SUPERDUPLEX and DUPLEX steels for the oil and gas industry

The special feature of duplex stainless steels is already clear from their name since it refers to the specific microstructure that distinguishes them at room temperature. It is not by chance that these steels are also known as duplex steels since they feature the coexistence of two phases: austenite and ferrite. The result is the best possible compromise between corrosion resistance on the one hand and mechanical features on the other hand. The number of phases present defines the product’s final characteristics. The most important aspects affecting the relationship between austenite and ferrite are the solubilization heat treatment that must be carried out following hot plastic deformation and the chemical composition, with specific reference to the chromium and nickel content.

Duplex steel features

Duplex steels have very high potential, yet they are characterized by considerable complexity from a structural point of view. If they are subject for a period of time to a temperature ranging between 350 to 1000 degrees Celsius, the precipitation of undesirable deleterious phases may occur, from which decays in toughness and corrosion may result. These steels are quite susceptible to embrittlement caused by the higher temperature, so they should not be subject to temperatures above 300 degrees Celsius. The versatility of duplex steels allows them to be used in a wide range of sectors, especially in those where there is a need for high mechanical properties and considerable corrosion resistance, such as, for example, the paper industry, but also the food and energy sectors, not forgetting the chemical, petrochemical and – above all – oil & gas industries.

Weaknesses and critical aspects of duplex steels

For duplex steels, forging is a complex process that proves to be a source of critical issues. If not handled correctly, cooling after forging can increase the risk of cracking, especially with significant thickness. Due to a complex phase balance, the mechanical properties are challenging to obtain; they depend, among other things, on the mass and thickness of the components.

Duplex steel types

Over time, various types of duplex stainless steels have been developed and fine-tuned, varying according to PRE (an acronym for Pitting Resistance Equivalent). In the case of a lean duplex, for example, the PRE is just over 25; in the case of a standard duplex, on the other hand, the PRE exceeds 35; for a super duplex, then, the PRE value is over 40; finally, for a hyper duplex, the PRE is just over 45. Forge Fedriga produces duplex steels with PREs above 35 (F51 and F60) and 40 (F53 and F55). In the case of F51 and F60, the same standard of raw material is used, but the mechanical properties change.

Why duplex steels are used in the oil & gas industry

One of the main areas where duplex steels are used is in the oil & gas industry. This is a sector where one has to deal with corrosive environments often associated with significant loads. In addition, the low temperature that usually characterizes oil & gas industry environments further aggravates the materials’ operating conditions.

The Norsok Specifications

The Norsok specifications were developed by the Scandinavian industry by the same name to regulate, in terms of cost and safety, the manufacturing of products for the oil sector; it is, in essence, a supplement to the general standards that already regulate the sector. For Forge Fedriga, the two Norsok reference standards are M-630 and M-650: the former includes various data sheets from the manufacturing process and material type, while the latter specifies the qualification required for manufacturing special materials. Regarding the Norsok M-630, the sheets are split into letters according to the family of materials: for example, MDS C stands for carbon steels, MDS D for duplex steels, MDS N for nickel alloys, MDS R for super austenitic, MDS S for austenitic steels, and MDS  T for titanium alloys. The manufacturing process and the manufacturer must be certified according to Norsok Standard M-650. This standard specifies the certification requirements for all types of materials that must be met to ensure that the manufacturer has the experience, skills, and facilities necessary to manufacture thicknesses and shapes with adequate mechanical properties.