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What Are The Different Classes Of Stainless Steel?

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Stainless steel is one of the most important metals on the market today thanks to its tremendous strength, corrosion resistance, and durability. Though it was first commercialized less than a century ago, it has quickly developed into one of the most versatile and essential metals and has transformed pretty much every modern industry, from architecture and engineering to the medical sciences.

The impact of stainless steel is due in large part to the large number of alloys that have been created, with more coming to market every year. Of course, this amazing diversity can also be a weakness, especially when it comes time to pick out which alloy might be best suited for a particular application. Thankfully, one way that manufacturers can educate themselves is to learn the different classes of stainless steel alloys, as broadly speaking, these will tell you a great deal about the specific alloys within each class.

With all this in mind, often the best course of action a manufacturer can take is to consult with an experienced technical professional. At Clinton Aluminum, we take great pride in helping our customers find the right material for any job. Contact us today to learn more.

Stainless steel alloys can be divided into five main classes

Stainless steels are generally divided into five main classes or families of stainless steel, according to the basic characteristics such as chemical composition. The most common stainless steel alloys, for instance 304 and 316, belong to the class known as austenitic stainless steel, which is characterized by the primary alloying elements chromium and nickel. They mainly fall into the 200 and 300 series of grades.

The identifying characteristic of austenitic stainless steels is their face-centered cubic structure, which means among other things that it is the most weldable of the stainless steels. Austenitic stainless steels generally fall into one of three categories, either the chromium-nickel stainless steel of the 300 series, the manganese-chromium-nickel stainless steel of the 200 series, or certain specialty alloys. Austenitic grades tend to be susceptible to sensitization of the heat-affected zone and hot cracking.

Following austenitic, ferritic stainless steel is probably the next most popular class of alloys. They are defined by the presence of alloying elements molybdenum, aluminum, and/or titanium in addition to the chromium and nickel found in austenitic grades. Ferritic stainless steel is known for its greater ductility, formability, and resistance to stress corrosion cracking, and these alloys are normally part of the 400 series.

These alloys are recognized for their ferrite microstructure that is similar to carbon steel, and characterized by their body-centered cubic crystal structure. They cannot be strengthened by cold work as well as austenitic stainless steels and they have similar magnetic properties to carbon steel. They are a poor choice for welding applications because of grain growth in the heat-affected zone. One other important weakness to note is they have relatively poor high-temperature strength.

The third class of stainless steel is known as martensitic. This family also falls into the 400 series of alloys and possesses several similarities to ferritic stainless steels. The best way to tell them apart is actually at the molecular level, as the martensitic steels can be recognized by their increased carbon content and reduced chromium.

These stainless steels use carbon and nickel for alloying agents, which is what transforms the austenite structure at high temperatures to martensite at low temperatures. They also have a body-centered cubic crystal and the carbon content means that preheating and post-weld heat treatment are necessary for welding applications.

The next class is known as precipitation-hardening stainless steels, and they are characterized by the use of aluminum, copper, and titanium as alloying agents. This class gets its name from the process used to produce these alloys, which entails the metal going through solution and aging heat treatment. This gives the metal a higher tensile strength than other families of stainless steel.

The precipitation-hardening stainless steels can be divided into three separate groups: martensitic, semi-austenitic, and austenitic. The main advantage of these alloys is they possess a level of corrosion resistance that matches austenitic steels, but because they are precipitation hardened, they are stronger than martensitic grades.

The final family of alloys is known as duplex stainless steels. They are the newest class of stainless steel, and they combine the properties of ferritic and austenitic alloys. Because they contain at least 22-25% chromium and 5% nickel, they exhibit increased yield strength and improved stress corrosion resistance, and are well suited for chloride environments.

Another benefit of duplex stainless steels is they have approximately twice the strength of austenitic stainless steels. They can be divided into five grades, known as lean duplex, standard duplex, super duplex, and hyper duplex.

What are the most common grades in each class?

As already mentioned, the most popular grades of stainless steel tend to be in the austenitic family. This includes 304 and 316, two of the workhorses in the industry, which are prized for their machinability, strength, and versatility. The one advantage that 316 offers is its increased corrosion resistance, in particular in marine environments. This is due to the added molybdenum.

In the ferritic family, type 430 is likely the most popular, due to its higher corrosion resistance when it comes to nitric acids, sulfur gasses, and organic acids. 409 and 410L are also common, thanks to their low cost. They are frequently found in applications such as auto exhaust systems, containers, and LCD monitor frames.

For martensitic stainless steels, grades 410 and 420 are solid choices. They are prized for their extreme hardness. Also, they have the ability to be hardened further through cold work and heat treatment. Popular applications include bearings, cutlery, dies, fasteners, and turbine blades.

The most well known of the precipitation hardening stainless steels is 17-4 PH. It gets its name from the fact that it has a chemical content of 17% chromium and 4% nickel, thought it also goes by the grade 630. Applications include engine components, boat propellers, molding dies, and gears.

Finally, the leading duplex stainless steel is probably 2205, which is composed of 22% chromium and 5% nickel. Thanks to its extreme corrosion resistance, it has become very popular in the oil and gas and chemical industries. Applications include piping, fitting and flanges, and other fixtures that are exposed to extremely corrosive substances.

What factors go into selecting a grade of stainless steel?

When it comes time to select a stainless steel alloy, you need to know what the requirements are for a particular application. A big factor is what environment the application will be exposed to. Another factor is the expected product life. Also important to alloy selection are pitting resistance, weldability, and magnetic properties.

Of course, one of the most important considerations is the desired strength of the material. Each stainless steel alloy is rated for strength according to a number of parameters, and it’s critical that you have a solid understanding of what the minimum strength requirements are.

Finally, stainless steel is best known for its corrosion resistance. As such, knowing the degree of corrosion resistance that will be necessary for your application will go a long way to informing you which of the five families of stainless steel you will need. Fortunately, the many highly specialized alloys that are available offer corrosion resistance for pretty much any scenario.

Summary

At Clinton Aluminum, we know that for every application, there’s a grade of stainless steel that’s a perfect fit. Fortunately, you don’t have to make this critical decision all alone, because our knowledgeable technical professionals have decades of experience when it comes to material selection. Our number one priority is to make sure our clients always have the perfect material for the job. Contact us today to learn more about how we can help your business succeed.

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