X80管线钢焊接热影响区组织的模拟研究

采用物理模拟和数值模拟相结合的手段,利用SYSWELD有限元计算软件和Gleeble-3500热模拟试验机模拟焊接热过程,对X80管线钢焊接热影响区单道粗晶区、临界区以及经历2次热循环的亚临界粗晶区、临界粗晶区和过临界粗晶区的组织进行对比分析,归纳了不同冷却速度下组织形态、M-A组元、晶粒尺寸及第二相粒子的转变规律.     

X80管线钢焊接热影响区不同区域的组织性能

通过夏比冲击试验和组织观察,研究了X80管线钢焊接热影响区不同区域的韧性分布及其原因.结果表明,焊接峰值温度低于950℃时,韧性变化不大;当峰值温度达到1300℃时,X80管线钢所对应的粗晶热影响区韧性最低,形成了局部脆化区域.引起粗晶热影响区韧性降低的原因为晶粒的显著长大和粗大M-A岛状组织的形成.     

X80管线钢焊接热影响区组织和性能分析

采用焊接热模拟技术和金相显微组织分析技术,对首钢研制开发的X80热轧板卷在不同焊接热循环下的组织和力学性能变化规律进行了深入分析.结果表明,粗晶热影响区是X80管线钢焊接热影响区中冲击韧性较差的区域,存在严重脆化.粗晶热影响区脆化是由于晶粒的粗化以及粒状贝氏体、上贝氏体、M-A组元等非平衡中低温转变产物数量增多造成的,且其冲击韧性随着t8/5的增加而降低.细晶热影响区是X80管线钢焊接热影响区的软化区域,软化程度随着焊接热输入的增加而增加.     

回火焊道热处理对X80管线钢热影响区组织性能的影响

采用焊接热模拟技术和显微分析方法.研究了回火焊道热处理对X80级管线钢焊接热影响区韧性的影响规律.结果表明,预备回火焊道处理可消除临界粗晶热影响区局部脆化现象.当预备回火温度为1000℃时,X80管线钢临界粗晶热影响区的韧度高于粗晶区的韧度,消除了局部脆化现象:补充回火焊道处理也可消除临界粗晶热影响区的局部脆化现象.当补充回火温度为650℃时.韧化效果最好.     

不同冷却速度下X100管线钢HAZ组织性能分析

利用热模拟技术模拟X100管线钢的焊接热过程,通过用光学显微镜、扫描电镜、透射电镜及显微硬度计对其显微组织观察及硬度测试,分析了其不同冷却速度下焊接热影响区(HAZ)的组织特征.结果表明,当冷却速度在0.05～5℃/s范围内时,HAZ显微组织主要以粒状贝氏体及块状铁素体为主,M/A岛主要以粒状或薄膜状存在;5℃/s时板条状贝氏体开始形成,M-A组元块状变大;冷速在15℃/s时,板条马氏体开始产生,板条贝氏体仍存在,边界处还有上贝氏体和下贝氏体存在;冷速在30～50℃/s时,显微组织主要以板条马氏体为主,M/A组元变大.     

X80管线钢焊接热影响区的韧性分析

X80管线钢在焊接过程中,热影响区由于受到焊接过程热的作用,其组织和性能会发生较大的变化.韧性是天然气长输管线的重要性能,采用热模拟技术、现代工程测试手段和显微分析方法,分析了不同热输入参数下X80管线钢焊接热影响区粗晶区(CGHAZ)韧性(夏比冲击功和CTOD)的差异及其原因.在一定范围内,较高焊接热输入下CGHAZ的韧性比较低热输入下CGHAZ的韧性明显高,超过一定范围,随着热输入的增加韧性激剧下降.造成不同热输入下韧性差异的根本原因是由CGHAZ显微组织的差异引起的.较低的热输入下CGHAZ中产生了一定量的低碳马氏体,从而导致韧性较差.     

X80管线钢的焊接性分析

对国产X80管线钢进行了手工焊焊接性能实验,并对其焊接接头的强度、低温韧性和冷弯性能进行了检验,结果表明,该管线钢焊接性能良好。     

焊接热循环对X80管线钢粗晶区组织性能的影响

采用焊接热模拟试验和冲击试验，研究了X80管线钢焊接热循环对粗晶区组织性能的影响。结果表明，当焊接热输入为20kJ／cm时，其热影响区粗晶区可获得最佳的韧性水平，其原因是产生了多位向分布的针状铁素体，它可作为X80管线钢埋弧焊的推荐焊接规范。在较高的焊接热输入下，X80管线钢的韧性降低，此时已开始出现对韧性构成损害的铁素体和珠光体。     

微合金X80管线钢焊接热影响粗晶区韧性与显微组织

利用Gleeble-1500模拟实际焊接条件下三丝纵列焊接热循环过程,通过冲击试验、扫描电镜(SEM)、透射电镜(TEM)以及电子背散射衍射(EBSD)对微合金X80管线钢焊接热影响粗晶区(CGHAZ)的显微组织、马/奥组元(M/A)分布及形态、冲击韧性和室温组织粗化程度进行了研究。结果表明,随奥氏体稳定性元素含量的降低,CGHAZ平均晶粒尺寸无明显变化,但晶粒尺寸离散度增加;原奥氏体向贝氏体转变温度升高,晶界渗碳体含量增加,且粒状贝氏体的晶粒取向选择过于单一,大角度晶界(15°)密度显著降低;M/A组元由块状向长条状转变且数量明显减少。上述原因使微合金X80管线钢焊后热影响粗晶区冲击韧性离散性增加。     

碳化钒析出对X80管线钢焊接热影响区韧性的影响

通过Gleeble-3500热模拟机对钒(V)含量分别为0.06%和0.02%的X80管线钢进行了峰值温度为1350℃和750℃的焊接热模拟试验,利用硬度计和光学显微镜(OM)分别分析了焊接热模拟试样硬度和试样中马氏体-奥氏体岛状组织(M-A)的数量、尺寸和形貌。利用透射电镜(TEM)对母材、粗晶区和部分相变区中粒子析出进行了表征。结果表明,与0.02%含V量X80管线钢相比,0.06%含V量的X80管线钢由于V含量较高,在受到多道次焊接热循环影响时,部分相变区有较大量碳化钒(VC)粒子析出,明显阻碍位错运动,使其强度明显提高的同时脆性增大、塑性变形难以进行,导致其焊接热影响区韧性下降。     

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.     

不同冷却速率下X80管线钢组织性能的变化规律研究

对常规X80管线钢采用油冷、水冷、风冷等3种不同冷却速率进行处理,采用显微分析法和力学性能测试手段对处理后的材料组织和性能进行分析.结果表明:随着冷却速率的变化,试验钢的组织和性能呈现出一定的变化规律.当常规X80管线钢在采用油冷的冷却速率下,可获得贝氏体+铁素体的双相组织,从而使得常规X80管线钢在保持原有高强度、高韧性的同时,又具有较低的屈强比、较大的均匀伸长率和较高的形变强化指数,从而满足了大变形管线钢的性能要求.     

冷却速度对连续油管焊接接头热影响区组织及性能的影响

本文以国产CT80连续油管作为研究对象,采用特定的焊接工艺对国产CT80连续油管进行管-管对接焊,对比两种不同冷却速度下的国产CT80焊接接头热影响区的显微组织和显微硬度。实验结果表明粗晶区金相组织以长条状铁素体组织为主;在水冷条件下进行焊接时,焊接接头的晶粒尺寸较小,显微硬度值较低。     

Influence of Welding Thermal Cycle on Microstructure and Properties in Heat-affected Zone of X80 Pipeline Steel

Single welding thermal–cycles with different input linear energies (ILE)(15, 20, 30, 40, 50 kJ/cm) and peak temperatures (PT) (900, 1050, 1200, 1300, 1350 ℃) were simulated by MMS-300 to study the correlation of toughness and microstructure in heat-affected zone (HAZ) of a X80 pipeline. The evolution characteristics of microstructure were investigated by OM, SEM and EBSD. The results show that numerous polygonal ferrites and grain boundary ferrites appear, and the sizes apparently decrease as the heat input decreases. Heat input in single welding should be less than 35 kJ/cm to ensure well Charpy impact toughness. The toughness of course grain zone is the lowest when welding heating temperature is 1350 ℃ and it is the weakness part in welded zone. The uniformity of prior austenite grain is worsened as increasing the heat input. Moreover, the characteristics of M-A constituents and high angle grain boundaries (HAGB) are influenced by heat input and PT. In the case of low heat input and PT, higher density of HAGBs, dispersed and fine M-A constituents were observed. Otherwise, with high heat input (≥40 kJ/cm), the effective grain size is almost the diameter of prior austenite grain, and it will decrease the density of HAGB, moreover, coarse M-A constituents which are prone to crack initiation will be generated, thus, the impact toughness of the coarse grain zone will be worsened obviously in welding HAZ.     

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粗晶区是焊接接头的薄弱环节.通过对X80管线钢进行热模拟、金相显微镜和透射电镜分析后表明,粗晶区的组织主要为板条束贝氏体和粒状贝氏体;X80管线钢焊接热影响区粗晶区冲击韧性较差,存在严重脆化,粗晶区脆化是由于晶粒的粗化以及M-A组元数量增多造成的;随着t8/5的增加,粗晶区的冲击韧性和硬度随之降低;峰值温度越高,X80级管线钢的组织越粗大、韧性越低;中间临界区是焊接热影响区中另外一个韧性较薄弱的区域.     

X80管线钢粗晶区在不同焊接热输入下的韧性

采用热模拟技术、力学性能测试手段及显微分析方法,研究了三种X80管线钢焊接粗晶区在不同焊接热输入(E)下冲击韧性和断裂韧性的变化规律.结果表明,当E＞30kJ/cm时,由于粗晶区中多边形铁素体和珠光体的形成,致使其韧性严重恶化;在E＜15kJ/cm条件下,由于马氏体的过饱和固溶作用以及较高冷却速度所形成的高应力状态,使得粗晶区的韧性受到损害;而在E=15～20 kJ/cm条件下,试验钢焊接粗晶区的主要组织为多位向分布的针状铁素体,表现出较好的韧性值.     

Microstructure and Mechanical Properties of Intercritical Heat-affected Zone of X80 Pipeline Steel in Simulated In-Service Welding

The intercritical heat-affected zone(ICHAZ) of X80 pipeline steel was simulated by using the Gleeble-3500thermal/mechanical simulator according to the thermal cycle of in-service welding.The microstructures of ICHAZ with different cooling rates were examined,and the hardness,the toughness and corresponding fractography were investigated.Results show that untransformed bainite and ferrite as well as retransformed fine bainite and martensite–austenite(M–A)constituents constitute the microstructure of ICHAZ.The two different morphologies of M–A constituents are stringer and block.Second phase particles which mainly composed of Ti,Nb,C,Fe and Cu coarsened in ICHAZ.Compared with normal welding condition,the toughness of ICHAZ is poor when the cooling time is short under in-service welding condition because of the large area fraction and size of M–A constituents that connect into chains and distribute at the grain boundaries.The Vickers hardness of ICHAZ that decreases with the increase in the cooling time is independent with the area fraction of M–A constituents.     

Microstructure and Mechanical Properties of X80 Pipeline Steel Joints by Friction Stir Welding Under Various Cooling Conditions

X80 pipeline steel plates were friction stir welded(FSW) under air, water, liquid CO_2 + water, and liquid CO_2 cooling conditions, producing defect-free welds. The microstructural evolution and mechanical properties of these FSW joints were studied. Coarse granular bainite was observed in the nugget zone(NZ) under air cooling, and lath bainite and lath martensite increased signifi cantly as the cooling medium temperature reduced. In particular, under the liquid CO_2 cooling condition, a dual phase structure of lath martensite and fi ne ferrite appeared in the NZ. Compared to the case under air cooling, a strong shear texture was identifi ed in the NZs under other rapid cooling conditions, because the partial deformation at elevated temperature was retained through higher cooling rates. Under liquid CO_2 cooling, the highest transverse tensile strength and elongation of the joint reached 92% and 82% of those of the basal metal(BM), respectively, due to the weak tempering softening. A maximum impact energy of up to 93% of that of the BM was obtained in the NZ under liquid CO_2 cooling, which was attributed to the operation of the dual phase of lath martensite and fi ne ferrite.     

大线能量焊接用钢模拟热影响区的组织与性能

研究了一种低碳低合金600MPa级耐大线能量焊接用钢模拟大线能量输入焊接粗晶热影响区（简称CGHAZ）的微观组织及力学性能。结果表明，大线能量模拟CGHAZ具有优良的强韧性配合（σs≥490MPa，σb≥610MPa，-20℃时AKV≥47J）。模拟热循环过程中CGHAZ中的高温稳定的复合夹杂物促使针状铁素体的形成，针状铁素体的数量与输入线能量有关。奥氏体和铁素体晶粒大小及针状铁素体数量显著影响焊接粗晶热影响区的力学性能。