An in-depth investigation into the implications of alleviating banded microstructures on crack-tip opening displacement test of API 5LX60 linepipe steel plate

Abstract

The present study seeks to fill the research gap regarding the correlation between crack-tip opening displacement (CTOD) and banded microstructures of high-strength low-alloy steel. For this purpose, a distinct casting procedure and thermo-mechanical controlled process route were implemented to produce an API 5LX60 MS linepipe steel plate free of banded microstructures and suitable for sour environments. According to the results of optical microscopy, the volume fraction of quasi-polygonal ferrite increased by 59% and the average grain size of the microstructure declined, whereas the volume fraction and size of pearlite constitute decreased compared to the API 5LX60 M linepipe steel plate, which is suitable for sweet environments. As opposed to the API 5LX60 M steel plate, no evidence of banded microstructures was observed, and in the CTOD test, the fracture was ductile with a surface exhibiting a higher percentage of side grooves. Moreover, no secondary cracks or river patterns were observed in the secondary electron imaging of the API 5LX60 MS steel plate. The CTOD and KIC fracture toughness increased from 1.32 mm to 1.73 mm and from 54.01 MPa m0.5–58.19 MPa m0.5, respectively. Moreover, the force vs. notch-opening displacement curves of both API 5LX60 M and API 5LX60 MS steel plates showed stable crack growth, with API 5LX60 MS having a higher plastic deformation prior to the final fracture. The rational correlation between the microstructure and CTOD test results of API 5LX60 MS steel plate approved improvements in this plate.