复合型碳氮化物强化铁素体耐热钢的组织热稳定性研究

Thermal stability of initial microstructure is critical to high temperature mechanical properties of heat-resistant steels. The direction of optimizing the microstructure of advanced 9-12Cr heat-resistant steels is to achieve the ferritic microstructure strengthened by thermally stable precipitates. This proposal targets the mostly developed ferritic heat-resistant steel 9CrWVNbCN strengthened by compound carbonitrides. In order to elaborate the detail of the microstructure of this steel, OM, SEM and TEM would be employed. The samples of this steel will be under creep, long-time aging and these procedures with pre-deformation. The microstructure evolution, the mechanism of how the compound resist the microstructure degenerate, and the optimum initial microstructure of the heat resistant steel will be addressed. Our previous work has found that the deformation would enhance the nucleation driving force and induce nitride precipitation so that nitrides could be further refined and dispersed. Lots of the fine nitrides could be the initial nucleus for the carbides to precipitate upon during the heat treatment. Therefore, plenty of compound carbonitrides formed after the two procedures. Moreover, the compound could also be formed through metallurgy by modification. As stated in many research papers, the compounds had a relatively low coarsening rate compared with M23C6 which was the most common carbides in heat resistant steels. Thus, they could delay the microstructure worsening efficiently. However, barely no research has been carried out on how the compound resisting the microstructure degeneration. Therefore, this proposal will focus on this resisting mechanism and optimize the initial microstructure of the heat resistant steel by investigating the compound carbonitrides evolution after creep, long-time aging and these procedures with pre-deformation.

初始组织的热稳定性决定着耐热钢的高温长时力学性能，由具有高热稳定性析出相强化的铁素体体组织是先进9-12Cr 系耐热钢组织优化的方向。本项目以最新研发制备的含有复合型碳氮化物强化的铁素体体耐热钢9CrWVNbCN为研究对象，拟通过光学电镜、扫描电镜、透射电镜等手段对高温时效组织、不同参数条件下的蠕变组织、及预冷变形后的高温时效和蠕变组织进行观察，总结复合型碳氮化物强化耐热钢在高温时效及蠕变条件下组织演变规律；探究复合型碳氮化物对组织退化的抑制机制；同时探索预冷变形对复合型碳氮化物抑制组织退化的影响。申请者的前期工作表明，通过变质处理、高温形变和后续热处理可以获得大量的复合型碳氮化物。目前尚无文献提及复合型析出相抑制组织退化的原因机理。因此本项目将通过研究复合型碳氮化物分别在不同环境下的演变，总结出复合型碳氮化物对组织退化的抑制机理，探索出热稳定最优的铁素体耐热钢的初始组织。

本项目将以实验室最新制备的复合型碳氮化物强化铁素体耐热钢9CrWVNbCN为研究对象，通过高温形变+热处理制备等轴晶复合型碳氮化物强化马氏体耐热钢、通过低温变形+再结晶退火制备再结晶小晶粒马氏体耐热钢、通过低温形变+去应力制备柱状晶马氏体耐热钢，研究其组织在高温、高温+应力环境下组织的演变机理。及EBW焊接接头组织及焊后性能。结果如下：.复合型碳氮化物强化铁素体耐热钢在加热过程中，晶格类型转变对温度敏感，可以居里点的出现作为晶格转变的表征。但冷却过程中晶格转变是与相变同时发生，对温度不敏感。随温度升高，组织的变化过程为：析出相的形成，晶格转变，马氏体晶界迁移，奥氏体晶粒形核及长大。奥氏体转变温度为770℃。而当冷速小于临界冷速时，在连续冷却过程中会形成无碳贝氏体，组织主要有位错结和贝氏体板条组成。而临界冷却速度测定为0.5℃/s。.对等轴晶复合型碳氮化物强化马氏体耐热钢在600℃进行冷变形+650-700℃再结晶退火后，变形量应控制在60%左右，保温时间2h。通过提高组织位错密度，组织高温稳定性得到极大的提升。新制备的再结晶小晶粒马氏体耐热钢在高温长时保温过程中，马氏体晶界发生迁移的时间明显延后。即新方案中“把服役初期的热能转化为析出相的析出动力学”的理念是可行的。.在600-650℃/100-230MPa测试条件下等轴晶复合型碳氮化物强化铁素体耐热钢在600℃的持久性能达到或超过NIMS公布的P92钢性能；但夹杂物为裂纹的主要萌生点，裂纹从晶界处不断地向外扩展，最终从晶体内部穿过产生孔洞，并且多个相邻的孔洞相互融合，从而产生裂纹。且组织变形严重。以位错运动失效机制为主。.而EBW焊实验结果显示，CCNS-2钢是一种冲击性能、拉伸性能、耐热性非常出色的低活化马氏体耐热钢，焊缝区主要为晶粒粗大的板条马氏体+少量δ铁素体。热影响区的马氏体晶粒尺寸从焊缝至母材方向逐渐减小。经760℃/30min保温，析出相仍然分布在原奥氏体晶界处，即马氏体晶内。