TEXTURE CONTROL FOR OPTIMIZING PROPERTIES IN X70 PIPELINE STEELS
Date
2022-04-26
Authors
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Journal ISSN
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ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Most pipelines are often designed to serve in rigorous environments for several decades. Failure
in pipelines can lead to environmental catastrophe and substantial economic losses. The importance of various microstructural characteristics of pipeline has rarely been considered, and these characteristics decide where cracks nucleate, how they propagate, or whether they can be arrested or not. Steel manufacturers have explored several methods, such as using micro-alloying
elements, the morphology of non-metallic inclusions, and the level of residual stresses’ to improve the quality of steels used in acidic environments. However, experiments have shown that these
strategies are not sufficiently effective. This study explores the alteration of texture and grain boundary structures in the manufacturing process of pipeline steel as a potential method for
improving fracture resistance in pipelines.
Different schedules of thermomechanical control processing were applied to API 5L X70 pipeline
steel. Rolling schedules were carried out at high and warm temperatures. Also, a multi-stage rolling
was carried out to determine the effect of the different processing stages on the final microstructure
and texture. All steel samples were then examined for microstructure and texture using EBSD, SEM, EDS, and XRD systems. EBSD analysis show that high finish-stage processing temperature played a vital role in developing the microstructure and influenced the mechanical properties of the steel. Also, fast cooling rate and accelerated cooling allowed the formation of more bainite, which increased the tensile strength of the steel. However, the texture at high-temperature rolling was weak and comprised of cubes Goss, brass, S, copper, R cubes and {331}<1-10>. The intensity and volume fraction of the desired {111}||ND were very diffuse and small compared to other texture components. On the other hand, a warm rolling schedule at 700 ⁰C produced higher intensities and volume fractions of the desired γ-fibre texture as the majority of the recrystallised grains were oriented towards {111}||ND. It was found that the recrystallisation of grains in steels rolled at 700 ⁰C favoured the development of the γ-fibre texture. Inhomogeneity of texture was observed in the investigated samples. The volume fractions and intensities of γ-fibre texture were higher at the mid-thick sections compared to the top and quarter thickness sections. In comparison to the surface, most grains at the central thickness were seen to be oriented in the {111}||ND and {001}||ND. The differences in texture across material thickness were related to recrystallisation and deformation modes across the thickness. Texture
measurements after two-stage thermomechanical processing indicated that although the volume
fraction and intensity of γ-fibre texture after the finish stage were lowered, this offers a promising
opportunity to retain the desired crystallographic orientations. Lastly, the tensile stress-strain curve
showed that the finish-stage processing produced the steel with higher yield strength. Thus,
reinforcing the fact that the finish-stage processing is a viable means for controlling the mechanical
properties of the steel.
Description
Keywords
Thermomechanical processing, pipeline steel, EBSD, Texture, microstructure
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering