Strain Aging Effects in High Strength Line Pipe Materials

Abstract

Strain aging behavior can occur in almost all steels, including micro-alloyed steels used in high-strength pipelines. The direct effects of strain aging on mechanical properties can include increased hardness, yield strength and tensile strength, and reduced ductility and toughness. Strain aging may take place in processes where the pipe material experiences thermal cycles, such as coating, welding and in-service heating, and may occur with or without additional plastic strain. The changes of material mechanical properties could seriously challenge the design principles and methodologies, so that these aging effects need to be taken into account. This is especially important for pipelines expected to see deformation-controlled loading conditions. This is not only because the difference in strain aging effects between a weld and the parent material can easily change the strength overmatch condition of the weld, leading to unpredictable girth weld flaw tolerance, but also because the return of Lu¨ders behavior on the stress-strain curves of these materials significantly reduces the pipe buckling load resistance. In addition, any change in fracture resistance due to strain aging may impact the fracture control design practice, particularly if the pipe material may be expected to experience plastic deformation during service. In this paper, a brief review of strain aging behavior in steels is presented, with an emphasis on the effects on the mechanical properties and toughness of three high-strength line pipe steels. Material strain aging mechanical test procedures of three high grade pipes will be described and the test results will be discussed.

Innovative analysis of Luders band behaviour in X80 pipeline steel

“…During the coating process the pipe is usually heated to a certain temperature, which modifies the mechanical properties of pipeline steel in a number of ways: (1) higher yield strength, (2) higher ratio of yield stress to ultimate tensile stress and (3) occurrence of Luders bands in the tensile test [3,4]. This phenomenon is known as strain ageing, which has the similar mechanism with the Luders behaviour [5,6].…”

Research of Strain Aging in Pipeline Steel with a Ferrite/Martensite Dual‐Phase Microstructure

“…During the coating procedure, the temperature of the pipe can reach 200–250°C for several minutes after cold expanded diameter, which causes the strain aging phenomenon. The direct effects of strain aging on mechanical properties can include the increase of yield strength and yield ratio, and the reduction of ductility and toughness . The changes of mechanical properties could seriously influence the use of the pipelines, especially for the pipelines with strain‐based design, so that these strain aging effects need to be taken into account for insuring security and stability during use process of the pipelines.…”