Department of Tool and Materials Engineering

1. Department of Tool and Materials Engineering Investigation of hot deformation characteristics of AISI 4340 steel usingprocessing map

2. Department of Tool and Materials Engineering Investigation of hot deformation characteristics of AISI 4340 steel usingprocessing map • ApichatSanrutsadakorn (M. Eng.) • Dr. Vitoon Uthaisangsuk (Dr.-Ing.) • Assoc. Prof. Dr. Surasak Suranuntchai (Ph.D.) • BorpitThossathappitak (M. Eng.)

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5. 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

6. 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

7. Flow curves at high temperatures AISI 4340 FE simulation - strain - strain rate - temperature 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

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9. The chemical composition (mass content in %) of the investigated steel AISI 4340 Procedure of the applied hot compression test Flow curves Investigated material + test procedure

10. 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

11. 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 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 • Determined flow curves

12. 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) stage IV (steady stage) Flow behavior at elevated temperature

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14. The Arrhenius equations (Zener-Hollomon parameter with an exponent-type equation) 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) 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 parameter T is the absolute temperature (K). is the strain rate (s-1). Qis 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

15. 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

16. 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

17. 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

18. (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

19. The values of material constants ( n , β , α,Q and lnA) 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

20. 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

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22. 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

23. 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 4340under different temperatures and strain rates of 0.01 and 10 s-1 are acceptable. Model improvement

24. 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

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26. 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. 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. Comparisons between experimental and predicted results were carried out. 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. Conclusion

27. ACKNOWLEDGEMENT Rajamangala University of Technology I-san Sakolnakhon Campus