【作者】 杨秀芝；

【导师】 余圣甫；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 高效焊接是现代焊接生产的发展趋势,然而低合金高强度钢对大热输入适应性差,普遍存在局部脆化现象,如何兼顾低合金高强度钢的高强韧性和焊接性,成为结构钢接头设计的关键因素。焊接热过程的准确分析和评定,是研究焊接接头组织转变、应力变形分析、焊接质量控制的基础。早期研究表明舰船用钢10Ni3CrMoV钢具有显著的热输入敏感性和脆化现象,本论文选用此钢为研究对象,采用数值模拟、物理模拟和焊接试验相结合的方法,对低合金钢大热输入下双丝焊热过程及其接头组织转变行为和韧化机理进行系统研究。主要研究成果如下：研制了多路焊接测温仪,研究了双丝焊接热循环规律及双丝焊热循环与熔池特征之间关系。在热输入为21.7-43.9KJ/cm的范围内,丝间距直接影响双丝焊热循环及其特征参数,双丝焊下800℃到500℃的冷却时间(t8/5)值大于前、后丝单独施焊时t8/5值之和(即t8/5双>t8/5前+t8/5后),调整丝间距可以控制热循环。用数值模拟方法,首次研究了双丝焊工艺参数与熔池存在形式、熔池尺寸以及峰值温度Tp和t8/5之间的关系。焊接熔池的大小和特征直接影响到接头尺寸和热影响区的宽窄,并最终影响到接头性能。研究表明,双丝焊工艺参数与两弧开始形成两独立熔池的临界位置存在对应关系：当前、后丝电流与电压分别为31A、37A与700V、500V时,焊速为1OOcm/min,丝间距为40mm时形成分离熔池,当焊速变为60cm/min,丝间距为60mm时开始形成分离熔池,即当焊接电流和电压不变时,随着焊速的增加,共熔池状态提前结束,两分离熔池提前到来；且前丝电弧形成的熔宽和熔深基本不随熔池特征变化,后丝电弧形成的熔宽和熔深都是在熔池即将分离的时候达到最大值。在丝间距一定,随着焊速增加,HAZ某一点的Tp逐渐降低；当E不断增加时,HAZ某一点的Tp值并不是直线上升,而是当前丝热输入低于后丝热输入(即E1<E2)出现一些下降点。在相同热输入下,离热源中心越远的点,其Tp越低但t8/5值却不断地增加；而双丝间距L的改变只影响某一点的Tp,几乎不影响该点的t8/5值；同一丝间距下,t8/5与E呈现显著的正向线性关系,且随着E的增加HAZ各点处的t8/5值越接近。用物理模拟方法,研究了双丝焊接热循环对10Ni3CrMoV钢接头组织转变行为的影响及接头脆化机理。表明大热输入下粗晶区脆化是由粗晶脆化、析出相脆化、组织遗传脆化引起。研究发现,粗晶区显微组织以大颗粒状贝氏体或板条贝氏体为主,TEM精细组织观察发现在粗大贝氏体板条内及板条间存在M-A组元,晶内存在高密度位错,晶界有碳化物析出钉扎位错,裂纹在这样的组织中极容易产生和扩展,引起粗晶区韧性恶化。在热输入为35.19KJ/cm,丝间距40mm时,粗晶区板条贝氏体间有残余奥氏体薄膜,宽度约为0.025-0.01nm,裂纹扩展受到阻碍使粗晶区韧性得以改善,是间距20mm时粗晶区韧性值的两倍。调整双丝焊总热输入、改变前后丝热输入比例和调整丝间距等,可以改变HAZ热循环以达到改善HAZ的组织和性能。对于10Ni3Cr-MoV钢来说,控制热输入在30KJ/cm-40KJ/cm范围,保证t8/3在80-350s之间,可以得到80%以上的贝氏体组织,同时通过调整丝间距L在40mm-100mm之间,控制二次热循环峰值温度Tp2(650℃≤Tp2≤980℃),以改善和细化粗晶区贝氏体组织。用焊接试验,研究了双丝埋弧焊工艺参数对10Ni3CrMoV钢焊缝成形及性能的影响,优化了双丝焊接工艺参数。在热输入为21.7KJ/cm-40KJ/cm,双丝串列方式且丝间距为20-100mm,焊速为60-100cm/min时,焊道成形和接头截面形貌良好且接头力学性能优良。丝间距是影响HAZ冲击韧性的主要因素之一,热输入为35.19KJ/cm时,10Ni3CrMoV钢对接接头-40℃的CTOD的特征值δ0.05和δi分别为0.185mm和0.250mm。用数值模拟方法,研究了丝间距对低合金钢焊接残余应力的影响。当焊接电流、电压和焊速一定,丝间距在0-100mm之间变化时,双丝焊最大焊接残余应力出现在焊缝中心和焊趾处。随着丝间距增大,焊缝中心残余应力值变化在482.70-633.428MPa范围内,且最大值没有超过母材的抗拉强度；焊趾处的焊接残余应力有减小趋势,残余应力值变化范围1298.21-775.27MPa,丝间距20mm以下残余应力已经超过母材的抗拉强度,因此在小间距共熔池焊接时应注意防止焊趾处开裂。更多还原

【Abstract】 High-effective welding is the development trend of modern welding production, however, high strength low alloy steel (HSLA) are unadaptable to high heat input. How to give consideration to the high Strength-Toughness and weldability of HSLA has become a key to design steel structure. Effect of welding thermal analysis and assessment of the accuracy are the foundation and prerequisite of metallurgical analysis, stress deformation analysis, welding process control and weld microstructure transformation.Early results showed that the ship with 1ONiCrMoV steel has significant correlation exists and heat input sensitivity. The subject take 1ONiCrMoV steel as research object, using combination methods of numerical simulation, physical simulation, welding manufacturability test, mathematical statistics to do a system research on the thermal effect of HSLA in double wire welding. The specific conclusions are as follows:Multi-channel welding colorimeter was developed. The laws of double-wire welding thermal and pool features of HSLA were studied. In the heat input for 21.7-43.9KJ/cm, the double-wire welding heat cycle and characteristic parameters were affected by wire spacing directly. The measured values of t8/5 were closely related to the distance between welding wires.Simulation program of twin-wire welding through the secondary development of ANSYS software is designed, Full use of basic operation of the array vector to provide new ideas for speeding up the calculation of simulation. Algorithm which is double-wire moving heat source to calculation loading was proposed. This algorithm can shorten the computing time as well as simplify the loading operation of moving heat source, mainly took an array of storage and related operations of vector as support. This calculation model can be easily extended to the calculation of multiple-wire welding.Analysis the numerical simulation results in weld temperature field of twin-wire welding, first study the rules among parameters of twin-wire welding, state of the molten pool, size of the molten pool, thermal cycling parameters such as peak temperature (Tp) and cooling times from 800℃to 500℃(t8/5):Firstly, there is a critical relationship between parameters of twin-wire welding and the position where the two arc of the twin-wire began to form two independent molten pool. With Current, voltage constant and the increase of welding speed, the common molten pool state is terminate early and the two separate pool is arrival early; the weld width and penetration that formed by the arc of first wire is maintained constant values; while the weld width and penetration that formed by the arc of second wire reach the maximum when the molten pool is going to be separated. Secondly, the tp gradually decreased with the increase of welding speed under the same distance between welding wires, and the impact of heat input (E) is much greater than the distance between two wires as to value of tp of the same point, while the tp value at the same point are not always rise with E increasing, however, some decline appeared when the heat input of first wire is lower than the heat input of second wire(E1<E2). The third, for points that far away from heat source center, their t8/5 is increasing continuously while tp is getting lower; The change of distance between welding wires only affect the tp and no affect on t8/5.The t8/5 showed a significant positive correlation to E under the same-distance between welding wires. The values of t8/5 at different points get closer with the increase of E.Analysis of mechanism and influencing factors of embrittlement on CGHAZ with the large heat input. The major microscopic reasons that result in the toughness of CGHAZ descending are austenite coarsening caused by high temperature, the increase of quantity and grain size of granular bainite, structure heredity, grain boundary effect and a large number of dislocation within intragranular and grain boundary. By using of physical simulation to enlarge and reproduction HAZ, Microstructure of coarse grain zone is be made up of granular bainite or lath bainite, with a large block of M-A island and high-density dislocation or grain boundary carbide precipitation distribution along the grain boundary and bainite lath, cracks in coarse grain area prone to cracks and expansion, which is the micro-mechanism of toughness deterioration in CGHAZ. In wire spacing 40mm, there are austenite thin sheet between lath bainite, crack hampered so as to improve the toughness of coarse grain area, that is twice of that in 20mm spacing. To 10Ni3CrMoV steel, by the control of heat input E at 30KJ/cm-40KJ/cm, ensure t8/3 at 80-350s, over 80% of bainite is get, at the same time by adjusting the wire spacing L (40mm≤L≤100mm), control of the second thermal cycle peak temperature Tp2 (650℃≤Tp2≤980℃), to improve and refine coarse grain zone bainite, so toughness is improved.The ways to improve toughness of the welding joints is explored. The post weld heat treatment (PWHT) can significantly improve the toughness of welded joints, but pay attention to controlling holding time to avoid carbide precipitate in thin section or precipitation along the grain boundary and avoid high temperature temper brittleness.With forming a good weld and joint cross-section morphology and excellent mechanical properties of joints, two-wire serial mode and the wire spacing of 20-100mm, welding speed 60-100cm/min is suitable for welding HSLA. The wire spacing is the more main factors to impact of HAZ impact toughness than welding speed, of which coarse grain zone embrittlement serious. The CTOD-40℃of 16mm steel of butt joints, with heat input 35.19KJ/cm, the values ofδ0.05 andδi are 0.185mm and 0.250mm.The results of stress field numerical simulation in twin-wire welding show that the maximum of welding residual stress of welded joint distribution along the weld toe and weld center when the distance between welding wires is range from 0-100mm. The welding residual stress of weld center is increases with the distance between welding wires increasing, while the maximum does not exceed the yield strength. Accordingly, when we formulate the final welding process, the main consideration is given to performance of welded joints and appearance of seam, unnecessary considering its impact on the residual stress of welding seam center with the changing of distance between welding wires.The welding residual stress of weld toe is decrease with the distance of welding wires increase, therefore we should pay attention to prevent cracking of weld toe when weld in common molten pool with small spacing.更多还原