The ASTM A312 standard includes seamless and welded austenitic stainless steel pipes used for high temperature and/or corrosive environments (the most common grades are 304/304L and 316/316L). In this article, the chemical and mechanical properties of the ASTM A312 requirements are discussed.
ASTM A312 Stainless Steel Seamless Pipes
are used for high-temperature applications where corrosion is a major issue. Stainless steel was produced at the beginning of the century to meet the growing demand, across many industries, for more robust steels that could survive challenging service environments. Stainless steel was developed as an evolution of regular carbon steel and was created by the addition of alloy elements to base iron such as nickel and chromium. The introduction of such alloy elements increases the tolerance of the steel to corrosion in rough environments.
Before moving through different grades, let’s look at the types of stainless steel available in the market and their classification.
Stainless Steel Types
Generally speaking, any steel alloy with a chromium content of at least 10.5 per cent may be called stainless steel. However a multitude of grades are available depending on the mixture of alloy elements (Nickel, Chromium, Moly, Titanium, Copper, Nitrogen, etc.). Each alloy has its own unique structure and chemical and mechanical properties.
The general feature of stainless steels is that they demonstrate resistance to corrosion through the outer layer of chromium oxide. These oxide serves as a microscopic layer of protection that reacts with oxygen and prevents corrosion. In addition, stainless steel alloys have higher performance in cryogenic applications than carbon steel, greater strength and endurance, increased ductility and relatively inexpensive.
Stainless Steel may be divided into a few families, referred to as series.
Austenitic Stainless Steel Pipes (Series 300)
They are the most popular grades of stainless steel. The microstructure of austenitic stainless steel is obtained by adding nickel, manganese and nitrogen, which give the alloy its weldability and formability properties. Corrosion resistance can be further enhanced by increasing the percentage of chromium, moly and nitrogen to the base alloy.
However, basic austenitic grades are prone to stress corrosion cracking (higher percentages of nickel are required to improve stress corrosion cracking). Even if the austenitic steels are usually non-magnetic, they can display certain magnetic properties based on the actual structure of the alloy and the work hardening performed during processing. Austenitic stainless steels are classified into 200 series (chromium-manganese-nickel alloys) and 300 series (chromium-nickel alloys 304, 309, 316, 321, 347, etc). Grade 304/304L is the most popular austenitic stainless steel pipe suitable for most corrosive applications. Any other grade in the 300 series improves the basic features of the SS304 class.
Martensitic Stainless Steel Pipes (Series 400)
Martensitic stainless steels are identical to ferric steels, both of which have an incredible chromium content, whereas martensitic steels have a higher carbon content of up to 1 percent. The high carbon content enables martensitic steels to be hardened and tempered as standard carbon and chrome alloy steels (but generally low weldability and corrosion resistance).
This type of stainless steel is specified for requirements in terms of high strength and fair corrosion resistance. The martensite grades are magnetic unlike typical austenitic stainless steels. Common grades of martensites are 410, 420, and 440C.
Ferritic Stainless Steel Pipes (SS430)
Ferritic stainless steels have a high chromium content but low carbon content (usually below 0.1 percent). The name of this stainless steel family comes from their microstructure being very identical to carbon and low-alloy steels.
These steels have a variety of applications, except for thin surfaces because they have low welding resistance or formability-requiring applications (ferritic steels have poor formability and ductility). Ferritic stainless steels cannot be heat-treated to harden. When applying moly to a ferritic grade the steel can be used as desalination plants and seawater in highly aggressive applications.
Such steels also show exceptional resistance to cracking corrosion stress. Ferritic SS is similarly magnetic to martensitic steels. The 430 (17 percent chromium) and 409 (11 percent chromium) are the most common ferritic grades, mostly used in the automotive sector.
ASTM A312 PIPE DIMENSIONS
The basic dimensions of stainless steel pipes are set according to the specification ANSI ASME B36.19.
Stainless Steel Seamless Pipes
are available in the size range 1/8′′ through 24′′, welded stainless steel pipes are made in the range 2′′ through 36′′ (ASTM A312, ASTM A358, i.e. electric-fusion-welded austenitic chromium-nickel stainless steel pipe, or as rolled).