What is stainless steel?

What is stainless steel? Stainless steel is a large group of materials, which all have in common that the main element is iron (from this “steel”) and the contents of chromium (Cr) is at least 10,5%. Besides chromium and iron, the steel can contain a wide range of other alloy elements. That could, for instance, be nickel (Ni), carbon, (C), and molybdenum (Mo). The different alloys serve the purpose of either enhancing the mechanical and/or the attributes regarding corrosion of the steel. Besides enhancing the steel, the alloy elements have significance for which type of grade the stainless steel is. The same goes for the amount of chromium. It is namely the contents of chromium and the alloys, which is of significance to determine which type of group or stainless steel the product belongs to. In total, there exist 5 types – or grades – of stainless steel: Austenitic, Ferritic, Martensitic, Duplex, and Super Duplex.

Types of stainless steel

The lowest alloyed types of stainless steel qualities (e.g. EN 1.4003) only contain 10,5% chromium while the remaining content is iron and was invented back in 1912. Already in 1913, in Germany, the tillegering of nickel was discovered for the sake of mechanical qualities and in 1920, it was discovered that the tillegering of even small amounts of molybdenum (Mo) benefits the resistance to corrosion tremendously. Thereby, the “acidproof” types of steel were born. Stainless steel can by crystalline structure be divided into different types of stainless steel spread out across five main groups:

Austenitic stainless steel

Austenitic stainless steel is mainly characterised by a high content of chromium (Cr), a high content of nickel (Ni), a low content of carbon (C) and often the addition of molybdenum (Mo). This group of steel is by far the largest and most significant and both the regular 18/8 and the “acid-proof” belong to this group. It is normally non-magnetic but becomes faintly magnetic by cold deformation. 

From a mechanic perspective, austenitic steel has a long “elongation at rupture” = great toughness. Austenitic steel is relatively soft and particularly suitable for plastic moulding for instance, in the deep drawing of kitchen sinks. Compared to the remaining types, austinites are nearly “bubble gum steel” and it is exactly the great formability, weldability and the corrosion resistance which makes the austinites the most used group by far. Anything from door handles to enormous brewery tanks can be made of austenitic stainless steel. 

Contrary to ferritic steel, austenitic steel does not become brittle at low temperatures and has better qualities against shrinking at high temperatures. Austenitic steel generally possesses good corrosion resistance but is vulnerable to chloride-induced stress corrosion. Therefore, austenitic steel is not always suitable for highly heated components in aqueous environments. 

Among stainless steel, it is still the 4301 and 4401-classics that is predominant globally. These pipes are “regular stainless” meaning 4301. 

Martensitic stainless steel

Martensitic stainless steel typically contains 12-16% Cr, a low amount of Ni, rarely Mo and a relatively high amount of carbon (C, 0,12-1,2%). It can be hardened by rapid cooling to over 1000 HV and due to the extreme hardness, it is particularly suitable for cutting tools such as surgical equipment and high-quality kitchen knives. 

After being hardened, martensitic steel can neither be shaped plastically nor be welded. The steel will lose its hardening if welded or exposed to other heat treatments. The martensitic steels are heavily magnetic and, due to their low Cr contents and high C contents, they generally have poor resistance to corrosion. This is often seen after an expensive kitchen knife has been washed in a dishwasher. 

Surgical equipment can be produced by the use of martensitic stainless steel. This gives a high strength but unfortunately a relatively poor resistance to corrosion. 

Ferritic stainless steel

Ferritic stainless steel typically contains 12-18% Cr, a low amount of Ni and a low amount of carbon (C ≤ 0,12). The ferrites have the same metallurgical structure as mild steel but due to the low carbon contents, they are not hardenable. It is relatively soft but has a poorer toughness compared to austenitic steel. Ferritic steel can be cold deformed but not to the same degree as the austenitic “bubble gum” steel. The stabilised types are weldable (the 45XX types) and all of them are heavily magnetic. 

The lowest alloyed (e.g. 4003) have relatively poor resistance to corrosion (especially in acid), while the higher alloyed (e.g. 4521) is comparable to acidproof steel regarding pitting corrosion and partially crevice corrosion. Additionally, ferritic steel is far superior compared to austenitic steel regarding the serious stress corrosion. 

Due to the low content of nickel, the ferrites are relatively cheap and are increasingly used for purposes where the formability and weldability of the austenitic steel are irrelevant or when a beautiful and magnetic surface is desired, for instance, refrigerator doors, kick plates or door handles. For the same reasons, the global consumption of the ferritic stainless steel types is significantly increasing. 

Moreover, ferritic steel has major possibilities regarding heated components where there is a risk for stress corrosion in both regular and acidproof stainless steel types and also where thermal conductivity is required, which is better compared to the austenitic steel. The thermal longitudinal expansion corresponds to that of black steel which is roughly 2/3 of that of austenitic steel. 

Ferritic stainless steel can advantageously be used for materials with thin goods, high material costs, and simple processing. This Syrian produced jug is made of 4016 (AISI 430) which is a widely used material in the catering industry. This is, by the way, a fantastic example of the fact that ferritic stainless steel can be deep drawn.  


“Duplex” is a term for a very special group of stainless steel. The unique part of the duplex steel is that it is unlike the austenitic (e.g. the 4301 and 4401-classics) or the ferritic chromium steel (e.g. 4016 and 4509) not one-phased. Duplex steel is two-phased. Put simply, this means that a bit over half (typically 55%) of the microstructure of the steel is ferritic while the rest is austenitic. In this way, the duplex steel can be categorised as a “crossroad” of the two most common groups of stainless steel. 

Precipitation Hardening (PH) stainless steel

Precipitation Hardening (PH) stainless steel is a two-phased martensitic-austenitic high-strength steel. It typically contains 15-17% Cr, 4-8% Ni, a low amount of Mo and up to 5% copper (Cu). The precipitation hardening takes place at increased temperatures through the separation of alien phases which generally makes PH-alloys strong, but less corrosion resistant. The most common types of PH stainless steel are “15-5 PH” and “17-4 PH”, which at times are used for the production of stainless chain links and golf clubs. Besides these examples, precipitation hardening steel is a rarely used group of stainless steel. 

The above examples are illustrations of brilliant use of the relatively rare stainless 17-4 PH-alloy (15-5 PH, EN 1.4542). Both the impact surface and the “body” of the given golf club is produced by the use of precipitation hardening stainless steel. 

Do you want to learn more?

Claus Qvist Jessen

Chemical Engineer, PhD
+45 87 94 40 45

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