Analysis of Fracture in JIS SCM435 Alloy Steel After Heat Treatment
Abstract
This paper describes an analysis of the fracture of a JIS SCM435 alloy steel after heat treatment. To understand the cause of the fracture, physical and mechanical properties of the material were determined. The results of the metallographic examinations, scanning electron microscopy with energy dispersive spectroscopy, and hardness tests for pre- and post-treatment samples were then compared. These tests revealed that the presence of pores caused a brittle fracture and that non-uniform hardening resulting from an improper heat treatment process was the major cause of the fracture.
Introduction
Steel engineering components undergo a variety of manufacturing processes to change their shape, improve their physical and mechanical properties, and enhance their performance. Heat treatment is one such process by which the properties of material are changed to enhance the performance of the part. Many components, especially those used in automotive and aircraft applications, are subjected to heat treatment to improve their strength and resistance to fatigue and impact loads.
JIS SCM435 is a kind of alloy steel that is frequently used in automotive and aircraft applications. This paper discusses the fracture analysis of a JIS SCM435 alloy steel after heat treatment. The specific heat treatment involved in this study is quenching, followed by tempering. After the heat treatment is finished, physical and mechanical properties of the material are tested to identify the materials’ performance and/or potential performance-related problems.
Material and Method
The material used in this study was a JIS SCM435 alloy steel. It was provided as a round bar of 35mm diameter and 350mm length, with a smooth surface finish.
The heat treatment process consisted of quenching and tempering. The quenching was performed at 890℃ for 1 hour and cooling was done in oil at a rate of 15℃/second. The tempering was done at 420℃ for 2 hours with a cooling rate of 30℃/second.
Tensile tests were conducted on both pre- and post-treatment samples. The results of these tests were compared to determine the influence of the heat treatment on the mechanical properties of the material.
The surfaces of pre- and post-treatment samples were examined using optical microscopy. Metallography was conducted to examine the microstructure of the samples.
Prior to the fracture analysis, the hardness of the samples was measured using the Vickers hardness test. The hardness results were compared to those of the pre- and post-treatment samples.
Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was employed for a detailed analysis of the fracture area.
Results and Discussion
The results of the tensile tests revealed that the heat treatment resulted in an increase in both the strength and elongation of the material.
The optical microscopy examination revealed that the surfaces of the pre- and post-treatment samples were similar.
The microstructures of the samples revealed that the microstructure of the pre- and post-treatment samples were different. After heat treatment, the microstructure consisted of martensite, temper martensite and retained austenite, indicating that the heat treatment was successful in inducing a martensitic transformation in the material.
The results of the hardness tests revealed that the heat treatment had resulted in an increase in the hardness of the samples. The hardness of the pre-treatment sample was 134 HB and 175 HB for the post-treatment sample.
The fracture area of the post-treatment sample was examined using SEM/EDS, which revealed the presence of pores. It was concluded that the fracture was due to brittle fracture, which was caused by the presence of pores.
The cause of the fracture was further investigated by microstructural examinations. It was revealed that there were numerous pores present in the heat affected zone (HAZ). It was found that these pores were the result of excessive heat input during the heat treatment process, which had caused non-uniform hardening of the material. This had resulted in a brittle fracture of the sample.
Conclusion
This study investigated the cause of fracture in a JIS SCM435 alloy steel after heat treatment. The results of physical and mechanical property tests, microstructural examinations and SEM/EDS analysis revealed that the fracture was due to a brittle fracture caused by the presence of pores. Further investigation revealed that the presence of pores was a result of non-uniform hardening that was caused by an improper heat treatment process.
Thus, it was concluded that an improper heat treatment process had caused a non-uniform hardening and ultimately led to the fracture of the sample. This finding highlights the importance of controlling the heat treatment process to ensure that the desired properties of the material are attained.
