双相不锈钢焊接接头腐蚀与断裂性能研究

发布时间：2018-03-10 16:03

本文选题：双相不锈钢焊接接头　切入点：应力腐蚀开裂　出处：《华东理工大学》2015年博士论文　论文类型：学位论文

【摘要】：双相不锈钢具有高的屈服强度、抗拉强度和优异的抗氯离子点蚀和应力腐蚀性能,在石油化工、造纸、化肥、海水淡化等工业的应用越来越广泛。母材在固溶处理条件下具有良好的性能,但双相不锈钢焊接引起的残余应力、残余应变、两相比例变化、合金元素的偏析或再分布以及析出相等大大降低其服役性能。本文以双相不锈钢反应器失效分析为背景,在获得反应器失效原因的基础上,调查了应力应变对双相不锈钢腐蚀性能的影响,研究了双相不锈钢焊接接头的氢脆敏感性和氯离子应力腐蚀敏感性,分析了归一法测试双相不锈钢焊接接头断裂韧性的可行性。本文研究的主要内容和结论如下：(1)反应器接管焊接接头失效的根本原因是焊接过程中出现的结晶裂纹在室温下出现了脆性裂纹扩展以及由点蚀引发的应力腐蚀开裂。不合理的焊接工艺导致蒸汽盘管接管焊缝的两相比例发生严重失衡,铁素体含量高达80-90%,在高拘束度下发生了结晶裂纹。而中间搅拌器接管焊缝应力腐蚀裂纹则是由于不合理的焊接工艺导致焊缝和热影响区出现大量的。相和二次奥氏体,严重降低了影响区的局部腐蚀抗力,在有水进入反应器非正常工况下,引发严重的氯离子的点蚀和应力腐蚀开裂现象。(2)弹性应力一方面降低了双相不锈钢的点蚀电位,改变了钝化膜稳定性,另一方面降低阳极电流密度,提高其抗均匀腐蚀性能。随着预应变的增加,SAF2205双相不锈钢的点蚀抗力轻微下降,阳极电流密度显著增加,均匀腐蚀抗力下降明显。塑性变形可以降低钝化膜的稳定性,SAF2507由于拥有更高的合金元素,即使受到显著的塑性变形,钝化膜的稳定也没有遭到破坏,腐蚀抗力基本没有改变。(3)应力加速双相不锈钢时效过程中沉淀相的析出速度,改变析出相的数量、形态和分布。应力加速了双相不锈钢的中合金元素的扩散速度,降低了析出相中合金元素的含量,提高了二次奥氏体中合金元素的含量,从而延缓了腐蚀性能的降低。而且由此改变了腐蚀模式,使时效后的组织呈现均匀腐蚀,而不是未加应力时效后的严重局部腐蚀。(4)双相不锈钢焊接接头氢脆敏感性基本取决于组织的两相比例,铁素体含量越高氢脆敏感性越大。尽管焊接接头区域非常狭窄,利用小冲杆测试技术也可有效的区分焊接接头不同位置氢脆敏感性大小,研究结果显示,热影响区是氢脆敏感性最大的区域。。(5)慢应变速率试验测试表明双相不锈钢焊接接头的应力腐蚀性能较母材低,断裂位置大部分出现在热影响区。由于受到焊接热循环,热影响区组织和力学性能发生明显变化,但仍保留一部分母材的性质,其应力腐蚀性能仍受到母材组织和力学性能各向异性的影响,沿轧制方向的焊接接头应力腐蚀敏感性明显小于垂直于轧制方向的焊接接头。(6)断裂韧性测试中的归一法和卸载柔度法获得的J-△a阻力曲线在裂纹扩展量小于0.5mm的情况下具有非常高的重合度,说明采用归一法获取双相不锈钢的断裂韧性参数是可行的。双相不锈钢焊接接头属于高韧性材料,具有高的硬化程度和屈服应变,需要更高的钝化线斜率匹配J-△a阻力曲线。分别采用GB/T21143-2007和ASTM E1820-01-2001方法研究了两种不同钝化线斜率对J0.2的影响,结果显示选取GB/T 21143-2007中定义的3.75Rm钝化线斜率获得的Jo.2误差在10%之内,说明用归一法选取3.75Rm钝化线斜率研究双相不锈钢断裂韧性是合适的。
[Abstract]:High yield strength of duplex stainless steel, tensile strength and excellent resistance to chloride pitting and stress corrosion resistance, in the petroleum chemical industry, papermaking, chemical fertilizer application, such as the desalination industry more and more widely. The parent material has good performance in the solution treated condition, but the double phase stainless steel welding induced residual stress stress, residual strain, ratio of phase change, or the redistribution of the segregation of alloy elements and precipitates greatly reduce its service performance. Based on the failure analysis of the background of dual phase stainless steel reactor, based on reactor failure, investigated the influence of stress and strain should be corrosion of duplex stainless steel, a double phase stainless steel welding joint embrittlement and chloride stress corrosion sensitivity analysis, the normalization method to test the feasibility of duplex stainless steel welded joint fracture toughness. The main contents and conclusions Are as follows: (1) reactor over the root cause failure of the welded joint is the crystallization of cracks in the welding process at room temperature appeared brittle crack propagation and pitting caused by stress corrosion cracking. Unreasonable welding lead over steam coil weld occurrence ratio of phase imbalance, the ferrite content is as high as 80-90% that happened in the high restraint degree of crystal crack. And the middle agitator nozzle weld stress corrosion cracking is due to the incorrect welding process of welding seam and heat affected zone. And the emergence of a large number of two austenite, severely reducing the effects of local corrosion resistance area, in the water into the reactor under abnormal conditions, lead to severe chloride pitting and stress corrosion cracking phenomenon. (2) the elastic stress on one hand reduces the pitting potential of duplex stainless steel, change the passive film stability. On the other hand, Yang Most current density, improve the anti corrosion performance. With the increase of pre strain, pitting resistance of duplex stainless steel SAF2205 decreased slightly, the anodic current density increased significantly, uniform corrosion resistance decreased significantly. The stability of plastic deformation can reduce the passivation film, SAF2507 with higher alloy elements, even under significant plastic deformation the passive film stability is not damaged, corrosion resistance is basically not changed. (3) stress accelerated precipitation phase velocity of duplex stainless steel in the aging process, the changes in the number of precipitates, the morphology and distribution of stress. Accelerate the diffusion rate of alloy elements in duplex stainless steel, reducing the content of the precipitation phase of alloy elements, improve the content of alloy elements in austenite two times, thus delaying the decrease of corrosion performance. And thus change the corrosion pattern, make the organization appear even after aging Corrosion, but not with serious local corrosion force after aging. (4) of duplex stainless steel welded joints depends on the Hydrogen Embrittlement Susceptibility organization, ferrite content is higher the greater susceptibility to hydrogen embrittlement. Although the weld zone is very narrow, the distinction between small punch test technology can also be effective in different the position of the Hydrogen Embrittlement Susceptibility size of welded joint, the results showed that the heat affected zone is a region. The largest hydrogen embrittlement (5) slow strain rate test test showed that the welded joint stress corrosion resistance than the parent material of low duplex stainless steel, the fracture position of most in the heat affected zone. Due to the welding thermal cycle, thermal effect and the mechanical properties of tissue changed significantly, but still retains the properties of a part of the parent material, the stress corrosion performance is still affected by the microstructure and mechanical properties of the base metal anisotropy, welded joints along the rolling direction Stress corrosion sensitivity is significantly less than the welded joints perpendicular to the rolling direction. (6) a very high degree of coincidence with less than 0.5mm under the condition of J- a resistance curve obtained fracture toughness test of the unitary method and unloading compliance method in crack growth, the normalization method to obtain the fracture toughness of duplex stainless steel is feasible. Duplex stainless steel welding joint belongs to high toughness materials, with a high degree of hardening and the yield strain, need higher blunting line slope, the J- Delta a resistance curve. Using the GB/T21143-2007 and ASTM E1820-01-2001 method to study the effects of two different passive line slope of J0.2, results show that the selected GB/T 3.75Rm blunting line the definition of the 21143-2007 slope obtained Jo.2 error within 10%, with normalization method selection on the fracture toughness of 3.75Rm duplex stainless steel passivation line slope is appropriate.

【学位授予单位】：华东理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG457.11

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