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Non-destructive Testing for Surface Hardness and Case Depth
Report No. 2013-‐01
1. Project Statements
1.1. Objective:
The objective of this project is to identify and develop a nondestructive hardness
and case depth measurement technique that can be applied accurately, rapidly,
and directly to carburized steel parts. The inspection technique should be capable
of accurately measuring hardness within a range of 35 to 65 HRC to a maximum case
depth of 2.54 mm. It is also desired that the technique have minimal manual
preparation or data interpretation, allowing for the part to continue as a production
asset after inspection.
1.2. Scope:
Identify candidate nondestructive hardness and case depth measurement
techniques and experimentally evaluate their effectiveness. Validation of a
successful non-destructive or minimally invasive measurement technique will require
correlation with traditional destructively tested traveler test specimens and part cutups.
Candidate techniques will be evaluated for three steel alloys, 1018, 8620 and 4140,
for three part geometries representing a range of gear tooth radii and spacing. Gage
reliability and repeatability assessments shall be conducted to determine the capability
of the candidate measurement systems.
1.3. Strategy:
To address the goals of this project, the following strategies are adopted:
Research Team:
Richard D. Sisson Jr. (508) 831-5335 [email protected]
Mei Yang (508) 353-8889 [email protected]
Lei Zhang (508) 241-1260 [email protected]
2
• Perform literature review on nondestructive techniques for hardness and case
depth
• Identify the most promising nondestructive technologies to be tested
• Identify the alloys and heat treat conditions to be tested
o Alloys –1018, 8620, 4140
o Range of hardness
o Range of case depths
o Surface conditions
o Geometries
• Fabricate heat treated alloy standards to be destructively and nondestructively
tested
• Use the carburization model (i.e. CarbTool©) to determine the process
parameters
• Use the integrated heat transfer, mechanics, and phase transformation software
(i.e. DANTE) to predict the hardness and residual stress
The project work is divided into the following tasks:
• Task 1 – Literature review
o Nondestructive testing techniques
§ Physics of measurement
§ Sensitivity to microstructural features
§ Sensitivity to residual stresses
§ Depth of penetration for measurement
o Microstructure à hardness relations
o Microstructure à NDE measurement relations
o Residual stress à NDE measurement relations
• Task 2 – Select nondestructive techniques for testing
o Select nondestructive testing techniques
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o Obtain access equipment
o Test on standard parts and components
• Task 3 – Select Alloys and Heat treating conditions for testing
o Surface hardness
o Case depths
o Temper conditions
• Task 4 – Select test part geometries and surface finishes
• Task 5 – Fabricate and verify metallurgy of standard test parts
o Surface Hardness (Rockwell C)
o Microhardness profiles
o Carbon concentration profiles (OES)
o Microstructure –characterization and analysis for Martensite and
carbide distribution (Optical, SEM and XRD)
• Task 6 – Conduct nondestructive tests and determine the correlations between
the destructive test results and the known results in the standards
• Task 7 – Determine the correlations among nondestructive test
measurements, hardness and microstructure for standard test parts
• Task 8 – Verify effectiveness of nondestructive test technique at CHTE
member companies
4
2. Executive Summary
The heat treating industry is in need of an accurate, rapid, and nondestructive
technique for the measurement of surface hardness and case depth on carburized
steels for process verification and control. Current methods require destructive test of
“traveler” specimens that are not always representative of production part
configurations and the associated subtleties of thermal history, carbon atmosphere,
and geometry. Traveler specimens are often accompanied by a requirement for
periodic production part cut-ups to validate carburization hardness and case depth
profiles of parts, particularly for critical shaft and gear teeth configurations.
Preparation of traveler specimens and part cut-ups is labor intensive, expensive, and
sensitive to operator error. In addition, the process of using traveler coupons and
periodic destructive production part cutup can be time consuming and costly.
This project will focus on the identification, development and verification of
nondestructive techniques for surface hardness and case depth measurement for
selected carburized steel. The initial nondestructive techniques to be evaluated include
Eddy Current, Meandering Winding Magnetometer (MWM), Barkhausen Noise and
Alternating Current Potential Drop (ACPD). The goal is the identification of
techniques that are sensitive to hardness microstructure and residual stress. A
challenge of this project will be the ability to distinguish among microstructure,
hardness and residual stress.
3. Achievements in this semester
3.1. Standard samples
3.1.1. Samples design
The 1018, 8620 and 4140 steels were selected to fabricate into the standard samples.
The standard chemical compositions are presented in Table 1. The samples were heat
treated to obtain appropriate case depth. The techniques that will be used to measure
the case depth are sensitive to the case depth that will be determined by the hardness
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distribution as well as the stress distribution [1]. A series of metallurgy
characterization techniques are performed to analyze the properties of the steels. Table 1．The chemistry composition of 1018, 4140 and 8620 steel measured by OES
The geometry of the test samples is presented in Figure 1. Both cylinder and plate
geometries are applied with diameters of 1.0 inch, 1.5 inch, 2.0 inch. They are
designed after simulation by DANTE to determine the residual stress after carburizing
processes.
Unit: Inch Material: 1018, 4140, 8620 Steel
Figure 1. Geometrical design of standard samples
C Cr Fe Mn Mo Ni P S Si
1018 0.184 0.120 Bal. 0.745 0.0225 0.0810 0.0044 0.014 0.2150
4140 0.377 1.025 Bal. 0.695 0.1505 0.1565 0.0071 0.011 0.2265
8620 0.214 0.560 Bal. 0.805 0.1535 0.4525 0.0065 0.014 0.2355
6
3.1.2. The samples fabrication procedure
Samples were heat treated after machining and surface finishing. The heat treatment
processes includes gas carburizing, quenching and tempering. CarbTool© was used
to calculate the carbon concentration profile. The fabrication procedure is presented in
Figure 2. This process produced samples with several carbon case depths and residual
stress conditions.
Figure 2. Sample fabrication procedure
Y
1. Y
CarbTool© Simulation
Carbon profile test
CNC Machining
Carburizing
Quenching
Tempering
Results meet the requirements?
Surface machining
Recipes
Raw steel bars
Standard samples prepared
N
2. Y
7
3.1.3. Recipe of the carburizing process
CarbTool© was used to develop the carburizing process to achieve a case depth of 0.8
mm. The samples were heated to 1700 °F and held 180 minutes in endothermic gas
with a carbon potential of 1.0%, then keep it at 1700 °F for 105 minutes in an
atmosphere with a carbon potential of 0.7% as a transition process. In the ‘diffuse
process’ samples were held at 1550 °F for 30 minutes in an atmosphere with a carbon
potential of 0.7%, then quenched in oil between room temperature and 160 °F.
Figure 3. Carburizing recipe by CarbTool©
3.1.4. Carbon concentration and microhardness profiles of test samples
Carbon concentration profile and microhardness are studied experimentally with
destructive methods. The results are presented in Figure 4, Figure 5 and Figure 6. The
carbon profile is measured with Optical Emission Spectroscopy (OES).
Microhardness profile is measured with aVickers microhardness tester. There is good
agreement for the 1018 and 8620 steel data. 4140 Steel is typically not subjected to
180 min Cp=1.0
105 min Cp=0.7
30 min Cp=0.7
8
the carburizing process because of the high carbon concentration. There is not much
change in the microhardness for 4140.
Figure 4. Carbon concentration and microhardness profile of 1018
Figure 5. Carbon concentration and microhardness profile of 8620
0 100 200 300 400 500 600 700 800 900 1000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Carbon
con
centra6o
n %C
Depth mm
1018
Carbon concentra6on Carbon simula6on Microhardness
Microhardne
ss HV
100 200 300 400 500 600 700 800 900 1000 1100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Carbon
con
centra6o
n %C
Depth mm
8620
Carbon concentra6on Carbon simula6on Microhardness
9
Figure 6. Carbon concentration and microhardness profile of 4140
3.1.5. Retained austenite (RA)
Retained austenite exists in the surface region of the samples after carburizing with
martensite as the primary phase[3]. The retained austenite was measured by X-ray
Diffraction (XRD) simulated in DANTE.
No retained austenite was observed in the carburized 1018 sample in agreement with
the DANTE prediction.
XRD testing results: RA=0% Simulation results: RA=0%
Figure 7. Xray measurement of retained austenite of 1018 steel
0
100
200
300
400
500
600
700
800
900
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Carbon
con
centra6o
n %C
Depth mm
4140
Carbon concentra6on
Microhardness
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
60 70 80 90 100 110 120 130 140
1018
α(110)
α(200) 𝐹𝑒3𝐶 (110)
10
The carburized 4140 steel exhibits 17.2% retained austenite, while DANTE predicted
20%. This difference is being investigated.
XRD testing result: RA=20.8% Simulation by DANTE：RA=2.3%
Figure 8. XRD measurement of retained austenite of 4140 Steel
The carburized 8620 steel exhibits 19.8% retained austenite by XRD. DANTE
simulation predicted 6.4%.
XRD testing result: RA=19.8% Simulation by DANTE: RA=6.4%
Figure 9. XRD measurement of retained austenite of 8620 Steel
0
10000
20000
30000
40000
50000
60000
60 70 80 90 100 110 120 130 140
Coun
ts
2theta
4140
0
10000
20000
30000
40000
50000
60000
60 70 80 90 100 110 120 130 140
Coun
ts
2 theta
8620
α(200)
α(200)
α(110)
γ(200) γ(220)
γ(200) γ(220)
γ(111)
𝛾𝐹𝑒2𝑂3 (115) 𝛾𝐹𝑒2𝑂3 (044)
𝐹𝑒3𝐶 (110)
γ(111)
α(110)
𝐹𝑒3𝐶 (110)
11
3.1.6. Surface residual stress study
The samples with different diameters are designed to exhibit different stress
distributions. The DANTE simulation results were used to differentiate the hoop and
axial residual stress distribution for each test sample. Residual stress can also be
measured using XRD.
𝜎!: Axial stress 𝜎!: Radial stress 𝜎!: Hoop stress
Figure 10. Stress distribution on cylinder surface
For each stress measurement, axial stress and hoop stress will be measured as shown
in Figure 10. The hoop stress is plotted in Figure 11 which presents the comparison
between XRD measurement and the simulation results. The cylinder with the 1.5 inch
diameter has a smaller compressive stress than the 2.0 inch sample. XRD
measurement results are lower than the DANTE simulation results.
Figure 11. Hoop stress of the 1018 sample of XRD and simulation results
-‐350
-‐300
-‐250
-‐200
-‐150
-‐100
-‐50
0 d=1.5 d=2 d=1.5 d=2
Hoop
stress /Mpa
12
3.2. Meandering Winding Magnetometer (MWM)
3.2.1. Introduction of MWM technology
Meandering Winding Magnetometer (MWM) eddy current sensors provide a very
practical and robust capability to measure electrical and magnetic properties of alloys
on surfaces with complex geometries [7].
The MWM sensor is sensitive to flaw, stress, microstructure. It can be used for a
number of applications, including fatigue monitoring and inspection of structural
components for detection of flaws, degradation and microstructural variations well as
for characterization of coatings [6].
Carburizing process will change the surface concentration and stress distribution of
the sample [2]. With MWM sensor, correlation between electromagnetic field and case
depth will be studied in this project.
Figure 12. The MWM mechanism [1]
The sensor consists of a meandering primary winding which will be the input of the
current. The meandering secondary windings are used for sensing the response with
voltage attached on. [1] The primary winding is typically fabricated in a square wave
pattern with the dimension of the spatial periodicity termed the spatial wavelength as
13
Figure 12 presents. The magnetic vector potential produced by the current in the
primary can be accurately modeled as a Fourier series summation of spatial sinusoids.
There is software named GridStation to convert the sensor impedance magnitude and
phase response into material properties, such as the conductivity and permeability.
3.2.2. Equipment for MWM
The MWM system is produced by JENTEK to address the need for quality
assessment and control of high-value added processes, such as coating, welding, heat
treatment and shot peening. Figure 13 presents the JENTEK GridStation® styem.
Figure 13. MWM GridStation System
Measurement procedure is presented in Figure 14. The system needs to be set up
carefully. All the configurations are finished by the software GridStation developed
by JENTEK. For different material, the frequency should be set properly to avoid
saturation. For our measurement, the probe is calibrated in air.
14
Figure 14 Procedure of measurement with MWM probe
15
3.2.3. MWM measurement results The MWM system is used to measure the electrical conductivity and magnetic
permeability of the carburized samples. To study well the relationship between the
conductivity and material properties, a series of measurements are applied. For
cylinder samples, the measurement is applied to three directions as Figure15 presents.
Figure 15 The MWM measurement directions
a) Conductivity of carburized 1018 steel
Figure presents the conductivity of carburized 1018 steel test sample range from
2MHz to 50MHz. Higher frequency leads to higher conductivity. MWM employ high
frequency which leads to a lower skin depth. The relationship between conductivity
and skin depth is plotted in Figure 17.
Figure 16 The conductivity of carburized 1018 steel
0°
90° 45°
Con
duct
ivity
S/m
Frequency Hz
2M Hz 50M Hz
16
Figure 17 The conductivity of carburized 1018 steel verse case depth
Figure 18 presents the effects of the sensor direction on the conductivity. At lower
frequency, there is not big difference. At a higher frequency, difference start to exhibit
between two perpendicular directions.
Figure 18 The conductivity of carburized 1018 steel in three directions
-‐3.00E+08
-‐2.50E+08
-‐2.00E+08
-‐1.50E+08
-‐1.00E+08
-‐5.00E+07
0.00E+00 1 2 3 4 5 6
Cond
uc:v
ity S/m
Skin depth µm C
ondu
ctiv
ity S
/m
Frequency Hz
0 degree
45 degree
90 degree
2M Hz 50M Hz
17
b) Comparison between original and carburized test samples
Figure 19, Figure 20 and Figure 21 present the conductivity of all three original and
carburized samples. 1018 and 8620 present the carburized test samples with lower
conductivity than original one over the frequency range. Carburized 4140 sample
presents lower conductivity at lower frequency. Further investigation should focus the
performance at lower frequency.
Figure 19 The comparison of conductivity of carburized and original 1018 steel
Figure 20 The comparison of conductivity of carburized and original 4140 steel
Con
duct
ivity
S/m
C
ondu
ctiv
ity S
/m
Frequency Hz
Frequency Hz
Original 1018 steel
Carburized 1018 steel
Original 4140 steel
Carburized 4140 steel
2M Hz 50M Hz 1.0E08 1.2E08 1.4E08 1.6E08 1.8E08 2.0E08 2.2E08
2M Hz 50M Hz 1.0E08 1.2E08 1.4E08 1.6E08 1.8E08 2.0E08 2.2E08 2.4E08
18
Figure 21The comparison of conductivity of carburized and original 8620 steel
Figure 22 presents the conductivity of original and carburized steels at 2M Hz
frequency.
Figure 22 Conductivity comparison of carburized and original test samples at 2M Hz
-‐1.60E+08
-‐1.40E+08
-‐1.20E+08
-‐1.00E+08
-‐8.00E+07
-‐6.00E+07
-‐4.00E+07
-‐2.00E+07
0.00E+00
Carburized 8620
Original 8620
Carburized 4140
Original 4140
Carburized 1018
Original 1018
Cond
uc:v
ity S/m
Con
duct
ivity
S/m
Frequency Hz
Original 8620 steel
Carburized 8620 steel
2M Hz 50M Hz 1.0E08 1.2E08 1.4E08 1.6E08 1.8E08 2.0E08 2.2308
19
Figure 23 presents the permeability of original and carburized steels at 2M Hz
frequency. It is similar with the conductivity figure. Carburized samples have a
smaller permeability value than original one.
Figure 23 Permeability comparison of carburized and original test samples
c) Bent strip stress measurement
The bent saw blade measurement is to determine the effects of the stress on the
conductivity and permeability. Total length of the saw is 18 inch. Two ends are fixed
and the d value is measured to control the stress strength including 1, 2, 3, 4and 5 inch
as Figure 24 presents. The probe is applied inside of the arc where compressive stress
exists.
Figure 24 The setup of bent saw experiment
0
10
20
30
40
50
60
Carburized 8620
Original 8620
Carburized 4140
Original 4140
Carburized 1018
Original 1018
Rela:v
e Pe
rmeablity
Original length: 18 inch
d
20
Figure 25 reveals that larger stress lead to lower conductivity. The measurement of 6
inch reverse is applied on the surface outside of the arc where tensile stress exists. The
tensile stress leads to a higher conductivity than the compressive stress.
Figure 25 The conductivity of the bent saw measured by MWM
Figure 26 reveals that the stress has significant effect on the permeability. Higher
stress leads to a much lower permeability. Further tests are planned to determine the
relationship between the stress and the MWM results.
Figure 26 The permeability of the bent saw measured by MWM
Perm
eabi
lity
rela
tive
Con
duct
ivity
S/m
Frequency Hz
Frequency Hz
2M Hz 50M Hz
24
40
32
1.0E08 1.2E08 1.4E08 1.6E08 1.8E08 2.0E08 2.2308
2M Hz 50 M Hz
21
3.3. Barkhausen Noise Testing
3.3.1. Introduction of Barkhausen noise technology
The Barkhausen noise phenomenon found in ferromagnetic materials is shown in
Figure 27. The physical principle behind the method is that microstructural defects
(dislocations, small carbides, grain boundaries, etc.), which are responsible for
mechanical properties, are also responsible for magnetic behavior: shape of the
hysteresis loop, coercive force, and permeability [9]. When a ferromagnetic material is
subjected to magnetic excitation, the magnetization is not obtained continuously but
in discrete jumps owing to domain walls interacting and overcoming barriers in their
path. These jumps, due to sudden changes in magnetization, yield electromagnetic and
acoustic signals that can be detected by a coil or an acoustic transducer [12]. For
carburized and nitrided steels, the carbon or nitrogen content increases the number of
defects or pinning sites and hence improves hardness. These pinning sites also change
the behavior of block walls in magnetic domains and thus the Barkhausen noise
level[9].
Figure 27 Barkhausen noise phenomenon
Barkhausen noise is now one of the most popular magnetic NDE methods for
investigating intrinsic properties of magnetic materials such as grain size, heat
treatment, strain and other mechanical properties such as hardness[10].
To produce Barkhausen noise, it is necessary for the specimen to be subjected to
varying levels of magnetization to the point of magnetic saturation at a certain rate.
22
This can be achieved by employing a yoke, particularly in surface analysis. To
maintain constant induction, the yoke is fed by a bipolar variable source of triangular
waveform. The Barkhausen noise sensor consists of an air coil of copper wire with or
without a ferrite core. Usually, signals are amplified, filtered, rectified, and integrated
to produce a spectrum. The height, half height width, and position with excitation are
parameters that provide useful information [11]. Measurements were carried out at
different frequencies using electronic filters to distinguish between the signal from the
bulk and that from the surface.
Figure 28. The Barkhausen noise equipment[9]
Figure 28 presents the equipment used for MBN testing. Figure 29 shows the origin
MBN signal under different applied stress; the left image shows MBN signal without
applied stress, right shows MBN signal under a tensile stress of 60.44 Mpa.
Figure 29 The process of Barkhausen noise measurement
23
To collect all the information contained in the signal, the frequency spectrum of the
Barkhausen noise was recorded. To do this, the samples were magnetized, as shown
schematically in Figure 30, with a yoke at an excitation frequency of 3 Hz. The
signals were detected by a coil of copper wire with a ferrite core. These signals were
amplified and filtered by a low pass filter at a cut off frequency of 200 kHz. Based on
these data, a fast Fourier transform (FFT) was determined and used on the raw
Barkhausen data. The spectrum obtained was integrated on ten frequency bands, each
20 kHz wide, from 0 to 200 kHz. These ten values, for each sample, were plotted as a
function of the measured case depth of specimens. Based on this, it was possible to
determine which range of frequencies was the most appropriate for case depth
evaluation [11].
Figure 30. The process of Barkhausen noise measurement [11]
To conduct the Barkhausen Noise testing, the STRESSCAN 500C unit, developed by
American Stress Technologies, Inc., was selected. It employs dedicated, patented
sensor designs to activate and detect the magneto-elastic signal. It incorporates
sophisticated microprocessor technology to energize the sensors and process the
signals.
The STRESSCAN 500C calculates the magneto-elastic parameter by averaging the
peak values of Barkhausen Noise signals over ten magnetizing cycles. The output in
the non-calibrated mode is provided in a magneto-elastic parameter called MP.
24
3.3.2. Barkhausen Noise Measurement Depth
The device provides a magnetic field (relative value) from 0 to 150. And it also
provides three different penetration depth, 0.02mm, 0.07mm and 0.2mm
corresponding to the frequency range of 3-15Hz, 20-70Hz and 70-200Hz. [13].
In an electromagnetic field, the skin depth is thus defined as the depth below the
surface of the conductor at which the current density has fallen to 1/e (about 0.37). In
normal cases it is well approximated as:
δ = !!!"
(1)
Where,
ρ = resistivity of the conductor
ω = angular frequency of current = 2π × frequency
µ = absolute magnetic permeability of the conductor
The chart in Figure 31 can be used to approximate the depth of measurement. Since
after carburizing the case depth would be around 0.8mm, the smallest frequency is
picked.
Figure 31 Depth of nominal depth of measurement chart [13]
25
3.3.3. Barkhausen noise measurement results
(1) Barkhausen noise changes with magnetic field
To test how Barkhausen noise changes with magnetic field, a 1018 plate sample with
1.5 inch diameter was used. Figure 32 presents how Barkhausen noise changes with
magnetizing current (MAGN) where nominal skin depth is 0.2mm. Within a small
range 0 to 50 (relative value), Barkhausen noise increases as the magnetic field
increases. When the magnetic field reaches 50, the Barkhausen noise maintains a
constant value. During the measurement, the device will have a test run first to tell the
saturated point for the specific sample you measure. Following testing, the magnetic
field will be set less than, but close to, the limit as the instruction indicates [13].
Figure 32 Barkhausen Noise changes with magnetic field for carburized 1018 steel
Nominal case depth=0.2mm
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140 160
MP
Magne6c field /rela6ve value
26
(2) Barkhausen Noise changes with frequency
Nominal case depths provided by the STRESSCAN are 0.02mm, 0.07mm, 0.2mm
corresponding to the frequency range of 3-15Hz, 20-70Hz and 70-200Hz, respectively
Using the same Magnetizing current (MAGN) of 30 (relative value), the 1018 sample
is measured under different nominal case depth as Figure 33 presents. Smaller case
depth will result in a larger Barkhausen Noise because the frequency is larger. After
the carburizing process, the surface carbon increases, and at the same time the
resistivity of the material will increase. The case depth will be deeper than the
material without carburizing. This also provides us the potential application for the
case depth measurement on the carburized samples.
Figure 33 Barkhausen Noise changes with nominal skin depth for 1018 sample
MAGN=30
8.9
29.2
58.6
0
100
200
300
400
500
600
700
0.2 0.07 0.02
MP
NOMINAL SKIN DEPTH/MM
27
(3) Barkhausen noise changes with hardness
The samples without carburizing will reach saturated limit at a magnetic field of 50 or
lower. The magnetic field of 30 is picked to compare the change of MP after
carburizing. Figure 34 shows original and carburized samples of 1018, 4140, 8620. In
the figure, 1018C, 4140C, 8620C represents carburized samples with a much smaller
MP value than the non-carburized samples. After the carburizing process, the domains
will be pinned by the dislocation and impurities which also increase the hardness of
the samples.
Figure 34 Barkhausen Noise within original and carburized samples
Nominal case depth=0.2mm MAGN=30
Tempering was applied for the samples at 177℃ for 4h. The hardness decreases from
64 HRC to 59 HRC as Figure 35 presented. Barkhausen noise decreases for 1018
sample, but for other samples, there is little variation.
Figure 35 Barkhausen Noise within carburized and tempered samples
Nominal case depth=0.2mm MAGN=150
0
10
20
30
40
50
60
70
0
20
40
60
80
100
120
140
1018C 1018 4140C 4140 8602C 8620
HARD
NESS HR
C
MP
0
10
20
30
40
50
60
70
0 2 4 6 8
10 12 14 16 18
1018C 1018T 8620C 8620T 4140C 4140T
HARD
NESS HR
C
MP
28
(4) Barkhausen noise changes with stress
Barkhausen noise is also sensitive to the stress. The bend saw testing is performed to
study the effects. As presented in Figure 36, the original saw has a length of 18 inch.
Then the height of the arc d is adjust to obtain different stress. In the figure, d is set to
be 0, 1, 2, 3, 4 and 5 then converted to a respective stress value. The top of the stripe
has a tensile stress, while the bottom has a compressive stress.
Figure 36 Bend saw testing
As presented in Figure 37, the MP increases with compressive stress. But in
Figure 38, with the increasing of the tensile stress, the MP decreases. When the
tensile stress reaches a certain value, the MP remains unchanged.
Figure 37 Barkhausen noise changes with compressive stress
Nominal case depth=0.2mm MAGN=30
25 26
37 43
60
85
-‐
10
20
30
40
50
60
70
80
90
0.00E+00 4.68E+05 5.88E+05 6.86E+05 7.73E+05 8.71E+05
MP
COMPRESSIVE STRESS PA
Original length: 18 inch
d
29
Figure 38 Barkhausen Noise changes with tensile stress
Nominal case depth=0.2mm MAGN=30
(5) Barkhausen noise changes with stress for cylinders
The residual stress effects on Barkhausen noise is also studied. The cylinder is
measured by STRESSCAN 500C at different position presented in Figure 39.
Figure 39 Barkhausen noise changes with tensile stress
23 25
21 19 19 19
-‐
5
10
15
20
25
30
0.00E+00 4.68E+05 5.88E+05 6.86E+05 7.73E+05 8.71E+05
MP
TENSILE STRESS
A B C D E
A B C D E
A B C D E
30
The surface residual stress for 8620 has been modeled by DANTE, as presented in
Figure 40 and Figure 41. The main residual stress is hoop stress which has a
compressive residual stress around 300Mpa. The cylinder with a large diameter
should have a larger residual stress. The cylinder with a 2.0 inch diameter has about
10% higher hood stress than the 1.0 inch diameter cylinder.
Figure 40 The surface axial and hoop stress of 8620 cylinders
Figure 41 Surface stress of 8620 cylinders with different diameters
-‐350 -‐300 -‐250 -‐200 -‐150 -‐100 -‐50 0 50
0 1 2 3 4 5 6
Axial and Hoop stress of 8620 (Mpa)
Axial stress Hoop stress
-‐350
-‐300
-‐250
-‐200
-‐150
-‐100
-‐50
0 0 1 2 3 4 5 6
Surface hoop stress of 8620 (Mpa)
d=1.5 d=2 d=1
31
As presented in Figure 42, Barkhausen noise signal varies with position into the steel.
The case depth will not change much; it tells more about the residual stress
distribution. As discussed in the bending study, larger compressive stress will lead to
larger Barkhausen noise; position E has larger residual stress than position A. Also,
the cylinder with a 2.0 inch diameter has larger stress at C,D,E than the other two
cylinders.
Figure 42 Barkhausen noise distribute on the surface of cylinder
Nominal case depth=0.2mm MAGN=30
3.4. Future work plan
• Determine the correlations between the nondestructive tests results from the
MWM, Barkhausen testing and the hardness and case depth results of the
standard samples.
• Conduct the other nondestructive tests including Alternating Current Potential
Drop (ACPD) at ISU.
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
A B C D E
MP
POSITION
d=1.0 d=1.5 d=2.0
32
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