Molybdenum and tungsten in duplex stainless steels
In my previous column, I promised to give an overview of the role of molybdenum and tungsten in duplex stainless steels (DSS). They are neighbors of chromium in the periodic table (Group 6B) and even more potent than chromium in preventing pitting, which is shown in the familiar PRE-relation:
PRE = %Cr + 3.3×[%Mo + 0.5%W] + 16%N
The coefficient in front of tungsten compensates for the fact that the atomic weight of tungsten is about twice that of molybdenum. Molybdenum has been used from the very beginning in DSS. It was used in 453S launched by Avesta Jernverk in 1930 and has been used in virtually all DSS ever since. However, it was not until the 1980’s when Sumitomo launched DP3 and Weir launched Zeron 100 that tungsten was used more systematically as an alloying element.
Sumitomo DP28 with 2.5%W is probably the most tungsten-rich DSS. It is obvious that there is a strong driving force for adding the elements that contribute to improved corrosion resistance. However, there are side effects emphasized in my previous columns. Nitrogen close to the solubility limit promotes formation of nitrides while the Group 6B elements promote intermetallics. The intermetallic compounds in DSS are essentially σ-phase, Χ-phase and R-phase, all of which consume chromium, molybdenum and tungsten in various proportions. This leads to matrix depletion and reduced corrosion resistance. In addition, brittleness ensues. Despite the chemical similarities of Group 6B elements their influence on the kinetics of intermetallic phase formation show some important differences.
Potent promotion of intermetallics
In a paper by Hertzman et al (Mater. Sci. Technol., 604-613, 1997) it was concluded that tungsten is more potent in promoting intermetallics than the others. This is related to the thermodynamics and the diffusivity rate. However, another important effect was observed related to the ease of nucleation of Χ-phase. Tungsten stabilizes Χ-phase, which is crystallographically related to ferrite and therefore nucleates much easier than σ-phase. This can be seen in Figure 1, which is a transmission electron micrograph confirming the cube-cube orientation relationship between Χ-phase and the host lattice. Because Χ-phase facilitates nucleation of σ-phase (see Figure 2), which is thermodynamically the most stable of the intermetallic phases, the kinetics of intermetallic phase formation is enhanced significantly.
Chemists in the 1780's
We can learn from this that caution must be exercised when tungsten is used to improve the pitting corrosion resistance, because of its side-effects. I find it remarkable that most of the important alloying elements in DSS, viz. nickel, manganese, molybdenum and nitrogen, have been discovered by Swedish chemists. The only example when Swedish chemists were not involved is in the discovery of chromium for which Vauqelin is given the credit (October issue SSW). It is an indisputable fact that the Spanish brothers d’Elhuyar discovered tungsten in 1783, but they received so much inspiration from a visit to Sweden that it is tempting to mention the role of the pharmaceutical chemist Carl Wilhelm Scheele. One of the brothers spent a couple of days with Scheele in 1782 in his pharmacy in the small town of Köping. Scheele showed him a piece of tungsten acid (hydrated WO3) produced from the mineral Scheelite (CaWO4) in which he suspected that a new metal was concealed. He called it tungsten (heavy stone in Swedish) because of its remarkable weight. However, his furnaces were not hot enough so he was unable to produce the pure metal and left the remaining work to the brothers at the “Seminara of Vergara” in Spain. Scheele also made the preparatory work that paved the way for the discovery of molybdenum. Once again, due to limitations of his experimental equipment, he left a compound (viz. MoO3) to another chemist for the final work. This time he laid the table for Peter Jakob Hjelm in Stockholm, who uncovered the new element molybdenum by reducing the oxide with carbon. The year was 1781.
Because of his important contributions to the discovery of elements Scheele deserves attention. A statue in his honor is placed in the garden Humlegården in the center of Stockholm. He generously paved the way for other chemists in their quest for new elements. They, in turn, paved the way for steel makers 200 years later to explore the technical possibilities, not only in duplex stainless steels, but in all types of steels. This concludes my series of columns on DSS and their chemistry. In forthcoming columns, the theme will be entirely different and, hopefully, equally exciting.
This article was first published in Stainless Steel World Magazine in December 2016.