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Metallographic analysis of premature bearing failure
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First, the failure of a power plant in the assembly of the S195 diesel engine gearbox; the 6205 deep groove ball bearing inner ring successively or successively cracked (block) appearance; in the quantity of a certain share (four sets per gearbox) According to the scene, the total supply of the batch of bearings is 3,000 sets; 1000 sets have been installed in practice; the remaining 2000 sets are not ready to install due to the userâ€™s â€œheart and soulâ€.
It has been verified that the scale of the gearbox shaft meets the requirements of the drawings; the bearing installation method is correct; it is completed by a fixed skilled worker. The user has unpacked 40 sets to measure the hardness of the inner and outer rings of the bearing; and the DZ-2000 large-scale magnetic flaw detector is used for flaw detection. The hardness of the results is in the range of 61-65HRC; it conforms to the JB1255 specification rule; no cracks are found by the naked eye. In view of this; in order to plead for the cause of cracking of the bearing ring; the bearing that was damaged at the site is returned; the physical and chemical tests are carried out.
These ferrules are heated and quenched by box type resistance furnace 840 ~ 860 Â° C; 160 Â° C Ã— 2 ~ h low temperature oil tempering treatment.
Second, experimental methods and results 1. Microscopic survey Figure 1 shows the photos of the damaged bearing when installed.
After careful investigation of the matter, we can know:
(1) No damage marks left by the installation of the machine on all the damaged bearings; the steel ball, the support frame and other parts are intact.
(2) The cracking of the inner ring starts at the side of the middle of the raceway; it occupies about 1/2 or 1/4 of the diameter of the end face.
(3) There is a secondary longitudinal straight crack extending into the inner diameter surface at the fracture; the crack is shaped like a knife; the wall is penetrated.
(4) The shape of the entire crack is uneven; but the fracture surface is smooth and flat. There is no trace of plastic deformation; only a few tear seams.
(5) The fracture is gray and fine grained porcelain brittle fracture.
2. In the acid etching experiment, the cracked inner ring and the intact inner ring were respectively subjected to hot pickling with a 1:1 aqueous hydrochloric acid solution; the temperature was 60-70 Â° C; the time was 10 min; the results are shown in FIG. 2 .
(1) Strip-like grinding burns and grinding cracks in the adjacent raceway.
(2) The grinding crack is dendritic; spread along one side of the raceway; and parallel to the grinding direction; the cracking happens to occur along the grinding crack.
(3) The secondary longitudinal straight crack and the cracking fracture and the grinding crack are roughly "T" shaped straight.
(4) The entire ferrule is interspersed with loose black dark spots and pores; its shape is circular or elliptical. It is investigated under a 40-fold magnification stereo microscope; its shape is irregularly empty or round pinholes; They can be connected to each other in a grain-like shape; the appearance of a perfect ferrule is brighter; the above disadvantages are not seen.
3. Micro-hardness and micro-arrangement In order to further know the hardness distribution of the ferrule and the micro-arrangement of the quenching and tempering micro-arrangement and the surface layer; select some cracked inner ring and its fragments; and set the micro-hardness on the end surface; The longitudinal metallographic sample was taken adjacent to the crack at the cracking line by wire cutting; the result was:
(1) The micro hardness value is 64 to 65. 2HRC.
(2) The quenching and tempering arrangement is in accordance with the 3 to 4 grades of JB1255; the partial area is up to 5; it is finely crystalline martensite + cryptocrystalline martensite, ten fine needle-shaped martensite + a few residual carbides + residual austenite body.
(3) Investigation at 100 times; the martensite matrix is â€‹â€‹markedly a non-strip arrangement; if it is according to the YB9-59 specification, it can be rated as 4; as shown in Fig. 3.
(4) It is investigated that there is a layer of crescent-shaped secondary quenching white layer arrangement and a sub-surface high-temperature softening layer arrangement; that is, â€œover-tempering layerâ€. Its white layer depth (outer surface to crescent bottom) It is about 0. 033 ~ 0. 056mm; the depth of the over-tempering layer is about 0. 293mm; the degree of grinding burn is very severe; as shown in Figure 4.
Third, the results of analysis 1. The effect of grinding quality on premature failure of bearings can be judged from the above test results; grinding cracks and grinding burns on one side of the raceway are a direct cause of the cracking of the ferrule and the premature failure of the bearing. The microscopic shape of the crack and the scattering characteristics of the burned burn are clarified; the original orientation of the raceway is too biased or the R of the raceway is not round; the grinding allowance on one side of the raceway is increased; when the coarse grinding is large, the feed is large When the cutting speed is fast; the entire raceway will have uneven grinding. If the grinding wheel is not trimmed in time; it will promote the embedding or sticking of the grinding wheel to form the grinding wheel pore blockage and passivation; thus, the grinding wheel imbalance will be increased; the spindle oscillation Aggravation; even the machine oscillates and attacks the grinding flutter. The significant grinding force changes periodically; it constitutes a continuous burn band.
In this way, the rubbing force and the grinding force of the grinding zone will inevitably increase; the high grinding heat will occur. When the temperature of the grinding zone is as high as or exceeds Acm, the raceway portion will be austenitized. At this moment, the volume is swollen; while the low temperature of the inner layer prevents it from swelling; the appearance is tight and the inner layer is the thermal stress of expansion. When the grinding wheel is out of the grinding appearance. In the quenching condition of the external coolant (even if it is air-cooled); the austenitized thin layer on the outside is subjected to the second bonfire from the head; the secondary hardening arrangement of high hardness is formed; that is, "grinding the white layer". Together Because the grinding heat inward and its adjacent layer of the soaking diffusion effect; make it constitute a layer of lower hardness black "over-tempering layer"; at this moment the volume is shortened; thus in the grinding of the outer surface layer has a higher Temperature gradients; then incur a series of changes in arrangement and changes in mechanical function. The "grinding white layer" is in the state of compressive stress; the "over-tempering layer" is in the maximum tensile stress state; thus the crack center is the simplest to germinate here; especially the "grinding white layer" and the "over-tempering layer" ; on the border; because this is the most significant change in the arrangement; the tensile stress is most concentrated; the strength is the weakest; when the residual tensile stress is greater than the breaking strength of the data; the crack is enlarged in the radial direction; Cracking; that is, grinding cracks.
2. The characteristic of the secondary quenching white layer arrangement is that the white layer is a multi-phase high dispersion arrangement in which austenite, martensite and carbide coexist. There is a coherent pseudo-equilibrium system between austenite and martensite and the lattice constant is different; therefore, when martensite is formed; the austenite of the second austenite is intensely darkened; this leads to the white layer. Arrange the increase in hardness. Also, the surface energy at the austenite-martensitic interface tends to zero; under the effect of the corrosive agent; the electrochemical cell is not arranged; therefore, the white layer is difficult to corrode; White layer formed under grinding conditions.
3. The influence of steel quality and quenching and tempering quality on induced grinding cracks. The martensite matrix is â€‹â€‹non-concentration difference when quenching and tempering; the higher micro hardness value and looseness defects; it is also a potential for inducing grinding cracks. The presence/absence of the difference in concentration indicates that there is a severe banded carbide in the original arrangement; thus causing segregation of the carbon concentration between the band and the band; thus, the quenching increases the tendency of overheating between the bands and the unevenness of the Ms point. Different; constitutes the quenching arrangement and the dispersion non-uniformity of the residual stress and the added matrix brittleness; weakens the bonding force between the grains and the grains, the grains and the matrix, and the matrix and the substrate. Although the ferrule is now tempered by low temperature oil; however, the tempering is not sufficient or the martensite matrix is â€‹â€‹tempered under such conditions; its tempering resistance is high; the quenching martensite tempering transition is incomplete; It still adheres to the quenching condition of high hardness and high internal stress. These elements are beneficial to the gathering of grinding stress; it is not easy to disperse; it is easy to overlap with the residual stress of the matrix; especially in the super-scale banded carbide with the mesh and strip In the case of carbides, the grinding stress is most likely to be gathered; the strips and the residual network cracks that promote the surface of the parts to fall off and scatter are scattered. The significant looseness is not allowed on the bearing rings of the products. It will Decrease the density of the ferrule; enrich the interior with more micropores; low melting point impurities; gas and non-metallic inclusions. It is the same as the banded carbide; it has the effect of cutting the matrix connection; the composition is segregated; the matrix is â€‹â€‹brittle; the mechanical function of the ferrule is greatly reduced; and the frictional resistance and the notch sensitivity are increased during grinding; the grinding stress is promoted. Height gathering; it is easy to induce grinding cracks.
4. The characteristics of grinding cracks are analyzed and the grinding cracks are slender; only the shallow surface of the outer surface is formed; the opening pole is extended. (11) Zheng Huang and the value of the sneak sneak peeks into the salt and the tired yellow æ¹« å®å•µ å®å•µ å®å•µ ã“ ã“ ã“ ã“ : : : : : : : : : Xin Yanpu's fierce Oé’— capsule recovery. Often the external force effect makes the movement of the internal layout of the metal directional; sometimes even under the effect of small external force; only the external force effect is sufficient at all times; Cracking. Sometimes there is no external force at all; because of the diffusion of vacancies and plugs inside the metal; the release of residual stress; the expansion stress of the retained austenite martensitic phase; the aggravation of atomic motion; Under the temperature conditions; some gaps in the appearance; such as cracking, burns, cracks, typing marks, turning marks, sharp corners, loose holes and partial tensile stress gathering areas; the same will cause cracking (self crack); Cracking (disruption, knocking, shoulder removal and detachment); forming a large piece of shedding block.
There are two most common microscopic shapes for grinding cracks; that is, several parallel lines and tortoises at right angles to the grinding direction. The grinding cracks in this example are more special; they are parallel to the grinding direction. Because of this Delayed cracking of the ferrule during processing. Perhaps the crack is not fully formed; only the tensile stress is concentrated at this part. Or the crack is now formed; it is only slender; it is impossible to detect by magnetic force. Appearing; under the effect of thermal acid etching; grinding cracks similar to stress corrosion cracking (SCC) along the direction of maximum tensile stress; further cracking.
Undoubtedly; the impact of the knocking (shock) of the 6205 bearing during the installation and the elastic (plastic) deformation caused by the expansion between the shaft and the sleeve; it is the "natural" external force effect for the cracking of the ferrule. Moment; when the crack occurs, it will rapidly expand to a secondary crack in a straight method.
4. Conclusions (1) The uniformity and density of the data used in the ferrule are congenital elements that constitute the rupture of the ferrule.
(2) The ferrule is under the effect of intense grinding heat and coolant; the outer thin layer instantaneously constitutes a secondary bonfire to grind the white layer; a higher grinding tensile stress occurs; causing grinding cracks and grinding burn.
(3) The ferrule is not tempered; the martensite matrix still adheres to high hardness and brittleness; high residual stress; and micro-arrangement non-uniformity; it is easy to promote stress superposition and induce grinding cracks and grinding burns.
(4) It can be assumed that the premature failure of the 6205 bearing is the result of the inductive effect of the metallurgical defects, heat treatment defects and grinding cracks of the data; and the micro-arrangement non-uniformity (differential section arrangement) and the external part tensile stress are cracked. The root cause of this.
(5) The oscillation and expansion force of the bearing when it is installed is the "natural" external force effect and time for the cracking; the triggering ferrule suddenly cracks off the block.
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