镍基合金焊缝高温失延裂纹的微观机制研究

High chromium Ni-base alloy 690 and Ni-base filler metals FM-52 for joining alloy 690 are frequently used in nuclear power plant for its outstanding stress corrosion resistance in nuclear radioactivity and hot water environment. However, in highly restrained weldments, this filler metal for joining alloy 690 was found to be susceptible to ductility-dip cracking(DDC)..Safety and life extension requirements in nuclear applications drive the development of compositional changes in the FM-52 to improve the DDC resistance.A significant decrease in DDC susceptibility was reported by 4.0% Mo and 2.5% Nb additions in the experimental alloys.but the effect of Mo and Nb is complex and still unkown. A number of mechanisms of DDC have been proposed but not experimentally verified. The goal of this project was to gain a better understanding of the mechanism which causes DDC in Ni-base weld metals with the additions of Mo and Nb. Accordingly, the goals of this project are:.1．Prepare STF test specimens of Ni-Cr-Fe alloy with different combination of Nb and Mo additions..2. Observe solidification microstructure in the as-welded STF specimens,identify the distibution of Mo， Nb and other elements in the weld, identify intragranular precipitate type, size, distribution, and morphology by XRD、OM、SEM and TEM..3. Evaluate and compare the DDC cracking susceptibility of different Mo and Nb additions of Ni-Cr-Fe filler metals by using the strain-to-fracture (STF) Gleeble-based testing technique.Provide the optimum combination of Mo and Nb..4. Observe and compare the change of microstructure in the as-welded and STF tested specimens,identify the effect of temperature and strain on the distibution of Mo and Nb and intragranular precipitate type, size, distribution, and morphology by XRD、OM、SEM and TEM..5.Provide the mechanism of DDC in Ni-base weld metals with Mo and Nb addition according to the STF tests and microstructure Characterization.

镍基合金690在核辐射和纯水的环境中具有最佳抗应力腐蚀开裂的能力,是核电重大装备的关键材料。但是目前使用的690合金的配套焊材在焊接过程中会出现高温失延裂纹（也称高温失塑裂纹，简称DDC裂纹）,影响焊接接头的使用寿命。研究发现在镍基焊材中添加Mo和Nb可以较大程度地改善焊缝的热延性，降低焊缝的DDC敏感性。本项目拟对此展开微观机制的研究，通过对不同Mo和Nb含量的镍基焊缝试样进行STF试验（应变-裂纹试验），研究Mo以及Mo和Nb相互作用对焊缝热延性影响的规律，给出Mo和Nb比例的优化结果。并借助XRD、OM、SEM和TEM等方法，研究STF试验前后焊缝中晶内和晶界的成分分布,Mo的存在形式，以及晶界析出物-碳氮化物的种类、形貌、数量和分布等的变化，确定Mo和Nb对晶界析出物的影响，探明Ni-Cr-Fe-Mo-Nb合金焊缝中的DDC形成机制。

针对固溶强化镍基合金690焊接过程中容易出现DDC裂纹的现象，开展了DDC形成机理和提高DDC强度的研究，制备了三种Mo，Nb含量的镍基合金焊缝，即不含Mo，含Nb0.84%的焊缝52M；含Mo4%，Nb2.48%的焊缝52MSS和增加Mo粉(7.35%)的含Nb2%的152焊缝，并对其进行了微观结构分析，以及裂纹敏感性试验和分析。结果表明，Mo的数量达到7.35%以后，合金体系以Ni-Cr-Fe-Mo为主，焊缝的主要微观结构仍然是以奥氏体基体为主， r/第二相共晶组织为晶间相的结构，但是随着合金元素的添加，第二相数量明显增加，其形状和组分非常复杂。不含Mo，含Nb0.84%的焊缝52M的晶界平直化明显，DDC强度低。含Mo4%，Nb2.48%的焊缝52MSS增加了凝固温度范围，凝固裂纹敏感性增加，但是由于晶界曲折度增加，DDC强度得到提高。含Mo7.35%，Nb2%的焊条152焊缝在大角度晶界存在尺寸达1-6μm的粒子，对晶界的迁移起到了有效的机械钉扎作用，大大提高了DDC强度。其中Mo主要是以固溶的方式存在于奥氏体基体中，Nb主要存在于NbC和Laves相中。此外，STF试验结果表明，与载荷垂直的大角度晶界是DDC裂纹的起裂位置，因此，提高大角度晶界的曲折度是提高DDC裂纹的有效方法。