New approach to development and manufacturing technologies of duplex steel

Abstract

We conducted a brief review of current production and application of duplex and super duplex steels for manufacture of equipment exposed to the hazard of sulphide stress-corrosion cracking, sea water and other corrosive environment. The super duplex steel with enhanced corrosion-mechanical characteristics in comparison with the known steels of austenitic-ferritic class was developed. Based on the concepts of formation of a special structure of two-phase austenitic-ferritic steels in the process of crystallization, the possibilities of compositional, technological, thermal and special impact techniques are considered and advanced ways of controlling physical, chemical, structural homogeneity and properties of super duplex steels are developed. Electroslag remelting with the application of low-frequency alternating current provides effective control over the length of the two-phase area, the size of the primary dendrites of the austenitic and ferritic phases, the average distance between their axes, the parameters of the crystallizing cell, the development of liquation phenomena and the size of the growing non-metallic phases. Within framework of the proposed approach, the thermodynamic and kinetic conditions for the formation and growth of hardening phases are assessed, a new composition and a complex technology for the manufacture of corrosion-resistant super duplex steels for gas and oil production equipment has been developed. Thermodynamically stable, having sizes of 30-300 nm, niobium nitrides and carbonitrides are located inside the grains of the ferritic phase. At the same time, the sigma phase and chromium carbide precipitates at the intergrain boundaries are not observed. The results of the determination of mechanical and corrosion properties in accordance with the NACE TM 0177 standard (method A), tests of corrosion witness-samples in field conditions demonstrate the advantages and prospects of using new super duplex steel for the manufacture of oil and gas production equipment operating in an environment with high H2S content and CO2 under significant mechanical loads, without the risk of brittle fracture.