不同焊接工艺下B780CF钢焊接接头疲劳裂纹扩展速率试验研究

发布时间：2018-05-04 13:51

本文选题：B780CF钢 + 疲劳裂纹扩展速率　；参考：《昆明理工大学》2017年硕士论文

【摘要】：B780CF是一种低合金高强钢,具有冷裂纹敏感性低、强度等级高、综合力学性能好等特点,被广泛的应用于大型、巨型水电站以及高水头抽水电站水轮机埋件、高压钢管、钢岔管等方面。为了做好800MPa级高强钢的焊接工艺技术储备,需对B780CF钢进行了一系列焊接试验分析,确定该材料合理的焊接工艺方案。但目前对B780CF钢焊接接头裂纹扩展速率的研究很少见报道,尤其是不同的焊接工艺下的疲劳裂纹扩展速率的研究更是罕见,本文就以不同的焊接工艺作为切入点,对B780CF钢的焊接接头的疲劳裂纹扩展行为进行试验研究,寻找最佳焊接工艺参数,并与热影响区和母材裂纹扩展速率做横向对比,系统地为B780CF钢焊接结构工程应用提供可参考的理论依据及数据,并分析比较了不同焊接工艺下的疲劳断口,探讨其破坏机理。首先采用标准紧凑拉伸试件(CT试件),对B780CF钢进行不同焊接工艺下的疲劳裂纹扩展试验,得到不同焊接工艺下的疲劳裂纹扩展数据(a,N)和(da/N,△K)。以Paris公式为出发点,研究了不同的焊接工艺对疲劳裂纹扩展速率的影响,得到了不同焊接工艺下的da/N-△K曲线,以及Paris参数C,m与焊接工艺的关系,同时利用Paris公式定量反推疲劳扩展寿命,并对疲劳裂纹扩展数据进行了可靠性、置信度分析:1)基于(a,N)数据的疲劳裂纹扩展速率可靠性研究;2)基于(da/,△K)数据的疲劳裂纹扩展速率可靠性研究;3)考虑置信度的影响基于(a,N)数据的疲劳裂纹扩展速率可靠性研究。研究结果表明:1)焊接接头各区在相同裂纹长度下疲劳寿命有差异,总体上母材埋弧焊热影响区手工焊,也就是母材的寿命最短、手工焊的最长;2)不同的焊接工艺下的Paris参数是不同的,说明不同的焊接工艺对Paris参数C、m有一定影响;3)可靠度越高,B780CF钢的疲劳裂纹扩展速率越快。其次对B780CF钢在不同焊接工艺下的疲劳试件断口进行扫描电镜(SEM)分析,结果表明:疲劳往往从试样的表面或有缺陷处、或最大应力处起源,在外载荷和较大应力集中作用下,导致夹杂自身的断裂,形成微裂纹;在疲劳裂纹稳定扩展的第一阶段,开动的滑移数量少,且滑移主要是在滑移面内进行滑移;在裂纹快速扩展阶段,开动的滑移系数量多,滑移方式多,除了在滑移面内进行滑移外,还有交滑移和攀移等滑移方式,具有很大的灵活性,当位错在滑移面上受到阻碍时,位错可以离开原来的滑移面继续运动,裂纹扩展速率加快。最后对B780CF钢标准紧凑拉伸试件(CT试件)进行过载条件下的裂纹扩展试验,研究不同的过载比对疲劳寿命的影响,结果表明:过载比越大,达到同一裂纹长度的疲劳寿命N越大,也就是裂纹扩展速率越慢。当过载比为2.5,疲劳寿命运行到100000次左右时,裂纹长度仍保持不变,即裂纹扩展速率基本为零,裂纹不扩展。
[Abstract]:B780CF is a kind of low alloy high strength steel with low cold crack sensitivity, high strength grade and good comprehensive mechanical properties. It is widely used in large scale, giant hydropower stations and high head pumping hydropower stations, hydraulic turbine burying parts, high pressure steel tubes, etc. Steel bifurcations, etc. In order to make the welding technology reserve of 800MPa grade high strength steel, a series of welding tests and analysis on B780CF steel are needed to determine the reasonable welding process scheme of the material. However, the research on the crack propagation rate of welded joint of B780CF steel is rare, especially the research of fatigue crack growth rate under different welding processes. In this paper, different welding processes are taken as the breakthrough point. The fatigue crack propagation behavior of welded joints of B780CF steel is studied experimentally to find the best welding process parameters and to compare the fatigue crack propagation rate with the heat affected zone and the base metal crack growth rate. The theoretical basis and data for the engineering application of welded structure of B780CF steel are systematically provided, and the fatigue fracture surface under different welding processes is analyzed and compared, and the failure mechanism is discussed. First, the fatigue crack growth test of B780CF steel under different welding processes was carried out by using standard compact tensile specimen and CT specimen. The fatigue crack growth data of B780CF steel under different welding processes were obtained. Based on the Paris formula, the influence of different welding processes on fatigue crack growth rate is studied. The da- / N- K curves under different welding processes and the relationship between the Paris parameter Cam and the welding process are obtained. At the same time, the fatigue propagation life is quantitatively deduced by using Paris formula, and the fatigue crack growth data are reliable. Reliability study of fatigue crack growth rate based on the data of (1) the reliability of the fatigue crack growth rate based on the data of da-, K3) the reliability of the fatigue crack growth rate based on the data of "A" N) and the reliability of the fatigue crack growth rate based on the data of "A" Nine) (3) the reliability of the fatigue crack growth rate based on the reliability of the fatigue crack growth rate. The results show that the fatigue life of welded joints varies from one zone to another at the same crack length. On the whole, manual welding in the heat-affected zone of submerged arc welding of the base metal is the shortest, that is, the base metal has the shortest life. The Paris parameters of different welding processes are different, which indicates that different welding processes have a certain effect on the Paris parameter CfM) the higher the reliability, the faster the fatigue crack growth rate of B780CF steel. Secondly, the fracture surface of fatigue specimen of B780CF steel under different welding processes is analyzed by SEM. The results show that fatigue often originates from the surface of the specimen or at the point of the maximum stress, and under the action of external load and greater stress concentration. In the first stage of stable growth of fatigue crack, the number of moving slip is less, and the slip is mainly in the slip plane, and in the stage of rapid crack growth, the number of moving slip system is large. There are many slip modes, not only slip in the slip plane, but also the slip modes such as cross slip and climbing, which have great flexibility. When the dislocation is obstructed on the slip plane, the dislocation can continue to move away from the original slip plane. The crack growth rate is accelerated. Finally, the crack growth test of B780CF steel under overload condition is carried out. The effect of different overload ratio on fatigue life is studied. The results show that the greater the overload ratio, the greater the fatigue life N of the same crack length. That is, the slower the crack growth rate. When the overload ratio is 2.5 and the fatigue life reaches about 100000 times, the crack length remains unchanged, that is, the crack growth rate is basically zero and the crack does not grow.
【学位授予单位】：昆明理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG407

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