Dergach T. Theoretical and technological foundations of structure control for increasing corrosion resistance of tubes made of low-alloy and high-alloy steels

A composition of low-carbon low-alloy 06Х1-У steel with a regulated content and ratio of chemical elements and an energy-saving technology for production oil and gas tubes in API 5L X 42...X 56 strength group were developed. The obtained tubes are characterized by a fine-grained structure with up to 23 % of special low-energy grain boundaries of 3n in ferrite and a high resistance against corrosion in chloride-containing media, resistance against sulfide and hydrogen corrosion cracking and in operation in the oil-and-gas producing industry. Kinetics of corrosion of the tubes made of 06Х1-У steel in a model chloride-acetic solution. An extreme rate of their corrosion was established and subsequent decrease to 0.03 mm/year which is 43 times lower than the corrosion rate of steel 20 tubes. Passivation of 06X1-У steel tubes was justified by formation of corrosion products with creased chromium content on their surface. A mode of double short-term temper of low-alloy steels which provides a fine-grained structure with fine-dispersed evenly distributed chrome carbides and a high resistance of tubes against HCC and SSCC was developed. Innovative technologies for manufacturing these tubes based on the principle of grain boundary engineering which ensure creation of the most favorable structure with a high SB content and a high complex of corrosion, mechanical and operational properties of the tubes were developed. Effect of surface active elements such as carbon, boron, nitrogen on structure and resistance against intergranular corrosion of high-alloy austenitic steels was established. Requirements to their maximum allowable contents. The developed technologies were introduced at the tube plants of Ukraine and the tubes were introduced in the gas-and-oil producing and chemical industries with a significant economic effect.