ABSTRACT
The increasing need to transport large quantities of oil and gas in aggressive environments has led to a demand for high strength and low alloy steels (HSLA) of API (American Petroleum Institute) grade. No doubt, the main failure method of these steels is hydrogen-induced cracking (HIC), so there is a need to look for metallurgical alternatives to reduce the susceptibility of these materials to this phenomenon and some of these solutions is the warm rolling and plasma nitriding. The present study aims on evaluating the influence of the warm rolling on the microstructure and formation of the nitrided layer in API 5L X70 steel. For this, samples of the steel in question were submitted to controlled warm rolling with 49.4% reduction with initial temperature 863 °C and final temperature 689 °C, posteriorly, conventional plasma nitriding and using cathodic cage in parameters were performed: 450 °C, with a gas flow of 75% H2 - 25% N2 and 25% H2 - 75% N2, pressure 2.5 Torr. For the characterization, optical microscopy, Vickers microhardness test and X-ray diffraction were performed. The obtained results show that the microstructure presented a lower banding, in relation of the material as received, through a higher distribution of perlite caused by the warm rolling. The nitriding formed a thin layer of compounds of superficial hardness of up to 670.0 HV (HV 0.05) formed by nitrides of type (Fe3N) and γ '(Fe4N). These characteristics are important in the fight against hydrogen-induced cracking, and suggest that the combination of these two techniques may be promising.
Keywords
API 5L X70 Steel; warm rolling; plasma nitriding; cathodic cage
