Duplex stainless steels date back to the 1920s with the first cast being made at Avesta Steels in Sweden in 1930. However, it is only in the last 30 years that duplex steels have begun to “take off” in a significant way, particularly in the offshore oil and gas industry.
Duplex steel is the preferred material for offshore oil and gas applications for various reasons, including:
• Higher strength material leading to weight reduction
• Greater corrosion resistance, specifically stress corrosion cracking
• Good performance in sea water service
The structure and grade is developed by carefully controlling both the alloying elements and the heat treatment performed on the alloy to obtain a structure consisting of 50% austenite and 50% ferrite. The result is an alloy that combines the higher strength of ferrite with the improved toughness of austenite. The super duplex grades contain higher levels of chromium and molybdenum to enhance their resistance to pitting and crevice corrosion.
Virtually every oil and gas end user, and sometimes EPC Contractors, will have their own specification for materials and particularly Duplex and Super Duplex Stainless. Many companies have adopted the NORSOK standards from Norway as a basis but there is a growing divergence between the requirements, particularly in the areas of:
• Chemistry - % Nitrogen content
• Charpy V Notch requirements and test temperatures
• Microstructure requirements and acceptance criteria
• Corrosion Test acceptance criteria
When there are varying valve requirements, the specifications do not always follow through the supply chain to the sub vendors manufacturing trim components for the valves. HSP has experience with a number of cases where vanilla NOROSK material was used and the materials were not in compliance with the client specifications. Often, this does not become apparent until the final stage of supply causing deviations or delays in the re-supply of material.
At HSP, we have a team of experienced project managers who are very familiar with both client specification and standards such as NORSOK. In addition, we employ a Metallurgist who is involved in all aspects of the supply process from specification reviews, pre-inspections and material verifications. Gordon McNair, Engineering Director says, “Our rigorous processes and attention to detail ensures we supply engineered valves to meet difficult and demanding specification, and get it right the first time.”