Effects of Microstructure and Texture on DWTT Properties for High Strength Line Pipe Steels

Abstract

The crack arrestability for high strength line pipe steels with tensile strength of 650 to 850 MPa was evaluated using precrack DWTT (pc-DWTT). Moreover, the effects of microstructure and texture on pc-DWTT energy were investigated. The pc-DWTT energy was remarkably affected by tensile strength. The pc-DWTT energy of ferrite and bainite/martensite dual phase steels was much higher than that of bainite single phase steels in comparison with the same tensile strength. The {100} plane is a cleavage plane in iron, so the brittle crack mainly propagates along the {100} plane. Bainte single phase steels indicated a high intensity of the {100} on the plane rotated 40° from the rolling plane with the axis of the rolling direction. On the other hand, ferrite and bainite/martensite dual phase steels indicated not only a high intensity of the {100} plane rotated 40° from the rolling plane, but also a high intensity of the {100} plane parallel to the rolling plane. Slant fracture could be easily formed by the high intensity of the {100} on the plane rotated 40° from the rolling plane if local brittle areas such as martensite and austenite constituent (M-A constituent), which became the initiation point of brittle fracture, existed. In contrast, separation tended to be formed by the high intensity of the {100} plane parallel to the rolling plane that was caused by the formation of ferrite and bainte/martensite dual phase microstructure. Thus, pc-DWTT energy and shear area were remarkably affected by microstructure and texture. Therefore, to control microstructure and texture is vay important for the improvement of pc-DWTT properties.

Short review: Potential impact of delamination cracks on fracture toughness of structural materials

“…secondary cracks parallel to the rolling plane, have been observed on high strength steels [1][2][3][4][5][6][7][8][9][10], aluminum-lithium alloys [11][12][13][14][15], Ti-stabilized stainless steels [16,17] and ultrafine-grained steels [18]. Different types of delamination cracks in combination with different orientations of the specimen direction had been identified [13,19].…”

“…It can be seen that delamination cracks opened the interface of elongated clusters of grains with different crystal orientation components, demonstrating that the delamination cracks spread along the boundaries of grains having high-angle grain boundary misorientation. Hara et al [3,8] made a relationship between the microstructure, texture and CTOA (crack tip opening angle) results. Table 4 shows the results of CTOA related to the phases present inside different steels and the texture.…”

Quantitative Investigation of Brittle Out-of-plane Fracture in X70 Pipeline Steel

“…Brittle out-of-plane cracks are commonly observed during impact tests along the transverse direction of high strength pipeline steels, and particularly for temperatures within the ductile to brittle transition range, Hara (2006). These fracture modes are delamination fracture and brittle tilted fracture (BTF), also called brittle slant fracture by Hara (2006).…”

“…These fracture modes are delamination fracture and brittle tilted fracture (BTF), also called brittle slant fracture by Hara (2006). Delamination fracture, which for these steels involves brittle (cleavage) cracking parallel to the rolling b a plane, has been investigated by many authors such as Shin (2009) and Joo (2012), but underlying mechanisms have only scarcely been investigated (e.g.…”

“…For a specimen tested along the rolling and the transverse direction, BTF and delamination interact with the main crack and thus affect the steel toughness. From Hara (2006), BTF reduces the shear percentage of the fracture surface after Battelle drop weight tear tests (BDWTTs). The present study aims at quantitatively investigating the anisotropy in resistance of cleavage fracture and its relevance to delamination issues, for a pipeline steel.…”