Duplex and super duplex steels are widely used in the petrochemical industry, as well as oil and gas refineries and marine rigs. They are prized for their superior corrosion-resistance, strength and durability.

We at Chemipetro are increasingly supplying duplex flanges and pipe fittings to demanding 21st century environments, such as deep-water oil wells and petrochemical plants. However, duplex steels were actually developed more than 70 years ago, for the Swedish paper industry.

The first generation duplex steels were specifically designed to be resistant to the chemical process fluids used in the paper pulp industry, such as chloride-enriched cooling waters. The second generation alloys now in use have improved upon this, having superior stress and pressure-resistant qualities, and enhanced resistance to pitting. The chemicals used in the modern petrochemical industry are far more pressurised and corrosive than the Swedish pulp mills were.

Duplex is so-called because it has a mixed microstructure of roughly equal proportions of austenite and ferrite. To this, chromium, nickel and molybdenum are added in varying amounts to improve resistance to pitting and weldability. Super duplex steels were developed in the 1980s in response to the increased use of highly corrosive and high-pressure industrial environments.

The phrase, “the whole is better than the sum of the parts” could have been written for duplex. Its resistance to stress-cracking through corrosion is superior to type 316 steel alloys, and its mechanical strength is roughly double that of singular austenitic steel. The one criticism of duplex pipe flanges has been embrittlement at extreme temperatures. However, this is being addressed by development of cool-welding techniques, and alloys that withstand sub-zero and arctic conditions.