Characterization of Anisotropy of Strength in API-X80 Line Pipe Welds Through Instrumented Indentation
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TECHNICAL ARTICLE
Characterization of Anisotropy of Strength in API‑X80 Line Pipe Welds Through Instrumented Indentation A. R. H. Midawi1,3 · N. Huda1 · C. H. M. Simha2 · A. P. Gerlich1 Received: 11 June 2020 / Revised: 30 September 2020 / Accepted: 13 October 2020 © ASM International 2020
Abstract In this work, an instrumented indentation technique with a nearly flat tip indenter was used to measure the yield strength of API-X80 line pipe weld. Using this technique, the yield strength can be estimated directly from the indentation load–displacement response. The yield strength of the weld metal was measured in the transverse (hoop) and longitudinal (long) directions of the weld, and the results indicated that the weld metal exhibits anisotropy in terms of the yield strength by as much as 100 MPa, which could lead to a non-conservative estimate when strength mismatch is considered. Conventional tensile testing obtained strength values to within 4.6% of the indentation measurements and confirmed the results. Hardness maps also show a 20 H V0.5 difference in the two directions of the weld, which supports anisotropy of yield strength. The microstructure analysis using SEM reveals a slight difference in the grain size, which partially explains the differences in properties. Keywords Pipeline Weld · X80 · Yield strength · Anisotropy · DIC · Weld metal microstructure
Introduction Isotropic behavior is usually assumed in line pipe specifications especially when the design is based on strain criteria. However, owing to the manufacturing processes, such as those manufactured by forming such as the UOE, this may not be the case [1]. In addition, most of the welding processes such as GMAW create residual stresses attributed to the thermal expansion and contractions, which may lead to anisotropic behavior in the weld metal. There are many requirements to be met when producing line pipe steels including homogeneity of the microstructure, mechanical properties, and weldability [2]. The thermo-mechanical * A. R. H. Midawi [email protected] 1
Center for Advanced Materials Joining (CAMJ), Department of Mechanical and Mechatronic Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada
2
School of Engineering, University of Guelph, Guelph, ON N1G 2W1, Canada
3
Mechanical Engineering Department, Faculty of Engineering, University of Benghazi, Benghazi 21861, Libya
controlled process (TMCP) along with the pipe making procedure, such as the UOE process usually creates anisotropy in the line pipe steels due to the asymmetric deformation imposed. The longitudinal direction of the pipe, in most cases, has a lower yield strength compared to the transverse direction of the pipe, resulting from the strains induced during the manufacturing process [2, 3]. Characterizing heterogeneous microstructures like pipeline welds using conventional methods such as tensile testing becomes more difficult when a narrow groove design is used. It is impractical to machine standard full size specime
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