| Fatigue wearis a type of surface failure occurred on contact surface, which is resulted from the repetitive contact stress under rolling fatigue contact. Therefore, the service life of material is intensively affected by the q uality of contact surface. For the machine tool guide, the occurrence of fatigue defect mayaffects production precision or result in scrapping of components as well. More seriously, potential safety hazard are also caused by the propagation of invisiblefatigue crack in the interiorof material.In order to resolve this, steel disc is mounted into gray cast iron(GCI) column as working surface in industrial. Thus, it not only reducesthe probability of the occurrence of fatigue defects, but also possesses the good shock resistance of GCI, which could decrease the possibility of occurrence of fatigue defects in the large extent.Although inlaying steel guide-ways can prominently improve the service life of rolling guide way, it needs to join pieces of steel disc together by special way to accod to travel path standard for the long guide way, and therefore it leads to the fatigue failure prefer to emerge at the joint. Additionally, it can be not absolutely combination between steel disc and GCI surface, which causes the obvious reduction of shock absorption. Hence, the corresponding disadvantages of inlaying steel guide-ways significantly restrict the more effective improvement of fatigue wear resistance. In light of this, it is necessary to explore a special method for the reinforcement of GCI surface, which maks the bulk GCI to replace or even transcend traditional inlaying steel guide-ways. According to previous studies, surface treatment, such as PVD, C VD, ion sputtering, etc., could effectively enhance the surface property, and subsequently improve the fatigue wear resistance(FWR) of material. However, the combination be tween the reinforced films produced by these methods and substrate is significantly weak, and it ignores the effect of material’s nature defects as well.In terms of g CI, as amounts of graphite disperses in matrix,stress concentration is ad t to emerge at the tip of graphit, which result in the mechanism of faitigue failure to be more complex. Therefore, for the guidance of production and security process, it is vital to improve the FWR of service componentunder rolling contact fatigue condition and infer the mechanism of formed fatigue defects.Basing on the bionic theory,GCI surface was locally modified by laser to formthe surface feature of wear-resistant creature, to improve the FWRof bulk GCI.What’s more, the mechanisms of surface bionic shape and characteristic quantity, content of carbide and alloying element were experimentally investigatedon the improvement of FWR. The resuts were listed as follows:1. With respect to the way of bionic treatment, the degree of FWR improvement was significantly affected by the characteristic quantity of bionic unit for bionic treated sampel.(a) Indicating the effect degree of various bionic unit shapes(punctate, striate and reticulate) on the FWR improvement of GCI. For the sample with different types of reticular unit, not only the roller was always supported by bionic unit, but also the combination of unit on sample surface exhibited the prominent bionic modern of alternative soft-hard phase along rolling direction, which resulted inthe best improved degree of FWR, attaining 53%in comparison with original sample.(b) Revealing the relation of the included angle between the orientations of unitand rolling direction on the FWR of sample. When the orientation of unit was inclined to the rolling direction, the created contact shear stress could be depersed into various shear stress plain,which the most effectively decreased the phenomenon of stress concentration, and the responding sample performs the optimum FWR.(c) Establishing the regression equation to indicate the effect regular of unit space on the FWR of material: when the unit space was more than 2 mm,FWR of sample increased by the decreased unit space owing to the fact that the area of reinforced region was predominant on the improvement of FWR; on the contray, because the disadvantage effect incurred by uniform deformation was more remarkable, the FWR of sample decreased by the decrease of unit spacefor unit space less than 2 mm.2. After laser carburizing, the content of carbide in laser carburizing unit(LCU) obviously increased, as high as 60-70%, in comparison with that in remelting unit of 40-45%.By the decrease of laser energy density, the cross-section area of processed bionic unit decreased, and the content of carbide in LCU was correspondingly decreased, which leaded to the reduction of FWR improvement degree. Additionally, the thickness of pre-coating had minor effect on the volums of units. However, the area of transition zone(TZ) decreased by the increased pre-coating thickness, and it was absolutely disappearance until the pre-coating thickness attained 0.3 mm. What’s more, grain size was proportional to pre-coating thickness, and it demonstrated the outstandingdeformation resistance ability of both shear and axial for the sample with finer grain.The improvement degree of sample ’s FWR decreased in inverse proportion to pre-coating thickness.3. Reinforced mechanisms of GCI on the improvement of FWR affected by alloying elements wererevealed. By the treatment of laser alloying, the mixedalloying elements of Cr+W penetrated into remelting pool, and subsequently transformed into the complex eutectic carbide, such as(Fe, Cr,W)x Cy,CrxCy,WxCy, etc..Distribution regular of alloying elements was suggested that Cr distributed on the both crystal and grain boundary while W was mainly detected along the boundary. In comparison with the addition of single alloying element, the mixture of Cr and W could lead to the further enhancement of crystal and grain boundary. By the reinforcement of crystal and grain boundary, the propagation of fatigue crack in unit can be significantly hindered, which leaded to the FWR of sample be effectively improved.4. The general regular of fatigue wear processwas suggested. The reasons of fatigue failure were listed as follows: removal of metal particals by the propagation of fatigue crack, crushing in high-stress region, pitting by the adhesive wear and the metal removal attached to crack or graphite. With respect to the region without fatigue damages, the apparent surface plastic deformation was taken place and the phenomenon of surface hardening was incurred as well under the repetitive contact stress. Additionally, the roughness of contact surface significantly decreased until the range of constant value of 285 nm-325 nm. Therefore, the sample indicated a steadymass loss at the long-termfatigue wear, and the mss loss in steady wear period apparently demonstrated the less mass loss in comparison with that in early fatigue wear.5. For the miacrostructure of laser treated region, crystal density obviously increased, and the crystal also showed the finer grain size; what’s more, the content of carbide was prodominatly increased and amounts of reinforced phase weredetected. In light of this, the deformation resistance of bulk material was significantly improved which resulted in the extension of cycle period for the occurrence of fatigue defect. Furtheomore, the existence of bionic not only released the contact stress emerged on untreated region which can effectively postpone the initiation and propagation of fatigue crack, but also improved the strength of high-stress region which decreased the possibility of fatigue defect. |
PDF Full Text Request