The elastic, plastic, and thermal strain components are obtained based on standard constitutive relations, and the phase transformation and transformation plasticity components are defined as follows: (10)

where βIJ is the transformation expansion coefficient for the case when phase I transforms to phase J, X·IJ is the time differentiation of the transformation volume fraction from phase I to J, and sij is the deviatoric stress. βIJ can be calculated from the difference in density between phases I and J. In the present problem, I is always the austenitic phase, and J can be any other phase obtained by transformation from the austenite phase.

To simulate the hot press forming process based on the phase transformation rule described above, the Microstructure Module was added to DEFORM-2D and utilized in the present investigation.  The material data for 22MnB5 were calculated by JMatPro and used as an input into the DEFORM simulation using the Microstructure Module.

The material data for the die used for the FEM analysis are shown in Figure 5 for flow stress, Young’s modulus, and the thermal expansion coefficient.

The forming step of the hat-type part predicted by the FEM simulation is shown in the sequential order in Figure 6. This result is for the first cycle of the hot press forming

operation. In general, the actual forming process is composed of multiple operations that are repeated continuously. Thus, it is necessary to examine the effect of multiple operations on the temperature as well as stress evolution for the die during the entire hot press forming process.

Figure 7 shows the evolution of temperature and the austenite phase ratio during the forming step as predicted by the FEM simulation. As the austenite phase ratio decreased, the ratios of other phases including martensite increased. However, the amount of martensite generated during the forming step was predicted to be rather small because the time duration of forming step was very short (about 1.3 s). Most phase transformation into martensite seemed to occur during the holding step (5 s) and the air cooling step when no die pressing is required. This implies that the stress of the die was not affected significantly by the martensite phase transformation of the workpiece during the forming step.

Assuming that the above forming step is repeated continuously, FEM simulation was performed to predict the stress evolution in the die. For simplicity, the time interval between cycles was assumed to be negligible, and only five cycles of forming were simulated in the present investigation. As most industrial forming processes consist of continuous cycles, the following results from the five-cycle simulation need to be used to estimate the result for continuous cycles.

Figure 8 shows the temperatures of the top die during the first cycle and the five cycles of hot press forming. The results show that the peak temperature occurred around the die corners that contacted the heated workpiece consistently during the hot press forming process. In the case of the thermocouple location of “tc1,” the peak temperature increased gradually as the cycle number

LIFE ESTIMATION OF HOT PRESS FORMING DIE BY USING INTERFACE HEAT TRANSFER COEFFICIENT 289

Figure 8. Temperature history of top die during hot press forming operation.

increased. That is, the peak temperature increased up to about 80oC in the fifth cycle, whereas it was about 30oC in the first cycle. Approximately, the temperature of “tc1” ranged from 13oC to 30oC in the first cycle and from 65oC to 80oC in the fifth cycle.

Two types of stresses were considered in this investigation: the von Mises stress and the maximum principle stress. The maximum principal stress is a good measure for material failures because most fracture-type failures are caused by the action of tensile stress on internal micro voids or cracks. In addition to the maximum principal stress, the von Mises stress was also considered in order to examine the overall stress effect on failures