The role of nitrogen in duplex stainless steels
Some nitrogen has been present in steel as long as steel-making has taken place. This has been unintentional and, in practice, there have been no means of removing or controlling the nitrogen concentration. It was not until the advent of modern process metallurgical techniques such as AOD, VOD and CLU during the 1970’s that nitrogen could be added to the steel melt in a controlled way and nitrogen, therefore, became an important alloying element.
The suffocating element
This was an inspiration in the development of steels in which the influence of nitrogen on the mechanical and corrosion properties was explored. In particular, further development of steels in the duplex family accelerated. The Scottish physicist Daniel Rutherford is regarded as the discoverer of the element nitrogen in 1772. However, at about the same time, Carl Wilhelm Scheele in Sweden investigated nitrogen quite independently and referred to it as “burnt air” or “phlogisticated air”. It is, therefore, fair to give credit also to him for his contribution to fundamental chemistry. He gave nitrogen the name “kväve” in Swedish, the meaning of which is “the suffocating element”. The name refers to his spectacular experiment in which flies were allowed to consume oxygen in a closed glass jar until they eventually died from oxygen deficit. Little did he imagine that the gas nitrogen was going to be used successfully as an alloy element in steel about 200 years later.
PRE as a working tool
Like nickel and manganese - the subject of my previous column - nitrogen is an austenite stabilizer. Moreover, it gives a substantial strength increment by solid solution hardening. It was soon realized that nitrogen together with chromium and molybdenum improves the resistance to pitting corrosion (see formula below). Lorenz and Medawar at Thyssen in 1969 defined a parameter called the pitting resistance equivalent (PRE) and presented the following relation:
PRE=%×Cr + 3.3%×Mo + k×%N
The formula turned out to correlate so well with pitting corrosion experiments (e.g. the ASTM G48 test) that it has become a working tool in alloy development and used for the purpose of ranking alloys. The prefactor “k” is commonly 16 indicating that the effect of nitrogen on the pitting corrosion resistance is quite significant. In early types of duplex stainless steels the austenitic phase was often more susceptible to pitting than the ferritic phase owing to the fact that chromium and molybdenum were enriched in ferrite. Because of the preference of nitrogen for the austenitic phase this unbalance could be remedied by alloying with nitrogen.
Sandvik SAF 2507 was the first alloy to be fully balanced, aided by the computer program Thermocalc and the high mobility of nitrogen (SSW, August, 2015). As pointed out by Hertzman et al, a high mobility of nitrogen is also essential during welding when reformation of austenite takes place in the heat affected zone. The expected microstructure is shown in Figure 2. Nickel alone is not sufficient to guarantee enough austenite because of the short time available. However, nitrogen with its higher mobility, enhances the process of austenite reformation. Without nitrogen the volume fraction of ferrite would be too high and brittleness would become a severe problem as in the early types of DSS.
Furthermore, the desired PRE-balance of the weld would not be obtained and pitting corrosion would become a potential problem. It needs to be reiterated that there is no remedy without side-effects and nitrogen is no exception. Despite the fact that nitrogen is quite mobile it has difficulties to escape from ferrite under very rapid quenching conditions. Owing to the low solubility of nitrogen in ferrite the formation of nitrides is a likely scenario. As a consequence brittleness ensues. The other extreme is too slow cooling that gives rise to formation of intermetallic phases, which also cause brittleness.
The point I want to make is that both too slow and too rapid cooling during welding leads to problems. Provided the welding instruction be followed these extreme conditions are avoided. However, I have now anticipated some of the content of the coming columns in which the roles of chromium and molybdenum are treated. It is more or less a law of nature that these elements also have side-effects. But they are indispensable in the development of duplex stainless steels!
This article was first published in Stainless Steel World Magazine in July/August 2016.