The term duplex stainless steel describes steel with microstructure containing austenite and ferrite in which the lesser phase is at least 30% by volume. First generation duplex stainless steels (typically 26% Cr, 4.5% Ni, 1.5% Mo) were about 75 to 80% ferrite.
Unlike the common-grade austenitic stainless steels, duplex stainless steels are highly resistant to chloride-ion stress corrosion cracking (SCC); they have excellent resistance to pitting and crevice corrosion, and have approximately twice the strength of the common austenitics.
In general, poor weldability was a characteristic of the first generation duplex stainless steels. Corrosion resistance and toughness of the base metal heat-affected zone (HAZ) were poor due to the effects of the welding operation. However, HAZ problems were greatly decreased with the advent of the argon-oxygen decarbonization process used in steel making which made it possible to precisely alloy with nitrogen. Nitrogen, which is a strong austenite former, permitted lower nickel contents and improved tensile properties and resistance to pitting and corrosion. These alloys are typically higher in chromium than the common-grade austenitics. Utilization of molybdenum as high as 4.5% accounts for resistance to pitting and crevice corrosion.
The duplex stainless alloys are characteristically stronger than either of their two phases considered separately. The coefficient of expansion and heat transfer characteristics are, as would be expected, intermediate between the ferrite and austenitic stain- less steels. Many duplex stainless steels, as with most of the other stainless steels, are proprietary alloys.
The second generation duplex materials, especially alloy 2205, have found increasing uses in the brewery business, chemical process industry and various chemical shipping containers including tankers and barges. This use includes heat exchangers, pressure vessels, tanks, columns, pumps, valves, shafts and pulp digesters where the increased resistance to chloride ion SCC pitting and crevice corrosion, and increased strength give advantages over the molybdenum-bearing austenitics..
Because of the large quantities of ferrite in the microstructure, the duplex stainless steels are subject to embrittlement when exposed to the 704 to 927°C (1300 to 1700°F)temperature range due to the formation of chi and sigma phases. For minimization of this form of metallurgical reaction, high levels of nitrogen are beneficial and high levels of molybdenum are detrimental.
Weldability. Acceptable techniques have been developed for SMAW, SAW, GTAW, GMAW and PAW of duplex stainless steels. The complexity of welding these alloys is related to the increased concern for hydrogen-related embrittlement due to large percentages of ferrite, the ability to reform austenite on the welding thermal cycle, and concern for formation of embrittling intermetallics.
Machinability. Special tool angles, low speed and heavy feed are required to machine duplex stainless steels. Due to the combination of high strength and toughness, machinability is considerably poorer for these steels than the common-grade materials.