To get harder kitchen pots it is important to consciously balance the different metals used in the steel, as well as the additives nitrogen and carbon. This shows research results from Giulio Maistro, Chalmers.
In a recently published doctoral thesis, Giulio Maistro presents studies of methodologies to make austenitic stainless steel harder, without losing the "stainless" properties. The results show that it is important to consciously balance the different metals used in the steel, as well as the additives nitrogen and carbon.
Austenitic stainless steel is a specific type of stainless steel alloy that is used for kitchen pots and many industrial applications. This type of material is very good to use with strong acids or salty water because it is resistant to corrosion.
Unfortunately, today’s stainless steel has the drawback of being very easy to scratch and damage. It is too soft. This is not crucial for our kitchen ware, but is a big problem for jewellery or for industrial applications. In industrial sectors like the oil, gas, food and nuclear industries, the surface has to be smooth like a mirror.
When making stainless steel, it is the combination of the material in itself and the surface treatment that defines how good the result is. The result of a surface treatment can be radically different depending on the formula the material is composed of. Giulio Maistro says that this can be both a good and a bad thing.
– Nowadays, we have reached a stagnation in the application of surface treatments like plasma, gas nitriding or carburizing. More or less everyone in the field knows "when it is worth to use them and when it is not".
According to Giulio Maistro, companies keep their processes secret which makes process development hard and almost completely abandoned in academia. Giulio Maistro’s research is welcomed. Not much research has been done earlier on the optimization of the materials to fit the treatment. Instead of trying to change and over-optimize the treatment parameters, it could be easier and more effective to tailor-make a new material that better matches the treatment.
This tailor-making involves Nickel and Molybdenum, two metals that typically are added into the steel to improve resistance against corrosion.
– In my research I show that by adding Nickel it is possible to decrease the unwanted formation of carbides, which are bad for corrosion. However, when too much Nickel is used, the material cannot be hardened very much. This is because carbon and nitrogen do not like Nickel and vice versa. If you use the metal Molybdenum, the opposite effect is shown.
To harden the steel, it is common to introduce nitrogen or carbon in it. The more nitrogen or carbon you have, the harder the steel gets. This relates to Nickel and Molybdenum. Depending on how much of those metals you have in the steel, you can change how much nitrogen or carbon you can introduce in it.
However, if you introduce too much nitrogen or carbon, chemical compounds called nitrides and carbides are formed. When they form, the stainless property of the steel gets lost. In general, Molybdenum increases the amount of nitrogen or carbon you can insert. Nickel limits the amount but also limits the formation of nitrides or carbides.
– This new knowledge shows that companies that manufacture products made of stainless steel need to find a balance between Nickel and Molybdenum to get the maximum hardness while maintaining the stainless properties, upon introducing nitrogen or carbon, says Giulio Maistro.
Gas nitriding or carburizing are methods to introduce nitrogen or carbon to the steel.
Low-temperature carburizing/nitriding of austenitic stainless steels - Influence of alloy composition on microstructure and properties.
Read more in this scientific article:
Text: Nina Silow
Photo in the article: Marcus Folino