摘要
金属密封球阀作为介质流动环节的重要部件,由于其具有流体阻力小、操作方便、启闭迅速、密闭性好和可靠性高等优点,被广泛应用于电力、水利、化工、油气、冶金等行业。为满足所在工况的苛刻要求,阀芯球体往往需要进行表面强化处理。然而,通过HVOF喷涂硬质合金、氮化处理等方法强化表面(HRC≥65),却给后续砂轮磨削带来了难度。阀芯球体的大小不同,使用杯形砂轮进行球面磨削时有两种方式:一是针对大型球面的杯形砂轮无摆动磨削;二是针对中小型球面的杯形砂轮回转摆动磨削。本论文研究的是使用金刚石颗粒杯形砂轮进行两种球面磨削时的表面质量与参数选择之间的联系。
磨削表面质量指标包括表面粗糙度、表面轨迹纹理、表面硬度和表面残余应力及微裂纹。针对这四个方面,具体研究内容如下:
首先,基于球面成型磨削原理和坐标变换理论,推导了杯形砂轮无摆球面磨削和回转摆动球面磨削时的轨迹方程。分别对两种磨削方式下轨迹方程所表现的单个砂轮块的轨迹纹理进行理论分析和实验佐证,发现杯形磨盘转速、主轴转速、摆动速度等加工参数的两两关系直接影响磨削轨迹纹理的形态。
其次,多个砂轮块磨削时轨迹纹理相互叠加。定义轨迹点密度对磨削落点进行采集,研究了随杯形砂轮尺寸变化的轨迹点密度分布。同时,用实验方法获得了随杯形砂轮尺寸变化的球面粗糙度分布。结论表明,考虑粗糙度分布和加工时间因素,回转摆动式球面磨削时选取砂轮半径区间为[0.6L, 0.8L]。又对因初始误差引起的多个砂轮块轨迹“缺相”问题进行了研究,提出了对初始误差极值的控制要求。
最后,为研究加工参数对磨削表面微观特征的影响,针对主轴转速、砂轮转速、摆动速度和进给量四个参数,使用扫描电子显微镜(SEM)对9组WC-Co涂层球面磨削试样观察。分析表明,进给量对微观表面的影响最为显著,WC-Co涂层的球面磨削缺陷形式为材料呈颗粒状或片状的脱落,烧伤形式为WC的脱碳及生成碳质结点。又用X射线衍射(XRD)法研究了四种磨削进给对表面残余应力的影响,结论指出适宜进给量应取ap = (1~1.5)μm。
As the crucial component on media-flow process, metal sealing ball valve is widely applied in industrial areas such as electric power, water conservation, chemical engineering, oil gas, metallurgy, etc. It has the advantages of less fluid resistance, convenient operation, quickly opening and closing, good sealing and high reliability. In order to meet the strict requirements in working condition, the surface strengthening is obliged to ball valve core, always using HVOF hard alloy, nitriding process (HRC≥65). However, it also brings some difficulties to the subsequent grinding. For different sizes of valve core sphere, two methods spherical grinding with cup wheel are usual: the first is cup-wheel spherical grinding without swing (CSGNS) for the large-scale workpiece, and the second is cup-wheel spherical grinding with swing (CSGS) for the medium and smaller one. This work presents the relationship between surface quality and parameters selection with diamond abrasvie wheel for two methods of spherical grinding.
Indexes of grinding surface quality include the surface roughness, surface exture on trajectores, surface hardness, and surface residual stress and cracks. According to the four aspects, this paper launched some researches:
Firstly, two trajectories equations for CSGNS and CSGS were derived based on the spherical surface forming grinding principle and coordinate transformation theory. Some analyses and two experiments were carried out based on its texture formed by trajectories of a single grinding block in two conditions. It reveals how relationship between some key parameters (i.e., cup-wheel rotation speed, spindle speed, and swing speed, etc.) is to affect the trajectories texture.
Secondly, due to the multi-block superposition of grinding trajectories, density of trajectories points (DTP) was defined and quantized in subsection. Relationship between distribution of DTP and distribution of surface roughness was studied among different grinding wheel radiuses. Conclusions show that the appropriate area for cup wheel radius is [0.6L, 0.8L] under the consideration of roughness distribution and machining time. In addition, the "lack-phase" problem of trajectories caused by the initial error was researched, and the initial error control and the maximum value requirements were proposed.
Finally, grinding surface micro-characteristics on WC-Co coating were watched out via scanning electron microscopy (SEM) aiming at spindle speed, cup-wheel rotation speed, swing speed and feed amount. It comes out a conclusion that the amount of feeding is above the other three priorities in the respect of grinding surface quality. The defect in the form of granular or flaky fall-off material is shown in spherical grinding on the WC-Co coating, and WC is vulnerable to decarburization and carbonaceous nodes are generated. Furthermore, the residual stress of surface was analysed through X-ray diffraction (XRD) method based on four ways to feed having an influence on it. It shows that the appropriate feed amount is ap = (1~1.5)μm.
引文
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