Tempering microstructure and mechanical properties of pipeline steel X80

The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF)+granular bainite (GB), and partial ferrite laths in BF merge and broaden. The interior sub-lath boundary of some GB begins to disappear due to merging, the M/A constituent (a mixture of martensite plus retained austenite) in GB is orbicular. At the two tempering temperatures the tested X80 steel shows a certain degree of tempering stability. After being tempered at 650 ℃, the microstructure of X80 steel is GB+quasi-polygonal ferrite(QF), and the original BF laths have merged to form smaller GB crystal grains. The reason is that the steel shows better match of strength and toughness. After being tempered at 700 ℃ , the microstructure of X80 steel is composed mainly of QF, which can improve the plasticity but decline severely the yield strength of X80, and the M/A constituent assembles and grows up at the grain boundary of QF, resulting in excellent lower low-temperature toughness of X80.

Microstructure and Property of Coarse Grain HAZ X80 Pipeline Steel

The coarse grain HAZ microstructure and property of X80 pipeline steel with different carbon content was investigated. The weld thermal simulation test was carried out on Gleeble 1500 thermal mechanical test machine. The Charpy tests were completed at --20 ℃ for evaluating the toughness of coarse grain heat affected zone （CGHAZ）. The microstructure was examined by optical microscope （OM） and transmission electron microscopy （TEM）, and the austenite constituent was quantified by X-ray diffraction. The results showed that the ultra-low carbon can improve the toughness of CGHAZ by suppressing the formation of carbide, decreasing the martensite austenite （M-A） constituent and increasing the residual austenite in the M A.

Effect of cooling temperature on the microstructure and corrosion behavior of X80 pipeline steel

Dual-phase accelerated cooling(DPAC) was applied to X80 pipeline steel to obtain its microstructure with different amounts of bainite and ferrite. The microstructure, hardness, and polarization behaviors of the steel, cooled to different temperatures, were investigated. Results showed that, with decreasing cooling temperature, the amount of polygon ferrite(PF) increased while that of acicular ferrite(AF) decreased. The amount of bainite correspondingly decreased, except when cooled to 760°C. Moreover, the grain size of ferrite increased. The corrosion behaviors of different phases were distinct. Martensite/austenite(M/A) islands presented at the grain boundary of the PF phase caused small pits. Numerous micro-corrosion cells were formed in the AF and bainite phases, where micropores were prone to form. X80 pipeline steel cooled to 700°C had the best corrosion resistance in the simulated seawater. The decreased amount of the PF phase reduced the area of cathode, resulting in slight corrosion. About 40 vol% of the bainite phase provided strength while the PF phase provided adequate ductility to the X80 steel. It was concluded that the appropriate cooling temperature was 700°C for ideal corrosion resistance and mechanical properties.

Microstructure Characteristics and Mechanical Properties of X80 Pipeline Steels

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels was investigated using optical microscopy, scanning electron microscopy, etc. It is shown that the structure consists of polygonal ferrite （PF）, quasi-polygonal ferrite （QPF）, acicular ferrite （AF）, and granular bainitic ferrite （GF）. With increasing volume fraction of M-A islands （below 3%）, the yield strength increases. With increasing content of higher angle grain boundaries（HAGBs）, the yield strength, elongation, and DWTT properties at -15 ℃ increase, and the volume fraction of M-A islands reaches its highest point in the steel containing the most volume fraction of GF.

Effect of anodic dissolution and passivation on X80 steel in NaHCO_(3) solution

The effects of extreme stray current on the anodic dissolution and passivation of X80 steel in NaHCO3 solution were investigated using measurements of polarization curves and EIS,AFM and SEM techniques.Under the interference of anodic current(i=0~200 A/m2),main constituents of corrosion products of X80 steel were FeO(OH)and Fe3 O4.A double-layer film formed at i=00 A/m2,in which FeOOH was in outer and Fe3 O4 lied in inner.The formation mechanism of Fe3 O4 was confirmed and described by the electrochemical reaction in various regions on anodic potentiodynamic polarization curve.

Effects of Carbon on the CG HAZ Toughness and Transformation of X80 Pipeline Steel

X80 pipeline steel produced by TMCP has high strength and high toughness with ultrafine grain microstructure. The mi-crostructure coarsens and the toughness worsens at the coarse grained (CG) HAZ apparently after weld simulation. The experimental results indicated that the bainitic ferrite and the second phases formed at cooling are differently as the variation of carbon in base metal. In low carbon steels, the bainitic ferrite laths are long and narrow, the second phases are complex including residual austenite, martensite, the M-A constituent and the Fe3C carbide. The formation of Fe3C carbide is the main reason of the poor toughness in CG HAZ. The ultralow carbon in base metal, however, can improve the CG HAZ toughness through restraining the formation of carbides, decreasing the M-A constituent, increasing the residual austenite content, which are beneficial to the CG HAZ toughness.

Microstructures and Plane Energy Spectra of X80 Pipeline Steel Welded Joints by Submerged Arc Automatic Welding

X80 pipeline steel was welded with submerged arc automatic welding, the microstructures, cavity sizes, fusion depths and plane scanning of chemical elements in the welded zone, fusion zone, heat affected zone and base steel were observed with OM（optical microscope） and SEM（scanning electron microscope）, respectively. The experimental results show that there is main acicular ferrite in the base steel and welded zone, the microscopic structure of fusion zone is a blocked bainite, and the heat affected zone is composed of multilateral ferrite and pearlite. M-A unit of the welded zone is the main factor to strengthen the welded zone, composed of acicular ferrites. The percentage of cavities in the welded joint is less than that in the base steel, which is beneficial to increasing its mechanical performance and corrosion resistance. The fusion depth in the fusion zone and welded zone is 101.13 μm and 115.85 μm, respectively, and the distribution of chemical elements in the welded zone is uniform, no enrichment phenomena.

Influence of different microstructural features on impact toughness and crack initiation behavior of coarse grain heat-affected zone in X80 pipeline steel

Coarse grain heat-affected zone samples of X80 pipeline steel under different heat inputs were obtained through thermal welding simulation experiments with Gleeble 3500.Charpy impact tests and a combination of multiscale characterizations were conducted to investigate the influence of various microstructural features on impact toughness and crack initiation behavior.The results prove that, as the heat input increases, the number of M/A components increases, thereby degrading toughness and increasing hardness.Meanwhile, more M/A constituents tend to aggregate on prior austenite grain boundaries(PAGBs),and the overall dimensions of M/A and the width and volume fraction of the lath martensite substructure inside M/A islands would increase as well.These changes make intersections between boundary M/As and PAGBs become one of the preferred sites for crack initiation.In addition, only large-sized grotesque inclusions can act as a direct inducement of crack initiation.

Analysis of inclusions and precipitates in tough weld metal of X80 steel

Submerged arc welding(SAW) and gas metal arc welding(GMAW) experiments of Nb-bearing X80 steel were conducted with high-toughness wires.The inclusions in weld metals were analyzed in terms of their types and sizes.In GMAW,the inclusions are primarily Ti, Ca, Si, Al, and Mg compounds with no Nb and are generally less than 0.8 μm in size, whereas, in SAW weld, the inclusions are larger, mostly approximately 2-5 μm in size, and are cored with Ca and Ti, exhibiting obvious oxidation metallurgical features.The SAW joint was hot-deformed, and Nb-bearing nano precipitates were newly found in the weld metal through transmission electron microscopy, and Nb-free core-shell inclusion was found through scanning electron microscopy.The inclusions and precipitates were dispersed in or on the boundaries of acicular ferrite, contributing to acicular ferrite nucleation and grain refinement.

Microstructure evolution and corrosion resistance in simulated CGHAZ of X80 high-deformability pipeline steel

In this study, the microstructure evolution and corrosion resistance in O. 5 M Na2CO3 - 1 M NaHCO3 solution of X80 high-deformability （ X8OHD ） pipeline steel coarse-grained heat-affected zone （CGHAZ） with several heat input levels were investigated. It is shown that the microstructure of CGHAZ changes from bainite ferrite to granular bainite as the heat input increasing. In addition, the corrosion resistance and the stability of passive film of base material are better than those of CGHAZ with several heat input levels. Too small or too big heat input is inadvisable and better corrosion resistance of CGHAZ is attained when heat input is 30 kJ/cm.

Experimental investigation on ablation characteristics of coated and uncoated steel under 30/80 μs impulse current

The ablation tests on coated and uncoated Q235 B steel sheets were conducted under 30/80 μs impulse current simulating the lightning first return stroke current, aimed at further understanding the ablation characteristics of steel and investigating the impact of anti-corrosion coating on these characteristics. Ablation characteristics were investigated through the macroscopic morphology and x-ray diffraction patterns on the surface of damaged zones, the microstructure and micro Vickers hardness on the cross-section of damaged zones, and the maximum rear-face temperature of sample sheets. It can be concluded that the ablation areas of uncoated sheet consist of the melted layer and the heat-affect layer. These ablation areas include not only the area ablated directly by the arc root, of which the depth is deeper, but also the area forming due to the splashing of molten steel, of which the depth is shallower and decreases when the area’s distance from the arc attachment point increases. For coated sheet, coating materials have decomposed and evaporated forming an ablation pit on the sheet surface, in which the steel surface is exposed, and zinc filler of coating primer has infused into the exposed surface. The ablation diameter of uncoated sheet relates to the amplitude of the 30/80 μs impulse current in quadratic function, while for coated sheet, the relation is linear. In general, under the 30/80 μs impulse current, the coating can decrease the energy injected from the arc to the steel sheet and reduce the melting and splashing of steel. As a result, the ablation severity of uncoated sheet is severer than that of coated sheet.

高Nb X80管线钢的屈强比在塑性变形过程中的变化规律

在Gleeble-3500热模拟实验机上,采用预拉伸方法,对高Nb、X80管线钢在JCOE大直径管线钢管成形过程中的塑性变形进行模拟,研究其屈强比的变化规律和机理。研究结果表明,受预拉伸塑性变形的影响,两种高Nb、X80管线钢的屈服强度和抗拉强度均得到不同程度的增加。但由于其屈服强度增加的梯度大于抗拉强度增加的梯度,从而导致材料的屈强比升高。在一次变形量超过8.2%,或一次变形量为6.2%并叠加2%二次变形的情况下,材料的屈强比将达到或超过管线钢管标准规定的0.95极限值。塑性变形过程中,由于位错密度增加而导致的加工硬化现象,是使屈服强度增加的主要原因;材料的初始屈强比及其组织均匀性,是影响变形过程中抗变形能力的主要因素。

低碳高铌X80管线钢焊接性及工程实践

低C高Nb高Mn的合金设计已广泛应用于X80管线钢,X80管线钢钢管的直缝焊接以及螺旋缝焊接性能对该合金体系的应用极其重要。热模拟研究结果表明,该合金体系X80管线钢的最佳热输入量应低于30 kJ/cm。相比于普通Nb含量管线钢,高Nb含量设计管线钢的粗晶区(CGHAZ)晶粒尺寸更细;临界粗晶区(ICCGHAZ)并没有粗大的析出物,Nb(CN)的尺寸≤30 nm。直缝焊及螺旋焊X80钢管的大量工业生产数据表明,高Nb X80管线钢热影响区的力学性能,特别是韧性能达到较高的水平,完全满足西气东输二线的技术指标,该合金体系的应用为西气东输二线的成功建成提供了有力保障。

高Nb X80管线钢析出物及析出行为研究

以高Nb X80管线钢为研究对象,分析了析出粒子的大小、形貌、成分及其析出行为。研究结果表明,高Nb X80管线钢中除了圆形的NbC和方形的TiN析出粒子外,还存在形貌不规则且尺寸较大的Nb的析出物,后者降低了钢的性能和Nb的利用。降低N含量并进行Ti/N比值的调整,可以提高钢中固溶Nb的含量,充分发挥高Nb的作用,才能生产出综合性能最优、成本控制较低的高Nb X80管线钢。

冷却速度和Nb含量对X80管线钢MA岛的影响

以高Nb X80管线钢为研究对象,利用金相显微镜、扫描电镜和透射电镜等实验设备对钢中MA岛的尺寸、形貌及分布特征进行了研究和分析.研究结果表明,高Nb X80管线钢热轧后,适当提高冷却速度可使钢中MA岛的形貌和分布更为细小和均匀,高的Nb含量促进了条状MA岛的生成.通过调整高Nb X80管线钢的合金成分和热轧后的冷却速度,均可减少MA岛对钢性能产生的不利影响.

HTP工艺生产X80管线钢的微观组织分析研究

以低碳高铌X80管线钢为研究对象,分析了钢的微观组织和析出的二相粒子。研究结果表明:高温轧制工艺(HTP)生产高铌钢的微观组织主要为针状铁素体,析出物大部分为附着在高热稳定性方形Ti N粒子上的圆形Nb(C,N)所形成的不规则复合析出。降低N含量并进行Ti/N比值调整能提高钢中固溶铌的含量,充分发挥高铌含量的作用。

高Nb成分X80级管线钢焊接性分析

通过高Nb+Cr、高Nb+Mo两种成分钢板的制管试验,从夏比冲击韧性、焊接接头硬度、焊缝化学成分等方面对两种成分钢板的焊接性进行了全面的对比。结果表明,Mo元素对焊缝和热影响区的性能改善起着重要作用,其细晶强化作用能有效提高焊缝的强度和韧性。在数据分析的基础上,提出了高Nb成分X80级管线钢焊接工艺的优化建议。

CO_(2) corrosion of X80 steel welded joints under micro-turbulence induced by welding reinforcement height

Corrosion failure accidents owing to flow erosion and pipeline corrosion frequently occur during transportation.The welding reinforcement height(WRH)can induce locally micro-turbulent flow field,which aggravates local corrosion of welded joints.A high wall shear stress(WsS)experimental setup was established to conduct the online electrochemical corrosion test.The influence of WRH sizes on local corrosion of welded joints was studied at different flow rates.The electrochemical signals of the local corrosion of X80 welded joints at different flow rates were monitored in real time using electrochemical impedance spectroscopy and wire beam microelectrode.In addition,the corrosion products composition and properties were analyzed.The results show that the micro-turbulent flow fields induced by the WRHs can enhance ion mass transfer near the welded joints.The corrosion products on the WRH surface also present different microscopic morphologies at different flow rates.In strong flow fields,the locally enhanced wsS can peel off the dense corrosion product partially,leading to the electrochemical distribution of large cathode and small anode,which accelerates the occurrence and development processes of the local corrosion of welded joints.The scientific guidelines for the corrosion protection of long-distance oil and gas pipelines can be potentially provided.

X80钢级φ1219×18.4mm螺旋焊管的研制与应用

X80钢级φ1219mm×18.4mm螺旋埋弧焊管、X80管线钢焊接材料(焊丝、焊剂)等新产品,是为满足西气东输二线工程的急需而开发的。该产品应用了低碳、高铌钢的合金体系,实现了18.4mm热轧卷板的大批量工业化生产。研制成功的X80钢级φ1219mm×18.4mm螺旋埋弧焊管,各项技术指标均满足Q/SYGJX0102-2007《西气东输二线工程用螺旋埋弧焊管技术条件》要求。目前,该产品已应用于西气东输二线管道工程。

X80管线钢管环焊缝二级安全评定研究

根据英国结构完整性评定方法BS 7910:2005标准,在给定裂纹尺寸、载荷条件下,针对国产高铌X80管线钢焊接接头焊趾处的表面裂纹、埋藏裂纹进行二级安全评定。根据母材、全焊缝拉伸试验和CTOD(裂纹尖端张开位移)试验结果,建立了高铌X80管线钢管环焊缝接头的二级评定曲线。结果显示,评定点均在评定曲线范围内,说明该结构是安全的。研究结果为国产高铌X80钢管的工程应用提供了依据。