An optimum balance between austenite and ferrite in duplex stainless steels is accomplished by a clever use of the alloying elements. First of all, nickel, manganese and nitrogen are austenite stabilizers while chromium and molybdenum stabilize the ferritic phase. However, the fascinating interaction between these elements has also other consequences, which will be discussed in this and future columns.
With a background in Swedish industry I feel called upon to mention that four of these elements were discovered by Swedish chemists during the 18th century. The only exception is chromium, which was discovered by the French chemist Vauquelin in Siberian red lead (in 1797).
In 1751 Axel Cronstedt found a mineral in the cobalt mine at Loos in Sweden which was assumed to contain copper because of the apple green colour. The mineral was most likely Annabergite, which is an arsenate mineral and can be seen in the museum of Loos (Figure 1). Very disappointingly, no copper was found so it was called “Kupfernickel”, which is German for “false copper”. However, he suspected a new metallic element, which was confirmed in his experiments and the new element was later termed nickel. A long time elapsed before nickel was used in engineering applications but today, nickel is an important alloying element and even provides the basis of industrial companies such as INCO, which erected a monument in Loos in 1974 (Figure 2).
The nickel monument in the mining village
Loos, Sweden, erected by INCO in 1974
in memory of the important discovery of nickel
by the Swedish chemist Cronstedt in 1751.
(Photo by the author)
Spurred by the assistance of the great Swedish chemist Carl Wilhelm Scheele, Johann Gottlieb Gahn , a chemist at Bergskollegium in Falun, discovered the element manganese in the mineral pyrolusite (MnO2) in 1774. It so happens that there are some similarities between manganese and nickel. For instance, they are both austenite stabilizers although manganese is less effective than nickel.
One important role of nickel in DSS is to counteract the ferrite stabilizing effect of chromium and molybdenum to achieve a balanced duplex structure with roughly equal amounts of ferrite and austenite. Early DSS such as type 329 contained about 5% nickel. This is also the case for the work horse 2205. With the advent of super DSS with PRE values above 40 typically 7% has to be added to balance the effect of chromium and molybdenum.
Soaring nickel prices culminating in 2008 stimulated the development of lean DSS with reduced nickel levels. For instance, the lean DSS LDX 2101 produced by Outokumpu contains only 1.5% nickel with maintained phase balance. This is only possible with the help of other austenite stabilizers. Fortunately insufficient nickel can be replaced in part by manganese, which also stabilizes austenite. Because manganese is only about half as effective as nickel in this respect correspondingly more manganese is required to obtain the same effect. In the case of LDX 2101 5% manganese is needed. The austenite stabilizing effect of manganese is further enhanced because it enables a higher solubility of nitrogen (0.22% in LDX 2101).
However, in addition to stabilizing austenite, nickel has other beneficial effects that cannot be fully compensated by manganese. When investigating a wide range of duplex steels, Wessman et al at the research institute KIMAB in Sweden found that a minimum amount of nickel was required to reach acceptable pitting corrosion resistance and toughness. Entirely nickel-free DSS were more susceptible to pitting than those containing nickel and also showed tendencies to cleavage fractures.
In the present moment the nickel price is below USD 10,000/tonne. This implies that the economic incentive for developing lean DSS is less today than in 2008 when the nickel price exceeded USD 50,000/tonne. One lesson to be learned is that even if the price of nickel would start soaring again we need a minimum amount of nickel to maintain sufficient corrosion resistance and appropriate mechanical properties. Both Cronstedt and Gahn would be pleased if they had known about the intricate interplay between nickel and manganese in duplex stainless steels and how we harvest the fruits of their discoveries in modern metallurgy!
Header image: A piece of the mineral Annabergite on display at the museum of Loos in Sweden. The apple green substance is hydrous nickel arsenate, from which is assumed that Cronstedt isolated the new element nickel in 1751. (Photo by the author)