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PRCI PR-202-010
- Effect of Defect Size and Yield to Tensile Ratio on Plastic Deformation Capacity Pipeline Steels
- Report / Survey by Pipeline Research Council International, 01/10/1993
- Publisher: PRCI
$448.00$895.00
L51686e
University of Gent-Belgian Welding Institute
Need: 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.
Benefit: 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.
Results: The present work has enabled the following conclusions to be drawn :The round bar yield strength depends on specimen diameter and sampling location. Sub-surface tensile specimens gave higher yield strengths than plate mid-thickness specimens because of the hardness gradient in the plate through thickness direction. The higher yield strengths produced a corresponding difference in Y/T ratio. The plate mid-thickness round bar specimens gave lower Y/T ratios than rectangular full section specimens. Sub-surface round bar specimens gave higher Y/T ratios. The yield criterion used (0,2% or 0,5% offset) greatly affects the Y/T ratio. The 0,5% offset yield strength which is the value normally used, was found to be less sensitive to specimen cross sectional area. Each of the steels investigated was susceptible to strain aging when loaded to three percent (3 %) straining and aged at ambient temperature for 6 months. Steel having an acicular-ferrite (AF) microstructure (or a continuous stress-strain behaviour) were found to age more rapidly than a ferrite-pearle (polygonal ferrite) (PF) microstructure having a discontinous stress-strain behaviour. The strain aging produced a substantial increase of Y/T ratio. The ratios increased from about 0,90 to 0,95 (PF microstructure) and, from about 0,86 to 0,99 (AF microstruc-ture). The notched wide plate test results showed that the yielding behaviour, strain capacity and defect tolerance are closely related to Y/T ratio. Increasing Y/T ratios gave a reduction of the maximum through thickness defect length, agy , for Gross Section Yielding (GSY). Moreover, the results of this investigation have shown that a correlation agy and Y/T ratio does exist, and that this correlation can be used to define a maximum Y/T ratio for a given tolerable defect size for GSY.
University of Gent-Belgian Welding Institute
Need: 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.
Benefit: 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.
Results: The present work has enabled the following conclusions to be drawn :The round bar yield strength depends on specimen diameter and sampling location. Sub-surface tensile specimens gave higher yield strengths than plate mid-thickness specimens because of the hardness gradient in the plate through thickness direction. The higher yield strengths produced a corresponding difference in Y/T ratio. The plate mid-thickness round bar specimens gave lower Y/T ratios than rectangular full section specimens. Sub-surface round bar specimens gave higher Y/T ratios. The yield criterion used (0,2% or 0,5% offset) greatly affects the Y/T ratio. The 0,5% offset yield strength which is the value normally used, was found to be less sensitive to specimen cross sectional area. Each of the steels investigated was susceptible to strain aging when loaded to three percent (3 %) straining and aged at ambient temperature for 6 months. Steel having an acicular-ferrite (AF) microstructure (or a continuous stress-strain behaviour) were found to age more rapidly than a ferrite-pearle (polygonal ferrite) (PF) microstructure having a discontinous stress-strain behaviour. The strain aging produced a substantial increase of Y/T ratio. The ratios increased from about 0,90 to 0,95 (PF microstructure) and, from about 0,86 to 0,99 (AF microstruc-ture). The notched wide plate test results showed that the yielding behaviour, strain capacity and defect tolerance are closely related to Y/T ratio. Increasing Y/T ratios gave a reduction of the maximum through thickness defect length, agy , for Gross Section Yielding (GSY). Moreover, the results of this investigation have shown that a correlation agy and Y/T ratio does exist, and that this correlation can be used to define a maximum Y/T ratio for a given tolerable defect size for GSY.
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