Failure analysis of large press die holder
Introduction
Press die holder is the core component of large die forging equipment, which will bear formidable mechanical impact load and high thermal cycling load in service life. In consideration of the extreme working condition and large size, the design and manufacture of large die and die holder are quite difficult. Moreover, the production period is long, and the cost is high [1], [2]. In order to assure that the die holder works well during the service life, the hot forging die steel should have outstanding comprehensive mechanical properties, such as high strength, hardness, toughness, wear resistance, fatigue resistance and plastic deformation resistance [3]. In fact, though the material is of excellent properties, the properties degeneration of die and die holder usually happens due to the high thermal cycling during forging procedure [4].
Even though the failure of hot forging die has been studied for several decades, the reliability of hot forging die is not so satisfactory. The failure of die is still one of the most significant factor which leads to the high cost of large forgings. The failure analysis for large hot forging die and die holder is fairly difficult because of the following two reasons. Firstly, the influence factors causing hot forging die failure vary and are hard to control. These factors consist of improper die design, die materials, die manufacturing and forging operations [5]. Secondly, the failure mechanisms of hot forging die may be one of thermal fatigue, mechanical fatigue, fracture, wear, corrosion and deflection as well as several kinds coupling of the above mechanisms [6]. Therefore, finite element analysis is usually introduced to simulate the stress distribution in die during failure analysis as well as the design and manufacturing procedure [7].
This paper studied the failure mode and mechanism of the die holder which is typical but draws little attention in recent researches, due to that both the die holder and the fracture surface are very huge and complex.
Section snippets
Material and experimental procedure
The large press die holder is made up of the upper die holder and the lower die holder. The upper die holder produced a crack in the forging procedure. As shown in Figs. 1 and 2. The crack extended as long as 2316.7 mm in the inner cavity bottom and penetrated the bottom and right side. The die holder was made of 55NiCrMoV7 hot forging die steel by forging process. The die holder was austenitized at 850 °C for 15 h, then quenched in the water for 30 min, after that cooled in the oil to room
Macroscopic inspection
Fig. 2 shows the whole fracture surface. The thick radial ridges, which indicate the fast fracture, can be seen obviously on the fracture surface. The radial ridges are convergent to the bottom surface of the die holder inner cavity. The fracture surface shows yellow–blue–black feature, which implies that it has suffered high temperature and oxidation. Specimens cut from the left side of section 5 were kept at the temperature of 200 °C, 250 °C, 300 °C, 350 °C, 400 °C and 450 °C for 40 min,
Discussion
The failure of the die holder was caused by high stress under the condition of material embrittlement. The white semicircle fatigue was caused by cycling compressive stress from the die.
The material embrittlement was the result of the serious degeneration of impact toughness and hardness. The degeneration of impact toughness and hardness was caused by the long-term thermal cycling. Under the regular service condition, the temperature of the die holder could reach 200 °C–350 °C. However, the die
Conclusions
- (1)
The failure of the die holder was caused by high stress under the condition of material embrittlement.
- (2)
The main crack was originated from the fillet of the die holder. The surface cracking zone is the place where the main crack initiates. The main fracture mode is a fast brittle quasi-cleavage fracture.
- (3)
The serious impact toughness degeneration at the bottom of the die holder results in the material embrittlement. The material properties degeneration is related to the coarsening precipitates of M
Acknowledgment
This research was supported by the national fund (No.JSZL2014601B004). The authors wish to thank Wuxi Turbine Blade Co., Ltd. for the technology and equipment support. The guidance and help of Hongxiang Jing is greatly acknowledged.
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