Cracks Pattern Formation and Thermomechanical Fatigue in Die Steels

Author Name(s): Ilnar R. Mukhametzyanov, Mikhail S. Kolesnikov, Gulnara F. Mukhametzyanova, Vladimir I. Astaschenko
Author Email: ilnarr116m@gmail.com

Abstract

Investigations are given of the influence of thermal, thermochemical treatment of die steels on the development of thermomechanical fatigue cracks depending on the temperature-force effects characteristic of hot deformation dies. As the evaluation criteria for the thermomechanical fatigue resistance of surface hardened die steels, the moment of nucleation, their quantity, and the depth of propagation were used. The study of the laws of the development of thermomechanical fatigue cracks in die steels in copper melts is carried out in an analogous manner, which provides a similarity to the operational loading of the material according to the local stress-strain state and the conditions of interaction with the molten melt. Dependences of the depth and number of cracks formed in 4X4VMFS DI22 die steel, hardened by various methods, on the number of loading cycles are established. It has been established that the performance of a stamping tool in a variant with diffusion hardening is determined by the specific laws of softening and destruction of contact zones. Moreover, the occurrence of cracks in the initial stages of operation is not decisive. The positive effect of boriding and nitriding on the performance of the dies is associated with increased structural stability of the diffusion saturation zones, which ensures the maintenance of a high level of mechanical characteristics of the tool working surfaces – hardness, strength, and as a result of high wear resistance. The analysis of the results of studies of the patterns of crack formation in steel 4X4VMFS DI22, subjected to various hardening methods, indicates the promising use of nitriding and boriding to increase the life of dies.

Introduction

The high pace of development of engineering and other industries determine the ever-increasing need for highly resistant technological equipment for stamping and injection molding processes, which are experiencing a complex loading complex. Numerous factors affecting the durability of dies can be divided into metallurgical, technological, structural, operational. A variety of factors determines the complex nature of die wear – thermomechanical fatigue, crushing, wear, etc. The results of studies of tool materials using standard testing methods for special characteristics (resistance to crack development of thermomechanical fatigue, etc.) and mechanical properties do not provide sufficient information to predict the technological resistance equipment. This is due to the fact that the loading conditions of the samples do not correspond to the loading conditions of the equipment. In this regard, a systematic study of the laws of behavior of tool materials for dies of solid-liquid die stamping under specific conditions of temperature-force impact and interaction with melts characteristic of operational ones for a specific tool group is an urgent task [1 – 5]. Hardening of the working surfaces of a heavily loaded tool for hot deformation and injection molds using nitriding, cyanidation, etc. methods have received limited distribution. The latter is associated with low resistance to thermomechanical fatigue of diffusion saturation layers. The adverse effect of thermochemical treatment on the occurrence of thermomechanical fatigue cracks is explained by a high level of initial stresses in the layers due to microstructural heterogeneity, high hardness and, as a consequence, increased fragility [1, 2, 6 – 10]. In the present work, when studying the thermomechanical fatigue resistance of surface-hardened steel samples 4X4VMFS DI22 as applied to the loading of Avtoforge dies, a complex evaluation criterion is proposed that takes into account the number of cracks (n) and the average depth (Havg) due to the duration of the cyclic temperature force impact. The purpose of the work is to establish the laws of formation of thermomechanical fatigue cracks in surface-hardened die steels in various ways to improve the performance of the die tool.

Conclusion

The analysis of the results of studies of the laws of crack formation in steel 4X4VMFS DI22, subjected to various hardening methods, indicates the promising use of nitriding and boriding to increase the life of dies.

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