Mechanical Characterization of Artificially Aged Steels Used in Pipelines for Hydrocarbon Transport

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Abstract
The aging process affects the mechanical properties of steel used in oil and gas pipelines worldwide, causing them to decline the more they are used. The mechanical properties of pipelines that have been in service for more than 20 years are particularly affected. This paper presents a comparison of the mechanical properties of tensile strength, Charpy (CVN) values, hardness, and microstructure under isothermal heat treatment at 500 °C for API 5L grade B and X52 low-carbon steels at different times—0 h, 500 h, 1000 h, and 1500 h—simulating the natural aging process. It is important to note that the literature reviewed indicates that the corrosion rate of API 5L steels can be influenced by the aging condition of the material. The microstructure of the steels was observed using scanning electron microscopy (SEM). The mechanical properties of yield stress (σY), ultimate tensile stress (σUTS), Charpy energy, and hardness decrease as the aging time increases. Charpy specimens show a decrease in toughness because low-carbon steels become embrittled as the aging time increases. The microstructure of Gr B steel remained constant across all times, at approximately 75% ferrite and 25% pearlite, whereas X52 steel was 77% ferrite and 23% pearlite. Finally, the literature data further indicate that aging can influence the corrosion behavior of API 5L steels.

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