Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline steels (original) (raw)
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Correlation of microstructure and fracture properties of API X70 pipeline steels
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2005
Effects of microstructure on fracture toughness and transition temperature of high-toughness X70 pipeline steels were investigated in this study. Three types of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact tests and pressed notch drop-weight tear tests (DWTT) were conducted on the rolled steel specimens. The charpy impact test results indicated that the specimens rolled in the single-phase region of the steel containing a reduced amount of C and Mo had the highest upper shelf energy (USE) and the lowest energy transition temperature (ETT) because of the appropriate formation of acicular, quasipolygonal, or polygonal ferrite and the decreased fraction of martensite-austenite constituents. Most of the specimens rolled in the single-phase region also showed excellent DWTT properties as the percent shear area (pct SA) well exceeded 85 pct, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled in the two-phase region. Thus, overall fracture properties of the specimens rolled in the single-phase region were better than those of the specimens rolled in the two-phase region, considering both USE and pct SA.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2006
Correlation between Charpy V-notch (CVN) impact properties, drop-weight tear test (DWTT) properties, and crack-tip opening angles for stable crack propagation (CTOAsc) in high-toughness API X70 pipeline steels was investigated in this study. Two-specimen CTOA test (TSCT) was conducted on the rolled steel materials to measure the CTOAsc, and the test results were compared to the CVN and DWTT data to find correlations between them. The CVN total energy density showed an almost 1:1 linear correlation with the DWTT initiation energy density. The TSCT results indicated that the materials rolled in the single-phase region had the larger CTOAsc as well as the higher CVN and DWTT energy density than those rolled in the two-phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The CTOAsc had a better correlation with the DWTT propagation energy density or the CVN total energy density than the DWTT total energy density. In particular, the value of sin (2CTOAsc) reliably represented a linear proportional relation to the DWTT propagation energy density.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
In this study, various types of drop-weight tear test (DWTT) were conducted on a high-toughness line-pipe steel in order to analyze abnormal fracture appearance occurring in the region impacted by a hammer. A pressed notch or a chevron notch was introduced into DWTT specimens, some of which had a back slot designed to decrease the inverse fracture area. After the DWTT, percent shear area, area of inverse fracture surface, and hardness and work hardening exponent of the hammer-impacted region were measured. The results indicated that the shear area of the pressed-notch DWTT specimens was larger than that of chevron-notch DWTT specimens. In the hammer-impacted region of all the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and its fracture area was correlated well with hardness and work hardening exponent measured by an indentation test. The formation of this inverse cleavage fracture raised the fracture propagation transition temperature (FPTT) by decreasing the shear area measured in accordance of the API 5L3 specification, which was confirmed by relating to the Charpy impact test data of pre-strained steel specimens. Thus, it was desirable to exclude the inverse fracture formed in the hammer-impacted region when calculating percent shear area of DWTT because compressive pre-strain induced by work hardening did not exist in actual applications of line-pipe steels.
Effective grain size and charpy impact properties of high-toughness X70 pipeline steels
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2005
The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures, effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process.
2014
Brittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20°C and-196°C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed.
Quantitative Investigation of Brittle Out-of-plane Fracture in X70 Pipeline Steel
Procedia Materials Science, 2014
Brittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20°C and -196°C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed.
Fracture toughness analysis in transition temperature region of API X70 pipeline steels
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006
This study is concerned with the analysis of fracture toughness in the transition temperature region of API X70 pipeline steels according to ASTM E1921 standard test method. Two kinds of steels were fabricated by varying finish rolling temperatures, and their microstructures and mechanical properties were examined. Elastic-plastic cleavage fracture toughness, K Jc was determined by three-point bend tests of precracked Charpy V-notch specimens, and then the measured K Jc values were interpreted by the three-parameter Weibull distribution. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing critical J value. Effects of martensite-austenite constituent (MA) and effective grain size on fracture toughness in the transition region were analyzed from the standpoint of cleavage crack initiation and propagation. The higher volume fraction of MA led to the lower resistance to cleavage crack initiation, and the larger effective grain size led to the lower resistance to cleavage crack propagation. Thus, the steel rolled in the two-phase region had the higher fracture toughness in the transition region and the lower reference temperature and energy transition temperature than the steel rolled in the single-phase region because of the lower volume fraction of MA and the smaller effective grain size.
International Journal of Fracture, 2018
This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called "abnormal" slant fracture, here denoted as "brittle tilted fracture" (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called "potential cleavage facets"). It was quantitatively demonstrated that the occurrence
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2005
Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2007
In the present study, effects of notch shape and specimen thickness on drop-weight tear test (DWTT) properties of API X70 and X80 line-pipe steels fabricated by varying hot-rolling conditions were investigated. The DWTT was conducted on the rolled steels, and the results were discussed in comparison with the Charpy V-notch (CVN) impact test and crack-tip opening angle (CTOA) test data. The DWTT results indicated that the steels rolled in the singlephase region had the higher upper shelf energy (USE) than the steel rolled in the two-phase region, because their microstructures were composed of acicular ferrite (AF). The DWTT energy transition temperature (ETT) of the steel rolled in the two-phase region was the lowest, because it had a finer effective grain size and the lowest volume fraction of hard secondary phases. Chevron notch (CN) DWTT energy density did not increase much with increasing specimen thickness, whereas pressed notch (PN) DWTT energy density increased by 1 to 3 J/ mm 2 , because the total energy of the CN DWTT increased less than that of the PN DWTT due to the larger stress concentration at the CN. The ETT increased with increasing specimen thickness due to the increase in constraint state, and the ETT of the CN DWTT was slightly lower than that of PN DWTT. The measured CTOA showed better correlation with the DWTT propagation energy than with the CVN or DWTT total energy because it was related to the crack propagation speed. The value of sin (2CTOA) reliably showed a linearly proportional relation with the DWTT propagation energy density.