Hot Stamping Process
Heating Phase
The process begins with the heating of the blank up to the austenitization temperature. This
temperature has a great influence on the part properties, the processing time and the cost-efficiency of
hot stamping. Heating of the blank may be isothermal, non-isothermal or both. Li et al. [1] have
investigated and characterized the effect of heating rate and temperature on the progress of austenite
formation under both non-isothermal and isothermal conditions. The fraction and distribution of
martensite in the formed part is determined by the extent of austenitization. Hence, the formation of
austenite during the heating process is of primary importance in determining the final properties of a
given part. Compared with the number of investigations on austenite decomposition during cooling,
studies on austenite formation have been few. This is because the observation of the progress of austenite
formation is difficult due to the difficulty in retaining the austenite at room temperature for inspection
and characterization [2,3].
The material used is 22MnB5 (Manganese-Boron) steel with the chemical composition and initial
microstructure information. Starting from a given initial microstructure, the heating rate and soaking
temperature are two critical factors which affect the kinetics of austenitization.
When a material undergoes a phase transformation, the lattice structure changes, which is in
principle accompanied by a change in specific volume. The formation of austenite involves the lattice
change of iron from a body-centered-cubic (BCC) structure to a face-centered-cubic (FCC) structure, which
results in a change in density, hence volume.
The hot stamping process begins with the heating of the blank op to the austenitization
temperature. For the determination of the process window regarding a homogeneous austenitization of
the blanks during a hot stamping as a pre-condition for fully martensitic transformation, lechler and
Merklein [4] performed annealing tests (investigations) considering austenitization temperature and time.
Starting from a given initial microstructure, the heating rate and soaking temperature are the two critical
factors which affect the kinetics of austenitization [1]. The investigations of Lechler [5] have shown that
the heating procedure of the blank has a great influence on the process time, the end mechanical
properties of part, and the cost-efficiency of hot stamping. Therefore, a homogeneous blank temperature
and a short heating time are the main demands on the heating system.
There are two stages of heating phase:
1. Stage 1 - Continuous Heating: This stage involves heating to raise the temperature and phase
transformation. This stage has two parts.
a. First: It involves the heating which causes linear expansion in the material (Boron Steel in
this case) with initial phase mixture.
b. Second: Here, the expansion curve/heating curve deviates from linearity at 1007K (Ac1).
This is because formation of austenite takes place. It is due to the competition between
the volumetric change induced by phase transformation and thermal expansion.
2. Stage 2 - Soaking: This stage also provides heating at constant temperature. The purpose is to
distribute the temperature uniformly throughout the material. At this stage, contraction is caused
by the phase transformation. The austenite transformation rate in the isothermal soaking period
decreases with the soaking time. Soaking time is usually 10-15 minutes. After soaking time, the
final microstructure could be a mixture of austenite and ferrite.
The phase transformation in general is controlled by two classes of factors: (1) Initial
microstructure, including phase composition, chemical composition, grain size and the presence of nonmetallic inclusions; that is, the intrinsic properties of an alloy. (2) External conditions, including heating
rate and temperature. Li et al. [1] performed two groups of experiments to investigate the effects of
thermal conditions on austenite formation. The results are listed in table 1. In the first group of
tests/experiments, soaking temperature was kept constant while heating rate and soaking time were
different for each experiment in the first group. Results of the first group are discussed below:
Effects of Heating Rate on Non-Isothermal Austenite Formation: It is apparent that less time is required
at a higher heating rate to achieve the same volume fraction of austenite than at a lower heating rate.
This is because higher heating rate stimulates a higher nucleation rate, which allows more nuclei for
austenite formation to be generated even within a shorter period of time, thus enabling a higher overall
growth rate of austenite [6]. Another effect of heating rate is that the temperature to attain a particular
amount of austenite increases with increasing heating rate. This is mainly because a higher heating rate
means less soaking time is available for diffusional transformation with a given growth geometry and the
transformation would be shifted to higher temperatures [7,8]. The quantification of the heating rate
effects on austenitization under continuous heating conditions is complex, since heating rate is coupled
with both temperature and time. Nucleation and grain growth are the two primary mechanisms operating
in the phase transformation process.
Effects of Heating Rate on Isothermal Austenite Formation: The isothermal transformation takes place
at a constant soaking temperature at which maximum austenite formation is possible. Results of
experiments from [1] reveal that, at the same stage of the transformation under the same isothermal
condition, less time is required for higher heating rates. It can be concluded that the thermodynamic
effect of heating rate proceeds from the heating step and continues through the subsequent isothermal
soaking step. This is because the transformation rate at any time depends on both the instantaneous
growth rate of new phase grains and the existing quantity of grains [9]. Since more nuclei are generated
during continuous heating at a higher rate, during the subsequent isothermal soaking at a given
temperature, a larger amount of pre-existing grains enables a higher overall growth rate of new phase,
i.e. a higher austenite formation rate.
In the second group of tests/experiments, heating rate and soaking time were constant while
socking temperature was different for each experiment in the group. Results of the second group are
discussed below:
Saturated Volume Fraction (ƒs): Heating rate affects the austenite transformation rate, but it doesn’t
affect the maximum achievable value of austenite fraction at a particular soaking temperature, i.e. ƒs is
only a function of temperature.
Effects of Transformation on Isothermal Austenite Formation: Results reveal that less transformation
time is required at a higher soaking temperature. Having the d=same heating rate, the same amount of
nuclei should be generated in the tests. Then the transformation rate is controlled by the growth rate of
the austenite phase. Thus, it is determined by only the soaking temperature under isothermal conditions.
Higher soaking temperature enables faster progress towards equilibrium.
The characterization of the dynamics of the austenite formation process is essential for the
thermal condition design, e.g. heating rate and soaking temperature, to enhance productivity and reduce
energy consumption in hot stamping process.
Table 1. Experiments and their Results
Test Group
Test Group 1:
Same Soaking
Temperature
(1173K)
Test Group 2:
Same Heating
Rate (5 K/s)
Thermal Conditions
Heating Rate (K/s)
Soaking Time (minutes)
1
10
5
15
25
15
Soaking Temperature Soaking Time (minutes)
1023
15
1073
15
1123
15
1173
15
Resulting Austenite Formed (%)
Start of Soaking
End of Soaking
77
100
61
100
54
100
Austenite Formed at the end of Soaking (%)
32
83
92
99
Table 2. Heating Phase Parameters (Input and Output)
Input Parameters
Thermal
Conditions
1
2
3
4
5
6
7
Heating Rate
(K/s)
1
5
25
5
5
5
5
Soaking
Temperature (T)
1173
1173
1173
1023
1073
1123
1173
Output
Soaking Time
(minutes)
10
15
15
15
15
15
15
Austenite Formed (%)
100
100
100
32
83
92
99
Forming
In order to avoid cooling of the part before forming, the blank must be transferred as quickly as
possible from the furnace to the press. Furthermore, forming must be completed before the beginning of
the martensite transformation. Therefore, a fast tool closing and forming process are the precondition for
a successful process control. After forming, the part is quenched in the closed tool, which is cooled by
water ducts to transfer the heat out of the tool system. In order to avoid the quenching of the blank
between the blank holder and the die during the forming process, most of the hot stamping tool systems
work with a distance blank holder.
Quenching
After the forming of the heated blank in the austenitic temperature range, the part is quenched
in the closed tool until the part microstructure is fully martensitic. A cooling rate of more than 27 K/s is
necessary for a full martensite microstructure of 22MnB5. Martensite evolution leads to an increase of
the flow stress. The transformation from austenite (FCC) into martensite (BCT) causes an increase in
volume, which influences the stress distribution during quenching. For the accurate prediction of the
resulting material properties, the volume fraction of different phases, the residual stresses, and the
distortion of the workpiece after cooling, a complete description/analysis of the transformation behavior
is needed [10].
References
[1]. Li N., Lin J., Balint D. S., Dean T. A., 2016. “Experimental Characterization of the Effects of Thermal Conditions on Austenite Formation for
Hot Stamping of Boron Steel”. Journal of Materials Processing Technology 231, pp: 254-264.
[2]. Reed A. C., Akbay T., Shen Z., Robinson J. M., Root J. H., 1998. “Determination of Re-Austenitization Kinetics in a Fe-0.4C Steel using
Dilatometry and Neutron Diffraction”. Materials Science and Engineering A 256, pp: 152-165.
[3]. Schmidt E. D., Damm E. B., Sridhar S., 2007. “A Study of Diffusion-and-Interface-Controlled Migration of the Austenite/Ferrite Front during
Austenitization of a Case-Hardened Alloy Steel”. Metallurgical and Materials Transactions A 38(4), pp: 698-715.
[4]. Lechler J., Merklein M., 2008. “Hot Stamping of Ultra Strength Steels as a Key Technology for Lightweight Construction”. In: Materials Science
and Technology (MS&T), Pittsburgh, Pennsylvania, pp. 1698-1709.
[5]. Lechler J., 2009. Grundlegende Untersuchungen zur Berchreibung und Modellierung des Werkstoffverhaltens von presshärtbaren BorManganstählen. Dr.-Ing. Dissertation, LFT, University of Erlangen-Nuremberg.
[6]. Savran V. I., 2009. Austenite Formation in C-Mn Steel. In: Materials Science and Technology. The Delft University of Technology.
[7]. Huang J., Poole W. J., Militzer M., 2004. “Austenite Formation during Intercritical Annealing”. Metallurgical and Materials Transactions A
35(11), pp: 3363-3375.
[8]. Cai J., 2011. Modeling of Phase Transformation in Hot Stamping of Boron Steel. Mechanical Engineering. Imperial College London.
[9]. Liu F., Sommer F., Bos C., Mittemeijer E. J., 2007. “Analysis of Solid State Phase Transformation Kinetics: Models and Recipes”. International
materials Reviews 52(4), pp. 193-212.
[10]. Neubauer R., Hübner K., Wicke T., 2008. “Thermo-Mechanically Coupled Analysis: The Next Step in Sheet Metal Forming Simulation”. In: 1st
International Conference on Hot Sheet Metal Forming of High Performance Steel, Kassel, Germany, pp. 275-283.
[11].