03. Duplex stainless steels- an interplay between ferrite and austenite

Figure 1: Microstructure of a super duplex stainless steel. Austenite in red and ferrite in blue contrast. Scanning electron micrograph.Duplex stainless steels (DSS) combine the properties of austenite with the properties of ferrite. While austenite is very sensitive to stress corrosion cracking ferrite is almost insensitive. Ferrite is rather brittle while austenite is tough. Work hardening is much less pronounced in ferrite than in austenite owing to the difference in atomic structure. Ferritic stainless steels are difficult to weld while austenitic stainless steels are readily welded. Combining ferrite and austenite of complementary properties results in a steel with a ferritic-austenitic or duplex microstructure (Figure 1) and entirely new properties. Some of these are the obvious results of a mixture of two phases.

However, as I emphasized in a previous column, there is sometimes a synergistic interaction leading to surprising properties that cannot be predicted from the properties of the constituents alone. The stress corrosion resistance and the mechanical strength are two examples of such synergism. Despite the fact that austenite is sensitive to stress corrosion the duplex structure is almost immune. This has been explained by the fact that the conditions for stress corrosion cannot be fulfilled in both phases simultaneously. Moreover, the mechanical strength is typically twice that of the constituents mainly because of the small effective grain size, which is difficult to obtain in a single-phase material.

DSS are suitable in applications where a combination of corrosion resistance, mechanical strength and weldability is required. DSS include both low alloy (lean) and highly alloyed (fat) variants. The lean ones are used in less severe applications where strength is more important than corrosion resistance. In highly corrosive applications fat DSS with PRE-values above 40, so called super DSS, are required.

Applications include heat exchangers, storage tanks, pressure vessels, umbilical tubing in the oil and gas industry (Figure 2), and various components in the urea industry. A critical stage of a urea plant is in the production of ammonium carbamate, which is a highly corrosive compound (Figure 3). The production of Urea is essential in modern agriculture and consequently for the survival of humankind. Table 1, which is far from exhaustive, shows a selection of DSS, their mechanical properties and their application areas.Figure 2: A bundle of tubes formed into a so-called umbilical tube used to control equipment on the bottom of the sea during oil extraction.

Figure 3: Urea plant for the production of fertilizers. The duplex stainless steel SAF 2906™ (Safurex®) is used successfully for carbamate condenser tubes in this application.

Figure 4: Bolts of SAF 2707 HD™ keeping the hull of the warship Vasa together. The ship foundered into her maiden voyage in 1628, was salvaged in 1961 and is now on display in the Vasa museum in Stockholm, Sweden.An odd but very interesting application of DSS is in the preservation of the warship Vasa, which is on display in the popular Vasa museum in Stockholm.The ship foundered into her maiden voyage in the Stockholm ström (the city harbor) in 1628 but was salvaged in 1961 in a surprisingly well-preserved condition. Owing to the water pollution the ship has been contaminated with various chemicals, of which sulphur is a major concern because of the continual production of sulphuric acid when exposed to air. The old steel bolts used to keep the hull together are rusting rapidly and need to be replaced by bolts of a corrosion resistant material. Bolts manufactured from the hyper duplex stainless steel SAF 2707 HD™ provided by Sandvik have been selected for the purpose (Figure 4). Corrosion tests promise that the bolts will keep the hull together throughout this century - at least!

This article was first published in Stainless Steel World Magazine in May 2017.