10 tips on stainless steel
- Stainless steel is not just one well-defined material but a common term for a wide range of alloys, all of which have three things in common:
* The main element must be iron (i.e. a “steel”)
* The steel must contain at least 10.5% chromium (Cr)
* The steel must contain a maximum of 1.2% carbon (C)
Aside from these three requirements, there is free play, and we can add a wide variety of elements because we want to take the physical or chemical properties in the desired direction. The only limitation is your fantasy and the periodic table.
- There are five main groups of stainless steels, and the most important of these are the austenitic ones. In this group, steel most often has 16-20% chromium (Cr), 8-25% molybdenum (Mo) and - most importantly - at least 8% nickel (Ni).
Nickel is an effective austenite stabiliser and contributes significantly to taking the steel over into the very face-centred cubic structure (FCC). The FCC structure has particularly fantastic mechanical properties, especially as good extensibility and weldability, and the corrosion properties are also not bad either. Among the austenites, we find both the “ordinary stainless” of the 18/8 grade (4301/AISI 304 grade) and the molybdenum-containing, “acid-proof” steels of 4404/316 grade.
The austenitic group covers approx. 90% of the Danish market for stainless steel - approx. 70% of the world market.
- If you do not add any nickel, the steel will most often remain body-centred cubic (BCC), which is the structure we call ferritic, and we know it from all the steel sheets used for cars, ships, building steels, etc. Admittedly, the steel’s corrosion resistance has been strengthened by adding at least 10.5% chromium, but the structure is the same.
Ferritic stainless steels are highly magnetic and are the easiest to find as thin sheets. We often see them in the kitchen as the fronts of refrigerators and other white goods, cooker hoods, colanders, stainless steel bowls, and - for the utility room - the drums of washing machines and tumble dryers. Ferritic steels are more common than people think.
- If nickel is kept at half the required for austenite, we get an interesting double structure: Duplex. Almost half of the steel’s microstructure eats all the nickel and becomes austenitic, while the other half has no nickel at all and becomes ferritic. Therefore, duplexes are a mixture of ferrite and austenite, typically 55% ferrite.
Mechanically, duplex steels are significantly stronger (and so less bendable) than both ferrite and austenite, and the corrosion resistance is excellent. Duplex steels are therefore most often used for purposes where exceptionally high demands are placed on the materials, for example, in the North Sea and the chemical industry.
- Certain stainless steels can be hardened. It is primarily the so-called martensitic steels, where the combination of a high content of carbon (C) and low content of chromium (Cr) means that the steel can be hardened by heating to a high temperature and rapid cooling.
The hardenable martensites are most often seen in the form of kitchen knives. They can be sharpened to a razor-sharp edge, and due to the hardness, they remain sharp for a long time. The stainless steel knives are even highly magnetic (the knife magnet) but not very corrosion resistant. We see this when they get “red dots” after being in the dishwasher a few times, which is often called “fly rust”, but this is not correct, as it is actually corrosion of the knife blades themselves. The lovely kitchen knives must be washed by hand under the tap.
- Stainless steel has an excellent ability to passivate due to a minimum of 10.5% chromium. The steel is covered by a thin (5-10 nm!) layer of chromium oxides, and this layer acts as an ultra-thin film and effectively protects the steel from mishaps. This is called “passivity”.
Unfortunately, the passive film is not resistant to everything - the steel can corrode in bad cases. This happens most often by a local breakdown of the passive film, while the rest remains passive. The corrosion attack is very local in the form of, for example, pinholes (pitting) or cracks (stress corrosion).
- The corrosion resistance of the steel (versus pitting) is often controlled by the steel’s content of chromium (Cr), molybdenum (Mo) and partly nitrogen (N). The higher the content, the better, so in practice, the optimal stainless steel choice is “just” a matter of choosing a steel with lots of chromium and molybdenum. This is because the “acid-proof” of the 4401 grade is more durable than “ordinary stainless steel” of the 4301 grade.
The disadvantage is, of course, that high-alloy steels tend to be more expensive, so in practice, you have to analyse each situation and choose the steel that provides the best balance between corrosion resistance and price.
- Corrosion is usually worst under fully submerged conditions, but it can also be a problem above the waterline. The main difference between the conditions above and below the waterline is the number of electrolytes. There are plenty underwater - above the water, there are “just” splashes and spray, but it can also be extremely bad on the coast or during a Danish, salty winter.
If you want to maximise the steel’s corrosion resistance above the water, it is necessary to think in terms of maximum drainability. Surfaces must be kept fine and smooth to ensure minimal adhesion, and horizontal places or areas with a backward incline must be avoided entirely. The less water, the less the risk of corrosion.
- There is no official “food-grade steel”. According to EU regulation 1935/2004, steel must not affect either food or the consumer (us!), but there is no positive list of approved types of steel.
Fortunately, the world is quite simple, and the Danish “Metal in Food” (2004-07) project clearly showed that when we “just” choose steel and processes so that there is no corrosion, no heavy metals will be released to the medium. Another good argument for choosing sufficiently corrosion-resistant steel and complying with Good Manufacturing Practice (GMP).
- You would not believe it, but stainless steel is one of the world’s most recyclable metals/alloys, and up to 80% of Europe’s new steel is made with scrap as a base material. It is the scrap that is collected at the individual company, from where it is sorted and driven to the steelworks, where it is melted and adjusted with extra chromium, molybdenum, nickel or whatever is needed to make steel type X:
* All the useful elements of the steel return to new steel.
* We limit the extremely environmentally harmful mining.
* We do not have to use energy to purify and refine the impure ores for unwanted elements such as S, P, etc.
* There is a good economy in recycling stainless steel scrap.
* We avoid stainless steel in nature.
The bottom line is that stainless steel is one of the most recyclable materials on the planet. No other commonly used alloy can provide the same degree of recycling.