1、99FTMI The Barkhausen Noise Inspection Method for Detecting Grinding Damage in Gears by: J .S. Ceurter, American Stress Technologies, C. Smith and R Ott, Harley-Davidson Motor Company American Gear Manufacturers Association TECHNICAL PAPER The Barkhausen Noise Inspection Method for Detecting Grindin
2、g Damage in Gears Jeffrey S. Ceurter, American Stress Technologies, Chad Smith and Roy Ott, Harley-Davidson Motor Company The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association.
3、Abstract Barkhausen noise is created by the abrupt changes in the magnetization of materials under applied ae magnetizing field. These cbanges are known to be affectcd by microstructure and the presence and distribution of elastic stresses. In the case of grinding damagc, both residual stress and mi
4、crostructural changes occur. Thc most common case of grinding damage results in a loss of hardness and compressive residual stress and is referred to as retempering. In this paper, the theory of Barkhausen noise will be reviewed with a focus on the application of non-destructively testing for grindi
5、ng damage on ferromagnetic materials. The theory of grinding will be reviewed, with respect to microstructural and residual stress changes, and examples will be given to show how Barkhausen noise is used to evaluate these changes. Following this introduction is a discussion of the success of the Bar
6、khausen noise technique-when measuring grinding damage in transmission gears using a semi-automated testing apparatus. The successes include the ability to quickly and easily obtain quantitative results, to eliminate subjective “ctching“ techniques, to dctect flaws early before a large number of dam
7、aged parts are produced, and to have a record of the results saved to hard disk. In short, it will be shown how Barkhausen noise can be used to easily detect grinding damage in surface-hardened steel components, specifically transmission gears. Copyright 1999 American Gear Manufacturers Association
8、1500 King Street, Suite 201 Alexandria, Virginia, 22314 October, 1999 ISBN: 1-55589-739-8 THE BARKHAUSEN NOISE INSPECTION METHOD FOR DETECTING GRINDING DAMAGE IN GEARS Jeffrey S. Ceurterl Chad Smith2 and Roy Ott2 IAmerican Stress Technologies, Inc. 267 Kappa Dr. Pittsburgh. PA 15238 2Harley-Davidson
9、 Motor Company W156 N9000 Pilgrim Rd. Milwaukee. WI 53201 Introduction When hardened steel components are ground. there is always the possibility for damage to the steel in the form of residual stress or microstructural changes. Methods for detecting this sort of damage have always had one or more d
10、rawbacks. such as cost. time. complexity. subjectivity. or the use of hazardous chemicals. A relatively new method. known as barkhausen noise analysis. meets the demand for measuring defects in ground steels in a very reliable. standardized and cost effective manner. Use of this technique is simple
11、and can reduce product failures to zero percent. Semi-Automated Gear Inspection Systems have been employed by gear manufacturers to take advantage of the capabilities of barkhausen instrumentation. Combined with dimensional inspection. hardness tests and periodiC metallographic analysis. the barkhau
12、sen noise analysis method can help close the loop on insuring product quality. Barkhausen noise analysis can be a strong link in the chain that ultimately leads to a long and reliable gear life. Measurement Techniques and Instrumentation Barkhausen noise analysis is a technique based around a relati
13、vely simple concept involving ferromagnetic materials and a magnetizing field. When a magnetizing field is placed near a ferromagnetic material. the material undergoes a net magnetization change. This change is a result of the microscopic motion of magnetic domain walls within the material. When a d
14、omain wall moves. it emits an electrical pulse that can be detected by a coil of conducting wire placed near the material. These discrete pulses are measured in a .bulk manner resulting in a compilation of thousands of electrical pulses referred to as Barkhausen noise.3.6.7.9 The amplitude of this s
15、ignal is sometimes referred to as the Magneto-Elastic Parameter (MP). The amplitude is affected by anything that impedes the motion of domain walls. Some factors to consider are inclusions. preCIpitates. dislocations. grain boundaries. and residual stresses. In the sense of macro-metallurgy. we may
16、sum up these factors into two categories. hardness and residual stress. In general. Barkhausen noise is increased with decreasing hardness and increasing tensile stress and Effect of Stress Effect of Hardness Compression ,- - Low -_. dIM. .11.1 I.IIIN- * Amplitllde I Higb Amplitude Low Hardneu Tensi
17、on 01 :.Itn:J:l o + AmpUtucie _ Stress Figure 1: Barkhausen Noise Amplitude vs. Stress. Figure 2: Barkhausen Noise Amplitude vs. Hardness. Instrumentation Central Unit Sensor Figure 3: Instrumentation Required For Barkhausen Noise Analysis. conversely, Barkhausen noise is decreased with increasing h
18、ardness and increasing compressive stress. This principle is illustrated in figures I and :!. The instrumentation required to detect Barkhausen signals is illustrated in figure 3. A magnetizing field is created and applied to a ferromagnetic material through the use of an electromagnet. The material
19、 reacts to the magnetic field as descrihcd above and emits Barkhausen bursts. which are captured by a sensor consisltng of a coil of conducting wire, The signal is then amplified and filtered. The amplitude is calculated using an RMS equation and the data is digitized for display and output to a com
20、puter. 2 The Nature of Material Defects Caused By Grinding Grinding damage is (he result of energy being convened to heat. This heat is concentrated in the surface layers and may cause undesirable effects if not properly managed. Some of the factors affecting the rise in temperature in the surface l
21、ayer include the coolant lype, coolant concentration, coolant age, coolant flow. grinding wheel type, grinding wheel speed, grinding wheel wear, feed rate and prior processing of material e.g. different heat treat batches. Wojtas, et. al.(9) explain that damage may stan with the panial relaxation of
22、 desirable compressive stresses at temperatures below 500C, As temperatures increase to near 600C. B class thermal damage. also known as re tempering bum. occurs. The effect will be an over-tempering causing a decrease in surface Comparison of the Inspection Methods Barkhausen Method Temper Etch Mic
23、ro Hardne Nondestructive Yes No Yes Use of Chemicals No Yes No Automated Yes No No Reliable Yes No No Evaluation Through Coatings Yes No No Danger of Hydrogen Embrittlement No Yes No Influenced by Both Stress and Microstructure Yes No No Figure 4: Comparison of Nital (Temper) Etch to Barkhausen Nois
24、e Analysis. hardness and the onset or materialization of tensile stresses. Further temperature increase to above 720C will cause D class thermal damage. also known as re-hardening burn. This defect will include regions of very hard and brittle material as well as surrounding areas of B class bum. so
25、ft material. The residual stresses will also be complex due to ranging levels of damage across the surface. Some areas will be compressive. while others will be highly tensile.2.9) The existing techniques for detecting the damage described above include visual inspection via nital etching 4, micro-h
26、ardness testing. residual stress profiling with x-ray diffraction and Barkhausen noise analysis. Each of the defects described above can be detected via Barkhausen noise analysis and can be done in a totally non destructive manner. The x-ray diffraction technique can also be used in each case. howev
27、er it is extremely time consuming. expensive. and it is destructive. The nital etching and micro hardness techniques are quick and easy. however. they can only detect B and D class damage. Furthermore. hardness testing is destructive and nita! etching is subjective. Figure 4 shows some of the featur
28、es of Grinding Damage: Retempering Grinding Burn MP Hardness Retempering -. Softening Amplitude Up MP Stress + Corl1lression -+ Tension Al11)litude Up Retempering Burn Increases Amplitude Figure 5: Effect of Grinding Bum on Barkhausen Noise Signal. 3 Barkhausen noise analysis compared to the most wi
29、dely used grinding bum detection technique, nital (temper) etching.2,9 It was seen in figures I and 2 that the Barkhausen signal increased for decreasing hardness and for tensile stresses. This is the exact scenario for re-tempering grinding bum as seen in figure 5. Since grinding damage affects the
30、 hardness and stress in ways which increase the Barkhausen Signal. detection of grinding damage by the Barkhausn nOi.se analysis is quite simple. If the aphtude ncreas.es. then there is burn. The exception to thiS rule IS for re hardening zones. In the case of re-hardening zones, the signal may decr
31、ease, however. these zones are always surrounded by severely re tempered zones, which exhibit large amplitudes of Barkhausen noise (MP). Semi-Automated Barkhausen Noise Evaluation Of Transmission Gears. Figure 6: Gear Inspection System. I- Gear Inspection Stand 2- Computer I ROLLSCANTM cabinet 2 4 O
32、utline of Instrumentatio and System Operation The inspection system, shown schematically in figures 6 and 7, consists of a linear x-y motion controlled sensor, a live center. a three-Jaw chuck with software controlled rotation, a Barkhausen Noise analyzer, a computer, and data acqulsltJon and analys
33、is software. Parameters for rotation and x-y motion are programmed into the computer for each individual gear type. The operator then installs a gear manually. selects the type of gear from the software and presses start. The remaining operations are all automatic. The sensor moves into place on the
34、 gear tooth. see figures 8 and 9. then axially scans the preset locations on the tooth. up to 4 radial locations per tooth. The gear then rotates slightly anowing the sensor to contact the opposite flank. which is then scanned r-i . . L. _ i j-i , I I -. - ,.-, - Figure 7: Gear Inspection Sland. 1.
35、Gearstand Frame 2. Sensor 3. Axial motion 6. Drive head 9. Sample 4. Horizontal motion 5. Live center 7. Control Logic box (SL) 8. Operation panel in the same manner as the first flank. Next. the sensor moves away from the gear and the gear rotates allowing the sensor to move in to test the next too
36、th. This continues for the preset number of teeth and the results for each scan are presented on the computer monitor with the status of ACCEPTED or REJECfED. based upon programmed rejection conditions. The system can be set up to measure I tooth or all teeth and it can be programmed to measure 1 ;i
37、gure 8: Sensor Contact With Gear Tooth. tooth. then skip 5 and test the sixth tooth and so on. A typical setup is to use 2 scans per flank and to measure a total of 4 teeth at approximately 90 degrees from one another. This type of setup drastically decreases the measurement time. compared io measur
38、ing each tooth. without sacrificing reliability. When the measurement is completed. the results can be saved to a file or output to a printer. Figure 9: Photograph of gear contact with helical type gear. (Photo courtesy of Stresstech Oy, Finland). 5 Getting Started With BarkhausenNoise Analysis In o
39、rder to get started with Barkhausen noise analysis one must frrst obtain a correlation between the Barkhausen noise signal and some other measure of the severity of bum. e.g. nital etching. Based upon the correlation data, a criterion for rejection can be established for the Barkhausen instrument. O
40、ne simple inspection method is based upon the fact that the MP values can be directly related to the results of a visual nital etch inspection. By measuring a variety of production parts and comparing to nital etch inspection, a correlation can be made and the level of burn can be quantified using t
41、he magnitude of the Barkhausen signal, or Magneto-elastic Parameter, (MP). By examination of the correlation, a criterion for rejection is established. The rejection criteria are then entered into the computer program for the type of gear being used. Once the rejection criteria have been entered, Ba
42、rkhausen noise analysis of production samples may begin. An example of the setup used for one gear type on a motorcycle transmission gear system is shown in figures lOa and lOb. The figures illustrate a correlation between the Magneto-elastic parameter, MP and the visual indication of bum from nita)
43、 etching. Figure J Oa is a correlation for the Maximum MP values measured on all scans of a gear, while Figure lOb is a correlation for the difference between Maximum and Minimum MP values measured on all scans of a gear. On each graph, a cross has been added to indicate the rejection criteria. In e
44、ach case, the lower left quadrant indicates acceptable samples, while the upper right quadrant indicates unacceptable samples. Therefore, by setting the Maximum rejection limit to 60MP and the Difference rejection limit to 20MP, all parts rejected by nital etching will also be rejected by the Barkha
45、usen noise analysis. The example given indicates some scatter in the data and it is only for a relatively small group of parts. Based upon this example, it would be wise to choose rejection criteria which are slightly lower than those indicated. This would be taking a conservative stance, however, y
46、ou would be 100% positive that no rejectable parts are passed. In order to increase confidence in results, aU final correlations are being done with 60 sample groups tested by 3 different users. This technique will further increase confidence in the 6 rejection criteria and limit the need for furthe
47、r analysis of the gears. Whena gear measures near or above the rejection criteria it is common practice to check the gear further using nital etching. This type of extra analysis is excellent for confidence building, especially if the rejection criteria was established using a small group of parts,
48、however, over time ?perators become more confident that the analYSIS system and the established rejection criteria is reliable and true. Benefits Of Automated Barkhausen Noise Inspection Early detection of damage Grinding damage usually occurs as a result of wheel wear, but may also occur due to inc
49、orrect feed rate, wheel speed, Or various other changes,“ as mentioned before. In a production cycle that outputs large numbers of parts every hour, it is essential that errors are detected quickly. Nital etching techniques can take several minutes and are not always conveniently located with respect to the grinders. Because of this. etching is typically done once an hour or less, therefore, detecting errors is by no means fast. This can be a huge loss in time and mone
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