管线钢落锤撕裂试验试样断口分离剪切面积的评定

结合生产实践,分析了管线钢落锤撕裂试验(DWTT)中出现的断口分离现象,对有争议的断口进行电镜扫描和定性分析,提出了分离断口的评定方法和基本原则以及断口评定的注意事项.     

高钢级低温管线钢的韧化机制及控制技术

针对高钢级管线钢在低温服役环境条件下韧性断裂止裂控制难题,分析了高钢级管线钢的低温断裂机理,明确了影响DWTT性能的两个重要因素。通过热模拟实验,讨论了提高DWTT性能的控制技术。采用粗轧再结晶区低温段大变形,增加粗轧阶段总累积变形量,及将精轧温度降低到临近Ar3的温度范围内,并施加足够的累积变形量,可获得一定数量的超细晶多边形铁素体,将板材心部的平均有效晶粒尺寸细化到2.5μm,使-60℃下DWTT断口剪切面积达到100%,为大口径高钢级管线钢的研发生产提供理论依据和技术参考。     

X70管线钢DWTT试样的分层裂纹及其断口评价

通过对管线钢不同温度的落锤撕裂试验和微观断口分析,研究了DWTT试样断口的分层裂纹及其对断裂的影响.结果表明,分层裂纹是受力变形时管线钢内部的薄弱界面受到三维应力作用的结果.分层裂纹出现于主裂纹起裂或加速之前,裂纹稳定扩展或减速时不会产生新的分层裂纹.分层裂纹的数量、张开程度和分层裂纹间距与主裂纹起裂或加速时的应力状态有关,而分层裂纹的长度与裂纹扩展时裂尖的应力状态有关.分层裂纹表面为解理形貌,解理面较大.韧脆转变温度以下的脆性断裂断口或韧脆转变温度附近混合型断口的脆性断裂区,不出现分层裂纹,仅在韧性断口或断口的韧性区出现宏观分层裂纹.研究表明,产生分层裂纹处的断口,分层裂纹无论是三角形分布或是其它形状分布,该区域均应评价为韧性.     

南钢HTP工艺试制高铌X80管线钢的组织性能研究

对南钢试制的低碳、高铌、不添加钼成分的高温轧制(HTP)的X80管线钢钢板的性能进行了分析,针对其显微组织特点进行了详细的观察和研究,并对DWTT断口形貌、夹杂物以及第二相21粒子的成分、尺寸及分布进行了分析,证实了采用HTP工艺技术生产的管线钢板能够完全达到X80的现行技术标准要求;而且低碳、高铌、不添加钼的成分可以大幅度降低生产成本,有助于提升管线钢的竞争力.     

试样减薄对大壁厚X80级管线钢落锤性能的影响

采用金相显微镜和扫描电镜对30.8 mm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。     

管线钢DWTT性能厚度效应的影响因素分析

研究了高级别管线钢DWTT性能厚度效应的影响因素,利用金相显微镜和扫描电镜,着重分析了中心偏析、带状组织、全厚度组织均匀性及夹杂物对DWTT低温止裂韧性的影响。     

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。     

HTP工艺试制高铌X80管线钢的组织性能研究

对南钢试制的低碳、高铌、不添加钼成分的高温轧制(HTP)的X80管线钢钢板的性能进行了分析,针对其显微组织特点进行了详细的观察和研究,并对DWTT断口形貌、夹杂物以及第二相粒子的成分、尺寸及分布进行了分析,证实了采用HTP工艺技术生产的管线钢板完全能够达到X80的现行技术标准要求。     

数据驱动的文献辅助管线钢产线落锤撕裂韧性内禀特征关联

管道运输是当前长距离输送石油、天然气等能源最经济的方式之一,具有优异的低温韧性是保证管线钢安全运输的重要特征.落锤撕裂试验（Drop weight tear testing,DWTT）是衡量管线钢低温韧性的最有效的方法.在目前的工作中,根据钢厂提供的产线数据集和文献收集的管线钢数据集,建立了基于机器学习的DWTT剪切面积预测模型.基于纯产线数据和文献数据辅助的产线数据构造了两种机器学习策略方案,测试了不同机器学习算法,效果最好的均是随机森林模型,策略一的纯产线数据模型的性能指标皮尔逊相关系数（PCC）为0.64,策略二的文献数据辅助的产线数据模型的性能指标皮尔逊相关系数（PCC）为0.92,文献数据的增加有效提高了DWTT剪切面积预测精度.机器学习技术为优化和预测DWTT剪切面积提供了一种新的思路.     

高强度管线钢落锤撕裂性能的试验

摘要：以X80管线钢为试验材料,采用金相显微镜和扫描电子显微镜,研究了显微组织、M/A、夹杂物对管线钢落锤撕裂性能的影响。结果表明,管线钢落锤撕裂性能（DWTT）随着多边形铁素体体积分数的增加而改善,当多边形铁素体的体积分数超过某一临界值(约28%)后,DWTT性能开始降低;组织中的M/A岛细小且弥散分布,其尺寸控制在纳米级别有利于改善DWTT性能;针对本研究试验钢,若其厚度方向上组织均匀、组织中铁素体体积分数为28%、M/A岛平均尺寸和比例分别为0.42μm和2.8%时,试验钢具有最好的落锤撕裂性能（韧性剪切面积率94.5%）。     

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.     

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.     

Normalization of the Cold Shortness of Plate Steel: II. Short-Brittleness Thresholds in Pipe Tests

The following serial curves are obtained for a pipe steel of strength class K65 (Kh80): strength and plasticity curves during tension in the temperature range from–80 to +20°C, fracture energy from drop weight tear test (DWTT) diagrams, and impact toughness KCV curves (from–180 to +20°C). The ductile–brittle transition temperature range determined from the DWTT energy is shown to be higher than that determined from a KCV diagram by 80 K. The KCV^–40 criterion is concluded to be more reliable than the DWTT^–20 criterion. KCV and DWTT tests are shown to reproduce the crack starting conditions in a pipe, and the resistance of a high-strength pipe steel to extended fracture should be estimated using other criteria.     

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.     

多边铁素体对X70管线钢DWTT性能影响分析

借助光学显微镜和扫描电子显微镜研究分析了多边铁素体对X70管线钢DWTT剪切面积百分比SA%的影响。分析认为：含有15%左右细小的多边铁素体在X70管线钢中可导致裂纹尖端产生较大的塑性变形,并使裂纹尖端的应力得到松弛,阻止裂纹扩展,从而提高剪切面积百分比。     

管线钢(X70)断口纤维率与显微组织关系的探讨

用扫描电镜和金相显微镜对X70钢DWTT断口纤维率差别较大的两类试样进行断口形貌和金相组织对比分析,结果表明:断口纤维率存在差别的本质原因是组织类型的差异.要提高X70钢的断口纤维率,应提高纤维组织中的针状铁素体数量,避免较大的成分偏析,并减少珠光体的数量.     

显微组织对管线钢(X70)DWTT性能的影响

通过对X70钢DWTT断口纤维率差别较大的两类试样进行断口形貌和组织对比分析,结果表明:断口纤维率存在差别的原因是带状组织、组织类型的差异。要提高X70钢的断口纤维率,需获得以针状铁素体为主的金相组织,提高组织的均匀性,避免成分偏析。