TECHNICAL PAPER

Development of Non-destructive Grinding Burn Detection
Technology
R. ITO T. AZUMA

Nitric acid etching, which is the conventional method used in manufacturing plants to detect grinding burn, has anumber of problems, described as follows: (1) detection level varies depending on the operator and the equipment
used; (2) the tested pieces must be scrapped as it is a destructive inspection; and (3) outflow of defective products
cannot be prevented perfectly as 100% inspection cannot be carried out. This study was performed to develop
a non-destructive inspection system to detect grinding burn, which can be used to perform quantitative 100%
inspection within a production line. For this inspection, an eddy current sensor is selected because of its shortmeasurement time and low cost. In this paper, a new method that simultaneously measures eddy currents of two
different frequencies is proposed. This method reduces the misdetection of grinding burn by eliminating the effectof variance in base material. Furthermore, a Grinding Burn Detection System has been prototyped, and it has been
confirmed that quantitative inspection is indeed possible.
Key Words: grinding burn, non-destructive inspection, eddy current, retained austenite, magnetic permeability

1. Introduction

2. Detection principle

Grinding burn is one issue faced in the grinding
process. Broadly speaking, grinding burn refers to
discoloration or transmutation of the grinding surface,
however this study particularly focuses on the change
in metallographic structure (re-quenched and tempered
layer) caused by grinding heat. These have a large
impact on the fatigue strength of workpieces. The nitricacid etching method conventionally used to inspect
for grinding burn on the production floor has several
disadvantages, including varying detection levels between
different people or equipment, and not being able toinspect all parts due to being a destructive test. As suchthere is a strong demand for the development of a nondestructive type of grinding burn detection system able
to inspect 100% of workpieces and unrelated to theoperator’s individual experience. There are a number of
1-4)
non-destructive inspection methods for grinding burnhowever, when considering implementation in the
production line for 100% of workpieces, measurement
time and equipment cost are important elements inaddition to detection sensitivity. In this study, it wasdeemed that a method using eddy current had potential so
we developed a Grinding Burn Detection System utilizing
an eddy current sensor.

Figure 1 shows the cross-sections and hardness of
two test pieces (material: SUJ2) with differing degrees ofgrinding burn. The dark, discolored portions are equivalent
to portions where the material has been softened due
to grinding heat (hereafter referred to as the “tempered
layer”). If there is a significant degree of grinding
burn, in addition to the forming of tempered layers, theoutermost surface layer may reharden (hereafter referred
to as the “re-quenched layer”). Although this portion has
higher hardness than the tempered layer, it is extremely
brittle, therefore both tempered layers and re-quenchedlayers are causes of reduced fatigue strength. Apart frommartensite, the metallographic structure of the test pieces
contains austenite (retained austenite) which did not
5)mutate following quenching and tempering. Figure 2
shows the results of measuring the retained austenite
for 3 types of test pieces (material: SUJ2). It is evident
that the amount of retained austenite does not change
in depth from the surface when there is an absence ofgrinding burn, however does decrease in the tempered
layer and increases in the re-quenched layer. In this way
it is possible to detect tempered layers by measuring thetretained austenite. Retained austenite is a non-magnetic
6)
structure therefore has a different magnetic permeability
to other structures.

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)5)27

−Development of Non-destructive Grinding Burn Detection Technology−

Hardness measurement position

Vcurrent ＝ Vm − Vw＝
Tempered layer

Hardness, Hv

Tempered layer
900
800
700
600
500
400
300
200
100
0

0

0.25

Re-quenched layer

0.50 0.75 1.00 0

Depth from surface, mm

0.25

0.50 0.75 1.00

Retained austenite, %

12
10
8
6
No grinding burn
Slight grinding burn
Heavy grinding burn

4
2

d＝

1

⑵

p f lr

0

50

100

150

E

V
Variable
resistance200

Depth from surface, µm

abFig. 2 Retained austenite content versus depth from top surface5)

Eddy current sensors use electromagnetic induction
generate an eddy current in a conductor and detect themagnetic field created by the eddy current as a change incoil impedance. The sensor makes it possible to detect
grinding burn as the flow of eddy current will change due
to differing magnetic permeability of the portions withgrinding burn as opposed to that of the portions without
grinding burn. Figure 3 is a simplified diagram of theeddy current sensor used in this study. As Fig. 3 shows, it
is a bridge circuit comprising of a coil for excitation anddetection, a calibration coil and two variable resistances.
Output is the potential difference V between terminals
ab . In the actual measurement, a master workpiece isprepared and its measured value made the benchmark tocompare with measured values from the test pieces andsee the degree of difference. Regarding the impedance ofthe coil for excitation and detection, Z S is used if nothingis measured (the test piece is far enough away from thesensor), Z S+DZ Sm is used for measurement of the masterworkpiece, Z S+DZ Sw is used for measurement of the test
piece, and furthermore if the output between terminals abat the time of master workpiece measurement is made V m
the output between terminals ab at the time of measuring
the workpiece for inspection is made V w the eddy current
output V current is defined in the following way.

28⑴

Here, f is the excitation frequency, l is the magnetic
permeability and r is the electrical conductivity.
Therefore, detection of an even higher accuracy ispossible by setting the excitation frequency to suit thedepth of the defect to be detected from the surface.

14

0

E

（ Z S＋D Z Sm＋Z C（）Z S＋D Z Sw ＋Z C ）Here, E is the power source voltage while Z C is the
impedance of the calibration coil. Moreover, a featureof the eddy current sensor is that the depth that eddy
currents penetrates into the material can be controlled bychanging the excitation frequency. The standard depthof penetration (the depth at which eddy current density
decreases to approximately 37% compared with that of
top surface) d is expressed in the following formula.

Depth from surface, mm

Fig. 1 Etched cross section of test pieces and hardness5)（ D Z Sm − D Z Sw）Z C

Calibration coil
(impedance: C)

Excitation and detection coil
Magnetic (impedance: S)
flux

Eddy current Test piece (conductor)

Fig. 3 A simplified diagram of the eddy current sensor system

3. Base material variance and the multifrequency measurement
Figure 4 shows the results of measuring the eddy
current output in the outer race of three ball bearings
from different manufacturing lots. As shown to the left
in Fig. 4, the eddy current output of the circumference
face was measured while the ball bearing outer trajectory
was being rotated. Test pieces 1 and 2 are free of grinding
burn, while grinding burn was intentionally created on
test piece 3 by changing the grinding conditions whenmachining the outer periphery face. There is practicallyno difference in the eddy current outputs of test piece 2(no grinding burn) and test piece 3 (with grinding burn),
therefore it is obvious that it is difficult to determine
the presence of grinding burn. This is because there isvariation in the amount of retained austenite in the basematerial, and in order to detect grinding burn, rather than
the absolute value of the retained austenite, it is necessary
to detect the decrease in retained austenite (difference
between the retained austenite in the base material

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

−Development of Non-destructive Grinding Burn Detection Technology−

Ball bearing
outer race
Eddy current sensor
Measurement
position

RotateEddy current output Vcurrent (250 kHz), V

compared to that in the tempered layer).
Therefore, as shown in Fig. 5 5), this study devises
a method to detect grinding burn by performing ameasurement with deeper penetration and lower
excitation frequency in order to suppress the impact ofvariation in retained austenite of the base material, andat the same time, perform as high excitation frequency
measurement to detect grinding burn, then correlating
the results of the two excitation frequencies. If there isa significant difference between the eddy current output
at low excitation frequency and the eddy current output
at high excitation frequency, there is grinding burn.
However, if the difference in eddy current output is small,
grinding burn is deemed to be absent. We selected 0.5
kHz as the excitation frequency to be sufficiently low, as
the eddy current output is impacted by the base material.
We selected 250 kHz for the high excitation frequency
so that tempered layers could be detected even if requenched layers occurred. Figure 6 shows an example
of measuring eddy current output using two frequencies
1.51.0(1) No grinding burn0.5(2) No grinding burn

0.0
-0.5
-1.0
-1.5

(3) Grinding burn
0

30

60

90

120

Phase, deg

Fig. 4 Influence of dispersion of material properties5)

Eddy current sensor
250 kHz(Re-quenched layer)Standard depth of penetration: approx. 30 µm→ Tempered layer impact is great
0.5 kHz

Tempered layerStandard depth of penetration: approx. 680 µm→ Base material impact is great

Base material

Small difference → No grinding burnBig difference → Grinding burn

Eddy current output Vcurrent (250 kHz), V

Fig. 5 Schematic view of multi frequency measurement5)
1.5(1) No grinding burn

1.0
0.5

(3) Grinding burn

0.0
−0.5
−1.0

(multi frequency measurement) on the same test pieces
as those used in Fig. 4. If there is grinding burn, the eddy
current output of 250 kHz will be greater in the negative
than the eddy current output of 0.5 kHz, and a signal willappear in the bottom right, as shown in Fig. 6. It can beseen that the grinding burn which could not be detected inFig. 4 can be detected by using this method.

4. Prevention of thermal drift
As Fig. 7 shows, the phase has been adjusted in thisstudy in order to suppress the thermal drift of the eddy
7)
current sensor . The output V of the eddy current sensor
is the vector quantity and can be expressed in a complex
plane, as shown in Fig. 7. The vertical axis shows theimaginary component of V (Im [V ]), and the horizontal
axis shows the actual component (Re [V ]). Here, it isassumed that the temperature fluctuation of the coil for
excitation/detection and the calibration coil occurs dueto the temperature of the test piece. Here, signals areshown for the respective measurements of V m, the masterworkpiece, V w-S, the test piece with only slight grinding
burn, and V w-L, the test piece with significant grinding
burn. As shown in the left of Fig. 7, V movements inthe complex plane differ depending on if the cause istemperature fluctuation or grinding burn. Here, because thecomplex plane movement due to temperature fluctuation islinear, as shown in the right of Fig. 7, by rotating the phaseso that the movement of the signal caused by temperaturefluctuation is parallel with the horizontal axis and only
making the imaginary component (Im [V ]) a valid output,
it is possible to suppress the impact of temperaturefluctuation on the output of the eddy current sensor.
The movement on the complex plane of eddy current
sensor output in relation to temperature fluctuation varies
depending on sensor specifications, excitation frequency,
test piece material and so on. As such, it is necessary tofind the rotation angle of Dh through experimentation
when performing phase adjustment.
In summary, through the method of suppressing
disturbance caused by test piece temperature fluctuation
explained in this section, and through a multi frequency
measurement method which suppresses the influence ofvariation in the base material structure, as discussed in
section 3, this research uses the evaluation function V d,shown in the following equation, as a parameter used in
detecting grinding burn.
Vd ＝ Im［Vcurrent（ f ＝ 250 kHz, Dh ＝ Dh 250 ）］］⑶− Im［ Vcurrent（ f ＝ 0.5 kHz, Dh ＝ Dh 0.5）

(2) No grinding burn

−1.5
−1.5 −1.0 −0.5 0.0

0.5

1.0 1.5

Eddy current output Vcurrent (0.5 kHz), V

Fig. 6 Results by multi frequency measurement5)

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

Here, V current is the eddy current output expressed inequation ⑴, f is excitation frequency, Dh is the rotation
angle of the phase and Dh250, Dh0.5 are constants
which suit the sensor specifications found throughexperimentation.

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−Development of Non-destructive Grinding Burn Detection Technology−

m

h

m
w-S

h+Dh

w-S

w-L

Signal movements
due to grinding burnw-L

Re [ ]，VRe [ ]，V

Fig. 7 Prevention of thermal drift

5. Prototyping a Grinding Burn Detection Device
5. 1 System overview
We prototyped a device to apply the grinding burndetection technology mentioned above. Figure 8 is aschematic of this device. This device is configured froman eddy current sensor, excitation/detection device, drive
device and data collection device. The tip of the eddy
current sensor is L-shaped and the preload mechanismof the measurement jig holding the sensor prevents thegap between the sensor and test piece from changing byapplying a consistent force.
Measurement jig
Excitation/
detection device

Arm

Data collection
deviceZ

ColumnX
PC
Test piece

Eddy current sensor
写 真 6. 1
Rotating table

Fig. 8 Schematic of grinding burn detection device7)

5. 2 Implementation examples
Figures 9 and 10 are examples of grinding burndetection using the prototyped grinding burn detection
unit. Figure 9 shows the detection results obtained byusing nitric acid etching (upper) and our prototyped
device (lower) respectively on the circumference of acylindrical test piece (material: SUJ2) which developed
grinding burn during the grinding process. In themeasurement results of our prototyped device, the greatera negative the evaluation function V d is, the blacker thecolor, hence the portions of the test piece with grinding
burn are displayed in black. Figure 9 clearly shows thatby using our prototyped device, it was possible not only
to detect grinding burn in places undetected by nitricacid etching (phases 0° to 90° and 180° to 270° of thetest piece), but also minute changes in the metallographic

30

structure undetectable using nitric acid etching.
Therefore, it can be said that a higher accuracyof grinding burn detection is possible using an eddy
current sensor, compared to the nitric acid etching formof detection. By comparing the results obtained from
nitric acid etching and the results obtained by using
our prototyped device, from here on we established thethreshold of grinding burn being present or absent as V d =-0.5 V. Figure 10 shows the results of evaluating grinding
burn presence/absence in the raceway surface of a ballbearing outer race (material: SUJ2) following the grinding
process. A total of 26 workpieces were measured here,
comprising of 25 workpieces from the production plantand 1 test piece ground in a way that would deliberately
cause grinding burn. All workpieces were demagnetizedprior to measurement. Moreover, measurements were
performed by putting the eddy current sensor in contact
with the bottom of the raceway surface as shown in Fig. 10
and doing one full circle while rotating the workpiecebeing measured. The prototyped device was placed ina non-air conditioned room within the plant and the
maximum temperature change of the measured workpieces
at the time of measurement was 3.9°C. The eddy current
output is 250 kHz excitation frequency on the vertical axis
and 0.5 kHz on the horizontal axis, with the results shownin the bottom section of Fig. 10. If the threshold is set as
V d = -0.5 V, it means grinding burn has occurred if themeasured value is plotted below the V d = -0.5 V line in
the graph, and no grinding burn has occurred if above theV d = -0.5 V line. Figure 10 shows that no grinding burnwas detected in the workpieces from the production line
measured in this sample however the test piece for which
grinding conditions were intentionally altered did havegrinding burn. Moreover, the output value of the eddy
current sensor for the 25 workpieces from the production
line had variation of around ±0.3 V however by using
the prototyped device, it was possible to detect grinding
burn without false judgment. In this experiment (Fig. 10),on top of suppressing the effect of disturbance due totemperature, the temperature change of the test piece
itself and the room temperature during the test was small,therefore it is believed that the impact of temperature on
the measurement results was sufficiently small.

14mm

Phase adjustment

Width, mm(Damage level: Small)(Damage level : Large)

Im [ ]

w-L

Im [ ]

w-S

14
0

0

①

②

③
①

④
②

30

60 ④ 90314mm (circumference length)⑥⑤

A

150 180 210 ⑦ 240
Angular position, deg.

B

330 360d (V)0.00
Upper: Detection using nitric acid etching (photo)−0.25
Lower: Detection using grinding burn detection system
−0.50
−0.75
③

120

⑦
⑥
⑤

270

300

Fig. 9 Grinding burn detection in circumference of a workpiece

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

Rotation

−0.5
−1.0

Re [

Measurement position( f =250 kHz, Dh=Dh250)]

Eddy current
sensorBall bearing
outer race

current−Development of Non-destructive Grinding Burn Detection Technology−

1.5
1.0

Current products
from plant(25 pieces)5)d= −0.500.5
0.0
Grinding burn6)

Deliberately changed grinding
condition (one piece)−1.5−1.5 −1.0 −0.5 0.0 0.5 1.0 1.5
Im [current( f =0.5 kHz, Dh=Dh0.5)]

7)

Fig. 10 Grinding burn detection in raceway surface of outer race

7)

6. Conclusion
This study involved the proposal and verification of agrinding burn detection method and unit using an eddy
current sensor as a technology to detect grinding burnthrough the non-destructive inspection of 100% of parts.
The following knowledge was obtained from this study.
(1) In regards to the excitation frequency, a highfrequency to detect the state of the test piece’s surface,and a low frequency to detect the state of the basematerial were used. By comparing the difference in themeasured values of these two frequencies it is possible
to suppress the impact of the variation in the basematerial structure.
(2) By ascertaining the change in output of the eddy
current sensor due to temperature change, it is possible
to suitably adjust the eddy current sensor output phase,and by making only either the actual component or
imaginary component valid outputs, it is possible tosuppress the impact of temperature change.
(3) In the grinding burn detection test on a ball bearing,by applying this detection technology, it is possible
to detect grinding burn quantitatively and at a higher
accuracy than nitric acid etching.

R. Ito, T. Azuma, S. Soma, S. Murakami, T. Kuriyagawa:
Development of non-destructive inspection system for
grinding burn (1st Report) Detection of grinding burnutilizing eddy current sensor, Journal of the Japan Society
for Abrasive Technology, 56, 8 (2012) 533 (in Japanese).
M. Mikami: Magnetic Studies on the Amount of Retained
Austenite in Quenched Steel, The Japan Society of
Mechanical Engineers, 37, 204 (1934) 217 (in Japanese).
R. Ito, T. Azuma, S. Soma, S. Murakami, T. kuriyagawa:
Development of non-destructive inspection system
for grinding burn (2 nd Report) Disturbance reduction
technique for eddy current sensor, Journal of the Japan
Society for Abrasive Technology, 56, 10 (2012) 677 (inJapanese).

References
1)

2)
3)

4)

Y. Ito, Y. Furuya: Nondestructive Material Evaluation by
Barkhausen Noise Analysis-Part 1:Basics of Barkhausen
Noise Method-, Journal of JSNDI, 45, 1 (1996) 3 (inJapanese).
Edited by Japan Society for Non-Destructive Inspection:
Eddy Current Flaw Detection I, (1995) 17 (in Japanese).
H. Shimoda: Influence of damaged surface layer on
X-ray stress measurement, Journal of JSNDI, 10, 5(1961) 239 (in Japanese).
T. Semba, Y. Tani, H. Sato: Quantitative Evaluation of
Damaged Layer of Ground Surface by Scanning Acoustic
Microscopy, SEISAN KENKYU, 39, 4 (1987) 145 (inJapanese).

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

R. ITO****

T. AZUMA**

Advanced Fundamental Research Dept.,Research &Development Headquarters, Doctor of Engineering
Advanced Process Innovation R&D Dept., Research &Development Headquarters

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