本文選題：B780CF鋼 + 疲勞裂紋擴展速率��； 參考：《昆明理工大學》2017年碩士論文

【摘要】：B780CF是一種低合金高強鋼,具有冷裂紋敏感性低、強度等級高、綜合力學性能好等特點,被廣泛的應用于大型、巨型水電站以及高水頭抽水電站水輪機埋件、高壓鋼管、鋼岔管等方面。為了做好800MPa級高強鋼的焊接工藝技術(shù)儲備,需對B780CF鋼進行了一系列焊接試驗分析,確定該材料合理的焊接工藝方案。但目前對B780CF鋼焊接接頭裂紋擴展速率的研究很少見報道,尤其是不同的焊接工藝下的疲勞裂紋擴展速率的研究更是罕見,本文就以不同的焊接工藝作為切入點,對B780CF鋼的焊接接頭的疲勞裂紋擴展行為進行試驗研究,尋找最佳焊接工藝參數(shù),并與熱影響區(qū)和母材裂紋擴展速率做橫向?qū)Ρ?系統(tǒng)地為B780CF鋼焊接結(jié)構(gòu)工程應用提供可參考的理論依據(jù)及數(shù)據(jù),并分析比較了不同焊接工藝下的疲勞斷口,探討其破壞機理。首先采用標準緊湊拉伸試件(CT試件),對B780CF鋼進行不同焊接工藝下的疲勞裂紋擴展試驗,得到不同焊接工藝下的疲勞裂紋擴展數(shù)據(jù)(a,N)和(da/N,△K)。以Paris公式為出發(fā)點,研究了不同的焊接工藝對疲勞裂紋擴展速率的影響,得到了不同焊接工藝下的da/N-△K曲線,以及Paris參數(shù)C,m與焊接工藝的關(guān)系,同時利用Paris公式定量反推疲勞擴展壽命,并對疲勞裂紋擴展數(shù)據(jù)進行了可靠性、置信度分析:1)基于(a,N)數(shù)據(jù)的疲勞裂紋擴展速率可靠性研究;2)基于(da/,△K)數(shù)據(jù)的疲勞裂紋擴展速率可靠性研究;3)考慮置信度的影響基于(a,N)數(shù)據(jù)的疲勞裂紋擴展速率可靠性研究。研究結(jié)果表明:1)焊接接頭各區(qū)在相同裂紋長度下疲勞壽命有差異,總體上母材埋弧焊熱影響區(qū)手工焊,也就是母材的壽命最短、手工焊的最長;2)不同的焊接工藝下的Paris參數(shù)是不同的,說明不同的焊接工藝對Paris參數(shù)C、m有一定影響;3)可靠度越高,B780CF鋼的疲勞裂紋擴展速率越快。其次對B780CF鋼在不同焊接工藝下的疲勞試件斷口進行掃描電鏡(SEM)分析,結(jié)果表明:疲勞往往從試樣的表面或有缺陷處、或最大應力處起源,在外載荷和較大應力集中作用下,導致夾雜自身的斷裂,形成微裂紋;在疲勞裂紋穩(wěn)定擴展的第一階段,開動的滑移數(shù)量少,且滑移主要是在滑移面內(nèi)進行滑移;在裂紋快速擴展階段,開動的滑移系數(shù)量多,滑移方式多,除了在滑移面內(nèi)進行滑移外,還有交滑移和攀移等滑移方式,具有很大的靈活性,當位錯在滑移面上受到阻礙時,位錯可以離開原來的滑移面繼續(xù)運動,裂紋擴展速率加快。最后對B780CF鋼標準緊湊拉伸試件(CT試件)進行過載條件下的裂紋擴展試驗,研究不同的過載比對疲勞壽命的影響,結(jié)果表明:過載比越大,達到同一裂紋長度的疲勞壽命N越大,也就是裂紋擴展速率越慢。當過載比為2.5,疲勞壽命運行到100000次左右時,裂紋長度仍保持不變,即裂紋擴展速率基本為零,裂紋不擴展。
[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.
【學位授予單位】：昆明理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG407

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