Anisotropy of the Stress-Strain Curves for Line Pipe Steels

Abstract

To suppress the appearance of Lu¨ders strain and to decrease yield to tensile strength ratio in the L-direction (longitudinal direction), as well as the C-direction (circumferential direction), have been more important for strain-based design. In this study, conventional UOE and ERW pipes were examined in terms of tensile properties in both directions. In the case of UOE pipes, yield point was clearly observed on the stress-strain curve in the C-direction. However, stress-strain curves in the L-direction showed the round-house type. This difference became prominent after heat treatment for the anti-corrosion. Namely, clear Lu¨ders strain appeared in the C-direction at a lower aging temperature compared with that in the L-direction. On the other hand, contrasting results were obtained in the case for ERW pipes. Thus far, it’s been thought that the difference between UOE and ERW pipe was caused by the direction of final strain during the pipe forming process. There are also differences in the occurrence of Lu¨ders strain between each grade. A stress-strain curve maintained the round-house type in X100 grade pipe after the heat treatment at 240°C for five minutes; however, X70 grade pipe showed the stress-strain curve in the L-direction with Lu¨ders strain after the heat treatment at the same temperature.

Effects of Finish Cooling Temperature on Tensile Properties After Thermal Aging of Strain-Based API X60 Linepipe Steels

“…Nagai et al [3] reported that discontinuous yielding behaviors occur in the API X80 linepipe steels. In addition, the uniform elongation decreases and the strain hardening exponent increases after thermal aging.…”

“…A pre-strain of 2 pct was given on the large-scale tensile specimens to simulate the strain state of the piping process. [3] The tensile tests were performed on the large-scale tensile specimens at room temperature at a strain rate of 0.003 s À1 using an Instron machine (model; 5567, Instron, Canton, MA, USA) with a 100 kN capacity. Heat treatment at 473 K and 523 K (200°C and 250°C) for 5 minutes was performed on the pre-strained largescale tensile specimens to simulate the epoxy coating process.…”

“…In this process, the 90 deg change in the direction of tensile specimens well simulated the actual situation inside the linepipe steel plates during the piping processes. [3,20] Tensile tests were performed on small-scale tensile specimens at room temperature at a strain rate of 0.003 s À1 using an Instron machine (model; 5567, Instron, USA) with a 100 kN capacity. From the stress-strain curves, the tensile properties, such as the yield strength, ultimate tensile strength, uniform elongation, yield ratio, and continuous yielding or discontinuous yielding behavior, were measured.…”

“…The total examined area was approximately showing how to prepare the large-scale tensile specimens for the pre-strain test and the tensile specimens for the tensile test after thermal aging processes. [3] 5080 lm 2 (=4 ea. 9 41 lm 9 31 lm).…”

“…[1,2] In particular, the uniform elongation of the linepipe steels are reduced during the coating process to prevent corrosion after the piping processes. [3,4] The epoxy coating temperature range of linepipe steels is 473 K to 573 K (200°C to 300°C), at which the Cottrell atmosphere occurs. [5] In the Cottrell atmosphere, the decrease in uniform elongation is caused by the interactions between the carbon atoms in ferrite and dislocations, which results in yield point phenomena during the tensile tests.…”