Characterization of Anisotropy of Strength in API-X80 Line Pipe Welds Through Instrumented Indentation

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 HV0.5 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.