Effective against aggressive media, the new breed of steel is ideal for high-performance tools in the oil and gas industry, explains Merlin Seifert
One of the most important things needed when developing new oil and gas fields is tools that reliably work in great depths under the sea. The problem is that these tools are subjected to rock formations of varying hardness and aggressive media, necessitating special high-strength steels that also exhibit a high corrosion resistance. This combination of properties proves to be a challenging proposition.
Whether for enhancing effectiveness in a car’s powertrain, extracting oil and gas safely and reliably or increasing durability of components in the aerospace sector, the use of special long steel is indispensable in many industries. Finished machined mandrel bars e.g. made from hot work tool steel and in lengths of up to 26m, are used in mills that manufacture seamless tubes and pipes for the oil and gas industry. Other applications for long steel products in the field of oil and gas extraction are drill collars and rotors in the drive unit and power section of subsea drill strings.
Special long steel generally allows a high product differentiation, yet it requires extensive expertise – especially in the highly sensitive area of oil and gas exploration. The drilling fluids used here often contain sour gas and chlorides. These aggressive media constantly attack the extraction equipment, particularly at increased temperatures. The reaction with the steel in use can lead to 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 leads to ongoing research and development of materials 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 to improve wear and corrosion resistance. Coatings can totally change the properties of a component, though can sometimes well adapt it to its requirements. As described above, alloy development is always subject to certain limits with regard to the desired combination of different properties. Whilst coating processes such as hard chrome-plating 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 for achieving required long steel properties
Deutsche Edelstahlwerke, a company in the Schmolz + Bickenbach Group, offers 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. With Corrodur 4418 Mod (also known as Super 13Cr) DEW has now qualified a stainless steel that 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 is an extremely high-performance steel characterised by high strength, outstanding toughness and good resistance to pitting and stress corrosion. With a tensile strength of at least 125ksi (862 MPa), a yield strength of at least 11ksi (758 MPa) and a toughness of at least 60ft-lbs (81J) at 14°F (-10°C), Corrodur 4418 Mod is far superior to conventional 13 % chrome steels.
Long steel laboratory tests confirm performance advantage
These properties were determined by extensive laboratory tests. The steel was austenitised between 1,778 and 1,886°F (970-1,030°C) and then quenched in water. Tempering was carried out at 1,022 to 1,202°F (550-650°C) subsequently. The best compromise between strength and ductility were achieved in the temperature range between 1,076 and 1,106°F (580-630°C). This temperature range was confirmed in real life during heat treatment of bars, proving that the required properties are met by industrial heat treatment in continuous furnaces as well.
The test results for Corrodur 4418 Mod are also impressive in terms of its corrosion properties. After extensive tests, no signs of intercrystalline corrosion according to ASTM A 262 Prac. A and Prac. E could be detected, 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 (formerly Statoil) as well as a growing number of other end consumers.
Merlin Seifert is with Deutsche Edelstahlwerke