Stainless steel pipe types
As a general rule, any steel alloy that has at least 10.5% of chromium may be considered stainless steel pipe. 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). Grade 304/304L 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%. 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%). 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 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 as 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% chromium), and the 409 (11% chromium), largely used in the automotive sector.
Duplex and Super Duplex Stainless Steel (UNS S32205, S31803, S32750/760)
Duplex stainless steels have a microstructure which is 50% ferritic and 50% austenitic. This composition gives them a better strength than either ferritic or austenitic steels. Further details about this topic are given below (see ASTM A790 pipes).