In situ TEM study of the effect of M/A films at grain boundaries on crack propagation in an ultra-fine acicular ferrite pipeline steel

Zhong, Yong; Xiao, Furen; Zhang, Jingwu; Shan, Yiyin; Wang, Wei; Yang, Ke

doi:10.1016/j.actamat.2005.09.015

Cited by 165 publications

(73 citation statements)

References 17 publications

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“…At higher crack growth rate the sigmoidal curves tended to converge. These behaviors can be explained by the crack closure phenomenon 13,14,43,44 . Figure 6b presents the crack closure levels of the two steels as a function of ∆K.…”

Section: Resultsmentioning

confidence: 99%

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Microstructure and mechanical properties of two Api steels for iron ore pipelines

et al. 2014

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This research compares the mechanical behavior of two API steels (X60 and X70) used in the longest pipeline in the world for the conveyance of iron ore. Tensile tests, Charpy impact tests, CTOD tests and fatigue crack growth tests are performed at ambient temperature. Metallographic examination showed a banded microstructure consisting of polygonal ferrite and pearlite in both steels, with smaller grain size and the presence of a small quantity of bainite in the X70 steel. All the mechanical tests revealed a ductile behavior for the two steels. The X70 steel is preferable for the pipeline project, due its better mechanical resistance, with no significant loss of fracture toughness and fatigue resistance. Its performance could be even better, if an appropriate combination of thermomechanical processing parameters were able to produce a microstructure with minor amount of pearlite, where acicular ferrite/bainite are present.

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Section: Resultsmentioning

confidence: 99%

“…However, little information is available on fracture toughness and fatigue, mainly on near-threshold fatigue crack growth behavior 13,14 . Therefore, the present study was undertaken to evaluate the behavior of two commercial microalloyed API pipeline steels manufactured by Brazilian steel plants.…”

Section: Introductionmentioning

confidence: 99%

Microstructure and mechanical properties of two Api steels for iron ore pipelines

et al. 2014

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“…AF dominant microstructures have a greater potential to improve the strength [5], toughness [5,6], H 2 S resistance [7] and fatigue behaviour [8] compared to a ferrite-pearlite microstructure through grain refinement, increased dislocation density and easily controlled precipitation [1]. Furthermore, the toughness associated with an AF microstructure is considerably higher than that of BF, due to a higher density of high angle grain boundaries (HAGBs), which are capable of arresting cracks or deflecting their propagation directions [9].…”

Section: Introductionmentioning

confidence: 99%

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

2017

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For the class of steels collectively known as high strength low alloy (HSLA), an acicular ferrite (AF) microstructure produces an excellent combination of strength and toughness. The conditions for the occurrence of the AF transformation are, however, still unclear, especially the effects of austenite deformation and continuous cooling. In this research, a commercial HSLA steel was used and subjected to deformation via plane strain compression with strains ranging from 0 to 0.5 and continuous cooling at rates between 5 and 50°C s -1 . Based on the results obtained from optical microscopy, scanning electron microscopy and electron backscattering diffraction mapping, the introduction of intragranular nucleation sites and the suppression of bainitic ferrite (BF) laths lengthening were identified as the two key requirements for the occurrence of AF transformation. Austenite deformation is critical to meet these two conditions as it introduces a high density of dislocations that act as intragranular nucleation sites and deformation substructures, which suppress the lengthening of BF laths through the mechanism of mechanical stabilisation of austenite. However, the suppression effect of austenite deformation is only observed under relatively slow cooling rates or high transformation temperatures, i.e., conditions where the driving force for advancing the transformation interface is not sufficient to overcome the austenite deformation substructures.

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“…On the other hand, bainitic ferrite with M/A interlayers is known to be one of the most desirable microstructures due to its high strength and good toughness and can be obtained by optimization of process parameters, without the need for significant concentrations of alloying elements. Bainitic ferrite, formed by mixed diffusion and shear transformation mode has a high dislocation density [21][22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking

Hejazi

Haq

Yazdipour

et al. 2012

Materials Science and Engineering: A

117

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The influence of composition and microstructure on susceptibility to hydrogen induced cracking (HIC) was investigated in high strength pipeline steels, with Mn contents of 1.2% (standard, X70), and 0.5% (medium, MX70). The HIC resistance of the simulated coarse grained heat affected zone microstructures and normalized X70 transfer bar was also investigated. Notched and fatigue pre-cracked samples were charged with hydrogen prior to three point bend tests. The conditional fracture toughness JQ was determined. The results are discussed in relation to grain size, microstructure, composition and the type and distribution of non-metallic inclusions and precipitates. Published as "Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking" AbstractHydrogen, even in very low concentrations, can diffuse to regions of high stress concentration resulting in a degradation of mechanical properties. However, the transport of hydrogen depends on the interaction between hydrogen atoms and microstructural traps which, in turn, is related to microstructure. The influence of composition and microstructure on susceptibility to hydrogen embrittlement was investigated by selecting pipeline steels with different Mn contents, i.e., standard Mn (X70) and medium Mn (MX70) strips. The HAZ (heat affected zone) microstructures of these strips as well as a normlised transfer bar (TB) were also investigated. HAZ simulations were conducted using Gleeble thermo-mechanical machine to simulate a thermal cycle typical of inservice repairs. Notched and fatigue pre-cracked samples were subjected to electrochemical hydrogen charging using a solution of H 2 SO 4 and NaAsO 2 to achieve 2 and 4 ppm hydrogen content. Three point bend tests were conducted on as-received and hydrogen-charged samples. The X70 strip exhibited a higher J Q than the MX70 strip before and after charging, whereas the TB displayed the lowest J Q . The fracture performance of both steels deteriorated almost linearly with an increase in the hydrogen content. Because of their coarser prior austenite grain sizes and the presence of relatively hard bainitic-ferrite structure with martensite/austenite islands, the simulated HAZ microstructures of the two steels demonstrated significantly lower fracture resistance both with and without hydrogen. The results obtained were compared and discussed in relation to the grain size, microstructure, composition and the type and distribution of precipitates in the samples.

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In situ TEM study of the effect of M/A films at grain boundaries on crack propagation in an ultra-fine acicular ferrite pipeline steel

Cited by 165 publications

References 17 publications

Microstructure and mechanical properties of two Api steels for iron ore pipelines

Microstructure and mechanical properties of two Api steels for iron ore pipelines

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking

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