形态、材料耦元对灰铸铁抗疲劳磨损性能的影响

发布时间：2018-09-18 21:57

【摘要】：机床导轨表面疲劳缺陷的出现严重影响加工产品精度而造成器件的报废,更为严重的是由内部不可见的疲劳裂纹扩展而引起的巨大安全隐患。为了解决这一难题,工业上一般采用在灰铸铁床身镶嵌硬质的钢片作为工作接触面,同时具备灰铸铁床身具有良好的减震性这一优异特点,从而最大程度上地降低疲劳缺陷出现的可能性。尽管镶钢导轨能显著地改善滚动导轨的服役寿命,但对于具有较长行径的导轨,需将多片钢片通过特殊方式拼接,以满足行径要求,导致在连接处疲劳失效更容易产生。同时,钢片底面与灰铸铁表面不能完全的契合,制约机床对震动吸收程度的进一步提高,因此,相应的缺点严重制约滚动导轨抗疲劳磨损性能更有效地改善。鉴于此,探索一种特殊的处理方式直接对灰铸铁材料进行强化处理,使其作为一个整体能替代甚至超越传统的镶钢导轨。对于灰铸铁而言,材料中弥散大量片状石墨,其尖端应力集中现象更易萌生疲劳裂纹,使灰铸铁材料疲劳失效机理更为复杂。因此,改善在滚动疲劳接触条件下服役产品抗疲劳磨损性能以及了解疲劳缺陷形成机理,不仅可提高经济效益而且对实际生产和加工安全至关重要。基于耦合仿生学原理,利用激光局部处理灰铸铁表面,形成与耐磨生物体相似的体表结构,提高其抗疲劳磨损性能,揭示不同的表面仿生形态及其特征量、碳化物含量及合金元素改善灰铸铁材料抗疲劳磨损机理。结果表明:1.仿生单元体形态及其特征量严重影响仿生灰铸铁材料抗疲劳磨损性能改善程度;(a)表明不同的仿生形态(点状,条状及网状)对材料的抗疲劳磨损性能的影响程度。当单元体呈网状形态时,仿生单元体对辊子起到连续支撑,并在滚动方向形成明显软硬相间的仿生结构,仿生试样有最优的抗疲劳磨损性能,与基体相比其抗疲劳磨损性能改善程度达53%。(b)揭示了仿生试样抗疲劳磨损性能与单元体取向和滚动方向的关系。当单元体与滚动方向呈60°时,能将所产生的接触切应力分散到无限个切应力平面,最为有效地降低应力集中现象,从而具有最佳的抗疲劳磨损性能。(c)建立回归方程,揭示单元体间距对材料抗疲劳磨损性能的影响规律:当单元体间距大于2mm时,强化区域面积起主导作用,材料的抗疲劳磨损性能随间距的减小而增大;当单元体间距小于2mm时,不一致变形对疲劳寿命的影响更为显著,材料的抗疲劳磨损性能随间距的减小而减小。2.对碳强化仿生单元体对灰铸铁抗疲劳磨损性能的影响进行实验性研究。相对于熔凝单元体碳化物含量40-45%,渗碳单元体碳化物含量明显提高,高达60-70%。随着激光加工能量密度的降低,所制备的仿生单元体截面积尺寸减少,渗碳单元体中碳化物含量也相应地减少,仿生试样抗疲劳磨损性能也随之降低。不同预涂层厚度对渗碳单元体截面积尺寸影响较小,但过渡区组织随着预涂层厚度增加而变化,当厚度为0.3mm时,其组织完全为索氏体;晶粒尺寸随预涂层厚度减小而减少,并且整体材料的轴向和切向抗变形能力与晶粒尺寸密切相关,因此,仿生试样抗疲劳磨损性能的改善程度与预涂碳层厚度成反比。3.阐明了合金元素强化对仿生处理灰铸铁抗疲劳磨损性能的强化机制。通过激光合金化处理,使混合合金元素Cr+W渗入到熔池内部,形成较为复杂的共熔碳化物组织,例如(Fe,Cr,W)xCy,CrxCy,WxCy等。探明了Cr和W元素在单元体中的分布规律:Cr元素同时分布在晶界和晶粒上,而W元素则主要分布在晶界上。相对于单一合金元素的添加,混合合金元素Cr和W的添加使单元体内部晶粒和晶界很大程度上的增强。随着单元体的强化,单元体对疲劳微裂纹的扩展的阻碍作用更为明显,使整体材料抗疲劳磨损性能有效提高。4.揭示了仿生处理材料疲劳磨损过程的一般规律。接触疲劳缺陷产生的原因有:疲劳裂纹的扩展形成金属颗粒的移除,高应力区颗粒的压碎,粘着磨损形成的点蚀以及依附裂纹或石墨片而形成的金属移除。对于未出现疲劳缺陷的区域,在循环应力的作用下,表面发生明显的塑性变形,出现加工硬化现象,而其表面粗糙度明显降低,直到一恒定范围(285nm-325nm)。因此,在长期的疲劳磨损过程中,接触表面的磨损失重率趋于平稳并明显低于前期磨损失重率。5.对于激光处理局域,其内部组织晶粒致密度明显提高,晶粒尺寸明显降低,碳化物含量明显增多,并且出现大量的增强相,整体材料抗变形能力得到很明显的改善,疲劳缺陷出现的循环周期也随之而延长。此外,单元体的存在不仅降低基体区域所产生的接触应力,延缓疲劳微裂纹的萌生和扩展,而且增加高应力区强度,降低疲劳缺陷出现的可能性。
[Abstract]:The appearance of fatigue defect on the surface of machine tool guideway seriously affects the precision of machining products and results in the scrap of the device. What is more serious is the huge hidden danger caused by the fatigue crack propagation inside the machine tool. In order to solve this problem, the hard steel sheet embedded in the grey cast iron bed is generally used as the working contact surface in industry, and the tool is also used. Grey cast iron lathe bed has excellent shock absorption, thus reducing the possibility of fatigue defects to the greatest extent. Although steel-encased guideways can significantly improve the service life of rolling guideways, for guideways with longer trajectories, it is necessary to splice multiple pieces of steel in a special way to meet the trajectory requirements, resulting in fatigue defects. At the same time, the bottom of the steel sheet and the surface of gray cast iron can not fully fit, which restricts the further improvement of the vibration absorption of the machine tool. Therefore, the corresponding shortcomings seriously restrict the fatigue wear resistance of the rolling guide to improve more effectively. For gray cast iron, a large amount of flake graphite is dispersed in the material, and the stress concentration at the tip of the material is easier to initiate fatigue cracks, which makes the fatigue failure mechanism of gray cast iron more complex. Therefore, the service production under rolling fatigue contact condition is improved. The fatigue wear resistance of gray cast iron and the formation mechanism of fatigue defect can not only improve the economic benefit but also is very important to the safety of production and processing. The results show that: (1) the morphology of bionic unit and its characteristic amount seriously affect the improvement of fatigue wear resistance of bionic gray iron materials; (a) the fatigue wear resistance of gray iron materials is improved by different bionic forms (dot, strip and mesh). When the element is in a network shape, the bionic element acts as a continuous support to the roll and forms a bionic structure with obvious soft and hard phases in the rolling direction. The bionic specimen has the best anti-fatigue wear performance, and its anti-fatigue wear performance improves by 53% compared with the matrix. The relationship between fatigue wear resistance and element orientation and rolling direction is studied. When the element and rolling direction are 60 degrees, the contact shear stress can be dispersed to infinite shear stress planes, and the stress concentration phenomenon can be reduced most effectively. Thus the fatigue wear resistance of the element can be optimized. (c) The regression equation is established to reveal the relationship between element spacing and material. The influence law of fatigue wear resistance is as follows: when the space between elements is larger than 2 mm, the area of strengthening zone plays a leading role, and the fatigue wear resistance of materials increases with the decrease of space; when the space between elements is less than 2 mm, the effect of inconsistent deformation on fatigue life is more significant, and the fatigue wear resistance of materials decreases with the decrease of space. The effect of carbon-strengthened bionic unit on the fatigue wear resistance of gray cast iron was studied experimentally. Compared with the carbide content of melting unit body 40-45%, the carbide content of carburizing unit body increased significantly, up to 60-70%. With the decrease of laser processing energy density, the cross-sectional area of bionic unit body decreased and the carburizing unit body decreased. The content of carbide in the cementite decreases correspondingly, and the fatigue wear resistance of the bionic specimen decreases accordingly. Different pre-coated thickness has little effect on the sectional area size of the cementite element, but the microstructure in the transition zone changes with the increase of the pre-coated thickness. When the thickness is 0.3 mm, the microstructure is completely sorbite. Therefore, the improvement of fatigue wear resistance of bionic specimens is inversely proportional to the thickness of pre-coated carbon layer. 3. The strengthening mechanism of alloying elements on the fatigue wear resistance of bionic gray cast iron is clarified. The laser alloying treatment can mix the bionic gray cast iron. The alloying elements Cr+W penetrate into the molten pool and form more complex eutectic carbide structures, such as (Fe, Cr, W) xCy, Cr xCy, W xCy, etc. The distribution of Cr and W elements in the element body has been studied. The distribution of Cr and W elements in the element body has been found out: Cr elements distribute simultaneously on the grain boundaries and grains, while W elements distribute mainly on the grain boundaries. The addition of Cr and W greatly enhances the internal grain and grain boundary of the element. With the strengthening of the element, the inhibiting effect of the element on the fatigue crack propagation becomes more obvious, and the fatigue wear resistance of the whole material is improved. 4. The general rule of the fatigue wear process of bionic materials is revealed. The reasons are as follows: the propagation of fatigue crack leads to the removal of metal particles, the crushing of particles in high stress zone, the pitting corrosion caused by adhesive wear and the removal of metal formed by attaching cracks or graphite flakes. As a result, the wear weight loss rate of the contact surface tends to be stable in the long-term fatigue wear process and is significantly lower than that of the previous wear. 5. For the laser-treated locality, the grain density of the internal microstructure increases, the grain size decreases and the carbide decreases obviously. In addition, the existence of the element not only reduces the contact stress in the matrix region, but also delays the initiation and propagation of the fatigue micro-cracks, and increases the strength of the high stress region. Reduce the possibility of fatigue defects.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG502.4;TG143.2

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