Department of Tool and Materials Engineering
description
Transcript of Department of Tool and Materials Engineering
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Strain
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es
s (M
Pa)
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1000 C
1100 C
1200 C
Department of Tool and Materials Engineering
Investigation of hot deformation characteristics of AISI 4340 steel using processing map
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.)
Outline
Outline
Motivation
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. Production is mainly
based on experience and trial-and-error.Database of material properties at
high temperatures are still insufficient according to the metallurgical aspect.
Motivation
Computer simulation technique
Metallurgical aspect- Grain size- Phase transformation
Input Data- Material properties- Friction- Temperature- Heat transfer 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.
Objective
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True Strain, [-]
Tru
e S
tres
s, [
MP
a]
T = 1150°C strain rate = 0.001
T = 1150°C strain rate = 1
T = 1150°C strain rate = 10
T = 1200°C strain rate = 0.001
T = 1200°C strain rate = 1
T = 1200°C strain rate = 10
T = 1250°C strain rate = 0.001
T = 1250°C strain rate = 1
T = 1250°C strain rate = 10
FE simulation
Flow curves at high temperatures
- strain- strain rate- temperature
AISI 4340
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.
Outline
C Si Mn P S Mo Cr Ni Fe0.40 0.03 0.08 0.035 0.04 0.30 0.90 2.0 (bal)
The chemical composition (mass content in %) of the investigated steel AISI 4340
Investigated material + test procedure
Procedure of the applied hot
compression test
Flow curves
Deformation dilatometer
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
Determined flow curves
Examples of true stress-strain curves obtained from the deformation dilatometer
True stress-strain curves at a strain rate of 1.0 s-1 and different deformation temperatures
True stress-strain curves at temperature of 1050°C and different
strain rates
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 stress-strain 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
Flow behavior at elevated temperature
stage IV (steady stage)
Theory: flow stress behaviors of material at elevated temperature
stage I (work hardening stage)
stage II (transition stage)
stage III (softening stage)
shows the change of grain structure during these 4 stages.
Outline
Constitutive modeling of flow stress
The Arrhenius equations (Zener-Hollomon parameter with an exponent-type equation)
where
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 parameterT 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.
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)
Determination of material constants
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.
β and β are the material constants.′
n= 6.2267Β= 0.0610 MPaα=β/n = 0.0097 MPa ¹ˉ
Determination of material constants
Taking the logarithm
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.
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
DATA
(1)
Plot of ln[sinh(ασ)] and lnZ
Determination of material constants
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
The values of material constants ( n , β , α,Q and ln A) in the constitutive equations
Determination of material constants
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.
Predicted and measured flow curves
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.
Outline
Therefore, the constitutive equation for the flow stress was modified as :
Model improvement
5/1
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).
The comparisons between the measured and calculated flow stresses are now satisfactory.
Model improvement
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.
Evaluation of the accuracy of the proposed constitutive equations
Model improvement
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.
Outline
Conclusion
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 work-hardening 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 material-dependent parameter.
ACKNOWLEDGEMENT
Rajamangala University of Technology I-san Sakol nakhon Campus
Thank You for your attention.