Anisotropy of Charpy Properties in Linepipe Steels

Abstract

Pipes can be manufactured by welding spirals of hot–rolled linepipe steels. This process has a cost advantage relative to one in which the steel is seam welded after bending into a tubular shape. Even though properties such as the Charpy toughness and strength meet minimum specifications, the existence of anisotropy can compromise, for example, the stability of the pipe to buckling. In the case of spiral pipes, the least tough Charpy orientation also happens to coincide with the circumferential direction which experiences the largest stress.The aim of the work presented in this thesis was to examine the possible factors of the anisotropy of Charpy properties in the hot–rolled linepipe steels. Six alloys of grade API–X70 and X80 have been investigated. Regardless of their different chemical compositions and processing variables, all the steels show similar anisotropy features

the ductile–to–brittle transition temperature is highest for the Charpy specimens machined in a diagonal direction relative to the rolling direction.It is known that orientation–dependent properties in linepipe steels correlate with inclusions, microstructural anisotropy and unfavourable crystallographic texture. In the present study, the occurrence of delamination and preferential alignment of {100} ferrite cleavage planes are found to be key factors in determining the observed anisotropy in Charpy properties. Delamination is related to the presence of banding in hot–rolled steels, and the additional plasticity it entails during the process of fracture leads to an effective increase in toughness. As a consequence, the toughness is worst when the Charpy specimen is machined at 45° to the rolling direction because the extent of delamination at that orientation is minimal.The crystallographic texture also leads to a greater propensity of {100} ferrite planes parallel to the fracture surface for the 45° orientation, leading to a further decrease in toughness. Even though delamination was avoided since microstructural banding was eliminated after a quenching heat treatment, the crystallographic texture was retained due to the texture memory effect so that anisotropic mechanical properties resisted.This implies that the anisotropy in hot–rolled steel is the essence of the problem because the microstructural banding and the typical rolling and transformation textures, {112}<110> and {001}<110> of ferrite, cannot be avoided in hot–rolled steels.