Hyper-Duplex stainless steel helps resist chloride corrosion
Super-duplex stainless steels, such as UNSS32750, S32760 and S32520, have found widespread use in the oil and gas, petrochemical and chemical processing industries with their popularity being mainly attributed to an attractive combination of high corrosion resistance, excellent mechanical properties and a relatively low cost, compared with other higher performance materials.
Despite a very wide range of applications, there are areas, for example, in heat exchanger applications, where the corrosion resistance of super-duplex grades is insufficient for higher temperature service or for a long service life, and where materials, with even higher corrosion resistance, are needed.
Claiming to meet this need is a new material grade from Sandvik which was recently launched on to the market in the form of heat exchanger tubing.
Designated a ‘hyper-duplex’ material, UNSS32707 (SandvikSAF2707HD) has been developed to provide high chloride corrosion resistance, combined with improvements in mechanical properties. This increase in corrosion resistance will extend the use of duplex stainless steels in aggressive chloride environments, such as in hot tropical seawater.
The new material has a well-balanced composition and has been designed for use in acidic chloride containing environments. The combination of chromium, molybdenum and nitrogen increases resistance to localised corrosion, ie pitting and crevice corrosion.
Using the industry standard measurement of PRE (Pitting Resistance Equivalent) value, the new grade offers a nominal PRE value of 49, which is significantly higher than super-duplex stainless steels currently available.
The critical pitting temperature (CPT) of UNSS32707, defined by testing in accordance with a modified version of ASTMG48A, was 97.5˚C, compared with about 80˚C for the super-duplex grade UNS32750.
General corrosion can limit the service life of a stainless steel component in contact with a process solution. UNSS32707 is highly resistant to corrosion by organic acids, eg formic acid and acetic acid. It also remains resistant in contaminated acid. The alloy is, therefore, a competitive alternative to high alloyed austenitic stainless steels and Ni-base alloys in applications where standard austenitic stainless steels corrode at
a high rate.
UNSS32707 also has high mechanical strength. The duplex structure accounts for a yield strength about twice that of austenitic stainless steels with corresponding pitting corrosion resistance. The higher strength of the duplex material will in many cases allow substantial reductions in the thickness of tubular materials, lowering the weight and total cost of the installation. In spite of the high strength, ductility remains at a high level and fabrication procedures, such as bending and expansion, can be performed in the same way as for austenitic steels.
In order to verify the new grade and gain industrial reference information, Sandvik has evaluated heat exchanger tubes in a number of industrial heat exchanger applications.
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Martin Holmquist and and Kenneth Göransson are with AB Sandvik Materials Technology, Sandviken, Sweden. www.smt.sandvik.com
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