The Effects of Plate Stress-Strain Behavior and Pipemaking Variables on the Yield Strength of Large-Diameter DSAW Line Pipe

Recent stringent specifications for large-diameter double-submerged-arc-welded gas-transmission line pipe include requirements that limit the range of allowable pipe yield strengths instead of the previous requirement of a minimum yield-strength value. These restricted pipe yield-strength ranges require control of the range of the yield strength in the plate used to make the pipe, knowledge of the relationship between the plate and pipe yield strength, and the effect of pipemaking (forming) variables on this relationship. The present study was conducted to determine the interrelationships among plate yield strength, plate stress-strain properties, pipe-forming variables, and pipe yield strength. In the first part of this work, pipe-forming strains were measured after each forming operating during actual pipe fabrication and the strains compared to the calculated values. The experimental and analytical values were in good agreement; thus, the cyclic straining of the original plate material during pipe forming was determined. In the second part of the program, specimens of typical line-pipe steels were cyclically loaded in the laboratory according to the cyclic histories that sections in the plate would experience when fabricated into pipe. The results showed a significant effect of the plate stress-strain behavior, as well as the amount of straining (or forming) on the resulting yield strength. Because of the complexities of all these interrelationships and the strain gradients developed through the pipe wall during pipemaking, a series of pipe were fabricated from steels having different plate stress-strain properties and the plate and pipe yield strengths were compared. Varying amounts of sinking (compressive straining) in the pipe O press and of pipe expansion were examined. Correlation of the plate and pipe yield strengths showed that, as a first order approximation, the pipe yield strength equated to the flow stress in the plate at a strain level equal to the total accumulated strain that occurs at the neutral axis of the pipe during the pipemaking operation. This approximation can only be made if work hardening occurs in the plate material at that total accumulated strain level. Otherwise, it can only be stated that the pipe yield strength will be less than that of the original steel plate.