Stainless Steel Types and Grades
Stainless Steel¹ is an iron alloy containing a minimum of 11% Chromium and is resistant to rusting and corrosion. It also can contain other elements such as carbon, which give it various properties, making it a useful metal for a variety of applications. The reason stainless steel is resistant to corrosion is that Chromium creates a protective film that can self-heal in the presence of Oxygen.
There are several types and grades of stainless steel that are utilised for different environments and applications. Today we cover the types we most commonly manufacture and explain what they are, their composition, and what use they’re best suited for.
Types
There are five main types of stainless steel, that can be categorised by their crystalline structure, including:
- Austenitic
- Ferritic
- Martensitic
- Duplex
- Precipitation hardening
Austenitic Grades of Stainless Steel
Austenitic stainless steel is easy to work and weld, and is
- Ductile
- Strong
- Corrosion resistant
They feature a crystalline structure that is an austenitic microstructure, which is a face-centred cubic crystal structure.
300 series stainless steels are our most commonly utilised types. 300 series is also the largest group and the most widely used. They are chromium-nickel alloys that achieve their austenitic microstructure almost exclusively by nickel alloying; some very highly alloyed grades include some nitrogen to reduce nickel requirements.
Type 304
The best-known grade is 304, also known as 18/8 and is composed of 18% chromium and 8% or 10% nickel.
304 stainless steel is used for a variety of household plus industrial applications such as food preparation and handling, processing equipment, screws and machinery parts, utensils, and engine exhaust manifolds. It’s also widely used in architecture for exterior features such as water and fire applications.
Type 316
The second most common austenitic stainless steel is 316. It contains 2% molybdenum and provides greater resistance to acids and localised corrosion caused by chloride ions. We also utilise low-carbon versions of both types, such as 316L or 304L, that are used to avoid corrosion problems caused by welding.
Ferritic Grades of Stainless Steel
Ferritic stainless steels have a ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure containing between 10.5% and 27% chromium with little or no nickel. This microstructure is present at all temperatures due to the chromium addition, so they are not hardenable by heat treatment. They also can’t be strengthened by cold work, to the extent of austenitic stainless steels. They are magnetic. Ferritic stainless steels are cheaper than austenitic steels due to the near absence of nickel, nd are present in many products, which include:
- Automobile exhaust pipes (Type 409 and 409 Cb are used in North America; stabilised grades Type 439 and 441 are used in Europe)
- Architectural and structural applications (Type 430, which contains 17% Cr)
- Building components, like slate hooks, roofing, and chimney ducts
Martensitic Grades of Stainless Steel
Martensitic stainless steels have a body-centred cubic crystal structure, and have a wide range of properties that are used as stainless engineering steels, stainless tool steels, and creep-resistant steels. They are magnetic, and not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content. They fall into four categories (with some overlap):
- Fe-Cr-C grades. These were the first grades used and are still widely used in engineering and wear-resistant applications.
- Fe-Cr-Ni-C grades: Some carbon is replaced by nickel. They offer higher toughness and higher corrosion resistance, good weldability and good resistance to cavitation erosion.
- Precipitation hardening grades: Grade EN 1.4542, also known as 17-4 PH, the most well known grade, combines martensitic hardening and precipitation hardening. It features high strength and good toughness and is used in aerospace.
- Creep-resisting grades. Small additions of niobium, vanadium, boron, and cobalt increase the strength and creep resistance up to about 650 degrees Celsius.
Martensitic stainless steels can be heat treated to offer improved mechanical properties.
Duplex Grades of Stainless Steel
Duplex stainless steels feature a mixed microstructure of austenite and ferrite. With the ideal ratio being a 50:50 mix although commercial alloys may have ratios of 40:60. They have higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
Commonly used in oil, gas and paper industry, plus for structural applications in building and construction, concrete reinforcing bars, bridge plates and coastal works) and in the water industry.
Precipitation hardening Grades of Stainless Steel
Precipitation hardening stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than other martensitic grades. There are three types of precipitation hardening stainless steels:
- Martensitic 17-4 PH (AISI 630 EN 1.4542) contains approximately 17% Cr, 4% Ni, 4% Cu, and 0.3% Nb.
- Semi-austenitic 17-7 PH (AISI 631 EN 1.4568) contains approximately 17% Cr, 7.2% Ni, and 1.2% Al.
- Austenitic A286 (ASTM 660 EN 1.4980) contains approximately Cr 15%, Ni 25%, Ti 2.1%, Mo 1.2%, V 1.3%, and B 0.005%.
Summary
There are many types and grades of stainless steel available for fabrication, with our most commonly utilised types being 304 and 316. If you’re looking for a local stainless steel fabrication workshop, get in touch with our specialist team located just south of Sydney, NSW.