管线钢焊接临界粗晶区局部脆化现象的研究

采用热模拟方法和现代测试技术研究了两种典型管线钢焊接热影响区的韧度变化规律。结果表明,管线钢在多道焊接过程中,当二次热循环的峰值温度处于（α γ）临界区时,热影响区（HAZ）韧度最低,表现为临界粗晶区（IRCGHAZ）局部脆化。引起IRCGHAZ局部脆化的主要原因,是在多道焊二次热循环的特定热过程中形成粗大、富碳的M－A组织和组织遗传。     

高钢级管线钢中M-A组元的研究

利用金相显微镜和电镜观察了高钢级管线钢中M-A组元的形貌,并计算分析了其尺寸、百分比及显 微结构。结果表明：该材料以20℃/S的冷却速度终冷至410℃的工艺下得到的M-A组元尺寸一般小于3μm,Lapera试剂腐蚀测量的百分比与利用SEM 金相照片所测量的结果有所差别,TEM衍射斑点分析得出M-A组元为多个取向的马氏体组合体。     

X70管线钢在役焊接局部脆化区的组织及精细结构

采用焊接热模拟试验研究了X70管线钢在役焊接粗晶区和临界粗晶区的显微组织及其精细结构,并和常规焊接进行了对比.结果表明,X70管线钢在役焊接粗晶区和临界粗晶区的组织在类别上与常规焊接相差不大,主要都是粒状贝氏体和贝氏体铁素体,不同的只是数量的多少和尺寸的大小.透射电镜下,在役焊接和常规焊接的粗晶区与临界粗晶区的主要形貌都是铁素体板条和分布在板条之间或板条基体上的岛状物,铁素体板条的亚结构都是高密度位错.在役焊接粗晶区和临界粗晶区的铁素体板条宽度都比常规焊接的要小,位错密度都比常规焊接的要高.     

X100管线钢粗晶区组织和性能的研究

焊接热影响区粗晶区是焊接接头中薄弱的区域。本文采用焊接热模拟技术、显微组织观察和力学性能测试研究了X100管线钢焊接粗晶区的组织和性能的变化规律,讨论了冷却时间、组织和性能之间的关系。研究表明,焊接粗晶区组织主要由粒状贝氏体组成,随着冷却时间t8/5的延长,晶粒变粗大,贝氏体化铁素体粗化,M-A组元体积分数增大,并由条状变成块状,晶界上M-A组元也变得粗大,导致冲击韧性下降,抗裂性变差,冲击断口形貌脆性断裂的比例越来越大,且硬度、屈服强度、抗拉强度逐渐减小。     

X80管线钢焊接粗晶区的组织和性能

采用热模拟技术研究了X80管线钢粗晶区组织和低温韧性的变化规律,讨论了冷却时间、组织和性能之间的关系。结果表明,焊接粗晶区主要由板条束状贝氏体和粒状贝氏体组成。当t_8/5在6~8s时,粒状贝氏体含量较多,其分割作用使板条贝氏体的方向性减弱,使板条更为细小;M-A组元呈块状,尺寸较小且均匀分布在基体上,冲击韧度最优越;当t_8/5增大或减小时,板条束状贝氏体的含量均增加,板条粗大,M-A组元在冷却速度较快时呈长条状,冷却速度较慢时呈块状且相对含量增加,尺寸增大,均使冲击韧度显著下降。     

X120管线钢焊接热影响区的模拟研究

采用焊接热模拟技术研究了焊接热循环对X120管线钢焊接热影响区的组织和性能的影响。结果表明,经过一次热循环峰值温度1300℃后,随着t8/5的增加,韧性显著下降,发生严重脆化;当t8/5不变,二次热循环峰值温度为600℃和1200℃时,韧性得到明显改善;当二次热循环峰值温度为800℃时,在原奥氏体晶界处形成明显的"链状"结构的网状组织,韧性有所下降;而当二次热循环峰值温度为1000℃时,局部脆化现象严重,这与冷却时获得含有粗大M-A岛状组织的粒状贝氏体有关。     

Q460耐候钢焊接粗晶热影响区中M-A组元及其对韧性的影响

利用模拟试验机对Q460耐候钢不同热输入条件下的焊接粗晶热影响区进行模拟,分析粗晶热影响区中显微组织及其中马氏体-奥氏体(M-A)组元的数量、形态、分布及大小对韧性的影响。结果表明,t8/5较小时,组织由板条贝氏体和粒状贝氏体构成,M-A组元尺寸较小;t8/5为100 s时,组织中板条贝氏体量逐渐减小,粒状贝氏体含量逐渐增多,M-A呈块状,尺寸变大,有效晶粒尺寸增加,韧性降低;当t8/5达到150~200 s时,即使组织粗大,M-A组元的量有所减少,出现部分残留奥氏体,韧性也会增加。     

二次焊接热循环中X100管线钢粗晶区组织性能研究

采用焊接热模拟技术、冲击试验和显微组织分析方法,对X100管线钢在二次焊接热循环不同峰值温度条件下获得的组织性能进行了系统的研究,着重讨论了临界粗晶区韧性及组织的变化规律.结果表明,在X100管线钢的双面焊或多道焊中,当二次热循环峰值温度处于Ac1～Ac3临界区时,韧性急剧降低,表现为临界粗晶区局部脆化.粗大晶粒和粗大、富碳M-A组元的形成是临界粗晶区局部脆化的主要原因.     

铝对大线能量焊接条件下HSLAA焊接热影响区粗晶区M-A组元及冲击韧性的影响

采用热模拟试验方法分析了HSLA在100 kJ/cm大线能量条件下添加不同铝元素(0.027％,0.038％,0.070％)对焊接热影响区粗晶区的M-A组元内部组织、含量的影响.通过透射电镜分析M-A组元的内部组织转变,发现铝元素可以促进M-A组元内部残余奥氏体的稳定性,减少M-A组元中马氏体的含量.M-A组元统计和低温冲击试验结果表明,添加铝元素能够有效减少焊接热影响区粗晶区中M-A组元的含量,含有较高铝元素的试样冲击吸收功明显提升且较为稳定.     

铝对大线能量焊接条件下HSLA焊接热影响区粗晶区M-A组元及冲击韧性的影响

采用热模拟试验方法分析了HSLA在100 kJ/cm大线能量条件下添加不同铝元素(0.027%,0.038%0.070%)对焊接热影响区粗晶区的M-A组元内部组织、含量的影响。通过透射电镜分析M-A组元的内部组织转变,发现铝元素可以促进M-A组元内部残余奥氏体的稳定性,减少M-A组元中马氏体的含量。M-A组元统计和低温冲击试验结果表明添加铝元素能够有效减少焊接热影响区粗晶区中M-A组元的含量,含有较高铝元素的试样冲击吸收功明显提升且较为稳定。     

X70管线钢焊接性分析

针对管道建设中日益广泛采用的X70管线钢，分析了其碳当量与冷、热裂纹敏感性问题，探讨了防止X70管线钢产生冷裂纹的工艺措施，计算了不同板厚X70管线钢的预热温度，为实际的管道建设提供了理论依据与工程指导。     

X70管线钢连续冷却过程中的相变行为

利用Gleeble-3500热模拟机研究了X70管线钢未变形和经双道次变形后连续冷却过程的相变行为,采用热膨胀法结合金相法建立了静态和动态连续冷却转变曲线,分析了冷却速度和变形参数对组织转变的影响规律。结果表明,热变形加速针状铁素体和多边形铁素体相变,使相变的开始温度和结束温度显著提高,CCT曲线明显向左上方移动。实际中为获得针状铁素体组织,需相应增加变形后的冷却速度。与同一冷速下未经变形的连续冷却转变的组织相比,热变形可以显著细化组织,使组织中的岛状物更加细小弥散,通过变形可以在更宽的冷速范围内获得针状铁素体。     

Microstructure transformation of X70 pipeline steel welding heat-affected zone

The continuous cooling transformation curve of heat-affected zone(HAZ) of X70 pipeline steel was measured by Gleeble-3500 thermal mechanical simulator,optical microscope(OM) and hardness analysis. The microstructure transformation rule at different cooling rates and solution behaviors of microalloy carbonitride during heating process of simulated specimens were investigated.When the cooling rate changes from 10 to 20 °Cás-1,microstructures at HAZ are identified as granular bainite,lathy bainite, and quasi-polygonal ferrite. This microstructure is featured with fine ferrite grains, martensite/austenite islands dispersed, high-density dislocations, and fine carbonitride particles, resulting in improving the strength and toughness of HAZ. With the cooling rate increasing to above 40 °Cás-1, the microstructure is predominantly coarse lathy bainite with clear primary austenite grain boundary. While the cooling rate decreases to below 1 °Cás-1, a fairly small amount of pearlite can be observed at the boundaries. The strength and toughness of HAZ are deteriorated because of coarse grains among these microstructures. Most of microalloy carbonitrides in HAZ could be dissolved in the matrix during heating process. A few of TiN particles existing as residues in the matrix can prevent austenite grain from growing, and then improve the strength and toughness of HAZ.     

Decreasing pitting susceptibility of passive films on X70 pipeline steel in NaCl solutions by illumination

The influence of UV illumination on passivity and pitting susceptibility on X70 pipoeline steel in a borate buffer （pH=8.4 ） solution containing NaCl is described. It is observed that illumination of the sample leads to a decrease in its pitting susceptibility as indicated by pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the applied potential during passivation. The electronic properties of the passive films on X70 steel were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicated that illumination during passivation led to modifications in the electronic properties of the passive films, mainly to a decrease of the bulk doping and an increase in the surface state density. The cause for the decrease in the pitting susceptibility is preliminary explained in terms of such modifications of the passive flm.     

Study on the microstructure and hardness of in-service welded joint of X70 pipeline steel

Hydrogen induced cracking （HIC） is one of the main problems of in-service welding onto active pipeline. Microstructure and hardness of welded joint have a vital effect on hydrogen induced cracking. The microstructure and hardness of welded joint of XTO pipeline steel were studied using simulation in-service welding device. The results show that the main microstructures of in-service welded seam are grain boundary ferrite , intracrystalline acicular ferrite , as well as small amount of widmanztatten structure. The main microstructures of coarse grain heat-affected zone （CGHAZ） are coarse granular bainite, lath ferrite and martensite. Metastable phases such as martensite and lath ferrite are found in CGHAZ because of the too quick cooling velocity a＇nd the hardness of the CGHAZ is high.     

Mn-Nb-Mo系X70级管线钢奥氏体再结晶规律的研究

研究了Mn-Nb-Mo系XT0级管线钢的再结晶规律，分析了变形温度、变形量等工艺参数对变形奥氏体再结晶百分数的影响，为现场控轧工艺的制定提供了理论依据。     

电化学充氢条件下X70管线钢及其焊缝的氢致开裂行为

采用电化学充氢的方法研究了X70管线钢在不同浓度硫酸溶液中的氢致开裂(HIC)行为．结果表明,增大充氢电流密度、延长充氢时间以及降低充氢溶液的pH值能够促进氢进入X70钢基体．微观观察表明,X70钢中的非金属夹杂物如氮化物和氧化物等对其氢致开裂行为有不同的影响,氮化物夹杂并不是充氢裂纹的必然形核位置,而Mg,Al,Ca等的氧化物是更为有害的氢致裂纹源．通过氢渗透实验测得室温下氢在X70钢中的有效扩散系数为3．34×10-9cm2／s．对XT0管线钢基体及焊缝试样电化学预充氢后拉伸,焊缝试样的拉伸塑性较差,各项塑性指标在充氢前、后均低于X70钢基体材料．     

Influence of cooling conditions on XT0 pipeline steel in-service welding HAZ

The chamber device was designed and set up to simulate the in-service welding. The results show ： the t8/5 , t8/3 and inner wall peak temperature Tp decrease with the cooling rate increases. The welding energy is carried off by flowing medium, the cooling rate increases, and many unbalanced microstructures such as granular bainite, martensite and M-A generate ; it worsens the properties of HAZ. Under air-cooling, the cooling rate is slow, the austenite grain grows obviously, the lath ferrite crosses the whole austenite, and it causes the hardness value is also big. The change of HAZ width is not obvious with the increase of cooling rate; and burn-through is not susceptible to the cooling rate. The quench microstructures increase and the hydrogen does not outflow from the HAZ easily when increase the cooling rate, so the susceptibility of hydrogen cracking increases.