The flow response of modern thermo-mechanically processed structural steels, like those developed for transmission pipelines, reflects the effects of several strengthening mechanisms.  Among others, these include refined microstructural size, solid solution strengthening, and precipitation strengthening.  While aspects of their effects can be correlated with microstructural metrics, general first-principles models of the flow response based on these metrics can be badly scattered.  For this reason engineers in need of flow properties to support their numerical analysis during design, or regarding integrity management decisions, rely on parametric empirical stress-strain models.  For many decades engineers doing strength-based design have assessed the post-yield benefits of these strengthening mechanisms in terms of a flow-stress, or by reference to the yield to ratio for the steel.  With the advent of strain-based design motivated by concern for ground movement among others, there was a need to also consider the role of the strengthening mechanisms relative to the strain, using for example the strain hardening exponent, *n*.  Recognition that the burst-pressure of pipe also showed a dependence on *n* further motivated the need to correlate the seldom measured value of *n* to commonly available metrics like Y/T. 

This paper begins with a brief review of correlative approaches involving *n* and Y/T, and then transitions to consider standard practices to quantify *n*, and their evolution over time.  Next the results of about 200 tensile tests carried either to the UTS or beyond are introduced.  Results are presented for Grades A25 and A30, and Gr B, for steels removed in rehabilitation as well as those used during the calibration of ASMR B31E.  Likewise, X-Grade data that underlie the calibration of B31G are considered in reference to Grades X46 and X52.  Flow properties are also considered for vintage X52 through modern X52, with similar results considered for early production of Grades X60, X65, X70, X80 and X100 versus for these same grades in recent to current production.  These flow properties are trended relative to Y, T, and *n*, as well as in reference to the uniform strain corresponding to the UTS. 

A correlation is developed between Y/T and *n* that accurately tracks this comprehensive dataset, which shows that these empirical data diverge from existing correlations of n vs Y/T for the lower-strength grades.  Further, the results show that within a Grade the value of *n* is a rather strong function of the ratio of the actual yield stress (AYS) normalized by SMYS, with this dependence indicative of differences in the chemistry and processing used to achieve the Grade. 

The significant implications of Y/T and its dependence on the ratio AYS/SMYS are illustrated in regard to the predicted failure response of line pipes subject to increasing pressure.  These predictions are validated in reference to about 20 full-scale tests on pipe pups for which the corresponding full-range stress-strain response has been developed. 


The Effects of the Flow Response on the Failure Pressure of Line Pipe Steels

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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