【作者】 张天会；

【导师】 徐人平；

【作者基本信息】 昆明理工大学 ， 机械设计及理论， 2012， 博士

【摘要】 焊接接头中不可避免地会存在各种形式的焊接缺陷,使焊接接头成为整个焊接结构中最薄弱部位,易产生裂纹起裂、扩展甚至失稳断裂。焊接接头的疲劳断裂性能是焊接结构损伤容限分析、保证结构安全可靠运行,以及抗断裂设计的重要依据。新型低碳贝氏体钢是采用微合金化控轧控冷工艺生产的新一代焊接无裂纹低合金高强度结构钢,被称为当今“绿色钢材”。合金结构钢的发展方向越来越注重提高其使用的可靠性和经济性,即在满足结构强度要求的前提下提高安全性和寿命。因此,对新一代低碳贝氏体钢焊接接头疲劳裂纹扩展可靠性研究具有重要的理论意义和应用价值。本文对新型低碳贝氏体焊接接头进行了常规力学性能及其微观金相试验研究、焊接缺陷及其尺寸分布研究、疲劳裂纹扩展试验及其可靠性分析研究。本文的主要工作有：从常规力学性能和金相试验角度对新型低碳贝氏体钢焊接接头焊接工艺进行试验研究。采用不同的焊接方法和不同的焊接工艺参数对新型焊接无裂纹低碳贝氏体钢B610CF、ADB610和WDB620钢焊接接头进行焊接,对焊接接头进行常规力学性能和金相试验。对试验结果进行分析发现：采用合适的焊接工艺参数,焊接接头中不容易产生裂纹,焊接接头的抗裂性能优良,可以得到具有优良综合机械性能的焊接接头；焊接工艺参数对焊接接头抗拉强度、塑性以及对热影响区的组织和常规力学性能的影响不明显,但大的焊接工艺参数会使焊缝区的组织发生变化,冲击韧性明显降低。对新型低碳贝氏体钢焊接接头焊接缺陷及尺寸概率分布模型进行研究。通过对焊接缺陷在焊缝长度区间上产生的缺陷数符合泊松分布过程、焊接缺陷在一定的焊缝长度内缺陷产生的比率为常数的物理背景,推导建立了焊接缺陷尺寸的概率模型为指数分布。随机抽取ADB610钢焊接接头X射线检测出的焊接缺陷,进行概率分布拟合检验研究,发现：焊接缺陷尺寸以服从指数分布为最佳,从实验数据上验证了焊接缺陷尺寸较好地服从指数分布。通过排列图的绘制与分析,ADB610钢和WDB620钢焊接接头绝大多数焊接缺陷是气孔和夹渣,焊接接头中产生的裂纹缺陷很少。提出了基于两步七点递增多项式的成组计算方法(本文简称为成组法)用于疲劳裂纹扩展试验数据的计算,在多试样疲劳裂纹扩展试验过程中各个试样数据记录的裂纹长度不相同时,可以计算相同裂纹长度下各个试样的裂纹扩展速率和同组试样不同存活率下的裂纹扩展速率。针对ADB610钢焊接接头母材区、热影响区和焊缝区进行多试样疲劳裂纹扩展试验,并采用成组法计算三区域不同存活率下的裂纹扩展速率和基于Paris公式的裂纹扩展速率表达式,从可靠性角度研究焊接接头疲劳裂纹扩展性能。在常规力学性能和金相试验以及焊接缺陷研究基础上,选用CHE62CFLH焊条,采用手工电弧焊对ADB610钢进行焊接,在对焊接接头三区域取多试样进行了裂纹扩展试验的基础上,采用本文提出的成组法,计算各区域存活率为50%、90%、95%、99%和99.9%下的裂纹扩展速率,分别与应力强度因子范围△K拟合,得到不同存活率下Paris公式。对三区域考虑存活率条件下的裂纹扩展速率表达式Paris公式进行分析,发现：在相同存活率下,母材区参数C最大,焊缝区的C最小；而母材区参数m最小,焊缝区m最大。随着存活率的增大,母材区参数C呈递减、m呈递增趋势,热影响区和焊缝区参数C呈递增、m呈递减的趋势；而且母材区C递减速度最小,热影响区C递增速度最快；热影响区m递减速度最快,母材区m递增速度与焊缝区m递减的速度相差不大。对考虑存活率下三区域裂纹扩展速率及计算的相对差值进行分析,发现：三区域裂纹扩展速率分散性有明显差异,基本上母材区的最小,焊缝区的最大。在相同存活率下,裂纹扩展前期母材区裂纹扩展明显最快,焊缝区的明显最慢,母材区与热影响区的裂纹扩展差异明显比热影响区与焊缝区的差异大；在裂纹扩展中后期,三个区域的裂纹扩展快慢差异相对较小。随着存活率的增大,基本上母材区与热影响区、母材区与焊缝区裂纹扩展快慢相对差异逐渐递减,在裂纹扩展前期递减明显,裂纹扩展中后期递减相对前期不明显；热影响区与焊缝区裂纹扩展快慢相对差异变化不明显。采用统计假设检验方法对三区域对数裂纹扩展速率的数字特征进行分析,发现：三区域的对数裂纹扩展速率方差(即分散性)有显著性差异,基本上母材区的最小,焊缝区的最大。在裂纹扩展速率均值方面：基本上,在裂纹扩展中前期,母材区的显著大于热影响区和焊缝区的,裂纹扩展后期母材区与热影响区和焊缝区的相当,没有显著性差异；对于热影响区与焊缝区,裂纹扩展前期热影响区的显著大于焊缝区的,但裂纹扩展中期后两区域的相当。从金相组织角度对ADB610钢焊接接头母材区、热影响区和焊缝区裂纹扩展机理进行研究,发现：在裂纹扩展的中前期,晶界间的阻力提高疲劳裂纹扩展的抗力,因此在裂纹扩展中前期,晶粒细小的焊缝区和有网状组织的热影响区的裂纹扩展慢于晶粒粗大的母材区的,焊缝区的慢于热影响区；在裂纹扩展中后期,晶界间阻力的影响逐渐减小后,三个区域之间的裂纹扩展速率差异相比前期较小。而且相对于热影响区和焊缝区而言,母材区各部位的金相组织差异性较小。采用基于ΔK-da/dN一次七点递增多项式的方法(本文简称为单试样法)计算ADB610钢焊接接头考虑存活率的裂纹扩展速率及Paris公式,并采用直观分析和统计对比分析方法,与采用成组法得到的结果进行误差分析,发现相同区域之间采用单试样法计算的结果误差大,得到的焊接接头不同区域之间裂纹扩展快慢的规律也与采用成组法的不相一致。单试样法是在单独地、各个试样之间互不相关地计算出每个试样的Paris公式后,然后取一系列的△K带入每个试样的Paris公式中,计算每个试样在相同△K下的裂纹扩展速率后,再计算同组试样不同存活率下的裂纹扩展速率和Paris公式来研究裂纹扩展的可靠性问题。认为基于一次七点递增多项式的单试样法从物理意义上不符合根据裂纹扩展试验数据的分散性来分析材料固有的分散性问题。本文的主要创新点：(1)在计算方法方面提出了采用两步七点递增多项式的专用于疲劳裂纹扩展速率可靠性分析的成组方法,并指出应用一次七点递增多项式的单试样法在裂纹扩展可靠性分析中具有较大的计算误差。(2)在统计意义上发现了ADB610钢焊接接头三个区域(母材区、热影响区和焊缝区)疲劳裂纹扩展速率数字特征的差异性,并从焊接接头金相组织的机理上解释了这种统计现象。(3)在研究成果方面得到了ADB610钢焊接接头不同存活率下基于Paris公式的疲劳裂纹扩展速率表达式,并发现了ADB610钢焊接接头疲劳裂纹扩展速率Paris公式的两个参数(C和m)随存活率改变而变化的规律。(4)在数学模型方面从泊松过程出发进行了理论推导,建立了新型低碳贝氏体钢焊接接头焊接缺陷尺寸分布的概率模型,并进一步通过ADB610钢焊接接头x射线检测的缺陷尺寸数据的分布拟合,实验数据验证了这个数学模型的合理性。更多还原

【Abstract】 It is inevitable that there are various forms of welding defects existing in weld joints. So weld joints have become the weakest part of weld structure. In weld joints, crack is easy to appear, and with the extension of crack, the weld structure will eventually be fractured. So the fatigue and fracture design of weld structure is mainly focused on fatigue fracture of weld joints. The newly developed low-carbon bainite steel is currently known as green steel, which is produced by using microalloyed and controlled rolling and cooling processes. The research on alloy structure steel is more and more focused on raising its reliability and usage. That is to say that in addition to meeting the strength requirements, more attention should be paid to its safety and longevity. Therefore, it is significant to investigate the fatigue crack growth reliability of weld joint for newly developed low-carbon bainite steel. In this thesis, the studies on conventional mechanical properties test, metallographic test, weld defect and fatigue crack growth of newly developed low-carbon bainite steel weld joints were carried out.The main works of this thesis are as followed:Mechanical properties test and metallographic test were carried out to study the weld properties of the newly developed low-carbon bainite ADB610steel, B610CF steel and WDB620steel by using different welding methods with different welding parameters. Research shows when using SMAW and MAG methods crack is not easy to appear in weld joints of these three steels, and the weld joints have excellent mechanical properties and anti-crack ability. And the welding parameters have little effect on strength, the plasticity of the whole weld joint, and the mechanical properties of heat affected zone (HAZ). However it has obvious effect on the impact toughness and metallographic structure of welded metal. Larger welding parameters will decrease the impact toughness of welded metal, and increase the size of crystalline grain in metallographic structure of welded metal.Research on probability distribution model of welding defect size and welding defect of newly developed low-carbon bainite ADB610steel and WDB620steel was carried out. It is concluded that welding defect size obeys exponential distribution based on the occurrence of welding defect meeting Poisson process and welding defect being generated randomly and defect ratio being a constant. Mathematical statistics analysis shows that welding defect size also obeys exponential distribution, lognormal distribution, normal distribution,2-Weibull and3-Weibull distribution, and the exponential distribution is the best one. Through the drawing and analysis of Pareto diagram, it is concluded that most of welding defect type in ADB610steel and WDB620steel is air hole and slag, and the crack is quite less.In this thesis, group method which adopts twice fitting of seven data was put forward. It is used to calculate the crack growth rate and Paris formula with different probability for more than one sample, when the record for crack length of each sample is not the same during the crack growth test. By using the group method, the fatigue crack growth rates of each sample in the same crack length can be calculated. And the fatigue crack growth rates and Paris formula with different probability were obtained.After ADB610steel was welded using SMAW when chosen CHE62CFLH electrode, the samples for base material(BM), HAZ and welded metal(WM) of weld joints were cut from the welded plate. After machining, the fatigue crack growth tests were carried out. By using group method, the crack growth rates of each sample in the same crack length were calculated. And the crack growth rates and Paris formula of these three zones with50%,90%,95%,99%and99.9%probability were obtained.Studies on C and m parameters in Paris formula of BM, HAZ and WM shows that C of BM is the largest, and C of WM is the least, and m of BM is the least, and m of WM is the largest when probability is the same. With the increase of probability, C and m of BM increase, and C and m of HAZ and WM decrease. And the decrease rate for C of BM is the least, and the increase speed for C and decrease speed for m of HAZ is the largest.Through visual analysis and calculation for relative difference value among BM, HAZ, and WM, it was concluded that the dispersibility of crack growth rate of these zones was obvious, and dispersibility of BM is the least, and the dispersibility of WM is the largest. When probability being the same, during the early stage the crack growth speed in BM is larger than the ones in HAZ and WM, and the one in HAZ is larger than the one in WM. The relative difference value of crack growth rate between HAZ and WM is less than the ones between BM and HAZ, and between BM and WM. During the middle and last stage of crack growth, the relative difference value of crack growth rate among three zones was obviously less than the one in early stage. With the increase of probability, the relative difference value between BM and HAZ, and between BM and WM decreases gradually in general.Statistical research on standard deviation and mean value of lg(da/dN) shows that the standard deviation among three zones has significant difference. In general, the standard deviation of BM is the least, and the one of WM is the largest. For the mean value of lg(da/dN), the one of BM is significant larger than the ones of HAZ and WM during the early and middle stage of crack growth, and the one of HAZ is significant larger than the ones of WM during the early stage of crack growth. It has insignificant difference in the mean value of lg(da/dN) between BM and WM during the last stage of crack growth, and between HAZ and WM during the middle and last stage of crack growth.From the aspects of metallographic structure and welding residual stress, the mechanism of crack growth for ADB610steel weld joints was studied. Studies shows that during the early and middle stage of crack growth, crack closure caused by resistance of grain boundaries and welding residual stress can increase the resistance for crack growth. So the speed of crack growth of WM having tiny grain and HAZ having net structure are slower than the one of BM. During the last stage of crack growth, when the resistance for crack growth decrease gradually, the difference among three zones is less. And the difference of metallographic structure among several area of BM is less than the ones among several area of HAZ and WM. So the dispersibility of crack growth of BM is less than the ones of HAZ and WM.By using single method which adopts once fitting of seven data combined with AK-da/dN method, the crack growth rate and Paris formula of ADB610steel weld joints with50%,90%,95%,99%and99.9%probability were obtained. The relative deviation by using single method compared to by using group method was studied through visual analysis, calculation for relative difference value and statistical research. It was found that the relative deviation by using single method is larger compared to by using group method. And it is inconformity with the process according the crack growth test data to analyze the dispersibility for crack growth property of material by using single method.The main innovations of this thesis are as followed:(1) group method which adopts twice fitting of seven data was put forward to calculate the crack growth rate in different probability. And it is pointed out that relative deviation is larger by using single method which adopts once fitting of seven data.(2) it was founded that standard deviation and mean value of lg(da/dN) among BM, HAZ and WM of ADB610steel weld joints has remarkable difference, which was explained from metallographic structure statistically.(3) for the research results, the crack growth rate and Paris formula of ADB610steel weld joints with50%,90%,95%,99%and99.9%probability were obtained. And with the increase of probability, the change rule of two parameters C and m in Paris formula was founded.(4) the probability model of weld defect size for newly developed low-carbon bainite steel was set up from Poisson process. And it was verified by statistical analysis through ADB610steel weld defect data on X-rays negative.更多还原