[2014 Fall Creative Capstone Design]
Kwon, Jaehwan
Kim, Junhyung
Hyun, Seunghyeop
Kwon, Soonhyun
20071080
20091813
20091876
20111821
Laboratory for Multiphysics & Multiscale Systems
2014. 12. 01
Appendix
- The principle of ultrasonic measurement
- Why there is a drastic change in hardness
- Why the hardened depth is shallow
2
- Appendix
The principle of Ultrasonic tester
Ultrasonic wave reflected
at the Boundary surface
Martensite
Boundary surface
𝛼-ferrite
& Pearlite
Figure A-1. Part of the cut specimen
Hardened area and no-hardened area have different micro-structure
So each area have different property.
Therefore Ultrasonic wave reflected at the boundary surface.
3
Real depth
Ultrasonic measured depth
Hardened depth
Martensite
Why is there a drastic change in hardness?
4
- Appendix
Why there is a drastic change in hardness
1. Original material(44MnSiVS6) in this experiment
- Chemical composition
%
C
Si
Mn
P
S
Cr
Mo
V
N
0.42
0.49
0.15
0.80
1.20
1.60
<0.025
0.020
0.060
<0.30
<0.08
0.08
0.20
0.01
0.02
- Micro-structure
Pearlite & 5~25% 𝛼-ferrite
Figure A-2. Part of the equilibrium phase diagram
for the Fe-C system
5
- Appendix
Why there is a drastic change in hardness
2. Induction Hardening
Figure A-3. Internal magnetic field distribution of
induction heated specimen
Figure A-4. Current density distribution
With induction, subsurface of a specimen is heated,
And then it would be quenched to be hardened.
6
- Appendix
Why there is a drastic change in hardness
3. How martensite is made
Temperature(°C)
Temperature(°F)
Eutectoid temperature
A: austenite
P: pearlite
B: bainite
M: martensite
Time(s)
Figure A-2. Part of the equilibrium phase diagram
for the Fe-C system
Figure A-5. isothermal transformation diagram for an ironcarbon alloy of eutectoid composition
After heated, the alloy should be quenched rapidly to make martensite.
This similar procedure was applied to the specimen.
7
- Appendix
Why there is a drastic change in hardness
Real depth
Ultrasonic measured depth
Hardened depth
Martensite
What phase is it?
Why is there a drastic change in hardness?
8
- Appendix
Why there is a drastic change in hardness
4. No-hardened part
- Above the Eutectoid temperature,
(1) Phase(𝛼-ferrite+pearlite) is changed
to Austenite(𝛾)
(2) And then it would be quenched to
make martensite.
- Under the Eutectoid temperature,
(1) Phase(𝛼+pearlite) is not changed.
Eutectoid temperature
Figure A-2. Part of the equilibrium phase diagram
for the Fe-C system
The temperature gradient makes the phase difference
and the phase difference gives the hardness difference.
9
- Appendix
Why the hardened depth is shallow
Real depth
Ultrasonic measured depth
Hardened depth
About 2mm
Why is the depth shallow?
10
- Appendix
Why the hardened depth is shallow
5. Skin effect by induction
- If you make the current frequency of coil higher, the charged current will
concentrate on the surface.
- The specimen tries to stay in present state.
- And the higher charged current is in surface, the stronger the magnetic force.
- It is a resistance to a change, and similar to inertia force.
- So, the hardened depth is shallow
Figure A-6. A specimen & one coil induction Figure A-7. How frequency affects current penetration depth in hot steel
11
Q&A
12