Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： 2.25Cr-1Mo-0.25V steel is the most popular material used for pressure-vessel applied at elevatedtemperature in hydrogen environment. For higher process efficiencies in future coal- conversion plants, chemical processing plants, and petrochemical-refining plants, much thicker cross-section component are necessary for constructing much larger pressure- vessel for these plants. Because of the thick cross- section, the cooling rate in the central region of the component is insufficient to obtain low bainite during quenching treatment, and a large amount of granular bainite appears in the central region. Previous studies have shown that good impact toughness can be achieved by appropriate tempering for 2.25Cr-1Mo-0.25V steel with low bainite microstructure. However, the impact toughness of 2.25Cr-1Mo-0.25V steel with granular bainite after tempering always cannot satisfy the demanding requirement due to the unclear understanding of the evolution of microstructure and mechanical properties during tempering. In this work, the influence of tempering on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with granular bainite microstructure was investigated by OM, XRD, SEM, TEM and EPMA. The results show that the normalized 2.25Cr-1Mo-0.25V steel with granular bainite microstructure is composed of bainite ferrite and island of martensite and austenite (M-A island). Nanoindentation test indicates that MA island is much harder than that of metrix bainite ferrite, because of the high concentration of carbon in M-A is-lands. The synergistic effect of the decomposition of M-A islands and softening of bainite ferrite determined that Charpy absorbed energy at -18 ℃ increases first and then decreases with the increasement of tempering temperature. The degree of decomposition of M-A islands and the morphology, size and distribution of carbides in granular bainite, coupled with the softening effect of bainite ferrite recrystallization are the key factors determining low-temperature impact toughness of 2.25Cr-1Mo-0.25V steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： After quenching and proper intercritical tempering, ZG06Cr13Ni4Mo martensitic stainless steel is composed of tempered martensite matrix and reversed austenite. The deformation induced martensitic transformation of reversed austenite occurring during the deformation results in the transformation induced plasticity (TRIP) effect, which is beneficial to the mechanical properties of this steel. However, studies on the TRIP effect of reversed austenite are limited to description of phenomenon and mechanism behind is not clear. In order to reveal the mechanical stability and transformation induced plasticity of the reversed austenite during tension test in tempered ZG06Cr13Ni4Mo steel, a custom-built mini tensile instrument has been designed and installed on Shanghai Synchrotron Radiation Facility to conduct the in situ synchrotron high energy X-ray diffraction (SHXRD) experiment during the uniaxial tension. Three samples, which were tempered at 620 ℃ with different holding times and cooling rates in order to obtain different volume fraction of reversed austenite, were used to investigate the relationship between the deformation induced martensitic transformation and work hardening behavior. The integral intensityand the full width at half maximum of diffraction peaks of the reversed austenite and tempered martensitic matrix under different engineering stress were recorded. The gradual decrease in the integral diffraction intensity of reversed austenite with increase in tensile stress indicates that the reversed austenite has been induced to transform into martensite during the tension deformation. Furthermore, the volume fraction of reversed austenite during tension was quantitatively calculated by fitting the whole diffraction spectra of reversed austenite and tempered martensitic matrix with the Rietveld refinement method. The evolution of the reversed austenite fraction indicates that the deformation induced martensitic transformation initiates at the macro-elastic stage and through the whole deformation, which is different to the retained austenite in TRIP steel. Meanwhile, the work hardening exponents of three samples with different volume fraction of reversed austenite have been compared. It is found that the deformation induced martensitic transformation of reversed austenite increases the dislocation density of martensitic matrix and results in the increase in the work-hardening exponent during the plastic deformation, which enhances the ductility of ZG06Cr13Ni4Mo martensitic stainless steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Ni-based filler metal is one of the most important filler metals in building the key components of nuclear power plants, however, ductility- dip- cracking (DDC) and inclusion defects form easily in the weldment and need to be repaired afterward. The precipitation of M23C6 (M=Cr, Fe) at grain boundaries will promote the nucleation and propagation of DDC. Adding Ti can form Ti(C, N) and reduce M23C6 precipitate at grain boundaries, which reduces DDC in the weld metal. However, the increase of Ti content in filler metal will cause the inclusion defects. Nb replacing part of Ti in Ni-based filler metal is proposed in this work. The reduction of Ti in filler metal is to reduce the sensitivity of inclusion defects in the weld metal. Nb can form MX (M=Nb, Ti, X=C, N) precipitates to reduce the M23C6 and DDC in weld metal. The effect of Nb on the size, number, and location of MX and M23C6 in Ni-based weldment has been investigated systematically in this work. Phase diagram calculations show that Nb is an element forming high temperature MX precipitate, and its affinity with oxygen is poor and not easy to form oxide. According to the phase diagram calculations, five different filler metals are designed and made with 0, 0.4%, 0.7%, 0.85%, 1.1%Nb content. The results show that the intragranular precipitates are distributed along sub grain boundaries. The intragranular precipitate for the Nb-free weld metal is Ti(C, N), whereas the intragranular precipitate in the Nb-bearing weld metals is MX. For the increased Nb in weld metals, more MX is produced, and more C is fixed within the grain. As the Nb content increased in weld metals, the initial precipitation temperature of M23C6 decreases, the intergranular M23C6 precipitate decreases and M23C6 turns discreted at grain boundaries. As Nb content increases in weld metals, the total crack length of DDC decreases. When the Nb content is over 0.85%, little DDC is found in the weld metals. The addition of Nb can improve the tensile strength, plasticity and bending property of the weld metals.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-03-31 Cooperative journals: 《金属学报》
Abstract：合金中微量元素的偏析加剧了主元素的偏析，形成有害相，恶化性能。如何控制微量元素的偏析，得到性能均匀的高性能合金？师昌绪先生领导的铸造高温合金团队自八十年代初开始了低偏析技术的研究，他们在研究高温合金的凝固过程中发现，控制微量元素P、B、Si和Zr含量可以降低高温合金的凝固偏析。采用低偏析技术提高了高温合金的使用温度和性能，并发展了M17系列合金及GH738及125合金等低偏析合金。 低偏析技术所研究的铸造高温合金是在真空炉中熔炼经电渣或自耗炉重熔得到的，当时产品的最大重量不超过1吨。现在，我们欲探索在数十吨甚至数百吨常压电弧炉熔炼合金钢中低偏析技术的实用性。通过低压转子用30Cr2Ni4MoV百吨大钢锭试验，获得了控制微量元素O、Al含量，可以显著减少直至消除偏析。此外，用低偏析技术研究了核电蒸发器传热管用690形变高温合金，通过控制690合金中的微量元素S、N达到了成分和组织均匀，性能提高的目的。在具有放射性元素的U-Nb合金中控制C、N、O，在高碳U中控制C、O也能达到同样的效果。 低偏析技术具有普适性，但是不同合金或钢中需要控制的微量元素是不同的，应广泛地在其它金属结构材料中进行研究、查找和分析，得出不同合金中严重影响偏析的微量元素，在冶炼和制备工艺上加以控制，以获得高质量的低偏析钢种，这将大大地支撑工业制造2025的装备制造水平。
Subjects: Materials Science >> Materials Science (General) submitted time 2017-03-31 Cooperative journals: 《金属学报》
Abstract：采用相场法研究了Fe-C-Mn三元合金在临界区等温过程中发生的奥氏体-铁素体相变. 基于Gibbs自由能平衡理论，相场模型考虑了替换性元素Mn在奥氏体/铁素体相界面内扩散所导致的自由能耗散，因此本模型可描述Fe-C-Mn三元合金中因Mn在相界面偏聚所导致的相变停滞和相变不完全现象. 利用相场模型进一步研究了不同Mn含量对奥氏体-铁素体相变微观组织转变和相变动力学的影响. 结果显示：随着Mn含量的增加，铁素体相的转变速率和最终体积分数降低，Mn在奥氏体/铁素体相界面内扩散所导致的溶质拖曳现象越明显，相变不完全的转变程度加剧.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：运用相场法研究了Fe-C合金在临界区等温过程中发生的奥氏体-铁素体相变过程. 通过分析铁素体生长过程中C的扩散行为，发现奥氏体-铁素体相变表现为混合控制生长的特征，奥氏体/铁素体相界面处于非平衡状态. 进一步研究了不同等温温度（1010、1048和 1087K）下奥氏体-铁素体相变的微观组织和C浓度场的演化情况. 结果表明，随着等温温度的降低，铁素体形核率增加，铁素体相变平衡体积分数增加，但奥氏体内部C浓度分布的不均匀程度加剧，1010K等温时的微观组织呈现为不规则细小铁素体晶粒围绕分散分布的残余奥氏体的两相结构. 随着等温温度的降低，奥氏体-铁素体相变过程表现出由扩散控制生长模式向界面控制生长模式转化的趋势.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：缩孔疏松是铸件中最常见的缺陷之一，严重影响了铸件的质量。一般通过设置冒口加强液态补缩的方法来消除缩孔疏松。但是对于大高径比的铸件，在凝固后期液态补缩通道细长，冒口中金属液的流动阻力加大,仅凭钢水静压力难以实现长程液态补缩，补缩效果微乎其微。因此大高径比、大断面连铸钢锭，常常产生轴线贯穿性缩孔疏松缺陷。鉴于传统的液态补缩机制不能解决此类缺陷，本论文提出了固态补缩机制。通过分析金属液静水拉力对内孔形核和固相屈服变形的作用，阐明了固态补缩机制的原理，并得出了影响铸件固态补缩能力的主要因素为：液体破裂压力值、金属液气体含量以及固相屈服强度。在此基础上，提出了降低铸件在径向上的温度梯度是消除大高径比铸件缩孔疏松缺陷的有效方法。最后，设计了铸件在不同模具温度下凝固的验证实验，证实了固态补缩对于改善大高径比铸件缩孔疏松的作用。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：总结了宽砧径向压实工艺(WRF法), 该工艺可使应变集中于钢锭中心区域, 并满足孔洞闭合所需的最佳高径比条件, 因此可高效愈合钢锭的中心缩孔、疏松. 基于该方法, 针对压机压力较小的情况, 提出了单向大变形压实法. 该方法通过两个道次的单方向变形将坯料锻造成扁方状, 可以使用小压力实现较大钢锭内缺陷的有效愈合. 基于界面焊合原理, 针对管板类锻件探伤合格率较低的问题, 提出了管板类锻件的锻间保温锻造法. 利用该方法成功进行了含缺陷管板的修复实验. 以上各锻造工艺均经过工业实验验证, 证明其可以顺利愈合锻件内的孔洞型缺陷, 提高锻件合格率.
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