【作者】 樊恋；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2010， 硕士

【摘要】 YL20.3硬质合金系钨钴类硬质合金,为制造合成人造金刚石的硬质合金顶锤材料。深冷处理作为一种冷处理工艺能有效改变材料的组织结构,改善合金的性能,从而提高顶锤的质量和正常使用寿命。因此,将深冷处理工艺应用于顶锤用硬质合金材料是一个有实际应用价值的课题。本文通过改变深冷处理时间(0-72h)对YL20.3硬质合金烧结成品在-150℃下进行深冷处理,测试了深冷前后材料的维氏硬度、抗压强度、抗弯强度和冲击韧性等力学性能,结果表明深冷处理提高了硬质合金的硬度和抗压强度,并探索出一次2h深冷后显微硬度和抗压强度达到最大值。深冷后试样的抗弯强度均有较低程度的下降,不同深冷时间的冲击韧性值与深冷前相比较小范围内上下波动。随后,论文选择2h深冷试样进行多次循环深冷的探索性研究,结果表明深冷后试样的抗压强度值随循环次数的增加有所降低。采用X射线衍射对深冷处理前后的硬质合金相结构和表面宏观残余应力的变化情况进行了分析。结合材料力学性能和磁性能的检测结果发现,经过深冷处理,YL20.3硬质合金发生马氏体转变,Co粘结相发生α-Co向ε-Co的转变,合金表面残余压应力有较大程度的提高,相变和应力变化同时作用提高了硬质合金的硬度和抗压强度,相变使得材料饱和磁化强度下降,矫顽磁力升高。本文还对深冷处理前后硬质合金的断裂与常规机械疲劳性能进行了研究。疲劳实验结果表明,深冷处理2h有效的提高了合金的疲劳寿命。运用立式双筒显微镜观察了合金抗弯试样断口的宏观形貌,分析了材料的断裂行为,利用扫描电子显微镜研究了硬质合金的断裂方式,发现抗弯、冲击和低周疲劳试样断裂以沿晶断裂为主,同时存在Co相撕裂,而高周疲劳断口上有较多WC颗粒劈裂现象。疲劳断裂的断裂行为是材料在往复的受力作用下,相界面脱离产生空隙,从而产生裂纹,裂纹扩展汇集,直至产生疲劳条带,当达到材料疲劳极限时,瞬间断裂失效。更多还原

【Abstract】 YL20.3 cemented carbide, a kind of tungsten carbide-cobalt hard metal, is utilized in the manufacture of cemented carbide anvil that plays an important role in synthesis of artificial diamond. Deep cryogenic treatment (DCT) as a kind of cryogenic treatment helps to change the structure and improve the properties of the material in order to improve the quality and life span of anvil. Therefore, it is a valuable subject to do research on DCT in the field of anvil-use cemented carbide.In the present work, the cemented carbide samples are subjected to different time of DCT namely 0～72h at -150℃. The response of mechanical properties of YL20.3 alloy to such various hours of treatment is evaluated in terms of microhardness, compressing strength, bending strength and impact toughness. According to the result, after DCT microhardness and compressing strength enhanced and reached maximun after 2h of DCT while bending strength falls a little, and impact toughness fluctuates within a small extend compared to the value before DCT. There is also an attempt to do circular DCT at the base of 2h DCT. However, the compressing strength decreases with the increasing times.X-ray diffraction is used here to analyze the phase structure and the surface residual macrostress of the material before and after DCT. The experimental result indicates that martensite transformation occurs in the Co phase, that is, Co phase has a tendency of transformation from a-Co toε-Co, and the residual compressive stress on the surface obviously goes up. These changes give rise to the increasing of hardness and compressing strength. Phase transformation leads to the decline of magnetic saturation and the increase of coercive force.Apart from this, the study of fracture and fatigue of YL20.3 cemented carbide is involved in this paper. Sample after 2h of DCT shows higher fatigue strength of the alloy in cyclic compressing test. Macrostructure of bending test sample is obtained from vertical binocular microscope to analyze the fracture behavior. Scanning electronic microscope (SEM) is used to observe the fracture mode. The pictures show that as for the samples of bending test, impact toughness test and low-cycle cyclic compressing test, the brittle fracture of WC/WC boundaries and interfacial fracture of WC/Co boundaries are the main mode, and ductile tearing of Co binder phase also exists. But on the fracture of high-cycle cyclic compression sample, much cleavage fracture of WC particles can be found. The fracture behavior of cyclic compressing test is that, under a cyclic force phase interface divorces, then gap forms, and crack generates, expands, gathers until fatigue ribbon forms, and at the end the material fails.更多还原