Low temperature toughness of acicular ferrite pipeline steel and redefinition of “effective grain size”

doi:10.13251/j.issn.0254-6051.2020.07.022

Abstract

Abstract: Toughness of the tested steel was investigated by series temperature charpy impact test and drop-weight tear test (DWTT),themisorientation angles of the grain boundaries of various microstructures along the cleavage crack propagation direction were statistized, and the angles between{100} cleavage planes on the cleavage crack propagation path were calculated in five rolling directions (0°, 30°, 45°, 60° and 90°).The effective grain size defined as grain with misorientation angle greater than 15° for high grade acicular ferritic pipeline steel was redefined as the size of cleavage units consisting of grains with {100}cleavage plane less than 35° between grain boundaries, and the quantitative influence of the redefined “effective grain size” on the low temperature toughness properties from Charpy impact test and DWTT were synthetically analyzed. The results show that the microstructure presents a typical acicular ferrite (AF) characteristic with somepolygonal ferrite (QF) and M-A islands, the distribution of high angel grain boundaries mainly distributes in the range of 45°-65° in different directions. The impact absorbed energy(ASTM A370-2017)and percent shear area of DWTT in the five directions increase with the refinement of the above redefined “effective grain size”, the ductile-brittle transition temperature also decreases with the refining of redefined “effective grain size”. The redefined effective grain boundaries can strongly hinder the fracture propagation, and thus the redefined “effective grain” can act as the effective microstructure unit for cleavage.

Key words: acicular ferrite pipeline steel, low temperature toughness, grain size, cleavage unit, crack propagation, misorientation angle

CLC Number:

TG142

Niu Yanlong, Liu Yansong, Liu Qingyou, Jia Shujun, Tong Shuai, Wang Bing. Low temperature toughness of acicular ferrite pipeline steel and redefinition of “effective grain size”[J]. Heat Treatment of Metals, 2020, 45(7): 101-110.

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