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.     

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.     

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.     

Cleavage delamination in impact tested warm-rolled steel

Cleavage delaminations or separations have been observed on fracture surfaces of impact specimens made from rolled steel pipe and plate products. These delaminations appear as cleavage cracks lying parallel to the plane of rolling. Depending primarily on the test temperature and extent of rolling, four fracture modes have been reported: ductile, transgranular, ductile with cleavage delaminations, and transgranular with cleavage delaminations. A model is proposed which accounts semiquantitatively for rolling plane weakening with respect to test temperature, rolling strain, and rolling temperature. The four observed fracture modes and the manner of transition from one to another are delineated by the model.     

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.     

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.     

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.     

试样减薄对大壁厚X80级管线钢落锤性能的影响

摘要：采用金相显微镜和扫描电镜对30.8mm大壁厚X80管线钢不同减薄方式下DWTT试样截面微观组织和试验断口进行了分析，目的是研究减薄方式对大壁厚管线钢DWTT性能的影响。结果表明，随着试验温度的降低，单边减薄的DWTT性能优于双边减薄，而且双边减薄试样从表面至心部，贝氏体体积分数逐渐降低，多边形铁素体体积分数逐渐升高，晶粒尺寸变大，MA岛由细小的颗粒状变为尺寸较大的链状，且心部伴随着严重的偏析和夹杂。而单边减薄试样从表面至心部组织整体变化不大；裂纹优先沿着消耗能量最低的部位开裂，所以双边减薄试样心部出现比单边减薄试样严重的解理和逆解理断裂。     

X65管线钢落锤性能不合分析

针对唐山中厚板公司所生产20.8 mm厚X65管线钢板落锤试样性能不合现象,进行了金相组织及断口扫描电镜分析,发现不合试样的显微组织中针状铁素体含量较低,主要由于终冷温度高和冷却速率低造成,在后续的生产中优化了工艺,使产品落锤性能合格率得到明显提升。     

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.     

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。     

Anisotropy in Mechanical Properties and Fracture Behavior of an Oxide Dispersion Fe20Cr5Al Alloy

Anisotropy of fracture toughness and fracture behavior of Fe20Cr5Al oxide dispersion-strengthened alloy has been investigated by means of compression tests, hardness tests, and wedge splitting test. The results show a small effect of the compression direction on yield strength (YS) and strain hardening. The YS is minimum for longitudinal direction and maximum for the tangential direction. The transverse plastic strain ratio is similar for tangential and longitudinal directions but very different from that in normal direction. Hardness depends on the indentation plane; it is lower for any plane parallel to the L-T plane and of similar magnitude for the other orthogonal planes, i.e., the L-S and T-S planes. Macroscopically, two failure modes have been observed after wedge-splitting tests, those of LS and TS specimens in which fracture deviates along one or two branches normal to the notch plane, and those of LT, TL, SL, and ST specimens in which fracture propagates along the notch plane. Besides LT and TL specimens present delaminations parallel to L-T plane. Both, the fracture surface of branching cracks and that of the delaminations, show an intergranular brittle fracture appearance. It is proposed that the main cause of the delamination and crack branching is the alignment in the mesoscopic scale of the ultrafine grains structure which is enhanced by the 〈110〉-texture of the material and by the presence in the grain boundaries of both yttria dispersoids and impurity contaminations. An elastoplastic finite element analysis was performed to study what stress state is the cause of the branches and delaminations. It is concluded that the normal to the crack branches and/or the shear stress components could determine the crack bifurcation mechanism, whereas the delamination it seems that it is controlled by the magnitude of the stress component normal to the delamination plane.     

Further investigation of critical events in cleavage fracture of C-Mn base and weld steel

In this work, an investigation of the critical event in cleavage fracture of C-Mn base and weld steel was carried out. The fracture surfaces and the remaining cracks in the vicinities of notches and precracks in the double-notched specimens (DNBs) and precracked crack opening displacement (COD) test specimens unloaded prior to fracture were observed in detail. The results demonstrate that critical events are different in notched and precracked specimens even if they are made of identical materials. For the notched specimens with the radius of notch roots from 0.075 to 0.45 mm, critical events in cleavage fracture are the propagation of the ferrite grain-sized microcracks (FCs) into the neighboring ferrite grains. However, for the precracked specimens fractured at -110 °C, critical events are the propagation of the second-phase particle-sized microcracks (SCs) into matrix ferrite grains. For the precracked specimens tested at-70 °C, critical events could be the propagation of either SCs or FCs depending on the blunted width of precrack prior to fracture. The cleavage fracture at -196 °C is controlled by the nucleation of the microcrack; there were no remaining cracks found in specimens unloaded prior to fracture. With a drop in the test temperature or decrease in the radius of notch root, the length of the critical crack decreases.     

微观组织对高速车轮钢解理断裂应力的影响

利用不同奥氏体化温度和冷却速率对碳质量分数为0.54%高速车轮钢进行热处理,得到具有不同晶粒尺寸和珠光体片间距微观组织的试样.在-120~20℃温度下对具有不同微观组织的拉伸试样和三点弯曲（3PB）缺口试样进行测试;采用二维平面应变有限元计算三点弯曲缺口试样缺口前的应力分布;利用扫描电镜对3PB试样断口进行观察并测量解理起裂源的位置;测定不同微观组织车轮钢试样的解理断裂应力.在扩展控制断裂机制下,微观组织对车轮钢的解理断裂应力具有明显影响,晶粒尺寸和珠光体片间距越小解理断裂应力越高.细化晶粒使未扩展微裂纹的特征长度减小,细化珠光体片间距有助于提高珠光体的有效表面能,从而使得解理断裂应力提高.      

Cleavage fracture in B2 aluminides

Cleavage fracture of B2 aluminide single crystals, including FeAl, NiAl and CoAl, has been investigated at temperatures below their ductile-brittle transition temperatures. Single-edge notched bend specimens oriented along specific crystalline directions were tested by 4-point bending. The fracture resistance was highly anisotropic because of the existence of a preferred cleavage plane in these B2 aluminide crystals. With a deep through notch NiAl and CoAl crystals that have high ordering energies generally cleave on {110} planes, while substoichiometric FeAl having a low ordering energy shows {100} cleavage as do most b.c.c. metals. In the case of NiAl, a transition fracture region, composed of fracture facets on {511} transient planes, appears at the initial cracking stage, followed by final cleavage on {110}. Different stoichiometric effects on the fracture toughness of B2 aluminides are observed when the A1 concentration is reduced. A general discussion on different mechanistic models has been used to explain the preferred cleavage planes in B2 structures. The intrinsic fracture toughness of an aluminide crystal can be determined by an ideal fracture test in which the cleavage plane is parallel to the notch plane and is normal to the applied stress. Because of geometric constraints an increased fracture resistance is obtained when the natural cleavage plane is not parallel to the notch plane, and the anisotropy of fracture toughness can be explained by a simple approach of resolved normal stress intensity.     

本钢厚规格X70管线钢热轧板卷的生产试验

介绍了厚规格X70管线钢热轧板卷的成分设计与生产试验情况。对2种不同成分设计的碳加铬加钼和钢的精轧控制、轧制温度以及碳、铬、钼含量对热轧板卷强度和DWTT撕裂面积等性能的影响进行了对比分析,结果表明:用铬合金化,降低钼含量和控制适量的碳含量生产的厚度20.6mm热轧板卷,具有优良强度和韧性,综合性能指标达到API X70标准要求。     

Dynamic Fracture of a Zr-based Bulk Metallic Glass

In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m^1/2, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage.     

铁素体和珠光体含量影响变形过程的原位研究

在扫描电镜下原位观察了两种钢的拉伸变形过程,两种钢分别为以铁素体为主、含少量珠光体的纯净高强钢和以珠光体为主、含少量先共析铁素体的车轮钢.纯净钢拉伸时,不论试样厚度满足平面应变与否,均以铁素体的滑移变形为主,并最终导致韧性开裂,裂纹连续扩展,少量的珠光体对整个变形断裂过程几乎没有影响;断口呈现韧窝状.对于车轮钢,当试样厚度很薄不满足平面应变条件时,尽管先共析铁素体很少,拉伸时,仍以先共析铁素体的变形为先导过程,并在先共析铁素体与珠光体的界面处优先开裂,成为不连续微裂纹,断口呈现韧窝和准解理两种混合特征;当试样厚度满足平面应变条件时,则以珠光体中渗碳体片层的脆性开裂为主,断口呈现准解理特征.