Analysis and prevention of yield strength drop during spiral piping of two high-strength API-X70 steels
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It is notable that more deviation occurs as the volume fraction of cementite decreases. This is primarily attributed to the assumption that the pearlite morphology does not vary with the carbon content. However, as shown in Figure 1, the pearlite morphology does change with the carbon content; the cementite length in medium carbon steel is much shorter than that of eutectoid steel. Thus, the larger deviation between the measured and calculated values in medium carbon pearlitic steel would result from the morphological difference. To incorporate the morphological factor, it is worthwhile to consider the stress borne by each fiber. For a plate with the length l and the thickness t, the maximum stress imposed on the fiber is given by l sfr (max) 5 t t
[7]
where t is the shear stress. This means that sfr decreases with the decreasing aspect ratio (l/t) of cementite.[13] Since the yield stress of ferrite is far below that of cementite, the flow stress of pearlite is raised by the fiber strengthening effect in the early stages of deformation. In addition, the increase of flow stress by fiber loading in well-developed continuous pearlite would be higher than that in degenerate pearlite at the same amount of strain. This results in the difference in the work hardening rate. Therefore, it can be concluded that the work hardening rate of pearlite strongly depends on both the cementite volume fraction, which affects the dislocation density due to the variation of cementite/ferrite interfacial area, and the cementite morphology associated with the fiber loading effect. However, the ferrite thickness has little effect on the strain hardening behavior.
The authors are indebted to Dr. K.-T. Park for helpful discussions during the course of this work. METALLURGICAL AND MATERIALS TRANSACTIONS A
Analysis and Prevention of Yield Strength Drop during Spiral Piping of Two High-Strength API-X70 Steels CHEOL WOO CHOI, HYANG JIN KOH, and SUNGHAK LEE When strong carbide formers such as Nb, Ti, and V are added to high-strength low-alloy (HSLA) steels, they lower g → a transformation temperature by providing numerous ferrite nucleation sites, refine ferrite grains, and greatly improve strength by forming fine carbonitrides.[1,2] However, when line pipes are formed from HSLA steels, there are many occasions in which the final products do not meet the strength requirements due to a drop in yield strength. This is because fabrication of large-diameter pipes by the spiral forming process results in lowering the yield strength in the radial direction, which is 45 deg to the spiral forming direction. In API specifications,[3,4] strengths of pipes are evaluated by tensile tests after piping and flattening. During the processes of piping and flattening, the inner and outer walls of the pipe receive different strains, i.e., tension-compression strain on the outer wall and compression-tension strain on the inner wall. Due to this varying strain history, flatted pipes show lower yield strength than in the hot coil condition, which can be interpreted
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