【作者】 张春国；

【导师】 吕彭民；

【作者基本信息】 长安大学 ， 机械设计及理论， 2013， 博士

【摘要】 随着科学技术的进步和焊接技术的进一步推广，人们对焊接零部件的制造精度、抗疲劳性能和使用寿命提出了越来越高的要求，钢结构中很多关键结构件都是通过焊接连接并且在疲劳载荷下服役。广泛使用的高强度低合金钢焊接接头的力学性能明显低于母材，成为整个焊接结构的“薄弱环节”，当焊接结构在疲劳载荷下服役时，接头部分常常出现过早疲劳失效。目前对传统焊接接头的改善研究已经进入了“瓶颈”阶段，这严重影响了高强度低合金钢性能发挥。延长接头的疲劳寿命、提高接头的安全性和可靠性，成为学术界和工程界特别关注的一个热点问题，这也是提高焊接结构抗疲劳性能和延长使用寿命的关键基础问题。本论文针对高强度低合金钢焊接接头经常出现力学性能（尤其是抗疲劳性能）下降严重的问题，提出在焊接金属和母材金属之间引入相对较软的缓冲层形成具有“软+硬”双金属焊缝的新型焊接接头以改善焊接接头的力学性能。将相对较软缓冲层焊接性好、韧性好、导热系数高及可降低焊接冷裂纹产生机率的优点与高硬度焊接金属强度高及抗疲劳性能好的优点结合起来；充分利用焊接过程中缓冲层与两侧金属合金元素扩散机理、后面焊道热循环对已焊接缓冲层的“热处理”原理。本文采用理论分析和实验研究相结合的方法，系统研究了焊接接头的金相组织、硬度分布、残余应力变形和分布、超载塑性变形和疲劳裂纹扩展性能。创新性提出的具有“软+硬”双金属焊缝的新型焊接接头、超载塑性变形分析新方法、残余应力变形分析新方法和“考虑载荷幅值影响的疲劳裂纹扩展速率模型”取得了良好的效果，主要的研究工作与创新成果如下：(1)建立具有“软+硬”双金属焊缝的新型焊接接头：针对高强度低合金钢焊接性差、接头不均匀性强和薄弱环节力学性能下降严重等技术难题，将焊接过程中“软金属”和“硬金属”两者优点有机结合，提出在焊接金属和母材金属之间引入相对较软的缓冲层形成具有“软+硬”双金属焊缝的新型焊接接头。解决了高强钢常规焊接接头中采用超强匹配或等强匹配时焊接材料的焊接性及韧性差、采用低强匹配时焊后焊缝金属强度无法保证等难题，同时提高了缓冲层与两侧金属粘着力、降低了冷裂纹产生机率，有效提高了焊接质量。常幅载荷疲劳试验结果表明，对于本文所制作的焊接接头试样，选用4mm的缓冲层取得了良好的效果。焊接接头试样最薄弱区域的疲劳裂纹扩展速率达到母材水平，而在最大载荷25kN应力水平下的疲劳裂纹扩展寿命比常规焊接接头在最大载荷24kN应力水平下的疲劳扩展寿命增加了101%。(2)超载疲劳裂纹扩展寿命及塑性变形分析新方法：超载后裂纹尖端残余压应力和应变硬化源于塑性变形量，通过定量测量试样厚度沿疲劳裂纹扩展路径上的变化，找出了超载塑性变形量与疲劳裂纹扩展速率之间的关系，结合试样的疲劳断裂机理，合理诠释了超载条件下疲劳裂纹扩展速率的变化规律。在超载过程中，具有“软+硬”双金属焊缝的新型焊接接头中高强度焊接金属增加了抵抗超载断裂的能力，降低了超载断裂区对裂纹扩展的加速作用；较厚低强度缓冲层增加了超载塑性变形量，增加了残余压应力和应变硬化对裂纹扩展的抑制作用，从而增加了接头试样的疲劳裂纹扩展寿命，效果显著。超载条件下疲劳试验结果表明，对于本文所制作的焊接接头试样，选用10mm的缓冲层取得了良好的效果；第一次超载（70kN）后疲劳裂纹从10mm扩展到17.5mm过程中，具有10mm缓冲层的焊接接头试样的疲劳寿命与常规焊接接头试样相比疲劳寿命提高了约6倍。(3)残余应力变形及硬度分析新方法：通过改变焊接件刚度和边界约束条件的方法测量了残余应力变形量，根据计算的残余应力释放量定性比较了不同焊接接头中焊接残余应力的大小及符号。提出了根据接头硬度分布曲线来判定接头试样中残余拉应力区位置和宽度的新方法。疲劳与硬度试验结果表明，焊接软化区与垂直焊缝沿试样长度方向上的残余拉应力区成明显的对应关系，两个区域发生的位置和宽度基本相同。该测试方法操作简单、成本低，更具工程实用价值。(4)提出适用于接头非均质区的疲劳裂纹扩展速率模型：利用焊接接头试样的疲劳试验数据，提出了适用于焊接接头试样非均质区、考虑应力幅影响的疲劳裂纹扩展速率模型，合理诠释了焊接接头中存在“最薄弱区”的固有现象。从疲劳断裂机理角度解释了裂纹扩展速率在接头“薄弱区域”突然增加的原因，获得了不同焊接接头在疲劳裂纹扩展过程中疲劳断裂机理的变化规律。疲劳试验数据表明：选择“考虑应力幅影响的疲劳裂纹扩展速率模型”作为观测焊接接头试样非均质区疲劳数据的连续模型，曲线能真实反映出裂纹扩展速率的变化规律，拟合误差较Paris曲线降低了一个数量级，由原来的1.0295×10-12降低到2.8964×10-13，相关系数大幅提高，从原来0.7470提高到0.9316。更多还原

【Abstract】 Along with the advance of science and technology, and with the further extension andapplication of welding technique, the demand for manufacturing accuracy, fatigue resistanceand service life of welding component is increasingly high. Many key-components of steelstructure are connected by welding procedure, and need to be working under the condition offatigue loading. The mechanical properties of the widely used high-strength low-alloy (HSLA)welded joints are obviously lower than those of the parent metal, thus it become the “weaklink” of the whole welded structure. The welded joints often generate premature fatiguefailure when welded structures serve under fatigue loading. The study on improvement of thetraditional welded joints has stepped into its “bottleneck” stage which has seriously affectedthe performances of HSLA steels. As a result, prolonging fatigue life and increasing safetyand reliability of the welded joints is a hot topic being discussed in the academic field andengineering field, and is also a fundamental key problem for increasing fatigue resistance andprolonging fatigue life of welded structures.Aiming at the technical problem that the mechanical properties of the welded joints oftendeteriorate seriously, this thesis proposed an alternative welded joint with a bi-metal seam(soft+hard) to improve the mechanical properties of the welded HSLA steels byincorporating a relative soft buffer layer (BL) between the parent metal and the weld metal.The advantages of the good weldability, toughness, thermal conductivity and cold-crackresistance of the relative soft BL were combined with the advantages of the high strength andgood fatigue resistance of the hard weld metal. In addition, the alloy element diffusionmechanism between the BL and the two adjacent metals (weld metal and parent metal) wasmade full use together with the “heat treatment” effect of subsequent weld thermal cycle onthe BL. The microstructure, hardness profile, residual stress-induced deformation, residualstress distribution, overloading-induced deformation and fatigue crack growth performance ofwelded joints were systematically studied by combing the theory analysis with the experimentstudy. This thesis innovatively proposed an alternatively welded joint with a bi-metal seam(soft+hard), new methods for analyzing overloading-induced deformation and residualstress-induced deformation, and fatigue crack growth rate model taking into account load amplitude variation, which achived good effects according to the experimental results. Themajor research and innovative conclusions are as follows:(1) An alternatively welded joint with a bi-metal seam (soft+hard): Aiming at thetechnical problems of poor weldability, heterogeneity and mechanical properties deterioration(weak link) of welded HSLA steel, this thesis proposed an alternative welded joint with abi-metal seam (soft+hard) by incorporating a relative soft BL between the parent metal andthe weld metal to combine the advantages of soft metal with the advantages of hard metal.The alternative welded joint solved the problems that the weldability and the toughness ofovermatching or equalmatching weld joints were poor, and the fatigue strength ofundermatching weld joint was low. This kind of welded joint increased the adhesive forcebetween the BL and the two adjacent metals and reduced the welding cold crack, thusimproved the welding quality. The experimental results under constant amplitude fatigueloading showed that the welded HSLA with a4mm BL gained good results as for the studiedwelded joints. The fatigue crack growth rate in weakest zone of welded joint specimen wasreached to that of the parent metal, and the fatigue life of the welded HSLA specimen with a4mm BL under maximum load of25kN increased by101%as compared with the traditioinalwelded joint specimen under maximum load of24kN.(2) An alternative method for analyzing plastic deformation and overloading fatiguelife: Because the compressive residual stress and strain hardening at the crack tip resultedfrom the plastic deformation, the relationship between the overload-induced plasticdeformation and the fatigue crack growth rate was proposed by testing the thickness variationalong the fatigue crack growth path of the specimens. Based on fatigue fracture mechanism,the fatigue crack growth rate variation under overloading was explained reasonably. Duringoverloading test, as for the alternatively welded joint with a bi-metal seam (soft+hard), theweld metal with high strength increased the fracture resistance, thus reduced the acceleratedeffect of overload-induced fracture zone on fatigue crack growth; the thick BL with lowstrength increased the overload-induced plastic deformation, thus increased the retardationeffect on the fatigue crack growth by increasing the compressive residual stress and strainhardening. The experimental results under overloading showed that the welded HSLA with a10mm BL gained good results as for the studied welded joints. The fatigue life of the welded HSLA specimen with a10mm BL after application of the1stoverload (70kN) increased byapproximately6times as compared with the traditioinal welded joint specimen as the fatiguecrack propagated form10mm to17.5mm.(3) An alternative method for analyzing hardness and residual stress-induceddeformation: The residual stress-induced deformation was measured by changing the stiffessand boundary conditions of the welded HSLA steel. The amplitude and nature of weldingresidual stress in different welded joints were compared according the partial release(accumulated value) of residual stress. A novel method for determining the location and itswidth of the tensile residual stress zone was proposed by using hardness profile of weldedjoint. The experimental results of fatigue and hardness showed that welding softening zonecorresponded to the tensile residual stress zone in the direction of vertical weld and alongspecimen length, and that the locations and the widths of the two zones were similar. Thetested method is of practical engineering value because of easy operatioin and low cost.(4) A novel fatigue crack growth rate mode suitable for heterogeneous zone ofwelded joint: A novel fatigue crack growth rate mode taking into account the stressamplitude effect was proposed according to the fatigue date resulted from the welded jointspecimens. The mode was suitable for heterogeneous zone of welded joint, and explained theinherent phenomenon that welded joint existed “weakest link” zone of fatigue performance.The sudden increase in the fatigue crack growth rate within the “weak link” zone wasexplained based on the fatigue fracture mechanism, and the fatigue fracture mechanismvariation during fatigue crack propagatin was gained for different welded joints. The fatiguedata showed that the curve could reflect the variation of the fatigue crack growth rate if thenovel fatigue mode taking into account the stress amplitude effect was chose to descripe thefatigue data resulted from heterogenous zone of welded joint, and that the fitting error wasdecreased one order of magnitude as compare with Paris mode from1.0295×10-12to2.8964×10-13, the correlation coefficient was increased from0.7470to0.9316.更多还原