Bauschinger Effect in Microalloyed Steels: Part II. Influence of Work Softening on Strength Development During UOE Line-
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TRODUCTION
THE high-strength low-alloy (HSLA) steel large diameter pipe body’s mechanical properties are developed in two ways. The first is microstructure formation during plate rolling and its influence on the properties via grain refinement, solid solution and precipitation strengthening mechanisms, and work hardening. The second is the cold deformation schedule during pipe forming, which may result in a strength increase due to unidirectional loading (work hardening) or decrease due to reverse loading (the Bauschinger effect). The UOE forming process, used for production of large (>400 mm) diameter line pipes, includes several deformation stages: bending of a plate to U-shape, O-ing to form a circular cross section, and ‘‘killing’’ to close the plate edges before welding and expansion (E-stage) to form a final diameter after welding. The main feature of the UOE forming process, affecting the property distribution in the pipe cross section, is inequality of strain magnitude and deformation direction sequence for different areas of the plate cross section (Figure 1). The pipe segment opposite to the weld undergoes two full cycles of forward-reverse deformation and the plate edges (pipe segments close to the weld) one cycle of unidirectional and one of forward-reverse deformation. This may result in work softening from plate to pipe, ANDRII G. KOSTRYZHEV, Research Fellow, is with the School of Electronic, Electrical and Computer Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact e-mail: [email protected] MARTIN STRANGWOOD, Senior Lecturer, and CLAIRE L. DAVIS, Professor, are with the School of Metallurgy and Materials, University of Birmingham. Manuscript submitted December 28, 2009. Article published online May 25, 2011 3170—VOLUME 42A, OCTOBER 2011
especially in the segment opposite to the weld.[1,2] The amount of possible strength loss depends on the pipe geometry, microstructure, and initial mechanical properties of the plate material. If the pipe body mechanical properties are measured using test coupons cut from flattened segments, usually taken from the 180 deg position (opposite to the weld), additional deformation during flattening will occur and may result in the measured property value deviating from the real pipe value. This should be considered when assessing the pipe properties determined using the flattened specimen method. Recent studies of plate to pipe property changes showed a different response of longitudinal and transverse strength to the deformation applied (Table I). In the pipe longitudinal direction, the yield stress increases from plate to pipe due to work-hardening (YSpipe-YSpl, Table I). Flattening of samples prior to testing modifies this relation by an amount that depends on the steel grade (YSfp-YSpl, Table I). In the transverse orientation, a decrease in the yield stress from plate to pipe was observed for microalloyed steels (YSfp-YSpl, Table I). This can be explained by work softening during pipe forming and sample flattening prior to testing. To
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