Stainless steel is a versatile material with excellent corrosion resistance, strength, and durability. There are many types of stainless steel that are categorized into different classifications based on their metallurgical structure and alloying elements. Understanding stainless steel classifications is important for selecting the optimal grade for an application.

Stainless steel is generally divided into five different categories. Each element is identified by, and named after, the alloying element that affects its microstructure. His main categories are:

1. Austenitic – The most common and versatile class. Includes 300 series grades like 304 and 316.
2. Martensitic – Includes 400 series grades like 416. Heat treatable and can attain very high strengths. Magnetic.
3. Ferritic – Includes 400 series grades like 430 and 409. Less expensive but more difficult to weld and form. Magnetic.
4. Duplex – A hybrid with austenitic and ferritic structures. Very high strength and corrosion resistance. Includes 2205 and 2507.
5. Precipitation Hardening – Includes 17-4PH and 15-5PH. Can be strengthened through heat treatment.

Austenitic stainless steel

Austenitic stainless steel alloy is the most prevalent variety of stainless steel in use. It is non-magnetic and the most versatile of its kind. Due to their high chromium and nickel content, austenitic stainless steels are the most corrosion resistant of the stainless steel group. So, it is known for its corrosion resistance and ease of manufacture.

The most common austenitic alloys are iron-chromium-nickel steels, commonly referred to as the 300 series. Chromium (about 18%-30%) and nickel (about 6%-20%) are mainly added. It has very good mechanical properties as it retains its strength at high temperatures, is easy to maintain and has good formability. They can be cold worked, but not heat treated. It is commonly used in the manufacture of shafts, valves, bolts, bushings, nuts, aircraft fittings, brewing equipment and cryogenic vessels.

Type 304

One of the most commonly used (austenitic) stainless steel grades with a basic composition of 18/8 (18% chromium, 8% nickel) and a maximum carbon content of 0.07%, also known as A2 stainless steel. It possesses superior resistance to corrosion, is straightforward to fabricate, and has exceptional plasticity subsequent to computerized numerical control machining procedures.

Grade 304/304L possesses superb plasticity and weldability, rendering it optimal for assorted domestic and commercial enterprises. Because of its high chromium and nickel content, it is ideal for manufacturing process equipment for the chemical (mild chemical), food/dairy and beverage industries.

Type 316

It is the second most widely used steel after 304 and contains 16% to 18% chromium, 11% to 14% nickel and at least 2% molybdenum. These combine to improve corrosion resistance. In particular, molybdenum is used to help control corrosion pitting. This grade can withstand stains in temperatures up to 1600F.

It can be used in chemical processing, pulp and paper industry, food and beverage and surgical equipment, processing and distribution, and corrosive environments. It is also used in the marine industry as it is more resistant to chloride corrosion than 304. SS316 is frequently utilized in nuclear fuel reclamation apparatus. Grade 18/10 stainless steel typically satisfies this application benchmark.

Martensite

Martensitic stainless steel grades are a group of stainless steel alloys that are corrosion resistant and hardenable (using heat treatment). Martensitic grades are pure chromium steels that do not contain nickel. They are magnetic, can be hardened by heat treatment, and are not as resistant to corrosion as austenitic stainless steels. Martensitic grades are primarily used where hardness, strength and wear resistance are required.

It is often utilized to fabricate pump shafts, bolts and screws, valves, bushings, rivets, coal receptacles, eating utensils, jet engine components, aircraft parts, mining equipment, rifle barrels, and fire extinguisher internals. Frequent alloys encompass 410, 414, 416, 420, 431, and 440.

Ferrite

Like martensitic steels, ferritic stainless steels are nickel-free, pure chromium steels that resist corrosion and oxidation while still resisting stress and cracking. These steels are magnetic but cannot be hardened by heat treatment. They are cold worked and can be softened by annealing. They have higher corrosion resistance than martensitic grades, but generally not as good as austenitic grades. Commonly used in trim moldings, gutters and some automotive applications such as exhaust systems. Common grades include 405, 409, 430, 434, 436, 442, and 446.

Type 409

It contains 11% chromium, which is the lowest chromium content in all stainless steels. This is the least passive surface film that forms the corrosion resistance of stainless steel. It is one of the cheapest stainless steel grades. This type should only be used for internal or external parts in non-severely corrosive environments. Typical applications include silencers.

Alloy 409 has better corrosion resistance than carbon steel and can be used as a substitute for carbon steel in less corrosive environments. It has advantages due to its high corrosion resistance and high temperature oxidation resistance.

Type 430

430 stainless steel alloy is a general utilization steel possessing superb corrosion immunity.**It has better thermal conductivity than austenite, smaller thermal expansion coefficient than austenite, thermal fatigue resistance, added stable element titanium, and has excellent mechanical properties in welds.

430 stainless steel is utilized for architectural ornamentation, fuel combustion device components, home appliances and appliance constituents.

430F is a kind of steel, which increases the free cutting performance on the basis of 430 steel. Mainly used on automatic lathes, bolts and nuts. 430LX adds Ti or Nb to 430 steel to reduce C content and improve handling and welding properties.

Principally employed in hot water reservoirs, water distribution networks, lavatory fixtures, domestic permanent goods, bicycle flywheels, etc.

Duplex (ferritic-austenitic) grades

Duplex stainless steel is a modern stainless steel that combines austenitic and ferritic materials. Known for its extreme strength and resistance to stress corrosion cracking. These grades are approximately twice as strong as austenitic and ferritic grades. It has better toughness and ductility than ferritic steels, but not at the level of austenitic steels. Heat treating is easy, but cold forming is hard. It is commonly used in the manufacture of chemical processing equipment, pressure vessels and heat exchanger components.

Duplex stainless steels are divided into four categories:

The first is a low-alloy variety denoting the UNSS32304 (23Cr-4Ni-0.1N) grade, the steel does not encompass molybdenum, and the PREN value is 24-25. It can function as a substitute for AISI304 or 316 regarding stress corrosion endurance.

The second type is the medium alloy type, the representative grade is UNSS31803 (22Cr-5Ni-3Mo-0.15N), the PREN value is 32-33, and the corrosion resistance is between AISI316L and 6%Mo+N austenitic stainless steel. between.

The third variety is high-alloy type, which commonly contains 25% Cr, plus molybdenum and nitrogen elements, with certain alloys also integrating copper and tungsten.The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), and the PREN value spans 38-39.The corrosion resistance of this type of steel is higher than that of 22%Cr duplex stainless steel.

The fourth classification belongs to the super duplex stainless steel type encompassing elevated molybdenum and nitrogen, the standard grade being UNSS32750 (25Cr-7Ni-3.7Mo-0.3N), with some also incorporating tungsten and copper.With a PREN value in excess of 40 under harsh moderate conditions, it has excellent corrosion resistance and overall mechanical properties comparable to super austenitic stainless steels.

Precipitation Hardening (PH) Grades

Like martensitic, precipitation hardening stainless steels can be strengthened and hardened by heat treatment. Its strength, hardness and corrosion resistance are superior to those of martensitic chromium stainless steel. It is generally stronger and runs hotter than austenitic stainless steels. It can retain most of its strength. Commonly called PH-type stainless steels, both contain higher chromium content and are used in the manufacture of military equipment and aerospace structural components. Common grades include 17-7PH, PH15-7Mo, 17-4PH, 15-5PH.

The alloying elements used also help categorize stainless steel:

Chromium – Required for corrosion resistance. Minimum 10.5% in stainless steel.

Nickel – Austenitic grades have high nickel content (8-10%+). Improves corrosion resistance and ductility.

Molybdenum – Added for increased corrosion resistance particularly against pitting and crevice corrosion.

Nitrogen – Enhances strength and resistance to corrosion. Used in duplex and austenitic grades.

With so many types of stainless steel, it’s important to assess the needs of the application when selecting a grade. Environments, fabrication requirements, mechanical properties, and cost are factors to consider. Consulting our experienced CNC machining specialists can ensure the best grade is chosen.