【作者】 熊拥军；

【导师】 李溪滨；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 TiC钢结硬质合金硬度高、比重小、耐磨性好,可机加工和热处理,广泛应用于工业模具、量具和抗震零件,也是航天、航空、船舶和兵器等领域用耐磨零件的重要制备材料。目前,我国军工和高科技领域用TiC钢结硬质合金基本依靠国产,与国外同类材料相比,国产TiC钢结硬质合金常常存在孔隙率过高,材质不均匀及强度偏低等问题,致使材料在加工和使用过程中出现各种问题,如：加工成品率低,抗磨损能力不足等。粉末冶金技术可以制造原料比重差异大、成分复杂的金属复合材料,通过液相烧结的方法可使TiC相与钢基体牢固结合,并赋予材料良好的力学性能及抗磨损能力。本论文采用粉末冶金方法制备了TiC钢结硬质合金,并就制备工艺、羰基铁粉替代还原铁粉、添加稀土元素对合金性能的影响,以及合金致密化技术和组织结构等内容进行了研究,获得主要结论如下：真空(真空度<4.5×10-1pa)环境下,采用粉末冶金液相烧结方法制备了Fe-3.0Cr-3.0Mo-0.5Cu-0.4Ni-0.5C-33TiC合金,研究了粉末处理技术、成形工艺和烧结工艺等因素对合金性能的影响。高能球磨过程中,元素Fe与Mo、C、 Cu、Ni等组元发生合金化反应,形成Fe的固溶体,随着球磨时间的增加,合金组织细化,成分均匀性增加；压坯密度随着成形压力的增加而提高,但成形压力相同时,冷等静压成形合金的抗弯强度、硬度和密度均高于模压成形；在一定范围内,合金的抗弯强度、硬度和密度随最终烧结温度的升高和保温时间的增加而提高,当最终烧结温度为1420℃,保温时间1h时,合金综合性能最佳。以羰基铁粉替代还原铁粉制备了相同成分的TiC钢结硬质合金,研究发现：在TiC钢结硬质合金中添加羰基铁粉可以改善粉末成形性能,使压坯密度更高,促进烧结进程,使烧结温度降低。提出羰基铁粉粒度小,装模过程可充填大颗粒间隙,呈圆形,成形过程在粉末颗粒之间滚动,减小粉末流动及变形阻力,是改善成形性能的主要原因。比表面积大、原子迁移距离缩小是促进烧结进程的主要原因。经1400℃烧结后,与未添加羰基铁粉的合金相比,添加30%羰基铁粉的合金力学明显提高,硬度由77.5HRA提高到81.8HRA,抗弯强度由1123MPa上或到1447MPa。制备了含微量稀土(RE)的TiC钢结硬质合金,比较了Fe-RE中间合金和CeO2两种不同添加形式对合金性能的影响。实验结果表明：以Fe-RE中间合金的形式添加RE效果更好,在球磨和烧结过程中,RE对合金粉末具有明显的脱氧和保碳作用,有助于不同组元间润湿性的提高,从而有利于致密化进程,达到减小孔隙率的目的。提出在液相烧结过程中,稀土聚集于TiC颗粒表面,使其表面能降低,阻碍TiC在液相中的溶解-析出过程,抑制TiC晶粒的长大。Fe-RE添加量为0.8%时,合金综合性能最佳。研究了锻造及热等静压工艺对钢结硬质合金致密性的影响。研究发现：采用自行设计锻模锻造时,合金受力状态有利于抑制裂纹的形成与扩展,合金开裂现象显著减少。通过锻造,合金中的孔隙变小或消失,TiC及其它碳化物脆性相碎裂细化,分布趋于均匀,力学性能提高。通过极差实验,确定了该合金的理想热等静压参数为：压力：140MPa；温度：1260℃；保温保压时间：0.5h。经过热等静压处理,合金在烧结过程中产生的孔洞等组织缺陷减小或消失,材料更加致密,材料综合性能明显提高。将锻造与热等静压工艺结合,合金的组织及性能改善更加明显,硬度达到86.8HRA,抗弯强度达到1677MPa。组织结构研究发现：烧结和热等静压过程中,TiC在基体液相中存在溶解-析出过程,伴随这一过程,小尺寸TiC颗粒减小或消失,大尺寸TiC颗粒长大且形状变得更加圆滑。本研究制备TiC钢结硬质合金界面处存在成分过渡,且无界面反应物生成,TiC相与基体结合良好。断口分析发现：基体断裂面表现为具有脆性特征的准解理断裂,TiC相断裂特征受颗粒尺寸影响,尺寸较大的TiC颗粒(>2μm)以解理断裂为主,尺寸较小(<1μm)的表现出沿晶断裂特征。更多还原

【Abstract】 Steel bonded titanium carbide (SBTC), which has low proportion, high hardness, good high-temperature stability, advantageous machining and heat treatment performance, has been widely used in making industry molds, measuring tools, rigid shock-resistant parts. It is important materials of preparation wear-resistant parts that were used in aerospace, aviation, ships and weapons. At present, compares with the foreign productions, the SBTC of China’s military and high-tech areas mainly relies on domestic production, which has high porosity, uneven texture and low strength, often exists some problems such as low machining yield, low abrasion ability etc. during machining and application. Powder metallurgy (PM) technology can manufacture metal matrix composites with different proportions’raw powder or complex compositions, and liquid phase sintering technology can strongly combine TiC and steel matrix, therefore, endow the alloy with excellent mechanical and wear resistance properties. SBTC was prepared by PM here, and the influences that preparation process, carbonyl iron powder substituting of reduced Iron powder and adding rare earth into raw powder on alloy’s performance were studied, as well as alloy densification method and interface structure. Main results are as follows:The Fe-3.0Cr-3.0Mo-0.5Cu-0.5C-33TiC alloy was prepared by liquid phase sintering technology in the condition of vacuum degree below4.5×10-1pa. The influences of powder processing, pressing process and sintering process on alloy were evaluated. Alloying reaction happened between Fe and Mo, C, Cu, Ni, and the solid solution of Fe formed during high-energy ball milling. Billet’s density increased with pressing pressure rising, however, alloy’s bending strength, hardness and density by cold isostatic pressing (CIP) were higher than moulding pressing (MP) in same pressure. Within a determinate range, alloy’s bending strength, hardness and density improved with the final sintering temperature and heat preservation time increasing, and the best comprehensive performances acquired by1420℃/1h.SBTC was prepared with the same constituents in which carbonyl iron powder substituted reduced iron powder partly. The study found that adding Carbonyl iron powder can promote powder’s forming performance, pressure density, sintering process and decrease sintering temperature. The study revealed that carbonyl iron powder, which filled into the space among large particle in mold process and rolled in press process, can improve formability, and can promote sintering process because of specific surface area and reducing atom migration distance. Via sintering at1400℃, the hardness of alloy, which contained30%Carbonyl iron powder, improved to81.8HRA from77.5HRA, and the bending strength improved to1447Mpa from1123Mpa.SBTC containing micro rare earth (RE) was prepared, and the different influence that Fe-RE and CeO2on alloy’s properties was investigated, the results showed that adding Fe-RE was better than CeO2. RE made powder deoxygenation, holding carbon and improving wetting during milling and sintering, therefore improving the densification process and decreasing porosity. In the course of liquid phase sintering, RE gathered at the surface of TiC particle and reduced surfacial energy, therefore, the dissolving-precipitating process of TiC in molten iron was inhibited, furthermore, the growing up of TiC grain was inhibited. The performance of alloy adding0.8%Fe-RE was the best.The influence of forging and hot isostatic process (HIP) on SBTC was studied. The study showed that forging plastic was markedly improved and cracking significantly reduced using own designed forging die, by which alloy suffered multi-directional stresses. The size of pore in alloy decreased or disappeared by forging, TiC and the other carbides brittle phase were broken into refinement and distributed more homogeneous, therefore, alloy’s mechanical performance improved. The ideal parameters of HIP is140MPa/1260℃/0.5h by range analysis. The defects in alloy such as pore, which formed during sintering process, reduced or disappeared after HIP. Alloy’s structure and performance improved more remarkable such as hardness to86.8HRA, bending strength to1677MPa, by incorporation of forging and HIP.The study on interface structure showed that TiC dissolved and precipitated in molten iron during sintering and HIP. With heat preservation time increasing, fine TiC particles decreased or disappeared, large TiC particles grown larger and appeared more smoothly. Components transition happened at the interface without interfacial reactionts, and had a larger bonding strength. Fracture analysis revealed that matrix fracture with quasi-cleavage features, the features of TiC fracture depended on particle size, the larger(>2μm) with cleavage fracture, the smaller(<1μm) with intergranular fracture characteristics.更多还原