焊接接头中应力促进氢致裂纹形成理论分析与实验研究

发布时间：2018-04-05 03:25

本文选题：氢致裂纹　切入点：焊接接头　出处：《哈尔滨工业大学》2015年博士论文

【摘要】：焊接接头氢致裂纹是影响高强钢焊接结构使用安全性的常见焊接缺陷,目前该领域研究存在的主要问题是形成氢致裂纹三个主要因素之间的关系不明确,且未形成统一的裂纹形成机理。本文从裂纹形核的基本原理出发,考察了氢致裂纹形核时的能量变化,提出了氢致裂纹形核的能量及力学判据,推导了形成稳定氢致裂纹的临界尺寸,揭示了三个主要影响因素之间的关系,基于理论研究明确了应力对氢致裂纹形成的促进作用。系统的研究了应力与扩散氢及氢致裂纹关系,对应力促进氢致裂纹形成进行了实验研究。针对临氢环境焊接接头低应力脆断和氢致裂纹问题,提出了预充氢拉伸法评价焊接接头氢致裂纹敏感性的新方法,本论文的主要研究内容如下:设计了一种研究应力与扩散氢及氢损伤关系的方法及装置,中心圆孔拉伸试样氢鼓泡演变规律表明高拉应力区首先出现氢鼓泡,氢鼓泡形成过程中分布规律与应力分布规律相一致,为应力促进氢致裂纹形成提供了直接证据。研究了在梯度拉应力作用下试样扩散氢逸出分布及氢鼓泡分布规律,结果表明扩散氢易于在高应力区聚集,证明了应力对扩散氢的诱导富集作用。氢鼓泡密度与拉应力成正比,为定量分析应力与氢致裂纹关系提供了实验基础。应力促进氢致裂纹形成可表述为:扩散氢通过应力诱导作用在高拉应力区富集,进入显微空腔后复合成氢气产生内氢压力,该氢压与周围扩散氢浓度成正比。当局部位置内氢压力同外加载荷和内应力引起的拉应力叠加等于原子间键合力、晶界或第二相与基体结合力时,氢致裂纹形核。拉应力增加能够降低氢致裂纹形核时的临界氢压,从而促进氢致裂纹形核。通过预应变引入内应力,电化学充氢实验表明,相同充氢条件下,预应变试样的裂纹敏感率和氢致塑性损失均高于未应变试样;揭示了塑性变形促进氢致裂纹形成的微观机制,预应变产生的位错及其在第二相或碳化物周围塞积形成的内应力场导致氢在界面处富集导致裂纹易于在界面形核。拉伸试样断口分析表明,脆性断裂特征比例随充氢电流密度增加而增加。实验结果进一步证实了应力对氢致裂纹形成的促进作用。提出了表观溶解激活能的概念及获得方法,计算了不同预应变后的氢致开裂临界可扩散氢浓度及表观溶解激活能,采用该参量可以通过西华特定律估算内氢压力。研究了焊接接头氢致裂纹的产生机制。低碳钢TIG焊接接头焊缝区为晶粒粗大的魏氏体组织,晶界密度低于母材区。扩散氢易于在母材区聚集,氢在母材区的扩散系数较焊缝区小,晶界处晶格畸变引起的内应力场对扩散氢具有诱导富集作用,是氢扩散和聚集的主要位置。低碳钢焊接接头不同区域氢损伤特征存在较大的差异,母材区表面氢鼓泡密度较高,熔合区和焊缝区表面氢鼓泡较少,但内部存在较多的氢致裂纹。高强钢焊接区由于马氏体相变产生的内应力导致扩散氢易于在焊缝区和热影响区聚集,晶界为扩散氢扩散和聚集的优先通道。母材区氢致裂纹沿平行于板面方向扩展呈台阶状分布;焊缝区大部分裂纹沿柱状晶晶界垂直板面方向扩展。母材区裂纹优先在碳化物与基体界面处形核,焊缝区氢致裂纹主要在板条马氏体束界、板条界等各种晶界处形核,优先在马氏体条高位错区边界形核。提出了焊接接头预充氢拉伸实验法评价接头氢致裂纹敏感性的新方法,采用30Cr MnSiNi2钢焊接接头进行了可行性研究,并比较分析了预充氢电流密度对焊接接头力学性能的影响规律。结果显示预充氢拉伸试样断裂位置由未充氢前的母材区转变为HAZ粗晶区,证明该区为临氢环境工作时的薄弱环节,所得实验结果与传统插销实验一致。焊接接头的强度和塑性随预充氢电流密度的增加有所下降。断口分析表明随预充氢电流密度提高,断裂方式由解理加韧窝混合型向解理过渡,但断裂位置没有改变。该方法可以快速准确的确定临氢环境时焊接接头的薄弱环节。基于以上理论分析及实验研究,本文明确了应力对氢致裂纹形成的促进作用,给出了氢致裂纹形核的临界力学条件。提出了能够建立可扩散氢浓度与内氢压力之间关系的表观溶解激活参量。设计并验证了能快速评价焊接接头氢致裂纹敏感性的预充氢拉伸法。
[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.

【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG407

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