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?
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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?
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 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.
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.
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 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.
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