本文選題：氫致裂紋　切入點：焊接接頭　出處：《哈爾濱工業(yè)大學》2015年博士論文

【摘要】：焊接接頭氫致裂紋是影響高強鋼焊接結構使用安全性的常見焊接缺陷,目前該領域研究存在的主要問題是形成氫致裂紋三個主要因素之間的關系不明確,且未形成統(tǒng)一的裂紋形成機理。本文從裂紋形核的基本原理出發(fā),考察了氫致裂紋形核時的能量變化,提出了氫致裂紋形核的能量及力學判據(jù),推導了形成穩(wěn)定氫致裂紋的臨界尺寸,揭示了三個主要影響因素之間的關系,基于理論研究明確了應力對氫致裂紋形成的促進作用。系統(tǒng)的研究了應力與擴散氫及氫致裂紋關系,對應力促進氫致裂紋形成進行了實驗研究。針對臨氫環(huán)境焊接接頭低應力脆斷和氫致裂紋問題,提出了預充氫拉伸法評價焊接接頭氫致裂紋敏感性的新方法,本論文的主要研究內(nèi)容如下:設計了一種研究應力與擴散氫及氫損傷關系的方法及裝置,中心圓孔拉伸試樣氫鼓泡演變規(guī)律表明高拉應力區(qū)首先出現(xiàn)氫鼓泡,氫鼓泡形成過程中分布規(guī)律與應力分布規(guī)律相一致,為應力促進氫致裂紋形成提供了直接證據(jù)。研究了在梯度拉應力作用下試樣擴散氫逸出分布及氫鼓泡分布規(guī)律,結果表明擴散氫易于在高應力區(qū)聚集,證明了應力對擴散氫的誘導富集作用。氫鼓泡密度與拉應力成正比,為定量分析應力與氫致裂紋關系提供了實驗基礎。應力促進氫致裂紋形成可表述為:擴散氫通過應力誘導作用在高拉應力區(qū)富集,進入顯微空腔后復合成氫氣產(chǎn)生內(nèi)氫壓力,該氫壓與周圍擴散氫濃度成正比。當局部位置內(nèi)氫壓力同外加載荷和內(nèi)應力引起的拉應力疊加等于原子間鍵合力、晶界或第二相與基體結合力時,氫致裂紋形核。拉應力增加能夠降低氫致裂紋形核時的臨界氫壓,從而促進氫致裂紋形核。通過預應變引入內(nèi)應力,電化學充氫實驗表明,相同充氫條件下,預應變試樣的裂紋敏感率和氫致塑性損失均高于未應變試樣;揭示了塑性變形促進氫致裂紋形成的微觀機制,預應變產(chǎn)生的位錯及其在第二相或碳化物周圍塞積形成的內(nèi)應力場導致氫在界面處富集導致裂紋易于在界面形核。拉伸試樣斷口分析表明,脆性斷裂特征比例隨充氫電流密度增加而增加。實驗結果進一步證實了應力對氫致裂紋形成的促進作用。提出了表觀溶解激活能的概念及獲得方法,計算了不同預應變后的氫致開裂臨界可擴散氫濃度及表觀溶解激活能,采用該參量可以通過西華特定律估算內(nèi)氫壓力。研究了焊接接頭氫致裂紋的產(chǎn)生機制。低碳鋼TIG焊接接頭焊縫區(qū)為晶粒粗大的魏氏體組織,晶界密度低于母材區(qū)。擴散氫易于在母材區(qū)聚集,氫在母材區(qū)的擴散系數(shù)較焊縫區(qū)小,晶界處晶格畸變引起的內(nèi)應力場對擴散氫具有誘導富集作用,是氫擴散和聚集的主要位置。低碳鋼焊接接頭不同區(qū)域氫損傷特征存在較大的差異,母材區(qū)表面氫鼓泡密度較高,熔合區(qū)和焊縫區(qū)表面氫鼓泡較少,但內(nèi)部存在較多的氫致裂紋。高強鋼焊接區(qū)由于馬氏體相變產(chǎn)生的內(nèi)應力導致擴散氫易于在焊縫區(qū)和熱影響區(qū)聚集,晶界為擴散氫擴散和聚集的優(yōu)先通道。母材區(qū)氫致裂紋沿平行于板面方向擴展呈臺階狀分布;焊縫區(qū)大部分裂紋沿柱狀晶晶界垂直板面方向擴展。母材區(qū)裂紋優(yōu)先在碳化物與基體界面處形核,焊縫區(qū)氫致裂紋主要在板條馬氏體束界、板條界等各種晶界處形核,優(yōu)先在馬氏體條高位錯區(qū)邊界形核。提出了焊接接頭預充氫拉伸實驗法評價接頭氫致裂紋敏感性的新方法,采用30Cr MnSiNi2鋼焊接接頭進行了可行性研究,并比較分析了預充氫電流密度對焊接接頭力學性能的影響規(guī)律。結果顯示預充氫拉伸試樣斷裂位置由未充氫前的母材區(qū)轉(zhuǎn)變?yōu)镠AZ粗晶區(qū),證明該區(qū)為臨氫環(huán)境工作時的薄弱環(huán)節(jié),所得實驗結果與傳統(tǒng)插銷實驗一致。焊接接頭的強度和塑性隨預充氫電流密度的增加有所下降。斷口分析表明隨預充氫電流密度提高,斷裂方式由解理加韌窩混合型向解理過渡,但斷裂位置沒有改變。該方法可以快速準確的確定臨氫環(huán)境時焊接接頭的薄弱環(huán)節(jié)�；谝陨侠碚摲治黾皩嶒炑芯�,本文明確了應力對氫致裂紋形成的促進作用,給出了氫致裂紋形核的臨界力學條件。提出了能夠建立可擴散氫濃度與內(nèi)氫壓力之間關系的表觀溶解激活參量。設計并驗證了能快速評價焊接接頭氫致裂紋敏感性的預充氫拉伸法。
[Abstract]:Welding hydrogen induced crack is the common welding defects in the safety of the structure of high strength steel welding, the main problems of current research in the field of relationship is formed between the three main factors of hydrogen induced crack is not clear, and has not formed the unified crack formation mechanism. In this paper, from the basic principle of crack nucleation of investigated the energy change of crack nucleation when induced by hydrogen, proposed the hydrogen induced crack nucleation energy and mechanics criterion is derived form the critical size of stable hydrogen induced crack, revealing the relationship between three main factors, based on the theory of definite stress on hydrogen induced crack formation system of promotion. The crack caused by stress and diffusion of hydrogen and hydrogen, the stress to promote hydrogen induced crack formation was studied. Aiming at the welding hydrogen environment joint low stress brittle fracture and hydrogen induced crack problems, put forward the precharged pull Extension method to evaluate a new welding method of hydrogen induced cracking susceptibility of joints, the main research contents of this thesis are as follows: design a research method and device for force and diffusion of hydrogen and hydrogen damage, central hole tensile specimens hydrogen blistering evolution shows that high tensile stress zone occurs hydrogen blistering, hydrogen bubble formation in the process of distribution and stress distribution are consistent for stress promoting hydrogen induced crack formation. The research provides direct evidence in gradient specimens under tensile stress distribution and diffusion of hydrogen escaping hydrogen bubbles distribution, the results show that the diffusion of hydrogen in high stress area is easy to gather, to prove that the stress concentration the role of hydrogen induced diffusion. Hydrogen bubble density and the tensile stress is proportional to the stress and hydrogen induced crack relationship provides an experimental basis for quantitative analysis. The stress promotes the hydrogen induced crack formation can be expressed as: the diffusion of hydrogen through the stress induced by the The high stress zone of enrichment, enters into the micro cavity after composite hydrogen generated in the hydrogen pressure, the hydrogen pressure and peripheral hydrogen diffusion is proportional to the concentration. When the hydrogen pressure in the same local position of the external load and internal stress caused by tensile stress is equal to the superposition of atomic bonding force between the grain boundary, or second phase and substrate binding. Hydrogen induced crack nucleation. The tensile stress increase can reduce the hydrogen induced crack nucleation when the critical hydrogen pressure, so as to promote hydrogen induced crack nucleation. The pre strain introduced internal stress, showed that the electrochemical hydrogen charging experiments, the same hydrogen charging conditions, induced ductility loss were higher than the strain of the prestrain specimen crack the sensitive rate and hydrogen; reveal the plastic deformation and promote the microscopic mechanism of hydrogen induced crack formation, dislocation and pre strain produced in the second phase of stress or carbide around the pileup formation field leads to hydrogen at the interface crack in the interface is easy to lead to enrichment of pull nucleation. Tensile test specimen fracture analysis showed that the proportion of brittle fracture characteristics increase with the hydrogen charging current density increased. Experiment results show that the stress of hydrogen induced crack formation effect is proposed. The apparent activation energy and the concept of dissolution method, cracking the diffusible hydrogen concentration and the apparent activation energy of dissolution of different pre strain after the hydrogen calculation, the parameters through the hydrogen pressure estimation of Xihua law. On the specific mechanism of welded joint crack caused by hydrogen. TIG low carbon steel welds for the widmanstatten structure of coarse grain, grain density lower than the base metal. The diffusion of hydrogen is easy to gather in the base area, diffusion coefficient hydrogen in base metal of weld zone is small, the stress field caused by lattice distortion at the grain boundary diffusion of hydrogen induced enrichment, is the main position of hydrogen diffusion and accumulation in different regions. The joint hydrogen loss of low carbon steel welding There is a big difference between the injury characteristics of base metal surface hydrogen bubble density is higher, the fusion zone and weld zone of surface hydrogen bubble less, but there are many internal crack caused by hydrogen. High strength steel welding zone due to the internal stress of martensitic transformation caused by hydrogen diffusion to ring area gathered in the weld area and heat effect, grain boundary as the preferred channel diffusion hydrogen diffusion and aggregation. The base metal hydrogen induced cracks along the direction parallel to the surface of expansion is stepped distribution; most of the weld zone crack propagation along the columnar grain boundaries perpendicular to plane direction. The crack in the base metal preferentially at carbide matrix interface nucleation at the weld zone, the hydrogen induced crack mainly in lath martensite bundle, such as lath boundary grain boundary nucleation preferentially in the martensite lath boundary dislocation nucleation zone. Proposed a new welding method of induced crack sensitivity evaluation of joint precharged hydrogen joint tensile test method using 30Cr, MnSiNi2 steel The feasibility study of welded joints, and a comparative analysis of the pre charge current density of hydrogen influences on mechanical properties of welded joints. The results showed that precharged tensile specimen fracture position changed from not sufficient base material before hydrogen is HAZ CGHAZ, shows that the area is a weak part in hydrogen environment work, income the experimental results are consistent with the traditional bolt experiment. The strength of welded joints and plasticity with increasing pre hydrogen charging current density decreased. The fracture analysis shows that with the pre hydrogen charging current density increases, the fracture mode by cleavage plus dimple mixed fracture to cleavage transition, but did not change its position. This method can quickly and accurately the weak link determination of hydrogen environment of welded joints. Based on the above theoretical analysis and experimental research, this paper defines stress on hydrogen induced crack formation to promote the role of critical mechanics condition gives the hydrogen induced crack nucleation. The apparent activation parameter of hydrogen that can be used to establish the relationship between diffusible hydrogen concentration and internal hydrogen pressure has been developed. A pre hydrogen stretching method that can quickly evaluate the hydrogen induced cracking susceptibility of welded joints is designed and verified.

【學位授予單位】：哈爾濱工業(yè)大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG407

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