Abstract
Corrosion is one of key factors limiting the service life of oil and gas pipelines. To increase corrosion resistance steels are alloyed with elements permitting to obtain an oxide film protecting the metal from dissolution. But one may expect that corrosion rate of conventional carbon and low-alloy steels can be partially controlled through changing the structure by means of heat treatment. This research is devoted to studying the role of this factor for pipeline steels 20A and 20FA taken as examples. The steels were subjected to 18 different heat treatment regimes including normalizing at various temperatures, full quench hardening followed with high-temperature tempering, and quenching from intercritical temperature range followed with high-temperature tempering. Mechanical properties of heat treated steels were evaluated, as well as uniform (general) corrosion rate in a model medium containing hydrogen sulphide, with the aim to reveal possible effect of structural changes on steels’ resistance to corrosion. The worst corrosion resistance was provided by regimes including intercritical quench hardening, apparently because redistribution of carbon and alloying elements between a and g phases during holding in the Ac 1 ... Ac 3 range intensifies corrosion. But in general corrosion resistance of steels oscillated irregularly, with no appreciable correlation with grain size, type of structure, tempering temperature and level of impact toughness. It is therefore clearly demonstrated that resistance of pipeline steels to corrosion in hydrogen sulphide media is mainly determined not by heat treatment regimes and structural characteristics, but by the chemistry of metal dissolution process (e.g., formation of protective films) and, possibly, by non-metallic inclusions.
Highlights
Коррозия и коррозионное растрескивание являются основными причинами ограниченного срока эксплуатации нефте- и газопроводных труб
One may expect that corrosion rate of conventional carbon and lowalloy steels can be partially controlled through changing the structure by means of heat treatment
Bulletin of the South Ural State University
Summary
1. Распределение скорости общей коррозии в зависимости от балла зерна для образцов из стали 20ФА. 2. Распределение скорости общей коррозии в зависимости от ударной вязкости для образцов из стали 20ФА. 3. Скорость общей коррозии образцов стали 20А после различных режимов термической обработки. В этом случае можно было бы ожидать наибольшую концентрационную неоднородность и наибольшую скорость коррозии, но последняя оказывается такой же, как и после отпуска при 670 °C. Результаты испытаний на стойкость против общей коррозии термообработанных по различным режимам образцов из стали 20А приведены в табл. 4. Из них видно, что для образцов из стали 20А также отсутствует взаимосвязь структуры и скорости коррозии. Наибольшая скорость наблюдается после закалки от 880 °C и отпуска при 670 °C, Результаты испытаний образцов из стали 20А
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