Toughness and separation improvement of thick plate X70 pipeline steels

Abstract

The effects of microstructure, texture, and material constraint for four American Petroleum Institute (API) X70 grade steel plates of varying thickness (13.5, 15.5, 22, and 32 mm) on separations and the subsequent effects of separations on toughness and toughness anisotropy were evaluated. The effects of plate thickness and separations on impact behavior were also investigated. Tensile, Charpy V-notch (CVN), and drop-weight tear testing (DWTT) were used to evaluate the plates’ mechanical properties in three orientations: 0°, 45°, and 90° from the plate’s rolling direction. Microstructural characterization determined that the 13.5 and 15.5 mm plate microstructures are primarily fine polygonal ferrite, with the 13.5 mm plate containing some small colonies of pearlite, while the 22 and 32 mm plate microstructures are banded with non-uniformly distributed polygonal ferrite and a bainitic secondary constituent that increases in volume fraction along the plates’ centerline. Energy dispersive X-ray spectroscopy (EDS) results did not correlate the microstructural banding observed in the thicker plates to manganese segregation or chemical banding. The increased volume fraction of the bainitic secondary constituent along the thicker plates’ centerline was correlated to a reduction in strength along the centerline, but may improve toughness measured by CVN testing. Coarser ferrite grain sizes may significantly lower a plate’s toughness measured by both CVN and DWTT. All four plates exhibited mechanical property anisotropy, deleterious in the 45° orientation, which was primarily attributed to texture rather than microstructure. High volume fractions and intensity of the {113}<110> texture component apeared to promote anisotropy in strength. Toughness performance degredation was observed in the 90° orientation for some of the CVN absorbed energy results and was correlated to the increased presence of separations in this orientation. Separation index (SI) measurements were performed in attempt to quantify separation severity, but the SI measurement is dependent on plate thickness and could only be compared between different orientations or through-thickness locations for an individual plate. Separations formed in impact specimens tested mainly at transition temperatures. Microstructure could not be directly correlated to separation formation in all plates, but was found to affect the morphology of the separation crack. Separations in polygonal ferrite microstructures generally were spread across the plate’s through-thickness and had a well-defined crack tip, while separations in a microstructure with a high volume fraction of the bainitic secondary constituent at the plate centerline formed mainly along the centerline and then split into many thin cracks extending from the main separation crack tip. Texture components with low angles between the closest {001} cleavage plane and the normal plane of a texture component were found to promote separation formation in plates that had high intensities and volume fractions of these components, especially the {113}<110> component. Increased material constraint (i.e. thickness) was also correlated to separation formation at the centerline of the plates. The bainitic secondary constituent, high material constraint, and texture maxima were found to promote separation formation primarily at the centerline for the thicker plates. High intensities of the texture components that promote separation formation across the through-thickness were found to promote separation formation across the through-thickness of the 13.5 mm plate. Lower intensities of texture components that promote separation formation in the 15.5 mm plate were found to reduce separation formation. The 45° orientation was the least susceptible to separations and had the highest plastic strain ratio for all four plates. This correlation between resistance to thinning and reduction in separation formation in the 45° orientation suggests that separations are less likely to occur in materials that accommodate more strain in the width direction, because of the subsequent reduction in strain that would need to be accommodated in the through-thickness direction, which could lower the constraint in the through-thickness direction. Separation formation likely releases high through-thickness constraint in plates susceptible to thinning. High plastic strain ratios in the 45° orientation also correlate with increased ductile to brittle transition temperature (DBTT), since resistance to thinning may promote brittle fracture instead of ductile deformation and/or separation formation. Agreement between DWTT and CVN estimated toughness behavior only occurred for plates and orientations that exhibited lower toughness. Increasing plate thickness was found to affect the mechanics of the DWTT and promoted inverse fracture (IF) and work hardening in the DWTT hammer impacted region up to 40 mm into the specimen. Shear area (SA) rating methods designed to eliminate IF from the rating were tested and limitations were identified. Short and jagged instrumented DWTT force-time/displacement traces were correlated to specimens that exhibited an “arrowhead” separation pattern.