300M钢的脱碳行为演化及防护研究

300M钢是重要的飞机主承力结构用材. 然而, 该材料在模锻热处理过程中会出现不同程度的表面氧化和脱碳, 严重影响锻件成品率. 本文对300M钢在不同热处理条件下的脱碳行为进行了系统研究, 利用扫描电镜(SEM)、X射线能谱分析(EDS)等手段表征分析了热处理后试样表层组织形貌, 并采用硬度法统计了脱碳层深度的变化情况, 阐明了300M钢在不同热处理条件下的脱碳行为演化规律, 厘清了防氧化涂层对脱碳层深度的影响. 结果表明, 300M钢在热处理过程中近表面处生成的致密氧化层对碳和氧元素的扩散具有明显的抑制作用; 碳的扩散速度与表层氧化速度在不同温度下都会有一个平衡点, 而表面防氧化剂涂层的施加则会破坏原有平衡. 微观组织分析结果显示, 300M钢表层全脱碳区为珠光体或铁素体, 半脱碳区为马氏体及析出碳化物. 本文还阐明了300M钢表层微观组织随保温时间的演化规律. 最后本文从热力学角度揭示了300M钢表层脱碳-氧化耦合机制. 本文的研究成果有望为300M钢的模锻优化工艺和热处理防护技术提供重要的数据和理论支撑.

Study on the evolution and protection of decarburization in 300M steel

300M steel is an important material for fabricating main bearing structure of aircrafts. However, the surface oxidation and decarburization of this material will occur in varying degrees during the heat treatment of die forging process, which seriously affects the yield of forgings. Herein, the decarburization behavior of 300M steel under different heat treatment conditions is systematically studied. The morphology and thickness of the oxide layer are analyzed by scanning electron microscope (SEM) and the corresponding Energy Dispersive Spectrometer (EDS). The variation of decarburization depth is measured using microhardness method. The evolution of decarburization behavior of 300M steel under different heat treatment conditions is clarified, and the effect of anti-oxidation coating on the depth of decarburization layer is presented. The results show that the dense oxide layer formed on the surface of 300M steel during heat treatment has distinct protective effect, which can prevent the matrix carbon from diffusing to the surface, thus reduce the depth of decarburization layer. There is a balance between the diffusion rate of carbon and the surface oxidation rate at different temperatures, and the application of anti-oxidant coating on the surface of 300M steel will change the original balance. The results of microstructure analysis show that the full decarburization zone on the surface of 300M steel is composed by pearlite or ferrite, and the semi-decarburization zone is mainly martensite and precipitated carbide. We also expound the evolution of the near-surface microstructure of 300M steel with the increase of holding time. Finally, the coupling mechanism of decarburization and oxidation on the surface of 300M steel is revealed from the perspective of thermodynamics. The results of this paper are expected to provide important data and theoretical support for establishing optimization process of die forging and developing heat treatment protection technology of 300M steel.