SPECIAL STEELS
pitting and stress corrosion in the worst case. To prevent restrictions or even failure of the equipment, only exclusive materials are approved by oil and gas companies. Good resistance to pitting and stress corrosion is frequently required in this respect, combined with a strength that allows users a certain amount of freedom in terms of component design. The steadily increasing requirements on the material properties lead to ongoing research and development of materials in order to counter corrosion in particular. The focus often lies on coatings or modified alloy compositions. For instance, the rotors already mentioned are tungsten carbide or hard chrome-plated in order to improve wear and corrosion resistance. Coatings can totally change the properties of a component though sometimes well adapted to its requirements. As described above, alloy development is always subject to certain limits with regard to the desired combination of different properties. While coating processes, such as hard chromeplating, often generate considerable additional costs, alloy development represents a more cost-effective alternative. Another feasible alternative is the application-oriented qualification of materials. Here, the conditions encountered during use are analysed and transferred to appropriate laboratory tests. If a steel passes the tests required for the qualification process, it can be approved for certain applications or customers. This was also carried out at Deutsche Edelstahlwerke (DEW) for the special steel, Corrodur 4418 Mod, which has to comply with requirements from the industry standards NORSOK M-650 and NACE MR0175. Controlled heat treatment Deutsche Edelstahlwerke, part of the SCHMOLZ + BICKENBACH Group, offer a wide portfolio of steel qualities for the oil and gas exploration sector. These include low and high-alloyed structural steels, ferritic, martensitic, high-alloyed austenitic and duplex stainless steels as well as non-magnetic steels. Corrodur 4418 Mod (also known as Super 13Cr) DEW is now qualified as a stainless steel and is widely used in completion tools in the oil and gas exploration industry. This involves a material solution that exceeds conventional chrome steels in terms of performance. The required properties are specifically set by means of a controlled heat treatment. The result www.steeltimesint.com
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Mechanical properties of Corrodur 4418 Mod measured in the tensile test depending on finished diameter. Source: Deutsche Edelstahlwerke
is an extremely high-performance steel characterized by high strength, outstanding toughness and good resistance to pitting and stress corrosion. With a tensile strength of at least 125 ksi (862 MPa), a yield strength of at least 110 ksi (758 MPa) and a toughness of at least 60 ft-lbs (81 J) at 14 °F (-10 °C), Corrodur 4418 Mod is far superior to conventional 13% chrome steels. Performance advantage These properties were determined by extensive laboratory tests. The steel was austenitized between 1778 and 1886 °F (970 - 1030 °C) and then quenched in water. Tempering was subsequently carried out at 1022 to 1202 °F (550 - 650 °C). The best compromise between strength and ductility was achieved in the temperature range between 1076 and 1106 °F (580 - 630 °C). This temperature range was confirmed in real life during the heat treatment of bars, proving that the required properties are also met by industrial heat treatment in continuous furnaces. The test results for Corrodur 4418 Mod were impressive in terms of its corrosion properties. Extensive tests found no signs of inter-crystalline corrosion according to ASTM A 262 Prac. A and Prac. E, as was expected. In addition, tests performed according to NACE TM0177 and NACE TM0316 regarding sulphide-induced stress
cracking (SSC) and stress corrosion cracking (SCC) using application-oriented media were passed successfully. This enabled the steel to be qualified by Equinor ASA (formerly Statoil ASA) as well as a growing number of other end-consumers. At the beginning of 2020, DEW presented its new special steel at the Corrosion Conference & Expo, the world’s biggest trade fair for solutions to prevent corrosion. More than 6,000 corrosion experts from 60 countries visited the exhibition in Nashville/Tennessee, USA, including scientists, engineers and technicians, but also purchasers, meaning the new steel and its qualification could be presented at all levels of the supply chain. � For further information, log on to www.dew-stahl.com
Merlin Seifert
Digital Edition - February 2021
