ATI has developed ATI 642™† alloy, a new Ni-Cr-Mo corrosion-resistant alloy that displays significant resistance to sensitization from the effects of hot-clad bonding and welding operations. The new alloy is unique because of its substantial Fe content, which gives it a significant cost advantage over Ni-based alloys of similar performance, while still containing enough Ni to provide SCC resistance superior to 6Mo stainless steels. It has a PREN value in excess of 40, which provides useful resistance to chloride environments such as seawater.
This paper will present the results of chloride stress-corrosion cracking (SCC), intergranular attack (IGA), and critical pitting temperature (CPT) corrosion tests, as well as results of tensile tests and Charpy impact energy tests. Testing was conducted in both the solution-annealed condition and following a sensitizing heat treatment that simulates typical exposure during the hot-clad bonding fabrication process. The microstructures of the new alloy in the solution-annealed, sensitization-heat-treated, and welded conditions will also be shown and discussed. The test results and microstructures will be compared to those of other alloys traditionally used by the oil and gas and chemical processing industries.
UNS(1) N06625 (alloy 625) is one of the most commonly used materials in the oil and gas (O&G) and chemical processing industries (CPI) because of its excellent corrosion resistance and its long history of reliable service. However, alloy 625 is relatively expensive because its chemical composition was designed to be suitable for a wide range of end uses in both high-temperature and aqueous corrosion-resistant applications. When less corrosion resistance is required, N08825 (alloy 825) is often used for similar applications. Although alloy 825 is less expensive than alloy 625, there are several environments where it is not a suitable replacement, such as in aqueous environments with high chloride contents at elevated temperatures. This is because alloy 825 is more prone to pitting and crevice corrosion, and to stress corrosion cracking (SCC) under those conditions.