Detection methods and bearing failure characteristics analysis related to grinding burns

Grinding burn is one of the bearing failure causes for components. In order to quickly confirm that grinding burn is the cause of bearing failure, detection methods for the grinding burns and characteristics of bearing failure caused by grinding burns have been studied in this paper. The results show that cold acid pickling examination, microhardness test and Barkhausen noise analysis can be used in the detection of heavy grinding burns but have several disadvantages to hinder their application on the detection of slight grinding burns. A grinding burn detection procedure is recommended for the analysis of bearing failure induced by grinding burn. It has been found that the modes of bearing failure induced by grinding burns were contact fatigue spalling developed from the contact fatigue crack, the propagation mechanism of the contact fatigue crack was obtained. And the unique phenomenon of fracture or fragmentation with cracks distributed in the origin or surrounding the spalling pit of the grinding burn layer was also obtained. Based on the above results the future work on detection methods and influence on bearing failure mechanism and life of grinding burn are proposed.


Introduction
Grinding is a key technology for production of advanced products in a wide range of industries and usually the final step of the manufacturing chain [1,2].And many components have high quality requirements of grinding surface integrity, such as the advanced bearing and crankshaft.While grinding burn known as one of the major defects which destroys the grinding surface integrity is often encountered in the grinding process [3,4].The serious grinding burns will significantly decrease working performance of the components and result in their failure [5][6].In order to prevent grinding burn and failure induced by grinding burn the appropriate detection methods of grinding burn should be selected.And therefore the characteristics of failure induced by grinding burn should be researched.

Materials and methods
Two 6305 deep groove ball bearings with artificially prepared heavy and slight grinding burns respectively in its outer ring raceway and one 6305 deep groove ball bearing without grinding burns were selected for the present study.Structural diagram and appearance of the 6305 deep groove ball bearing is shown in Figure 1 and Figure 2. Inner and outer rings, rolling elements of the bearings were made of GCr15 steel produced from ingot by forging.And the microstructures were consisted of tempered martensite, spherical primary carbides (Fe,Cr)3C, tempered carbides and retained austenite.The chemical composition of the GCr15 steel is shown in Table 1.Table 1.Chemical compositions of GCr15 steel (wt.%).
Cold acid pickling inspection, metallographic examination, microhardness testing and Barkhausen noise analysis were used for grinding burn detection of the outer ring raceway.The Cold acid pickling inspection procedure is shown in Figure 3. Figure 3. Cold acid pickling inspection procedure [19,20].Metallographic examination was carried out by Zeiss Axio-observer type optical metallographic microscope through the following way.Metallographic samples were prepared by Wire-Cut Electrical Discharge Machine along the circumferential and axial directions of the spalling, and no spalling grinding burn areas of the outer ring raceway existed.Then, the samples were inlaid, polished and corroded (the corrosive solution was 4% nitric acid alcohol solution), the characteristics of grinding burn layer, spalling and crack propagation were examined by optical metallographic microscope.Microhardness was measured on grinding burns and the adjacent matrix of the metallographic sample by TUKON2500 type microhardness tester.Barkhausen noise analysis was carried out by Rollscan 250 type Barkhausen noise analyzer through the following way.First, the optimal magnetization voltage of the detected outer ring was adjusted with the following method: the magnetization voltage was gradually increased in unit of 5 from zero, after the detected ring was placed in the detection position of the Barkhausen noise analyzer ; when the magnetization voltage increased and the magnetoelastic value did not increase with the increase of magnetization voltage, the detected ring had reached its magnetic saturation state, and the magnetization voltage which was 5 smaller than the magnetization voltage corresponding to the magnetic saturation state is the best magnetization voltage of the outer ring.Second, the surface magnetic field strength of the outer ring should not be greater than 3 Gs and be demagnetized when it was greater than 3 Gs.Contact fatigue test of the bearing was carried out on ЦКБ-72 bearing rig.Inner ring of the test bearing was assembled on the rig shaft, which was divided into four stations.The inner two stations were equipped with the accompanying bearings, and the outer two stations were equipped with the test bearings.The rig shaft was driven by the electric spindle through the coupling.Outer ring of the test bearing was assembled on the shell of the rig.The radial load was applied to the test bearing through the rig shell, and the axial load was applied to the test bearing through the end caps on both sides.Schematic diagram of the experimental assembly and loading of the test bearings is shown in Figure 4. Experimental condition of the test bearings is shown in Table 2.The macroscopic damage morphologies of the bearings were obtained by visual and Zeiss Smartzoom 5 stereo optical microscope observation.Microscopic damage morphologies of the bearings were acquired by Zeiss-Sigma500 type scanning electron microscope.

Cold acid pickling inspection
Cold acid pickling morphologies of the heavy and slight grinding burns in the outer ring raceways are shown in Figure 5.The heavy and slight grinding burns were composed of re-hardening burns with white (pale) colour and over-tempering burns with dark (black) colour.Grinding burn morphology of the heavy grinding burns was more obvious than that of the slight grinding burns.The heavy and slight grinding burns were both unevenly distributed along the axial direction of the outer ring raceways and more obvious in the axial ends of the raceway.The heavy grinding burns was banded along the circumferential direction of the outer ring raceway with the features of white (pale), dark (black) strips, adjacent and continuous distribution of patch shapes.The slight grinding burns was banded along the circumferential direction of the outer ring raceway with white (pale), dark (black), adjacent and continuous distribution of patch shapes in the axial ends of the raceway, and distributed along the circumferential direction of the outer ring raceway with the appearance of several white (pale) and dark (black) strips (not very obvious) in the inner areas of the raceway./ (a) heavy grinding burns, (b) slight grinding burns Figure 5. Cold acid pickling morphologies of grinding burns with different severity.

Microhardness test
Typical microhardness test result of the heavy grinding burns layer and adjacent matrix is shown in Table 3.The hardness of the re-hardening grinding burn layer was higher than that of the matrix.And the hardness of the over-tempering grinding burn layer was lower than that of the matrix.The hardness change trend is that the hardness first decreased and then increased as the distance from the surface to the matrix increased, when the grinding burn layer was composed of re-hardening grinding burn layer and over-tempering burn layer.Table 3. Hardness test result from grinding burn layer to matrix (HV0.3).

Barkhausen noise analysis
Maximum magnetoelastic values of Barkhausen noise of the outer ring raceway with heavy and slight grinding burns were higher than those of the outer ring raceway without grinding burns, as shown in Table 4.And maximum magnetoelastic values of Barkhausen noise of the outer ring raceway with heavy grinding burns was higher than that of the outer ring raceway with slight grinding burns.

Failure characteristics
The bearing failures caused by the heavy and slight grinding burns were characterized by contact fatigue cracks and contact fatigue spalling of the outer ring raceway.The contact fatigue cracks first appeared and then developed into contact fatigue spalling during the contact fatigue test.Since the contact fatigue cracks and spalling of the heavy and slight grinding burns are similar, the characteristics of bearing failure induced by grinding burns are mainly described by taking the characteristics description of bearing failure induced by heavy grinding burns as an example.The contact fatigue cracks with relatively straight edge appearances were mainly located in the working track of the raceway and distributed along or inclined to intersect with axial direction of the raceway, as shown in the Figure 6.The contact fatigue spalling induced by heavy grinding burn of the outer ring raceway had the following morphology characteristics: ① contact fatigue spalling developed from the propagation of contact fatigue cracks which originated from the raceway surface; ② axial length of the spalling was greater than circumferential length of the spalling in the initial stage of spalling; ③ the spalling propagated along the circumferential, axial and radial direction of the outer ring; ④ fatigue striations were observed in the propagation area of the spalling with light or no wear; ⑤ there were cracks in the non-spalling area around the spalling pit, and some of the cracks connected with the spalling, as shown in Figure 7.

Comparation of grinding burn detection methods
In essence, grinding burn is the result of material phase transformation caused by grinding heat, grinding force and cooling medium.The phase transformation includes re-hardening and over-tempering.Microstructure of the re-hardened zone is mainly secondary quenched martensite with stronger corrosion resistance and higher hardness than those of the matrix.The microstructure of the overtempered zone is mainly composed of tempered martensite, tempered troostite, tempered sorbite or granular pearlite with poorer corrosion resistance and lower hardness than those of the matrix.Therefore, grinding burn can be detected by microhardness test and acid corrosion examination methods such as cold acid pickling and metallographic examination.And grinding burn can also be detected by Barkhausen noise analysis.Since Barkhausen noise is produced by irreversible changes of domain rotation and domain wall motion inside the ferromagnetic materials applied by a outside time-varying magnetic field.And Barkhausen noise is sensitive to microstructure composition, crystalline defects, residual stresses and hardness [21][22][23].According to grinding burn detection results of the paper, heavy grinding burns can be detected by cold acid pickling inspection, metallographic examination, microhardness testing and Barkhausen noise analysis.While, these methods are not always valid, because of their disadvantages.In practical application, it is necessary to formulate appropriate cold acid pickling procedure according to the requirements of material chemical composition, microstructure characteristics and grinding burn detection accuracy.Moreover, it is not easy to distinguish the slight grinding burn from the matrix due to the small difference in colour, such as the grinding burns shown in Figure 5b and Figure 13.And cold acid pickling examination is costly for large bearings required the longer examination times, compared with inspection of smaller size bearings [24].Metallographic examination and microhardness test of grinding burn require sample preparation by cutting, inlaying and polishing the grinding burn.The sample preparation process is relatively complicated.The grinding burn position needs to be roughly determined in advance.When the grinding burn area is small, it is not easy to prepare the examination sample.Because the microhardness measurement has certain requirement on size of the grinding burn, the microhardness method can not be used for detection of grinding burn with small size, such as of the re-hardened zone with maximum depth of 0.008 mm shown in Figure 7b and the re-hardened zone shown in Figure 8b.

Spalling Grinding burns
Barkhausen noise analysis has been used as a nondestructive testing method due to the correlation of some parameters of the Barkhausen noise with the microstructure of the testing material [25][26][27].While, Barkhausen noise analysis of the grinding burns is affected by these factors [19,29]: ①surface integrity of the investigated area, such as microstructure composition (phase type, impurities, dislocation density, grain size and orientation), the residual stress, the hardness as well as the profile; ②the measurement and analyzing settings, such as the magnetizing and analyzing frequency; ③micromagnetic parameters selected to interpret the degree of grinding burns, such as the mp-value, coercivity and peak position.These factors make it difficult to accurately evaluate grinding burns by Barkhausen noise, and hence hinder the industrial application of Barkhausen noise.In a word, grinding burns of bearing can be detected by Cold acid pickling inspection, metallographic examination, microhardness testing and Barkhausen noise analysis.While, these methods have their advantages and disadvantages, as shown in Table 5.When detecting grinding burns in the failure bearings, we should make rational use of the advantages of these detection methods to avoid wrong judgments caused by their disadvantages.It is suggested that grinding burns of the failure bearing are detected by the following procedure.First, the failure bearing is detected by Barkhausen noise analysis in order to obtain whether there are grinding burns.Second, the failure bearing is detected by cold acid pickling examination in order to confirm detection results of Barkhausen noise analysis and positions of the grinding burns.Third, the samples of the grinding burns and failure positions are prepared for metallographic verification.At last, the microhardness testing can be carried out on these metallographic samples.

Detection method Advantages Disadvantages
Cold acid pickling inspection easy to be operated, and convenient observation.slight grinding burns can't be found, destroy the surface integrity, result can't be fully reproduced, automation is limited and costly for large bearing component.

Metallographic examination
detect light grinding burns, measure depth of the grinding burns, examine microstructure of the grinding burns, observe the distribution of spalling and cracks in grinding burns layer.location of grinding burns should be confirmed before preparing metallographic samples, difficult to detect grinding burns with small areas, detection of slight grinding burns is vulnerable to surface contaminants during corrosion.
Microhardness testing obtain hardness of the grinding burns, determine the type of grinding burns.location of grinding burns should be confirmed before preparing samples, difficult to detect grinding burns with small areas.
Barkhausen noise analysis influence of human factors can be avoided, nondestructive, result is reproducibility.complex relationship between surface integrity of grinding burns and signals of Barkhausen noise analyzer, require calibration blocks for accurate detection, almost impossible to develop grinding burn with defined characteristics on bearing components in a repeatable way [30].
① Failure mode of the bearing is contact fatigue spalling of the outer ring raceway.② The contact fatigue spalling develops from the contact fatigue crack.③The contact fatigue cracks are located in the working track of the raceway, and shows relatively straight appearances along the axial direction of the raceway.
④The contact fatigue crack propagates in such way: First, the crack propagates to the inside of the ring raceway along an acute angle with the tangential direction of the raceway surface; when the crack reaches to bottom of the arc-shaped spalling pit, it propagates for a distance and then turns to the raceway surface, causes the material above the crack to fall off; and finally induce an arc-shaped spalling pit.
⑤Adjacent contact fatigue cracks expansion shows the continuous characteristic.⑥Fracture or fragmentation of the grinding burn layer can often be found in the process of spalling origin and propagation, showing the unique phenomenon of cracks distributed in the origin or surrounding the spalling pit.
⑦Fatigue striations can be seen in the propagation area of the spalling with light or no wear.The above characteristics of bearing failure induced by grinding burns show that the failure is surface originated contact fatigue spalling developed from surface contact fatigue crack.It is similar to the results of the surface contact fatigue crack propagation studied by fracture mechanics and experiments [31][32], the contact fatigue spalling forms in the following mechanism: First, the contact fatigue crack initiates in the raceway.Second, the contact fatigue crack propagates nearly along the original crack plane by shear mode (mode II).As the crack propagates, the crack growth mode changes to tensile mode (mode I).Then, the crack propagates in the tensile mode and results in a spalling.The tensile mode is considered to be caused by oil hydraulic pressure resulting from lubricant oil penetration into the crack.And there are two hypotheses on the oil hydraulic pressure, as shown in Figure 14.One hypothesis ( No.1 hypothesis ) is that the contact pressure is directly transmitted to oil hydraulic pressure (tensile stress ) of the crack by the oil to the crack faces when the mouth of the crack is open.And the other hypothesis ( No.2 hypothesis ) is that oil hydraulic pressure ( tensile stress ) of the crack comes from the pressure due to oil entrapment induced by a closing crack mouth.According to the fact that fatigue striations can be seen in the propagation area of the spalling, meaning that the crack is not seriously closed, and it is considered that the No.1 hypothesis should be more possible.Contact fatigue is a process of damage accumulation and development to failure on the surface layer of the contact component under repeated contact stress.Contact fatigue is influenced by several factors such as the contact stress, lubrication and microstructure.The microstructure and hardness of the grinding burns is different from the matrix.This leads to inconsistent deformation between the grinding burn layer and the matrix under contact fatigue stress, and it is easy to form high stress in the grinding burn layer and cause cracks.And the inconsistent deformation should be the cause of the unique phenomenon of cracks distributed in the origin or surrounding the spalling pit.On the other hand, the outer ring raceway is easy to form a large tensile stress along the tangent direction of the raceway surface under the action of the outer ring assembly stress and working stress, which is also one of the reasons for the formation of contact fatigue cracks in the grinding burn layer of the outer ring raceway.

Bearing failure characteristics induced by grinding burns
Bearing failure induced by grinding burn has the following characteristics.

Future work
Cold acid pickling standard should be established according to the specific bearing steel materials and testing accuracy, because the detection method is influenced by the material chemical composition and microstructure characteristics.The interface between the over-tempering grinding burn layer and the matrix is not obvious in the microstructure state.This affects the evaluation accuracy of grinding burn.Therefore, it is necessary to explore the corrosion method that can be used to accurately distinguish the re-tempering grinding burn layer and the matrix.There are several factors to decrease the accuracy of Barkhausen noise analysis on the grinding burns.And it is very difficult to prepare the standard sample with the same degree of grinding burn as the actual workpiece.Therefore, the main future work on Barkhausen noise analysis is to study the process of preparing standard sample and establish the relationships between the Barkhausen noise and the influence factors.Although several distinctly different modes of contact fatigue failure have been suggested, the factors which control the nucleation and propagation of each type failure are not clear [18,33].Basic mechanisms of the contact fatigue failure induced by grinding burns are needed to be well understood, especially on the initiation of the contact fatigue crack.And grinding burns influence of bearing life is also needed to be completely studied.

Summary 1)
Cold acid pickling, metallographic examination, microhardness test and Barkhausen noise analysis can be used in the detection of heavy grinding burns of bearing.These detection methods have several disadvantages to hinder their application on the detection of slight grinding burns.It is recommended that at first to determine whether there is grinding burn by Barkhausen noise analysis, then detect the possible grinding burns by cold acid pickling, and finally confirm the grinding burns by metallographic examination and microhardness test during the analysis of bearing failure induced by grinding burn.
2)Modes of bearing failure induced by grinding burns are contact fatigue spalling developed from the contact fatigue crack of the outer ring raceway.The contact fatigue cracks locate in the working track and approximately distribute along or obliquely intersecting with the axial direction of the raceway.The contact fatigue cracks originate from the raceway surface, firstly propagate in shear mode, then propagate tensile mode and finally induce the spalling.Fracture or fragmentation of the grinding burn layer can often be found in the process of spalling origin and propagation resulting in the unique phenomenon of cracks distributed in the origin or surrounding the spalling pit.
3) Cold acid pickling standard should be established according to the specific bearing steel materials and testing accuracy.The corrosion method that can be used to accurately distinguish the re-tempering grinding burn layer and the matrix should be explored.The preparation method of Barkhausen noise analysis standard sample and the relationship between Barkhausen noise and influencing factors shall be further investigated.Basic mechanisms of the contact fatigue failure induced by grinding burns and grinding burns influence on bearing life are needed to be completely studied. .

Figure 1 .
Figure 1.Structural diagram of the 6305 deep groove ball bearing.

Figure 2 .
Figure 2. Appearance of the 6305 deep groove ball bearing.

Figure 4 .
Figure 4. Experimental assembly and loading of the test bearings.

6 .
(a) heavy grinding burns (b) slight grinding burns Figure Typical cracks of the outer ring raceway with grinding burns.

Figure 7 .
Figure 7. Contact fatigue spalling of the outer ring raceway induced by heavy grinding burns.The contact fatigue spalling pit of the outer ring raceway with heavy grinding burns was artificially opened along the axial direction of the outer ring, as shown in Figure8.And fracture morphology of the contact fatigue spalling pit is shown in Figure9.It can be seen that the spalling originated from the raceway surface and propagated obliquely along the circumferential and radial direction of the outer ring.After the old spalling propagated to a certain extent, a new spalling formed at back zone of the old spalling area.The old spalling and new spalling connected through the tail of the old spalling to origin of the new spalling.The connection of the old spalling and new spalling show the continuous characteristic of adjacent spalling expansion.A relatively flat fan shaped fracture surface and fatigue striations could be seen in the propagation area of the spalling.

Figure 8 .
Figure 8. Spalling and the opening position in the outer ring raceway.

Figure 9 .
Figure 9. Spalling fracture morphology of the outer ring raceway ( side B in Figure 8 ).Circumferential metallographic samples are taken from the spalling area, as shown in Figure 10.And the cross-sectional morphology and crack propagation path of the spalling and cracks are shown in Figure 11 and Figure 12.It can be seen that the spalling developed from the contact fatigue crack

Figure 10 . 11 .
Figure 10.Circumferential metallographic samples are taken from the spalling pit of the outer ring raceway with heavy grinding burn.

Figure 13 .
Figure 13.Slight grinding burns in the failure outer ring raceway.Metallographic examination and microhardness test of grinding burn require sample preparation by cutting, inlaying and polishing the grinding burn.The sample preparation process is relatively complicated.The grinding burn position needs to be roughly determined in advance.When the grinding burn area is small, it is not easy to prepare the examination sample.Because the microhardness measurement has certain requirement on size of the grinding burn, the microhardness method can not be used for detection of grinding burn with small size, such as of the re-hardened zone with maximum depth of 0.008 mm shown in Figure7band the re-hardened zone shown in Figure8b.

Figure 14 .
Figure 14.Schematic illustration showing occurrence of two kinds of oil hydraulic pressure action[[31].

Table 2 .
Experimental condition of the test bearings with grinding burns.

Table 4 .
Barkhausen noise analysis results of the outer ring raceway with grinding burns and outer ring raceway without grinding burns (mp).