Stainless Steel Classifications

Stainless steels are in general grouped into

• martensitic stainless steels
• ferritic stainless steels
• austenitic stainless steels
• duplex (ferritic-austenitic) stainless steels
• precipitation-hardening stainless steels

Alloying metallic elements added during the making of the steel increase corrosion resistance, hardness, or strength. The metals used most commonly as alloying elements in stainless steel include chromium, nickel, and molybdenum.

Stainless steels are available in the form of

• plate
• sheet
• strip
• foil
• bar
• wire
• pipes
• tubes

Stainless steels are a iron-based alloy containing at between 10.5% to 30% Cr. Stainless steel achieve its stainless characteristic through the formation of an invisible and adherent chromium-rich oxide surface film.

Other alloying elements added to improve the characteristics of the stainless steel include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, and selenium.

Carbon is normally in amounts from 0.03% to more than 1.0% in some martensitic grades.

Selection of stainless steels are in general based on

• corrosion resistance
• fabrication characteristics
• availability
• mechanical properties for specific temperature ranges
• product cost

Since stainless steel resists corrosion, maintains its strength at high temperatures, and is easily maintained, it is widely used in items such as automotive and food processing products, as well as medical and health equipment. The most common US grades of stainless steel are:

TYPE 304

The most commonly specified austenitic (chromium-nickel stainless class) stainless steel, accounting for more than half of the stainless steel produced in the world. This grade withstands ordinary corrosion in architecture, is durable in typical food processing environments, and resists most chemicals. Type 304 is available in virtually all product forms and finishes.

TYPE 316

Austenitic (chromium-nickel stainless class) stainless steel containing 2%-3% molybdenum (whereas 304 has none). The inclusion of molybdenum gives 316 greater resistance to various forms of deterioration.

TYPE 409

Ferritic (plain chromium stainless category) stainless steel suitable for high temperatures. This grade has the lowest chromium content of all stainless steels and thus is the least expensive.

TYPE 410

The most widely used martensitic (plain chromium stainless class with exceptional strength) stainless steel, featuring the high level of strength conferred by the martensitics. It is a low-cost, heat-treatable grade suitable for non-severe corrosion applications.

TYPE 430

The most widely used ferritic (plain chromium stainless category) stainless steel, offering general-purpose corrosion resistance, often in decorative applications.

Austenitic Stainless Steels TypeEquivalent UNS 201 S20100 202 S20200 205 S20500 301 S30100 302 S30200 302B S30215 303 S30300 303Se S30323 304 S30400 304L S30403 302HQ S30430 304N S30451 305 S30500 308 S30800 309 S30900 309S S30908 310 S31000 310S S31008 314 S31400 316 S31600 316L S31603 316F S31620 316N S31651 317 S31700 317L S31703 317LMN S31726 321 S32100 330 NO8330 347 S34700 348 S34800 384 S38400 Ferritic Stainless Steels TypeEquivalent UNS 405 S40500 409 S40900 429 S42900 430 S43000 430F S43020 430FSe S43023 434 S43400 436 S43600 442 S44200 446 S44600 Martensitic Stainless Steels TypeEquivalent UNS 403 S40300 410 S41000 414 S41400 416 S41600 416Se S41623 420 S42000 420F S42020 422 S42200 431 S43100 440A S44002 440B S44003 440C S44004

Stainless steel is recognized for its strength, beauty, and resistance to corrosion. Not all stainless steel is alike, however. The metal comes in five distinct families and about 150 different grades.

What is stainless steel exactly?

Download Our Stainless Steel Spec Sheet Today

Kloeckner Metals is a full-line stainless steel supplier and service center. Download our stainless steel spec sheet and check what Kloeckner Metals routinely stocks.

Put simply, stainless steel is any one of the steel alloys that contains at least 10.5% chromium in the admixture of elements. Other elements can include nickel, molybdenum, or titanium to enhance a special characteristic of the alloy. For example, the addition of molybdenum increases the corrosion resistance of stainless steel and changes the alloy from grade 304 stainless steel to 316 stainless steel.

Stainless steel can be classified into one of five different types: austenitic, ferritic, martensitic, duplex (austenitic-ferritic), or precipitation. Each of these types can be subdivided into grades of stainless steel. Each grade features a different level of quality, durability, and temperature resistance.

What are the different types of stainless steel?

Austenitic

The most common grades of steel, grades 304 and 316, are austenitic. This type of steel contains 16-26% chromium and up to 35% nickel. Typically, austenitic stainless steel has the highest corrosion resistance and is not hardenable by heat treatment. This family of stainless steel is also nonmagnetic, tough, and ductile. (Ductile refers to a metal’s ability to change shape without losing its strength.)

The most common type of austenitic steel is grade 304, sometimes called 18/8 because it contains 18% chromium and 8% nickel. Austenitic steels are commonly used in the aerospace and food-processing industries.

Generally, the physical properties of austenitic steel are similar to those of ferritic and martensitic steels. It’s the chemical composition that is different.

Ferritic

Ferritic steels have a low carbon content, less than 0.2%, making them non hardenable by heat treatment. They are only moderately hardenable by cold working. As a result, they have less anti-corrosion ability than austenitic steels. Standard ferritic steels contain 10.5-27% chromium and no nickel. The most popular grades of ferritic stainless steel include 409, 430, 439, and 441. Ferritic steel is commonly used for architectural applications and auto trim.

Martensitic

Typically containing 11.5-18% chromium and as much as 1.2% carbon (and sometimes nickel as well), martensitic steels are hardenable by heat treatment. Strong and ductile, these steels are also magnetic and feature moderate corrosion resistance. They often appear in the 400-level grades. Martensitic steel shows up in cutlery, wrenches, turbines, and surgical instruments.

Austenitic-ferritic (duplex)

Combining austenitic and ferritic stainless steels equally, duplex steels contain 21-27% chromium, 1.35-8% nickel, 0.05-3% copper, and 0.05-5% molybdenum. The combination of elements makes duplex steels stronger than either austenitic or ferritic steel by itself. Consequently, duplex steel often gets used in chemical processing and in crafting storage-tanks and containers that ship chemicals.

Precipitation Hardening

Precipitation-hardening stainless steel is noteworthy for its strength. This type of steel contains 15-17.5% chromium and 3-5% nickel along with aluminum, copper, and niobium, each less than 0.5% of the total mass of the steel. Precipitation-hardening steel’s corrosion resistance compares to that of austenitic steel. This kind of steel can be hardened with an aging treatment at fairly low temperatures. Metal workers use it to create long shafts for pumps or for valve spindles.

What do the grades of stainless steel mean?

Stainless steel gets categorized into the types or the families outlined above, and then it is broken down further into grades.

What is the difference between a family and a grade of stainless steel?

The families of stainless steel are determined by the ratio of the metals that compose the alloy. The grades, by contrast, describe the properties of the type of steel, such as its toughness, magnetism, and corrosion resistance.

Many different organizations have developed their own systems for grading stainless steel. One of the most commonly used grading systems comes from the American Iron and Steel Institute (AISI). This organization employs a three-digit system to categorize stainless steel. When metallurgists and metal workers talk about grades 304 and 316, for instance, they are using the AISI grading model.

How do you determine the right type for your needs?

Out of the number of options, how do you choose the type of steel to go with for your project?

Consider these factors:

• Your Environment — The operating environment will affect the stainless steel you use. Choose a grade with the necessary strength, toughness, and resilience to withstand the temperatures, pH, stressors, and corrosion you will expose it to. A marine environment, for example, may require an alloy that includes molybdenum, which resists chloride ions.
• Formability or Weldability — If your application requires good formability, you’ll want to choose an austenitic or ferritic steel such as grades 304 or 430. Martensitic steel such as grade 410 is weaker and can crack. Also take into account the steel’s weldability if you plan to weld it to another metal.  
• Corrosion Resistance — The high levels of chromium in austenitic steels make them a good choice for harsh environments. Austenitic steels, particularly grade 316, offer the highest level of corrosion resistance. Less expensive martensitic stainless steel and ferritic stainless steel can also be good choices in corrosive settings.
• Strength, Ductility, and Toughness — How much weight will your application need to bear? How will it need to change shape without losing strength? What will it take to fracture the steel you choose? Stainless steel alloys containing chromium and nickel often provide the best combination of strength, ductility, and toughness.
• Cost and Availability — Austenitic stainless steels are usually the most expensive choices. That said, they’re also usually the best quality steels, so buyers may find they save money on maintenance and repairs in the long run.
• Magnetic Response — Some types of stainless steel are more magnetic than others. Adding chromium to an alloy tends to make it more magnetic, but adding nickel offsets or even eliminates the magnetism. Grades 316 and 310 are non-magnetic while grades in the 400 series are more magnetic.

Ultimately, the right type of stainless steel is the one you like. Find out more about the types of stainless steel and which may be right for your application.

Contact Our Qualified Team Today

Kloeckner Metals is a full-line stainless steel supplier and service center. Kloeckner Metals combines a national footprint with the latest fabrication and processing technologies and most innovative customer service solutions.