Stainless steel is continuing to develop in unexpected ways. A lot of attention has been focussed on duplex, ferritic and 200 series steels as a way of reducing the cost of stainless steel. Now, an even more revolutionary breakthrough is being developed at the University of Senderup in Scandinavia. Professor Olaf Pirlo has been researching there for a number of years. He has found that he can replace nearly all of the chromium in stainless steel with iron and leave the corrosion resistance properties intact. But it is not just any old iron. It is iron which is neither austenitic nor ferritic in structure but an entirely new crystal structure, tentatively called “frenetic” by Prof Pirlo. There is some way to go before the new steel becomes commercially available. However, he is “cautiously optimistic” or perhaps even “optimistically cautious” about the discovery.
Crystal structure is the key to understanding the properties of materials. For stainless steel, the well known structures of ferrite, austenite and martensite explain the physical, mechanical and corrosion properties of that most useful of materials.
Professor Pirlo’s discovery of a new iron crystal structure is nothing short of revolutionary. It is neither Body Centred Cubic nor Face Centred cubic but something quite different.
The nearest representation that Professor Pirlo has so far been able to provide about the atomic structure is shown on the left.
The mechanism by which this structure acts as a stainless steel is not the same as the passive layer produced by chromium. Rather it seems that the atoms are in such a state of energy that they are able to repel the usually aggressive chloride ion.
Here are some results comparing the new material with more traditional stainless steels.
Clearly, the normal relationship between PREN and Critical Pitting Temperature breaks down with the new steel as it only contains residual amounts of Cr Mo and N.
The new steel is approximately equivalent to type 1.4401 (316).
Similar results have been found in salt spray testing.
1.4401 (316) 2K “Frenetic”
The surface finish is not so critical in the frenetic steel.
These are remarkably close to the standard austenitic stainless steels and so there should be little problem in transferring the steel to be used in existing forming applications.
Obstacles to be Overcome
Although the results obtained so far are encouraging, there are difficulties which Prof Pirlo acknowledges will need to be overcome before commercial launch is possible. The main problem is the stability of the “frenetic” structure. So far, he has only been able to retain the structure for less than a week. Clearly, having a week-long stainless steel is not of much practical use. He has found that adding about 11% Cr stabilises the structure to the point where it becomes useful. Readers will quickly realise the drawback of this solution!
The graph shows the remarkable straight line relationship between the Chromium content and the duration of the period of frenetic stability.
Olaf Pirlo will no doubt continue to build on his discovery until a commercial use is found. No doubt the chromium and nickel producers will hope that he remains unsuccessful in his quest.