Austenitic stainless steels are divided into the series 200 (chromium-manganese-nickel alloys) and 300 (chromium-nickel alloys like 304, 309, 316, 321, 347, etc). Stainless Steel 304 Pipes is the most common austenitic stainless steel that suits most corrosive applications. Any other grade in the 300 series enhances the basic features of SS304.
Martensitic Stainless Steel (Series 400)
Martensitic stainless steels are similar to ferritic steels as they both have remarkable chromium content, however, martensitic steels have higher carbon content up to 1 percent. The high carbon content allows martensitic steels to be hardened and tempered as standard carbon and chrome alloy steels (but show generally low weldability and ductility). This type of stainless steel is specified in case of high strength and moderate corrosion resistance requirements. Different from standard austenitic stainless steels, martensitic grades are magnetic. Common martensitic grades are 410, 420 and 440C.
Ferritic Stainless Steel (SS430)
Ferritic stainless steels have significant chrome content but low additions on carbon (generally below 0.1 percent). The name of this family of stainless steels comes from the fact that their microstructure is quite similar to carbon and low alloy steels.
These steels have a wide range of application, except for thin surfaces as they have a low resistance to welding or applications requiring formability (ferritic steels show low formability and ductility). Ferritic stainless steels cannot be hardened by heat treatment. By adding moly to a ferritic grade, the steel can be used in highly aggressive applications like desalination plants and seawater. These steel show also remarkable resistance to stress corrosion cracking. Likewise martensitic steels, ferritic SS is magnetic. The most common ferritic grades are the 430 (17 percent chromium), and the 409 (11 percent chromium), largely used in the automotive sector.
Precipitation hardening (PH 17-4)
PH steels can feature remarkable strength due to the addition, in the alloy, of elements as copper, niobium, and aluminum. These steels can be machined to very specific shapes with high tolerances requirements before the final aging treatment. This is different from conventional hardening and tempering of martensitic steels that are subject to distortion during the treatment.
The resistance to corrosion of precipitation hardening steels is comparable to standard austenitic steels like SS304. The most common precipitation hardening stainless steel is the 17-4PH, which features 17 percent chromium and 4 percent nickel.
ASTM A312 PIPE DIMENSIONS
The standard stainless steel pipes dimensions are set by the ANSI ASME B36.19 specification. Seamless SS pipes are available in the size range 1/8″ thru 24″, welded stainless pipes are manufactured in the range 2″ thru 36″ (ASTM A312 Pipes i.e. electric-fusion-welded austenitic chromium-nickel stainless steel pipe, or as rolled).