DUPLEX STAINLESS STEEL

Duplex stainless steels are a family little known in everyday life but fascinating from a metallurgical standpoint. Their main characteristic —and what radically distinguishes them from other families— is the simultaneous presence of two crystalline structures in bulk, a phenomenon known as "multiphase". This structural duality is not common in traditional stainless steels, which usually present a single dominant phase: body-centered cubic (BCC) in ferritics, trigonal in martensitic, and face-centered cubic (FCC) in austenitics.

In duplex steels, however, an ideal 50:50 ratio is achieved between the ferritic (α) phase and the austenitic (γ) phase. This combination is obtained through a controlled reduction in nickel content, which decreases from the typical 8–10% in austenitics to values close to 4–5% in duplex steels. Thanks to this miscibility between phases, the result is an alloy that inherits the best of both: the mechanical strength and thermal stability of ferrite, along with the ductility and toughness of austenite.

The magnetic behavior of these steels is slight but perceptible, and their corrosion resistance is notably superior to that of more common grades. This versatility makes them suitable for a wide range of industrial applications, from mechanical components such as nuts, shafts, valves, and gaskets, to more complex structures like pipes, heat exchangers, and piping systems in aggressive environments. Their ability to be easily machined without losing hardness or toughness makes them high-performance materials.

The chemical composition of duplex steels includes a high content of chromium (up to 25%) and molybdenum (between 4% and 8%), which enhances their resistance to corrosive agents, especially in the presence of chlorides. Some variants incorporate traces of elements like tungsten (W), niobium (Nb), and tantalum (Ta), specifically designed to withstand extreme conditions, such as those found on offshore oil platforms or in demanding chemical facilities. Their P.R.E.N. factor —an indicator of pitting resistance— is considerably higher than that of conventional stainless steels, positioning them as materials of choice in environments where localized corrosion represents a constant risk.

As a collector of rare metals and alloys, it's understandable that you've been drawn to this family. Their high price reflects not only the complexity of their production but also their exclusivity and technical performance. Duplex steels are not simply a mixture: they are a structural synthesis that redefines what can be expected from stainless steel.

Super duplex steels represent the most advanced evolution within the duplex family, and just like the so-called 'super-austenitics', their designation is not merely commercial but technical. These are alloys that retain the characteristic multiphase structure —a balanced mixture of ferrite (α) and austenite (γ)— but with an even more enriched chemical composition. The chromium (Cr) content can exceed 25%, molybdenum (Mo) can reach up to 8%, and high-value strategic elements like niobium (Nb) and tantalum (Ta) are incorporated, designed to reinforce resistance against corrosion, thermal fatigue, and prolonged mechanical stress.

This combination of elements not only improves the material's structural stability but also makes it an insurmountable choice for applications in highly aggressive environments, such as offshore oil platforms, desalination plants, chemical reactors, heat exchangers subjected to intense thermal cycles, and piping systems exposed to saline or acidic solutions. Their performance against pitting, intergranular corrosion, and stress corrosion cracking in the presence of chlorides is exceptional, with P.R.E.N. factors well exceeding the thresholds required for prolonged immersion in seawater.

In terms of corrosion resistance, super duplex steels stand at the pinnacle of "normal" metals, surpassed only by nickel and cobalt superalloys, which belong to an even more specialized and costly domain. This privileged position is due not only to their chemical composition but also to the synergy between their crystalline phases, which allows for a uniform distribution of internal stresses and a balanced mechanical response to dynamic loads.

Although their price is high —as you well know from your experience as a collector— their technical performance justifies every gram. They are not materials for everyday use but for challenges where conventional engineering falls short. In short, these are steels designed to last where others fail, and to resist where others yield.

Would you like us to now explore nickel and cobalt superalloys, or would you prefer to conclude this series with a reflection on the future of stainless steels? I'm with you to the last atom of tantalum.