Failure analysis and countermeasures of the SCM435 high-tension bolt of three-step injection mold

Abstract

PurposeThe objective of this study was to perform damage analysis of SCM435 high-tension bolts connecting upper and lower parts of a three-stage injection molding machine.Design/methodology/approachDamage material used in this study was a SCM435 high-strength bolt connecting upper and lower molds of a three-stage injection mold. Causes of damage were determined by macroscopic observation. Microstructure observation was done using a metallic microscope, scanning electron microscope (SEM, S-2400, HiTachi, Japan), energy dispersive X-ray spectroscopy (EDS, Kevex Ltd., Sigma) and Vickers hardness tester (HV-114, Mitutoyo). Fatigue limit of the damaged material was evaluated using equivalent crack length.FindingsBolts were fractured by cyclic bending stress in the observation of ratchet marks and beach marks. The damaged specimen showed an acicular microstructure. Impurity was observed. Chromium carbide was observed near the crack origin. Both shape parameters of the Vickers hardness were similar. However, the scale parameter of the damaged specimen was about smaller than that of the as-received specimen. Much degradation occurred in the damaged specimen. Bolts should undergo accurate heat treatment to prevent the formation of chromium carbide. They must prevent the action of dynamic stresses. Bolts need accurate tightening and accuracy of heat treatment. Screws require compression residual stress due to peening.Originality/valueThis study conducted failure analysis of damaged SCM435 bolts connecting upper and lower parts of the three-stage injection mold. Fatigue limit of the damaged material was evaluated using equivalent crack length. In order to control this fracture, accurate tightening of bolts, accuracy of heat treatment and screws are required for compression residual stress due to peening.

Guest editorial

“…This Part II of the Special Issue on Structural Integrity and Failure Analysis of Metal Materials and Structures, resulted from a number of papers that were subjected to a rigorous peer-review process according to the high scientific standards of the International Journal of Structural Integrity. This special issue is composed of a total of eight papers proposed by engineers and scientists, covering subjects related to the aforementioned topics, such as Weibull statistical data based on Rockwell Hardness (Nam and Son, 2020), failure analysis of high-tension bolts (Nam and Yun, 2020), reliability analysis and safe operation of gas pipelines (Yang et al , 2021; Zhedulov et al , 2021), the relationship between the friction coefficient and interference in rail axles (Liu et al , 2021), efficient steel truss system using CFS sections (Dar et al , 2020), health monitoring and innovative reinforcement of bridges (Zhang et al ., 2021a, b).…”