Correlation of crack-tip opening angle for stable crack propagation with charpy and drop-weight tear test properties in high-toughness API X70 pipeline steels

Correlation between Charpy V-notch (CVN) impact properties, drop-weight tear test (DWTT) properties, and crack-tip opening angles for stable crack propagation (CTOA_sc) 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 CTOA_sc, 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 CTOA_sc 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 CTOA_sc 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 (2CTOA_sc) reliably represented a linear proportional relation to the DWTT propagation energy density.     

Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe Steels

This study is concerned with effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties and fracture modes of API X70 and API X80 low-carbon microalloyed linepipe steels. Detailed fractographic analysis of broken DWTT specimens showed that the fracture initiated in an initial cleavage mode near the specimen notch and that some delaminations occurred at the center of the fracture surface. The chevron notch (CN) DWTT specimens had broader initial cleavage areas than the pressed notch (PN) DWTT specimens. The larger inverse fracture areas (i.e., cleavage areas close the hammer impact side) appeared in the PN DWTT specimens, because their higher fracture initiation energy at the notch allowed a higher strain hardening in the hammer-impacted region. The number and length of delaminations were larger in the CN DWTT specimens than in the PN DWTT specimens, and increased with increasing specimen thickness due to the plane strain condition effect. As the test temperature decreased, the tendency of delaminations increased, but delaminations were not found when the cleavage fracture prevailed at very low temperatures. The DWTT test results such as upper shelf energy (USE) and energy transition temperature (ETT) were discussed with relation to microstructures and fracture modes including initial cleavage fracture, ductile fracture, inverse fracture, and delaminations.     

Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline steels

The effects of microstructure on inverse fracture occurring in the hammer-impacted region were analyzed after conducting a drop-weight tear test (DWTT) on high-toughness pipeline steels. Three kinds of steels were fabricated by varying the alloying elements, and their microstructures were varied by the rolling conditions. The pressed-notch (PN) or chevron-notch (CN) DWTT and Charpy V-notch (CVN) impact tests were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from CVN tests of prestrained specimens. In the hammer-impacted region of the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and the inverse fracture area correlated well with the upper-shelf energy (USE) obtained from the CVN test and with the grain size. The steel specimens having a higher USE or having coarse polygonal ferrite tended to have a larger inverse fracture area than those having a lower USE or having fine acicular ferrite. This was because steels having a higher impact absorption energy required higher energy for fracture initiation and propagation during the DWTT. These results were confirmed by the CVN data of prestrained steel specimens.     

Correlation of microstructure and fracture properties of API X70 pipeline steels

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.     

Correlation of microstructure and fracture properties of API X70 pipeline steels

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. are jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology.     

Effective grain size and charpy impact properties of high-toughness X70 pipeline steels

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.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

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.     

Relationship of failure energy and amount of ductile component in a fracture during the DWTT for rolled plate prepared by controlled rolling

Typical features and the difference in specimen fractures are described for rolled plate DWTT specimens manufactured by controlled rolling technology with final deformation in the two-phase γ+α-region, or controlled rolling with final deformation in the γ-region and subsequent accelerated cooling. The question is considered of the effect of the amount of absorbed energy on specimen failure mechanism with dropweight testing. Results are provided for evaluation of the ratio of the amount of ductile component in a fracture and total specimen failure energy during the DWTT for rolled plate of strength class Kh70.     

Evaluating the Dynamic Toughness Properties of Pipeline Steels

Abstract

The dynamic toughness properties of three plates of the same grade of a high strength low alloy pipeline steel were evaluated by impact testing DWTT and CVN specimens sampled from the plates at 45° to the rolling direction. Impact testing was performed on either a conventional pendulum-type Charpy test machine or a Dynatup 8000 instrumented Drop- Weight Test Machine, at temperatures down to - 80°C. The shear fracture mode was found to be dependent on the type of specimen tested, and the ratio of crack initiation energy to the crack propagation energy expended in breaking a standard DWTT specimen appeared to provide a more definite indication of the onset of brittle fracture than either the shear area or total energy by itself.

Résumé

Les propriétés de ténacité dynamique de trois plaques du même type d'acier microallié ont été examinées d'aprés l'essai par choc d'échantillons DWTT et CVN prélevés it 45° de la direction du laminage. Les essais ont été réalisés soit sur une machine Charpy conventionnelle soit sur un mouton vertical instrumenté par le système Dynatup 8000, ceci à des températures atteignant – 80°C. Le mode de rupture par cisaillement dépend du type d'échantillons, et le rapport de l'énergie d'initiation des fissures et de l'énergie de propagation dépensées pour briser un échantillon standard DWTT semble donner un meilleur indice de l'apparition de la rupture fragile que l'aire cisaillée ou l'energie totale.     

Effects of Dynamic Strain Hardening Exponent on Abnormal Cleavage Fracture Occurring During Drop Weight Tear Test of API X70 and X80 Linepipe Steels

In this study, drop weight tear tests (DWTT) were conducted on API X70 and X80 linepipe steels fabricated with various compositions and rolling and cooling conditions in order to correlate the strain hardening with the abnormal cleavage fracture occurring in the hammer-impacted area. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, Charpy impact energy, and strain hardening. All the steels consisted of fine acicular ferrite, together with some bainitic ferrite, granular bainite, and martensite-austenite constituent. As the volume fraction of acicular ferrite increased, the area fraction of DWTT abnormal cleavage fracture decreased because the toughness of acicular ferrite was higher than other microstructures. The area fraction of abnormal cleavage fracture was weakly related with strain hardening exponents obtained from the quasi-static tensile and compressive tests, but showed better correlation with those obtained from the dynamic compressive test. This tendency could be more clearly observed when steels having similar Charpy impact energy levels were grouped. Since the DWTT was performed under a dynamic loading condition, thus, the abnormal cleavage fracture behavior should be related with the strain hardening analyzed under a dynamic loading condition.     

X70管线钢微观组织结构对落锤性能的影响

结合12～33mm厚X70管线钢落锤试验结果,利用光学显微镜和扫描电镜研究和分析不同厚度落锤试样的组织演变规律及组织对落锤性能的影响。结果表明：随着钢板厚度的增加,钢的组织由彼此交织在一起的针状铁素体、多边形铁素体/准多边形铁素体演变成粒状贝氏体＋少量针状铁素体/多边形铁素体,碳化物的析出数量和析出尺寸随之增加。具有交织在一起的非等轴状AF＋PF/QF混合组织的试样落锤性能优于以晶粒粗大粒状贝氏体为基体组织的试样的落锤性能。通过控制M/A岛形态和分布可以提高钢的落锤性能。     

Effects of Strain Rate and Test Temperature on Torsional Deformation Behavior of API X70 and X80 Linepipe Steels

This study aimed at investigating effects of strain rate and test temperature on deformation and fracture behavior of three API X70 and X80 linepipe steels fabricated by varying alloying elements and hot-rolling conditions. Quasi-static and dynamic torsional tests were conducted on these steels having different grain sizes and volume fractions of acicular ferrite and polygonal ferrite, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and adiabatic shear band formation. The dynamic torsional test results indicated that the steels rolled in the single-phase region had the higher maximum shear stress than the steel rolled in the two-phase region, because their microstructures were composed mainly of acicular ferrites. Particularly in the API X80 steel rolled in the single-phase region, increased dynamic torsional properties could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. The possibility of the adiabatic shear band formation at low temperatures was also analyzed by the energy required for void initiation and difference in effective grain size.     

Effect of the Small Reaustenized Grains on the Mechanical Properties of a HSLA Steel

Grain refinement improves not only strength but also toughness.Small reaustenized grains were discovered to form along the grain boundaries and lath boundaries in an as rolled high-strength low-alloy (HSLA) steel when heated to double phase region.The number and size of the small reaustenized grains were revealed to depend mainly on temperature in the range of 700 to 760℃.These small reaustenized grains were deduced to form by consuming precipitates.The mechanical property tests showed that they resulted in slight changes to tensile properties.However,the sample heated at 740℃ produced a peak Charpy V-notch (CVN) impact energy of 248J when tested at-30℃.Fractography showed that the facture surface of the 740℃ heated impact sample exhibited the smallest brittle zone away from the notch root and a large dimple zone lay ahead of the notch tip,which demonstrated the highest toughness.     

Impact properties of sintered and wrought 17–4 PH stainless steel

Abstract

Charpy V notch (CVN) impact testing was conducted on full size and subsize specimens of sintered and wrought 17–4 PH stainless steel (17–4 PH SS) in the as sintered and H900 heat treated conditions. Test geometries correspond to the American Society for Testing and Materials (ASTM) and Metal Powder Industries Federation (MPIF) impact testing standards. Merits of a notched specimen compared with an unnotched specimen were analysed for both the wrought and sintered materials. The notched ASTM standard bars had a lower coefficient of variance for impact energy than the unnotched MPIF standard bars and displayed greater toughness. Porosity and grain size have a detrimental synergistic effect on impact toughness for the sintered material. Following a discussion about the differences in the wrought and sintered microstructures, it is recommended that impact testing of the injection moulded and sintered specimens should be evaluated according to the ASTM test specifications.     

Effects of Microstructure on CVN Impact Toughness in Thermomechanically Processed High Strength Microalloyed Steel

Investigation on the correlation between microstructure and CVN impact toughness is of practical importance for the microstructure design of high strength microalloyed steels. In this work, three steels with characteristic microstructures were produced by cooling path control, i.e., steel A with granular bainite (GB), steel B with polygonal ferrite (PF) and martensite-austenite (M-A) constituent, and steel C with the mixture of bainitic ferrite (BF), acicular ferrite (AF), and M-A constituent. Under the same alloy composition and controlled rolling, similar ductile-to-brittle transition temperatures were obtained for the three steels. Steel A achieved the highest upper shelf energy (USE), while large variation of impact absorbed energy has been observed in the ductile-to-brittle transition region. With apparently large-sized PF and M-A constituent, steel B shows the lowest USE and delamination phenomenon in the ductile-to-brittle transition region. Steel C exhibits an extended upper shelf region, intermediate USE, and the fastest decrease of impact absorbed energy in the ductile-to-brittle transition region. The detailed CVN impact behavior is studied and then linked to the microstructural features.     

An Assessment of Vacuum‐Heat‐Treated H11 Hot‐Work Tool Steel using the KIc/HRc Ratio

Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. K_Ic, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K_Ic test values are not widely used in toughness specifications. This is surprising since to the designer K_Ic values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness K_Ic ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K_Ic/HRc).     

Microstructure Development for Brittle Fracture Control in Nb Microalloyed Line Pipe Steel

EBSD characterization of density and dispersion of high angle boundaries was carried out in niobium microalloyed steels of HTP base chemistry with 0.09 wt % Nb,which were thermo-mechanically processed under laboratory conditions.Similar studies were carried out in higher grade (X-100 and above) line pipe steels with different chemistries,which were processed under simulation of industrial rolling conditions.The twin objectives are (i) to understand the effect of chemistry and processing parameters on the density and dispersion of high angle boundaries,and (ii) to correlate the microstructure and density of high angle boundaries with strength and fracture properties.The present studies confirm that refinement of austenite grain size prior to pancaking,large strain accumulation in austenite conditioning,alloy design with high hardenability and high cooling rates are essential to control high density and uniformity of dispersion of high angle boundaries in the final microstructure in order to achieve high strength,toughness,low DBTT and consistently 100% ductile shear in DWTT in thermo-mechanically rolled higher grade line pipe steels.