A Short History on the Origin and Development of Stainless Steel
By Mark Newman
Previously, I have written about some of the energy applications Sandvik is involved in, but the overall objective for our work at strategic research is to continue the materials evolution. This, to a large extent, can be achieved by continuing to develop new problem-solving alloys.
Developing new alloys
It is perhaps not entirely clear to everyone, but out of a group of stainless steel companies collectively producing close to 40 million tons of stainless steel each year, only a handful develop new alloys and materials. Most produce a relatively small program of materials and adding a new alloy to their program involves waiting for a patent to expire and then making a "generic" version of another company’s development. Sandvik then, is unusual in its ambitions, and unique in terms of its product portfolio.
In order to continue the materials evolution, it is very important to remember where we are coming from. It is also quite fun to look back at some earlier stainless steel developments.
The origin of stainless
Going right back to the beginning, it seems the first patents for chromium-containing steels that didn't rust were granted independently to Harry Brearley in Sheffield, England and Benno Strauss and Eduard Maurer in Essen, Germany between about 1912 and 1916. That makes "stainless steel" about 100 years old. Those early inventions were a serious breakthrough, but they wouldn't be much use in today's applications since they would have been virtually impossible to weld effectively due to structure and impurities. The development of new and useful grades of stainless steel has gone hand in hand with manufacturing technologies capable of smelting and refining them. The Argon Oxygen Decarburisation (AOD) process that Sandvik uses in Sandviken was only developed and made commercially available towards the end of the 1960s along with the similarly capable CLU (Creusot Loire Uddeholm) and VOD (Vacuum Oxygen Decarburisation) processes. It is not unreasonable therefore to suggest that the era of modern stainless steels began at the same time. The defining feature of all these converter technologies is the ability to remove the bulk of the carbon, which is a legacy from the pig iron produced in a blast furnace where iron ore is reduced using coke.
The alloy situation differs somewhat between strip, wire and tube products. But taking seamless tube as an example, in the early 1970's the core stainless steel alloys were mainly from the 300 series. Before the AOD converter came along, it was impossible to remove enough carbon to prevent the risk of sensitization. Sensitization is where chromium from the matrix reacts with residual carbon from around the grain boundaries to form chromium carbides. Since chromium is sucked from the matrix it is not able to perform its corrosion resistance duties and knife-edge corrosion can take place. To counter this, titanium was added as an alloying element that would tie up the carbon in the form of titanium carbides leaving the chromium to do its intended job. A good example of such a titanium stabilized material is 321, which is very similar to 304L but for the titanium addition. With the advent of converter technology (AOD, CLU, VOD) carbon levels could be refined down to less than 0.03% removing the risk of sensitization and the need for Ti stabilization, although some markets still prefer a touch of titanium despite its redundancy.
Combatting severe conditions
Both in the cases of wet corrosion and high-temperature corrosion, as conditions get more severe the necessary alloying level goes up (Cr for corrosion resistance and Ni to retain an austenitic microstructure), the pinnacle of the 300 series being TP310. Grade 310 is very interesting, and you see the basic footprint of 25% Cr and 20% Ni in many different places in the world of stainless steels. But at some point in the 1970s, it became clear that high or low carbon variants of 310 were not good enough to solve all of the problems that were being encountered and new solutions were required. We already talked about Stress Corrosion Cracking of 300 series stainless steels when discussing Duplex, but when discussing the best examples of where materials development facilitates a leap forward in chemical processing technology, Sandvik 2RE69 is a case in point. A close and enduring customer wished to implement stripper technology into the process for manufacturing urea, but the conditions were too aggressive for any available materials at the time. Much had been done with 304L and 316L within urea production to reduce corrosion caused by impurities and grain boundary ferrite in standard versions of the grades. The results were Sandvik 2R12 and 3R60 U.G. But stripper technology was a bridge too far and required a new corrosion-resistant solution. As a result, Sandvik 2RE69 was developed by basically adding molybdenum to TP310. More impurities were refined out in the AOD and a new era in urea production was born (a path that has continued with the development of Sandvik Safurex™ and the positive changes to the urea process it enables).
Another example of how TP310 has acted as the starting point for further development comes in the form of a non-Sandvik product from the high-temperature sphere. When supercritical power stations first appeared, it became clear that the incumbent austenitic superheater materials that were used in the subcritical fleet were not up to the job so a niobium alloyed version of TP 310 featuring dispersion strengthening was introduced by one of the few competitors also in the business of materials development. That material is UNS S31042 and will be familiar to anyone working with boiler tubing. In the next generation of advanced Ultra Super Critical plants, Sanicro® 25 will be able to elevate this application to the next level due to its further improved properties.
A key point of separation between stainless steel development and the reference points of the 300 series that had come before came with the launch of Sanicro® 28 during the 1970's. Originally developed to solve corrosion problems in heat exchangers in the production of phosphoric acid, Sanicro® 28 has become a true multipurpose material solving corrosion problems in a variety of different applications from both up and downstream oil and gas applications to fertilizer acids. It also inspired several subsequent alloys too. But even though Sanicro 28® broke the mold, it was still a development out of the "old school". What would happen after would clearly alter the available possibilities in stainless steel development.
The golden age
Two significant things happened that turned the 1980s in a golden age for materials development. Firstly, the discovery of the numerous benefits of using nitrogen as an alloying element. Nitrogen has become extremely important in high-performance stainless steel development for the following reasons:
- it is a strong austenite former and alternative to nickel
- it has a very strong positive effect in terms of pitting and crevice corrosion resistance
- it has the effect of slowing the nucleation of deleterious phases and last, but not least
- nitrogen is very cheap
The second factor was the availability of thermodynamic computer modeling which would allow much of the guesswork to be taken out of the previous trial and error method of materials development. The original poster child of nitrogen alloying was probably Avesta's 254SMO, but both tools were used to great effect in Sandvik SAF 2507® which was initially launched at the end of the decade. Both aspects continue to be key tools today. Other materials developed in the decade were Sandvik SAF 2205™, Sandvik SAF 2304™ and Sandvik SX, used in concentrated sulfuric acid.
Much has happened since this golden age, but it would be hard to maintain the "new grade" tempo of the 1980s. Nevertheless; the gap that existed between the 300 series stainless steels and the nickel alloys has been systematically whittled down. Our challenge at Strategic Research in the coming years includes developing the same record of success with nickel alloys that we already have in stainless steels.