L51686 Effect of Defect Size and Yield to Tensile Ratio on Plastic Deformation Capacity Pipeline Steels

Micro-alloyed low-carbon linepipe steels offer an advantageous combination of high toughness and a low carbon equivalent (CE or Pcm) for good weldability. The continuing improvements in pipeline steel manufacturing practices have also led to pipeline steels with higher yield to tensile (Y/T) ratios and a corresponding reduction in strain hardening capacity. Potential users of high Y/T ratio pipeline steels are somewhat reluctant to modify their existing specifications. This is because they do not have the required information to judge the performance characteristics of such steels under a wide range of service conditions. This is not surprising knowing that yielding behaviour, and defect tolerance in particular depends not only on toughness but also on the strain hardening capacity. Therefore, the interaction between toughness and strain hardening capacity (or Y/T) for a successful application of high Y/T ratio linepipe steels must be considered. For the pipe fabricator this means that not only the yield strength of the plate, used to make the pipe, must be carefully controlled, but also that the relationship between the mechanical properties of pipe and plate must be known.

This study examined the engineering significance of the yield-to-tensile (Y/T) ratio on yielding behavior and defect tolerance of 1-inch thick X70 steels in plate form. Stress-strain characteristics were measured by tensile testing of standard round-bar, full-section square, and wide-plate specimens. The strained condition was tested to determine the effect of cold forming on the Y/T ratio and yielding behavior. Finally, defect tolerance was determined by testing 8-inch wide notched-plate specimens.