Study of Bauschinger effect of acicular ferrite and polygonal ferrite through ex-situ interrupted bending tests in API X80 linepipe steels

Abstract

Linepipe steels complexly consisted of low-temperature transformation microstructures of bainitic ferrite, granular bainite, and acicular ferrite (AF) as well as polygonal ferrite (PF) which individually affect the Bauschinger effect occurring during the pipe-forming. In this study, microscopic analyses of electron back-scattered diffraction (EBSD) coupled with tension-compression and interrupted bending tests were performed for verification of the Bauschinger effect of AF and PF working as major microstructures in single-phase- and two-phase-rolled API X80 steels, respectively. With respect to microstructural effects on Bauschinger effect, the reduction in mobile dislocation density during the flattening was smaller in the AF than in the PF. However, the dislocation pile-up at low-angle substructures and high-angle grain boundaries was more frequently observed, thereby leading to the higher back stress and Bauschinger effect in the AF. Boundary kernel average misorientation (KAM) profile played a critical role in determining the Bauschinger effect because they were closely related with the back stress. Thus, the Bauschinger effect was higher in the single-phase-rolled steel than in the two-phase-rolled steel. The present ex-situ interrupted bending methods coupled with EBSD analyses are outstanding ones for the detailed explanation of Bauschinger effect and provide an important idea for the yield strength designs of linepipe steels.

Effects of finish rolling temperature and yield ratio on variations in yield strength after pipe-forming of API-X65 line-pipe steels

“…The grain size and total boundary density were measured from EBSD maps, and the results are also listed in Table 1. Here, the total boundary density is defined as the low-and high-angle boundary point number divided by the whole indexed point number 16,17 . In all the steels, the higher FRT causes the larger grain size and the lower boundary density as the fraction of coarse GB rises with increasing FRT.…”

“…11 a. This YD is useful for accurately simulating the outer wall via tensile-compressive-tensile cyclic test, instead of the previous YD definition method using the tensile-compressive cyclic test 16 , 17 . To simulate the inner wall, the specimen was compressive-tensile-tested, and then was tensile-tested for evaluating the yield strength.…”

The Evolution and Distribution of Microstructures in High-Energy Laser-Welded X100 Pipeline Steel

“…In order to obtain the texture type and distribution pattern of cubic crystal polycrystalline materials, the ODF (orientation distribution function) is used to measure and visualize the texture [34,35,36]. The texture index calculated by the ODF is an important parameter for describing the texture strength of polycrystalline metal materials [37,38,39].…”

Yield-strength prediction of flattened steel pipes by competing Bauschinger effect and strain hardening during pipe-forming

“…6(d)). This indicates that dislocations formed at low- and high-angle boundaries are mobile ones which can generate the back stress 28 . On the other hand, the PF deforms early in the low strain range, and then the yielding occurs, while most of dislocations are formed inside PF grains to induce a relatively low back stress 28 .…”

“…6(a,b). Detailed test and analysis methods are described in a previous study 28 . Line profiles of KAM value and misorientation obtained from a yellow-arrow line of Fig.…”