ABSTRACT−During the hot press forming process, the die experiences repeated thermal and mechanical loads owing to exposure to the heated workpiece. Such repeated thermal loads may cause deterioration in the mechanical properties of the die, leading to fatigue failure. Therefore, for the successful implementation of the hot press forming process in mass production, it is necessary to estimate the die life under hot press forming conditions. The fatigue life of a hot press forming die is estimated based on the stress history of the die during the forming process. Because accurate understanding of thermal behavior is essential for reliable analysis of fatigue life of the die at elevated temperatures, we characterized the thermal boundary condition, i.e., the heat transfer coefficient at the die–workpiece interface. For this purpose, die temperatures during a hot press forming process were measured as a function of time at select locations on the die. Inverse finite element method (FEM) analysis of the hot press forming process was performed to determine the interface heat transfer coefficient. The interface heat transfer coefficient was applied to the FEM simulation, and the temperature distribution and stress values for the die were determined. Considering the thermomechanical stress history, the fatigue life of the die was estimated based on the stress-life approach.

KEY WORDS : Hot press forming, Die life, Thermal stress, Heat transfer coefficient, Inverse analysis

1. INTRODUCTION

Hot press forming is an effective method for improving the strength of materials for automotive parts; the method is based on the phase transformation of steels (Altan, 2006; Lee et al., 2009; Park et al., 2013). During the hot press forming process, the die usually experiences repeated thermal and mechanical loads owing to interaction with the heated workpiece. The magnitude of mechanical loads in a hot forming process may be smaller than in the case of a cold forming process because owing to the softening behavior of metals, the flow stress of metallic workpieces decreases as temperature increases.

However, when the thermal and mechanical loads are considered together, the overall loads acting on a die under hot press forming conditions might be comparable to or even larger than those under cold forming conditions. In addition, die heating due to exposure to the heated workpieces can reduce the strength of the die material, making it more vulnerable to static or fatigue failures compared to a die under room temperature conditions. Considering the high tooling and equipment costs for the hot press forming process, it is necessary to estimate the die life for the hot press forming process in order to successfully design the forming process.

There have been several research investigations on the thermal fatigue of dies. Gronostajski et al. (2013) studied the thermal fatigue of the hot forging die. Concer et al. (2012) examined the thermal fatigue of the die-casting die, and Shibusawa et al. (1997) investigated the fatigue cracking of the die-casting die. Song et al. (1999) and Yoh et al. (2002) predicted the fatigue life of the STD61 steel die.

One of the main difficulties in predicting the fatigue life of dies for hot press forming is estimating accurate values of process conditions such as the heat transfer coefficient at the die–workpiece interface. The temperature distribution of the die has a critical dependency on the interface heat transfer coefficient. There have been numerous investigations to estimate the heat transfer coefficient for normal heat treatments or cooling processes (Kim and Oh, 2001; Kim et al., 2011; Seo et al., 2010). However, there has been little research on evaluating the heat transfer behavior in the hot press forming process. For example, Lee et al. (2012) and Kang and Lee (2008) conducted the finite element method (FEM) analysis of heat conduction

for hot press forming dies. Furthermore, only a few studies have been conducted to evaluate the interface heat transfer coefficients for hot press forming for die–workpiece or die–die interfaces (Kim et al., 2011; Lenhard et al., 2006). Most investigations failed to accurately determine the interface heat transfer coefficient for hot press forming; in most of these studies, the FEM analysis of heat transfer was performed using an assumed heat transfer coefficient.