AISI 304 and AISI 316: how are they different from other stainless-steel types?

Stainless steel is a unique material: resistant, hygienic, and also sustainable, as it is 100% recyclable. This iron-based alloy combines the properties of steels (hardness and tensile strength) with the corrosion resistance typical of noble metals. The stainless properties of steel derive mainly from the presence in the chromium alloy that determines the metal’s ability to passivate, that is to form a layer of oxides that protect it from rust formation.

However, not all stainless steels are equal. The different composition of the alloy they consist of affects the performance, as well as the cost of the material itself.

Classification of Stainless Steels

To distinguish among various types of stainless steel one of the most common ways is to refer to the nomenclature of the American Iron and Steel Institute (AISI). The AISI designation consists of a three-digit number, the first of which indicates the family or class of steel.

European countries also adopted the EN 10088 standard, which sets out the chemical composition, physical characteristics, and technical conditions for the supply of stainless steel. In this regard, it is essential to know the origin of stainless steel because the reference standards of other countries, such as China for instance, are not always compatible with the American and European ones and consequently with the reference market.

Therefore, knowing the classification and designation of stainless steels is particularly important both as guidance when choosing among the various products for their application, and as an objective comparison that may account for the ratio of qualitative characteristics to price.

Based on their chemical-physical structure stainless steels belong to three large groups: martensitic, ferritic, and austenitic. In particular, we can distinguish:

• chromium-nickel austenitic steels (300 series)
• chromium-manganese-nickel austenitic steels (200 series)
• straight chromium ferritic and martensitic steels (400 series)

Steel types and Corrosion Resistance

The most corrosion-resistant steels are the (300 series) chromium-nickel austenitic steels, in particular AISI 304 and AISI 316. We have already discussed the characteristics of these two types of steel and the main difference between the two (link to article).

Here, instead, we examine the difference between the 300 series austenitic steels and the 200 series austenitic ones.

The 200 series was introduced to partially replace the percentage of nickel with manganese, due to similar austenitizing properties. However, the use of manganese requires further modification of the alloy, namely a reduction of the chromium content. An 18% chromium content typical of AISI 304 and AISI 316 steels, when combined with low nickel values, leads to the formation of ferrite; for this reason, the chromium content in the 200 series steels is reduced to 15-16% and in some cases even to 13-14%. It is therefore not possible to confuse these 200 series steels, although austenitic, with those of the 300 series because their corrosion resistance is definitely lower.

Mondeo valves are made of AISI 304 or AISI 316 stainless steel and undergo an electropolishing treatment that further enhances the corrosion resistance typical of 300 series stainless steels.( Salt spray test: the superior resistance of electropolished AISI 316 steel).