Department of Tool and Materials Engineering
Investigation of hot deformation characteristics of AISI
4340 steel using processing map
250
900 C
Stress (MPa)
200
1000 C
150
900 C
1000 C
1100 C
1200 C
1100 C
100
1200 C
50
0
0
0.2
0.4
0.6
Strain
0.8
1
Department of Tool and Materials Engineering
Investigation of hot deformation characteristics of AISI
4340 steel using processing map
Apichat Sanrutsadakorn (M. Eng.)
Dr. Vitoon Uthaisangsuk (Dr.-Ing.)
Assoc. Prof. Dr. Surasak Suranuntchai (Ph.D.)
Borpit Thossathappitak (M. Eng.)
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour at high
temperatures
Processing map characterization
Conclusion
Outline
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour at high
temperatures
Processing map characterization
Conclusion
Outline
Hot forging process is mostly applied in Thai part making industries.
There is still a lack of technology for
effectively predicting and controlling
hot forming process.
Database of material properties at
high temperatures are still insufficient
according to the metallurgical aspect.
Motivation
Production is mainly
based on experience
and trial-and-error.
Input Data
- Material properties
- Friction
- Temperature
- Heat transfer
Computer simulation
technique
Metallurgical aspect
- Grain size
- Phase transformation
inaccurate
Although some companies have applied computer simulation techniques in their development
stage, but the metallurgical aspects such as grain size and phase transformation as well as material
properties like flow curves are still insufficient. They are especially important basic parameters for
the calculation.Also reliable data describing deformation behavior of material at hot working
temperature are absolutely required.
Motivation
Flow curves at high temperatures
Stress - Strain Curve
AISI 4340
120
100
- strain
- strain rate
- temperature
True Stress, [MPa]
T = 1150°C strain rate = 0.001
T = 1150°C strain rate = 1
80
T = 1150°C strain rate = 10
T = 1200°C strain rate = 0.001
T = 1200°C strain rate = 1
60
T = 1200°C strain rate = 10
T = 1250°C strain rate = 0.001
T = 1250°C strain rate = 1
40
T = 1250°C strain rate = 10
20
0
0
0.2
0.4
0.6
True Strain, [-]
The objective of this study is to investigate the
deformation characteristics of steel AISI 4340
depending on strain, strain rate, and temperature by
means of a hot compression test. A constitutive
model describing the relationship between flow
stress, strain rate, and temperature of the
investigated steel at high temperatures has been
proposed. At last, an optimization of the developed
flow curve model was done.
Objective
0.8
1
FE
simulation
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour at high
temperatures
Processing map characterization
Conclusion
Outline
The chemical composition (mass content in %) of the investigated steel
AISI 4340
C
Si
Mn
P
S
Mo
Cr
Ni
Fe
0.40
0.03
0.08
0.035
0.04
0.30
0.90
2.0
(bal)
Procedure of the
applied hot
compression test
Flow curves
Investigated material + test procedure
Deformation dilatometer used in this work is a dilatometer type DIL805 that can
measures the change in length of materials at various heating rate and deformation.
Components:
• Vacuum chamber
• Heating element
• Load cell
• Distance measuring system
• Welding equipment
• Gas flow system
Cylindrical
specimens
Height: 10 mm
Diameter: 5 mm
Deformation dilatometer
Examples of true stress-strain curves obtained from the
deformation dilatometer
True stress-strain curves at
temperature of 1050°C and different
strain rates
True stress-strain curves at a strain
rate of 1.0 s-1 and different
deformation temperatures
In this slide, examples of true stress-strain curves obtained from the hot compression test are depicted.
We can see that the effects of temperature and strain rate on the flow stress are very clear for all test conditions.
The flow stress decreased with increasing deformation temperature and decreasing strain rates. The true stressstrain curves showed a peak stress first at low strain values. Then, at higher strain the flow stresses decreased
and became saturated at the end. This is the dynamic flow softening of material
Determined flow curves
Theory: flow stress behaviors of material at elevated
temperature
shows the change of grain
structure during these 4 stages.
stage I
(work hardening stage)
stage II
(transition stage)
stage III
(softening stage)
Flow behavior at elevated temperature
stage IV
(steady stage)
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour
at high temperatures
Processing map characterization
Conclusion
Outline
The Arrhenius equations (Zener-Hollomon parameter
with an exponent-type equation)
where
were applied to describe the
relationship between flow stress,
strain rate, and temperature.
From the experimental hot
compression test, material
constants in the constitutive
equations can be directly
determined. According to this
equation the flow stress of
material can be expressed as
shown in equation (1) and (2)
σ is the material flow stress (MPa) for a given stain.
R is the universal gas constant (8.31 Jmol-1K-1). Z is the Zener-Hollomon parameter
T is the absolute temperature (K). is the strain rate (s-1).
Q is the activation energy during hot deformation (kJmol-1).
A, α and n are the material constants, and α = β/n.
Constitutive modeling of flow stress
Determination of material constants
for the constitutive equations
Following is an introduction of the solution procedure for determining the material
constants by taking the peak stress as an example.
DATA FLOW CURVE
For the flow stress level ( ασ < 0.8) and the high stress level (ασ > 1.2), the
relationships between the flow stress and strain rate can be expressed as the power
law and exponential law of F(σ) in Eq.2, respectively.
Determination of material constants
β and β′ are the material constants.
Taking the logarithm
n= 6.2267
Β= 0.0610 MPa
α=β/n = 0.0097 MPaˉ¹
The value of n and β could be obtained from the slope of these lines in the diagrams. For different
deformation temperatures a linear fitting method was used and a mean value of n and β were
computed as 6.2267 and 0.0610 MPa, respectively. Then, α=β/n is equal to 0.0097 MPa-1.
Determination of material constants
For all stress levels (low and high stress levels), Eq. (2) can be represented as
followed:
The values of activated energy (Q) could be easily calculated for different strain rates and
temperatures. The averaged value of the activated energy is therefore 348.104 kJmol-1 .
Determination of material constants
(1)
Plot of ln[sinh(ασ)] and lnZ
DATA
From the experimental results the relationship as
shown in the diagram could be determined. Then, the
values of lnA are the y-axis intercept and the value n is
the slope. Now, the values of A was calculated as
1.7910×10¹³ s-1 and the value of n was 3.8379
Determination of material constants
The values of material constants ( n , β , α,Q and ln A)
in the constitutive equations
By the same manner, the values of
material constants (Q, A, β, n, and
α) in the constitutive equations
were computed under different
individual strains with in the range
between 0.05 - 0.8 with an interval
of 0.05 The relationships between
Q, lnA, β, n, α and strain for steel
AISI 4340 can be represented in a
fifth polynomial form.
Determination of material constants
All determined material constants were substituted in this
equation and the flow stresses for all investigated strain
rates and temperatures could be computed.
. In case of strain rate of 1 s-1 the predicted results could
precisely represent the experimental curves. However, the
predicted flow stresses are higher than the experimental
ones for the strain rate of 10 s-1, while the predicted flow
stresses are lower than the experimental ones for the
strain rate of 0.01 s-1.
Predicted and measured flow curves
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour at high
temperatures
Processing map characterization
Conclusion
Outline
1 / 5
Therefore, the constitutive equation for the flow stress was modified as :
A modification of the Zener-Hollomon parameter by compensating the strain rate was done.
Multiplying both sides of equation (1) by έ⅕ , the modified Zener-Hollomon parameter ( Z′ )
can be expressed as equation (12). Therefore, the new constitutive equation for the flow
stress was revised as equation (14).
Model improvement
The comparisons between the measured and calculated flow stresses are now satisfactory.
From the comparisons between predicted and measured flow curves we can
see that with consideration of the strain rate compensation the flow stress
predictions for steel AISI 4340 under different temperatures and strain rates
of 0.01 and 10 s-1 are acceptable.
Model improvement
Evaluation of the accuracy of the proposed constitutive
equations
The average mean of 4.36% and the standard deviation of 5.19%
were found for the proposed model
It showed that the
introduced constitutive
equations provided a more
precise prediction of the
flow stress at elevated
temperatures for the
investigated steel AISI 4340.
Model improvement
Introduction
Material and experimental procedure
Constitutive modelling of stress-strain behaviour at high
temperatures
Processing map characterization
Conclusion
Outline
1. The deformation characteristics of steel AISI 4340 were
investigated for the practical range of temperature and strain
rate using hot compression test on a dilatometer.
2. Based on the experimental data, constitutive equations
incorporating effects of temperature, strain rate, and workhardening rate of material were proposed in order to describe
the flow behavior of material.
3. Comparisons between experimental and predicted results
were carried out.
4. It was confirmed that the modified constitutive equations by
compensating the strain rate provided a better prediction.
The compensation of strain rate concerns a materialdependent parameter.
Conclusion
ACKNOWLEDGEMENT
Rajamangala University of Technology I-san
Sakol nakhon Campus
Thank You
for your attention.