Alloy elements

The alloying elements in the stainless steel is a deciding factor for, among other things, the corrosion resistance of the steel.

The most frequently used alloy elements in stainless steel

Chromium, Cr

The main alloying element in all types of stainless steel grades and normally present 10-25%. The passive film of the steel primarily consists of chromium oxides and generally, the corrosion resistance of the steel is enhanced in most media (especially pitting corrosion and crevice corrosion) by the increased amount of exactly Cr.  

Mechanically, thbreaking strength is enhanced due to the increased contents of chromium. The same goes for the heat resistance and the resistance against oxide scales.  

Ferrite stabiliser, which is why increased content of chromium requires to be balanced by a corresponding increased content of nickel. 

Molybdenum, Mo

Is added 0,8-7,5%. Even better than chromium at “passivating” and even small contents of Mo will enhance the corrosion resistance remarkably – especially in acid anaerobic environments. Works beneficially against all forms of corrosion but is, unfortunately, an expensive alloy element. 

The price difference between regular stainless steel and acid-proof stainless steel can to a high degree be related to the minimum content of 2% molybdenum.  

Ferrite stabiliser, which enhances the mechanical strength of the steel and, like in the case of chromium, requires extra nickel to maintain the austenitic structure. 

Carbon, C

Harmful element, which like all other martensitic types is attempted to be kept as far down as possible percentagewise. Normally, < 0,08%; low-carbon < 0,03%. For martensitic steel grades, the content of carbon is typically 0,12-1,2% - the higher the percentage, the more hardenable 

C binds Cr especially at temperatures 500-580º C (= sensitisation), which can cause intergranular corrosion. Hence the frequent usage of low-carbon types of steel EN 1.4307 and 4404. C is a powerfuaustenite stabiliser which is why the low content in modern steel needs to be compensated with extra Ni if the structure is to be maintained. This is seen in 4306 and 4435. 

Nitrogen, N

Present 0-0,5%. Enhances the passivity even in extremely small quantities but is difficult to add to the melted metal in practice. Is often used in highly alloyed austenites and duplex grades of steel. It is the only austenite stabiliser that benefits the passivity of the steel and is especially effective against pitting corrosion and crevice corrosion.  

Silicon, Si

Is usually added as contamination from the crucibles of the steelworks. Austenite stabiliser and is normally present under 1,0%. No major effect on the corrosion resistance in the regular concentration area. 

Manganese, Mn

Like Si, Mn is normally present as contamination in the steel (1-2%, however, sometimes up to 5-6% in the AISI 200 grade). Enhances the hot-roll qualities of the steel and is moderately strength enhancing. Austenite stabiliser, which in itself does not have any great impact on the corrosion conditions but can bind sulphur to the extremely harmful manganese sulphides (MnS).  

Sulphur, S

Contamination and extremely harmful for the corrosion resistance. Normally S < 0,015% but stainless fine machining steels may contain 0,15-0,35%. Forms manganese sulphides (MnS), which make the steel short-chipped and reduces tool wear. Therefore, fine machining steels are far better for the purpose of machining in comparison to the “normal” tough austenites. Unfortunately, MnS is a catastrophe for the resistance against all types of corrosion and 4305 is in practice far less corrosion resistant compared to the regular 4301. Sulphur alloyed grades of steel are not suitable for neither welding nor pickling. 

Phosphorous, P

Like S, P is an undesirable contamination, but it is less catastrophic for the corrosion resistance. Is sought to be decreased to a minimum (< 0,045%) but is often even lower.  

Copper, Cu 

0-2%. Enhances the corrosion resistance in acid, anaerobic environments (e.g. sulphuric acid) by accelerating the hydrogen evolution and thereby making the material moroxidising (= anodic protection). 904L contains 1,2-2% Cu and is especially suitable for sulphuric acid. Cu has a strength-enhancing effect in the PH alloys.  

Titanium / Niobium, Ti / Nb

Significant elements especially because both Ti and Nb bind carbon and thereby counteract the harmful effect of C in austenitic steel grades (sensitisation and intergranular corrosion). 

The effect of adding Ti/Nb is roughly equivalent to using low-carbon steel, and 4541 and 4571 can usually be substituted with respectively 4307 and 4404. From a mechanic perspective, Ti-Nb steel is marginally stronger than low-carbon steel (especially under high temperatures). On the other hand, they are difficult to polish up due to Ti-carbides and the use of formier gas can result in the welding seam turning yellowish due to the formation of Ti-nitrides. 

In ferritic steel gradesTi and Nb contribute to the stabilisation of the steel and makes it weldable (e.g. 4512, 4509, and 4521).