Microstructural Evaluation of API 5L X80 Pipeline Steels Submitted to Different Cooling Rates

Abstract

During the life cycle of an API steel there are occasions when it will be submitted to thermal cycle. For tubes produced by the UOE process, for the oil and gas industry, severe thermal cycles occur during the welding procedure and when the tube requires hot induction bending. It is therefore of interest to learn about the influence of different cooling rates on the microstructure of these steels, produced by TMCP (Thermomechanical Controlled Process), since these changes can be reflected in a variation in the mechanical properties and hence in the performance of the pipeline. In this work three steels of class API 5L X80, with additions of, Nb-Cr, Nb-Cr-Mo and Nb-Cr-Mo-V, and otherwise similar alloy content, produced by TMCP without accelerated cooling were investigated. These alloys were submitted to austenitization at 900oC for 1 hour followed by three different cooling rates of 1.5oC/s, 30oC/s, 115oC/s, in air, oil and water respectively. It is well known that the main strengthening mechanism in TMCP, which allows a simultaneous increase in strength and toughness, is the reduction of the grain size. Other mechanisms such as solid solution hardening, precipitation and increasing dislocation density also contribute. For the steels in this study, all having a similar base chemical composition and yield strength, all the different thermal cycles applied promoted a reduction in the grain size, and only for the higher cooling rate (115oC/s) were there significant observable phase transformations. The Nb-Cr-Mo-V system exhibited a higher percentage of martensite than the Nb-Cr and Nb-Cr-Mo systems.