Classes of Stainless Steel

When it comes to steel types, stainless is a broad category. Any ferrous alloy containing at least 10.5% chromium can call itself stainless steel. While additional alloying elements may be included, chromium is the significant ingredient in transforming a steel into “stainless”. Chromium boosts the corrosion resistance, durability, and strength of steel. It also results in a distinctive shine and boosts the metal’s stain-resistant properties – which is where we get its name.

So we see that all steels within the stainless family share one common alloy in chromium. What then separates them into classes are the other alloying elements, such as molybdenum or nickel. The different chemical compositions affect the steel’s crystal structure, which is how they are divided into 5 basic classes:

  • Austenitic stainless steel
  • Ferritic stainless steel
  • Martensitic stainless steel
  • Duplex (ferritic-austenitic) stainless steel
  • Precipitation-hardening (PH) stainless steel

 Austenitic stainless steel

The most popular grades of stainless steel come from this group, which are alloyed with high levels of chromium, molybdenum, and nickel. This gives the metal a superior degree of corrosion resistance and malleability, along with excellent weldability. While this class of stainless steels cannot be hardened through heat processing, and are prone to cracking under heat, they can be successfully hardened through cold working. Another asset is its strength against corrosive elements, which means austenitic steel is used in varied applications from marine and aerospace environments to chemical and food processing.

Ferritic stainless steel

These steels are best known for having low levels of carbon, making them a more iron-centric (ferritic) type of steel. It contains high levels of chromium and nickel, and like austenitic stainless, is strengthened through work hardening or cold working. It may not be as strong or hard as austenitic steel, but ferritic stainless is very resistant to stress-induced corrosion cracking. This makes it a popular choice for any components encountering corrosive substances or environments, such as industrial or automotive parts.

Martensitic stainless steel

Martensitic steels make up the hardest class of stainless steel, with a high level of carbon content alloyed mainly with chromium. However, that high degree of hardness also limits its use, since it can be brittle and less tough than the other classes of stainless steel. This means martensitic steel use is reserved for parts requiring great tensile strength and impact resistance, without being exposed to corrosive elements. Surgical instruments, valves, and pumps are often made of these steels.

Duplex (ferritic-austenitic) steel

Also known as ferritic-austenitic stainless steels, duplex is engineered to combine the best properties of both classes. They are alloyed with over 20% chromium and 5% nickel, boosting their yield strength and corrosion resistance. It also has approximately double the overall strength compared to ordinary austenitic stainless steel, while still being less expensive than austenitic due to the lower amount of nickel. With these qualities, duplex is heavily used in the oil industry, especially for underwater oil and chemical processing. It has the toughness to stand up to the corrosion caused by chloride and other elements.

PH (precipitation hardening) stainless steel

Steels in this class are notable for including aluminum, copper, and titanium among their alloying elements. When these steels are heat treated, the aluminum and other alloys form precipitates within the metal, reinforcing its crystal microstructure. This natural precipitation-hardening means that after its forging, it requires just a single low-temperature hardening to be ready for use. PH steel ranks highest in tensile strength among all stainless steel classes, which makes it well suited for the stresses of industrial use. Turbines, nuclear power plants, and aerospace components often make use of this class of steel.

Grade 304 vs 316

Grade 304 vs 316: The Differences between Stainless Steels

304 Stainless Steel versus 316 Stainless Steel: to the average layperson, these will appear to be very similar materials. They both fall into the 300 Series class, made up of austenitic chromium-nickel alloys. They’re the same grayish-silver color, with a bright and lustrous shine. Neither type can be hardened through heat treatments, but can be work-hardened.
But as those with more knowledge of these steels realize, it’s the small details between the two grades that make for crucial differences. Knowing more about these difference can provide consumers with better insight into the right type of steel for their use.

300 Series Steel
The steels within the austenitic class are generally alloyed with three main elements: chromium, nickel, and molybdenum. They’re known as low-carbon steels, with less than 0.8% carbon making up their chemical composition. Alloying with chromium increases the strength, hardness, and corrosion resistance of iron. Nickel also boosts the metal’s strength and hardness, while helping to prevent against loss of ductility and toughness.
However, it’s the addition of molybdenum that is one of the most significant differences between the 304 and 316 grades. Grade 304 stainless steel contains 18% nickel and 8% chromium, while 316 has 16% chromium, 10% nickel, and 2% molybdenum. Like the other alloying elements nickel and chromium, molybdenum is used to strengthen and toughen steel. But its biggest asset in 316 stainless steel is in the prevention against chloride corrosion.

Molybdenum: The Difference Maker
The boosted percentage of nickel, along with the inclusion of molybdenum, means that 316 has better chemical resistance when compared to 304 steel. The most significant corrosion protection it offers is against chlorides. In applications where the steel will be exposed to salts or seawater, 316 steel outperforms other Series 300 steels in resisting the development of pits and crevices. This is also why 316 is known as “marine grade” steel.
That’s not to say that Grade 304 isn’t strong and resistant to general corrosion on its own merits. In fact, its overall attributes and reliable performance is why 304 steel is the most widely used of all stainless steels. One other great advantage is its cost: by containing less nickel, along with no molybdenum, Grade 304 is a more affordable choice of material. So as long as the steel will be used with milder acids only, and have little to no contact to salt, it will save a bit of money to construct items out of 304. For applications that include exposure to chlorides and powerful corrosives, 316 with its added molybdenum would be the better choice.
When it comes to formability and welding, molybdenum comes into play again. Formability refers to a metal’s ability to be formed into a particular shape without incurring damage. While 316 is regularly used to form metal parts and structures, the hardness coming from molybdenum means forming requires more effort than compared to 304. The same goes for welding; both grades are commonly welded austenitic steels, but 304 is generally more easily weldable.

Advantages and Applications of Grade 304 Stainless Steel
With its good formability, resistance against heat and corrosion, and affordability, it’s understandable that 304 is the most popular of all stainless steels. It’s a reliable and versatile alloy, and readily welded by all common methods.

Some of the most common applications of Grade 304 steel include:
• Kitchen equipment and cookware

• Cutlery and flatware

• Appliances such as refrigerators and dishwashers

• Piping and fasteners

• Equipment used in dairy, brewing, and pharmaceutical production

Advantages and Applications of Grade 316 Stainless Steel
While it does come at a higher cost, when an application requires protection against severe corrosion, 316 will be the recommended choice. Its strength and durability are a necessity for usage in harsh environments, including underwater. In some cases, 316 may last many times longer than a part made of 304 – giving extended usage life to your parts and structures.

Some of the most common applications of Grade 316 include:
• Marine vessels and structures in marine environments

• Chemical processing and storage equipment

• Medical devices

• Textile, pulp, and paper manufacturing

• Jet engine parts