0

50

100

150

200

250

0 0.2 0.4 0.6 0.8 1

Strain

Str

es

s (M

Pa)

900 C

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

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1

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.