新型铸造奥氏体耐热钢合金设计及其1000 ℃氧化和蠕变机理研究

Cast austenitic heat-resistant steels are potential candidates for automotive exhaust component applications, owning to their outstanding oxidation and creep resistance at 850 ℃. However, the conventional austenitic heat-resistant steels are unable to satisfy the improved service requirements, since the exhaust temperature is increasing to as high as 1000 ℃. In order to improve the creep strength and oxidation resistance at high temperatures, a series of novel cast alumina-forming austenitic (AFA) heat-resistant steels are designed through adjusting the alloying contents of Ni, Al, Nb and C. The multi-scale calculations and experiments are carried out by use of the CALPHAD (CALculation of PHAse Diagrams) method and the advanced characterization technologies, including 3DAP (Three-Dimensional Atom Probe), TEM (Transmission Electron Microscope) and EPMA (Field Emission Electron Probe MicroAnalyzer), et al. Firstly, the synergistic effects of the four elements on as-cast microstructures of the designed cast AFA steels are clarified, based on which a screening criteria of alloy compositions is built. Subsequently, the alloying effects on oxidation behaviors of these alloys are investigated at 1000 ℃, and the growth mechanism of Al2O3 scales is clarified. Furthermore, their effects on creep behaviors of these alloys are studied to illustrated the creep strengthening and failure mechanisms at 1000 ℃. Ultimately, a quantitative relationship among compositions, microstructures and oxidation/creep properties is established, and an alloy design method based on CALPHAD is developed. The achievements of present project will significantly deepen the understanding of this type of alloys and improve the efficiency, accuracy and economy of the design of related materials.

铸造奥氏体耐热钢具有优异的高温蠕变和抗氧化性能，是汽车排气部件重要的使用材料。但是，随着排气温度上升至1000 ℃，传统奥氏体耐热钢已无法满足服役性能要求。为提高蠕变和抗氧化性能，本项目通过调整Ni、Al、Nb、C含量设计在表面可自发形成Al2O3保护膜的新型铸造奥氏体耐热钢（AFA钢），并综合运用相图计算、3DAP、TEM等先进计算和实验手段进行研究。首先阐明四元素交互作用对铸造AFA钢铸态组织的影响，初步建立合金成分筛选标准。然后阐明对铸钢1000 ℃氧化行为的影响，揭示Al2O3保护膜的生长机制。还要阐明对铸钢1000 ℃蠕变行为的影响，明确超高温蠕变的强化和失效机制。最后建立合金成分、微观组织和氧化/蠕变性能的定量关系模型，并构建、完善基于相图计算的合金设计准则。研究成果对提高相关材料设计的效率、准确度和经济性具有实际的指导意义，同时对深化该体系材料的基础认识也有积极的学术意义。

为了应对越来越严苛的环保标准，汽车发动机的排气温度逐年升高，目前已达1000 ℃，以降低汽车尾气排放。传统汽车发动机排气部件用材料已无法满足升高了的服役性能要求，需要开发承温能力更高的新型材料。耐热铸造奥氏体不锈钢具有优异的高温力学性能和抗氧化性能，因而本项目选择其作为拟开发的新材料，以提高其1000 ℃蠕变性能和抗氧化性。研究结果表明，作为参考合金的商业合金CF8C-Plus钢和A3N钢的微观组织均由奥氏体枝晶干和枝晶间呈草书体状的NbC/Nb(C,N)组成。CF8C-Plus钢的枝晶干和晶界区域还发现少量MnS析出，而A3N钢的枝晶间区域还形成了大量的Cr23C6。新型实验钢经CALPHAD方法设计合金成分，其Nb含量高于CF8C-Plus钢而与A3N钢相当，但完全去除了W。显微组织表征结果显示，新型实验钢的枝晶间区域形成了草书体状的Nb(C,N)，析出量与A3N钢相当，但Cr23C6的析出量已大幅降低。1000 ℃的高温拉伸实验结果显示，CF8C-Plus钢、A3N钢以及新型实验钢的屈服和抗拉强度相当，但实验钢在拉伸颈缩后仍然表现出较高的组织强化效果。1000 ℃、50 MPa条件下的蠕变实验结果显示，A3N钢和新型实验钢均呈现典型的蠕变三阶段，稳态蠕变寿命较长；CF8C-Plus只有蠕变第一阶段和第三阶段，蠕变抗力极低。从蠕变后组织表征结果可知，草书体状NbC/Nb(C,N)的热稳定性非常高，其在晶界上的不连续分布可以有效钉扎晶界，阻止晶界在高温蠕变过程中的滑动，从而提高合金的蠕变性能。此外，在蠕变过程中铸钢的奥氏体枝晶干区域会析出大量纳米级的NbC/Nb(C,N)阻碍位错滑移，可以进一步提高枝晶干区域的强度。Cr23C6在1000 ℃的蠕变过程中非常不稳定，容易粗化长大，从而促进蠕变裂纹形核、扩展。Cr、W是Cr23C6的主要形成元素，降低其含量对控制Cr23C6析出，从而抑制裂纹扩展非常有效。此外，草书体状的NbC/Nb(C,N)可以促进Cr的扩散，提高铸钢枝晶间区域的高温抗氧化性能。基于以上研究可知，利用CALPHAD方法进行相关铸钢的合金设计是高效且可靠的，建立的合金设计准则可为下一阶段的合金成分优化和组织控制提供科学依据，对相关合金的开发具有积极的指导意义。