Duplex steels combine the positive properties of their structural components ferrite and austenite. While the ferrite phase is crucial in determining the strength level, the austenite portion serves to increase ductility. The two-phase structure is obtained by alloying ferrite and austenite stabilizing elements (e.g. Cr, Si, Mo and Ni, Mn, N) whereupon the two-phase range is stabilized down to room temperature and to some extent, well below it.

In addition, high alloy duplex steels offer good corrosion resistance. This assures them a large range of applications, above all in areas in which corrosion resistance is needed in combination with high strength and ductility; like heat exchangers, vessels for chemical engineering and pressure vessel constructions, oil and natural gas industry products.

The field of application of stainless duplex steels can be extended further if in addition to formed products cast structures are deployed. Casting is taken into consideration especially for geometrically complex designed components, since it prevents the need of forming and assembling. The automotive industry stands as a potential customer for cast duplex components, particularly for thin walled steel castings for light weight construction.

With castings, however, there are serious constraints on achieving a desired structural morphology due to the absence of a deformation-assisted microstructure transformation. That is why it is critical to obtain an optimal microstructure already during solidification. With castings, it is also necessary to ensure equally high properties throughout, regardless of variations in wall thickness.

In a research project the Technische Universität Bergakademie Freiberg (University of Mining and Technology) in Germany and Evosteel GmbH investigated the microstructure and the resulting mechanical properties of thin-walled duplex steel castings. Evosteel is a foundry specialized in manufacturing thin-walled cast steel components located in Leipzig.

The test specimens were duplex alloys with varying chemical compositions, casted with a wall thickness within 2 and 7 mm. Two systems were employed to melt the alloys. An industrial-scale low pressure pouring system was used to investigate the effect of wall thickness and the location of the test specimen within the casting. A second laboratory-scale pouring system served to determine the effect of chemical composition. The content of silicon, chromium, molybdenum and nickel was varied. Wall thickness and thereby cooling rates were held constant.

Both systems poured metal into pit-iron sand molds that had not been preheated. After solidification, the material was prepared for metallographic and mechanical investigations. To investigate the effect of a subsequent heat treatment on the microstructure and mechanical properties, annealing was carried out at temperatures between 950 and 1050°C while the duration was varied.

The researchers found that the chemical composition and the cooling rate are the main parameters affecting the properties. The chemical composition of the duplex steel affects the thermodynamics and thus the adjustable relative proportions of austenite and ferrite. Ferrite solidifies first from the melt. On further cooling, it transforms into austenite. The variation of wall thickness affects the kinetics of the phase transformation by affecting the cooling rate.

The austenite vs. ferrite proportion as well as the structural dispersion are influencing factors determining the strength and ductility properties. An increasing ferrite content reduces the impact toughness severely at temperatures below the ductile-to-brittle transition temperature of ferrite. With an appropriate heat treatment technique, in particular the ductility properties can be further increased.

L. Petzold, D. Minnich, T. Kreschel, and D. Peisker ; DOI: 10.1002/srin.201000004