Bauschinger effect in API X60 linepipe steels

Abstract

As part of a programme to evaluate changes in mechanical properties occurring during U-O-E pipe forming, laboratory tests were carried out to assess the Bauschinger effect in 12·7mm thick, controlled-rolled Nb–V API X60 steels. Assuming a stress-strain relationship of the form σ = Coεⁿ_a for the initial strain cycle, empirical relationships are established between C1 and n1 of the reverse cycle, with prestrain. A Bauschinger-effect factor is defined for linepipe steels, and a method is developed to calculate this factor using Co, no, and prestrain values. The Bauschinger effect was found to increase with increasing prestrain up to ∼ 0·03, and it was more prominent when the reverse-flow stress was measured at small offsets (0·005) in the transient region than at high strains, which tend to eliminate the effect. In the context of pipe testing, the Bauschinger effect resulting from the cold flattening-tensile testing operation is practically unaffected by the preceding strain cycle.