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Bearing premature failure of metallographic analysis

Time:2016-06-27 15:24:22  From:Website    Back

DUF bearing in the job, as a result of a variety of complex reasons, such as the structure unreasonable, the material quality is poor, poor performance, work on the surface defects, shock, vibration, improper installation and bad lubrication etc, all can cause bearing early failure. However, in some cases, some bearing was not put into use (run), but users in machine process the original complete appearance is often damaged, lost the applicability, cause bearing premature failure. This paper tries to reveal is belong to the failure form of a practical example.

One, failure background

A certain power plant in assembly S195 diesel engine gear box, the 6205 deep groove ball bearing inner ring gap or continuously happen break-up (block) phenomenon, in quantity of certain proportion (per gear box four sets). According to the investigation on the spot, this batch of bearing total extreamely (-a ly for 3000 sets, the actual have been loaded with 1000 sets, the remaining 2000 sets for user "was" dare not installed.

After the verification, gear box axial dimension conforms to the drawing requirements, bearing installed method correctly, is the fixed journeymen finish. The user had unpacking extraction 40 sets of measurement bearing internal and external ring of hardness, and the DZ - 2000 type large magnetic inspection machine testing, the result hardness in 61 ~ 65 HRC range, in line with the JB1255 standard, visual check not found crack. In view of this, in order to beg a split bearing ring, the reason of the site would be damaged bearing return, the conventional physical and chemical test.

These ring system by box type resistance furnace 840 ~ 860 ℃ heating quenching, 160 ℃ x 2 ~ h low temperature oil tempering treatment.

Second, the test methods and results

1. The macroscopic observation

Figure 1 is installed in the damaged bearing real photos.

After careful observation physical known:

(1) in all of the damaged bearing not seen for machine knock and left traces wounded, the ball and cage the spare parts such as intact.

(2) inner ring raceway of split starting at one side of the center, occupy end diameter 1/2 or 1/4.

(3) fracture in stretch into inside the second longitudinal straight crack, crack is similar to knife cut, through wall.

(4) the break-up appearance rugged, but fracture surface smooth level off. No signs of plastic deformation, only a small amount of tearing crack.

(5) fracture with grey fine grain porcelain shape of brittle fracture.

2. Acid corrosion test

Respectively will split inner ring with complete inner ring with the 1-1 hydrochloric acid aqueous solution of the thermal pickling, temperature is 60 ~ 70 ℃, time 10 min, the results shown in figure 2.

(1) in the break-up place nearby raceway exist inside banded grinding burn and grinding cracks.

(2) grinding cracks are dendritic, along the raceway side distribution, and with grinding direction parallel, just burst along the grinding crack development.

(3) secondary longitudinal straight crack and burst fracture and grinding cracks is roughly "T" type vertical.

(4) the entire ring surface distribution with loose shape black scotoma and pore, its shape is round or oval. In the amplification 40 times the stereoscopic microscope observation, its shape is irregular hole or circular aperture, they can be connected into each other like grain boundary shape, and complete ring surface is quite bright and clean, could not see the above defects.

3. Apparent hardness and microstructure

In order to further understand ring of hardness distribution and quenching tempering microstructure and surface of the microstructure feature, and selected parts of the inner ring and burst fragment, in its upper end fixed macroscopic hardness, and linear cutting method in split place near interception longitudinal metallographic specimen, the result is:

(1) the macro hardness value for 64 ~ 65.2 HRC.

(2) the organization JB1255 quenching and tempering with standard 3 ~ 4 level, local area up to 5 level, for small crystal martensite + aphanitic martensite ten fine acicular martensite + a small amount of residual carbide + residual austenite.

(3) in 100 times and observed under martensitic matrix is obviously black and white banded organization, if according to YB9-59 standard can be judged to be 4 level, as shown in figure 3 shows.

(4) observed raceway surface exist a layer of a crescent shape of the secondary hardening white layer organization and sub-surface of high temperature softening layer organization, namely "after tempering layer". The white layer depth (the surface to link bottom) is about 0.033 ~ 0.056 mm, tempering depth about 0.293 mm, visible grinding burns very serious degree, as shown in figure 4 shows.

Three, result analysis

1. The grinding quality on bearing premature failure of the influence from the above test results can be judged, raceway side of the grinding cracks and grinding burn is a cause of ring break-up, cause bearing premature failure of a direct cause, this example reveals macroscopic crack form and grinding burn the distribution characteristics of instructions, raceway of the original position after partial or roller way R not round, one side of the raceway grinding allowance increases, when the coarse grinding feed stage, grinding speed, the whole raceway is produced not uniform grinding. If not timely grinding wheel dressing, will promote grindings embedded or adhesive cause wheel pore jams and passivation, which will cause the grinding wheel unbalance degree high, the spindle vibration increased, even machine vibration and produce grinding flutter. Obviously grinding force occurs cyclical change, form discontinuity burns banding.

This grinding area rate wipe force and grinding force will inevitably will heighten, produce high grinding heat. When grinding area surface temperature as high as moment or exceed the Acm, raceway local be ground surface will be back austenitizing. The volume expansion, and the low temperature of inner part to restrict its expansion, produce surface compression and lining for expansion of the thermal stress. When the wheel from grinding after surface. In external cooling liquid quenching conditions (even empty cold), the surface has austenitizing thin layer, again carries on the second Cui fire, forming high hardness of the secondary hardening organization, namely "grinding white layer". At the same time because grinding heat inward and its adjacent layer on the penetration of the diffusion effect, it can form a layer of hardness lower black "tempering layer", the volume shrinkage, so in the grinding surface layer is produced in the high temperature gradient, which cause a series of organization change and mechanical properties change. "grinding white layer" system in a pressure stress state, and "tempering layer" is in the largest tensile stress state, so the most easy to crack the core in the initiation, especially "grinding white layer" and "after tempering layer, on the border, because it


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