Designing a steel that is stainless was considered impossible in the beginning of the 1900’s. A renowned German chemist, G Mars, maintained the opinion that creating a stainless steel is impossible because iron is not a noble metal and its oxides are thermodynamically more stable than the pure metal. The year was 1911 but, by the irony of fate, the two first stainless steels were launched the following year.
What was not known at the time was the fact that chromium present in sufficient quantity (above about 10 wt%) can form a stable oxide on the steel surface. As opposed to haematite (Fe2O3), eskolaite (Cr2O3) is almost defectless and therefore capable of forming a protecting surface layer. Although chromium oxide is more stable than metallic chromium, its growth in standard atmosphere ceases when the thickness reaches just a few nanometres for kinetical reasons. This provides the basis for the so called passive layer on stainless steels.
Chromium was discovered in the mineral Siberian red lead (PbCrO4) by the French scientist Vauquelin in 1797. The name chromium (derived from the Greek word for colour; chromos) was found suitable because of the various colours in which it appears in nature. Somewhat later Berthier (1821) and the famous scientist Faraday (1822) noted the positive effects of chromium on corrosion, but it was not until 1912 that the first stainless steels were patented.
In duplex stainless steels the effect of chromium is at least two-fold; It stabilizes the ferritic crystal structure and it provides corrosion resistance, in particular pitting corrosion resistance. Among several types of corrosion, pitting corrosion is the only type that is directly related to the chemical composition. Pitting is a localized type of corrosion involving the formation of a pit more or less hidden below the surface and in which the environment becomes increasingly acidic. Once having started, the process may continue in an autocatalytic way and, therefore, difficult to stop. Among elements preventing pitting corrosion in duplex stainless steels chromium is the alloy element that gives the major contribution because of its high concentration. The processes occurring inside the pit are complicated and not fully understood because of the experimental difficulties in analysing the local chemistry. The beneficial effect of the above-mentioned elements is, however, empirically very well established.
One would assume from the PRE-formula that gradually higher concentration of these elements would give better protection. This is true up to a certain limit. However, as was discussed in my previous column on the effects of nitrogen, there are also side-effects that cannot be neglected when this limit is exceeded. While nitrogen in too high concentrations tends to lead to nitride precipitation, chromium increases the risk of forming intermetallic phases (here used as a generic term for sigma-phase, chi-phase and R-phase). All intermetallic phases are brittle themselves. This alone leads to embrittlement, but the process is enhanced in the case of sigma-phase due to its lack of coherency with the surrounding matrix. While some intermetallic phase (about 1% by volume) can be tolerated before the corrosion resistance drops, toughness is extremely sensitive. This is illustrated in Figure 1 showing the toughness as a function of intermetallic phase. As shown in the same diagram, hardness is only weakly influenced. If hardness is used as an indication of intermetallic phase (this has been used by some producers!) misleading results are inevitably obtained. Of all indirect methods used to estimate intermetallic phase in duplex steel, impact toughness is therefore recommended.
An example of intermetallic phase formed after 9 min at 850°C in a super duplex steel is shown in Figure 2. In this particular case a heat treatment was performed under controlled conditions in the laboratory aiming at analysing the kinetics of sigma phase formation. Once having mapped the sensitivity for intermetallic phase formation at various temperatures in the interval 600-900°C recommendations can be given about cooling rates required to prevent its formation (e. g. Wilson and Nilsson, Scand. J. Metall. 1996). This helps to prevent intermetallic phase formation during production and fabrication.
Molybdenum and tungsten are neighbours of chromium in the periodic table and even more potent than chromium in preventing pitting. You are welcome to read about these elements in duplex steels in my next column.