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  Title Onset and susceptibility to separations in modern high strength line pipe steels
  Author(s) Dr Guillaume Michal, Dr Bradley Davis, Prof. Cheng Lu, C.Y.Huo and Prof. Frank Barbaro  
  Abstract THE PIPELINE INDUSTRY has long recognized that the Battelle Two Curve Method (BTCM) is unable to predict ductile fracture arrest in high strength pipe grades above API grade X65. Full-scale burst tests have demonstrated that significant correction factors, in terms of specified Charpy toughness, are required to ensure ductile fracture resistance. Fundamentally, the Charpy test is insufficient to define ductile fracture resistance in high strength pipe grades. It also appears that there is no arrest criterion based on standard mechanical property criterion able to predict a pipe performance under full-scale ductile fracture test conditions.

It has been observed that pipes exhibiting good resistance to ductile fracture develop larger plastic strains during propagation and so are believed to contribute to arrest via increased strain energy absorption. Parameters relevant to work hardening alone do not adequately quantify arrest capacity. Other factors such as microstructure, steel cleanness and texture are well known to affect strain distribution. As the alloy design and microstructure of modern high strength steels differ significantly from the lower strength steel grades used to develop the original BTCM for fracture control, it appears plausible that a metallurgical solution is required to determine the material resistance to ductile fracture.

The observation of fracture surface’s separations in modern high strength steels is common, particularly in severely controlled rolled high strength steels. Although the mechanism for the formation of separations is the subject of significant debate in the literature, it is known that separations can influence the through-thickness distribution of strain and so may be part of the metallurgical solution to improve prediction of ductile fracture arrest. Determination of susceptibility and factors controlling the onset of separations is an avenue proposed to understand the mechanical property response and the strain distribution associated with fast running ductile fracture.

The purpose of the study is the screening of metrics to identify indicators of pipe arrest performance and susceptibility to the formation of fracture surface separations. The dataset fundamentally relatives to arrest and propagation API grade X80 pipes from 3 independent full- scale burst tests. Through-wall thickness data on either side of the fracture faces is presented for one of these tests. The thickness is considered in light of its associated fracture velocity. Through thickness Charpy impact transition temperature curves, notched in the plane of the pipe wall, are obtained from a minimum of one arrest and one propagate pipe in each test. It is shown that through-thickness Charpy transition temperature and through-wall thinning can serve as indicators to pipe arrest performance.

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