Metallography of Vacuum Tempered Steel
Metallography is the study of the microscopic structure of metals, and the basic principles of metallography have been discussed in various materials research studies. Vacuum tempered steel, which is subject to a process of hardening by tempering in an evacuated chamber, is a relatively recent and increasingly popular material used in a variety of applications. In this article, we will discuss the metallographic processes and results of vacuum tempered steel, and summarize the advantages and limitations of this tempering technique.
The vacuum tempering process involves the heating of the steel to its tempering temperature, which is slightly below its full hardening temperature, in an evacuated environment where all internal gas pressure is released. This allows for uniform heat transfer across the sample and prevents oxidation of the material. After achieving the desired tempering temperature, the steel is quenched rapidly in an appropriate cooling medium, usually water. The rapid cooling rate reduces the opportunity for carbide precipitation along the grain boundaries, thus resulting in a more homogeneous microstructure.
Following the vacuum tempering process, metallographic examinations of the material are usually done in order to evaluate the resulting microstructure. The sample is typically mounted, ground and polished. Once the desired surface finish is achieved, the sample is etched and the metallographic results are evaluated. The most common etchant used for examining the microstructure of vacuum tempered steel is nital (1% nitric acid diluted in alcohol), which selectively etches ferrite, pearlite and carbide phases. Using light optical microscopy, it is possible to observe the grain size and boundaries of the material, as well as the distribution and type of precipitates, precipitate stability and connectivity.
Metallographic analysis of vacuum tempered steel reveals a more homogeneous microstructure than that of conventional quench and temper processed steel. The microstructure consists mostly of finely dispersed, randomly oriented ferrite grains, with little micro-segregation of certain alloying elements. This results in increased material strength and toughness, as well as improved fatigue properties. Moreover, vacuum tempering eliminates the potential for micro-structural inhomogeneities due to non-uniform heat transfer, and also reduces the possibility of oxidation, as the steel enters the tempering chamber in an almost totally deoxidized state.
However, metallography of vacuum tempered steel is not without certain limitations. The process can only be conducted on thin sections (up to 5 mm thick) due to the associated long tempering times. Additionally, the technique requires special equipment and conditions which may not be readily available at all processing facilities. Furthermore, due to the high degree of stress relief produced by tempering at low temperatures, it may not be possible to produce the highest levels of strength and hardness.
In conclusion, metallography of vacuum tempered steel reveals a homogeneous microstructure with improved material properties. However, the process has some limitations such as the difficulty of tempering thicker sections and the potential for reduced strength and hardness. Despite these drawbacks, vacuum tempering remains a popular method of processing steel due to its numerous advantages, including improved material properties and uniform heat transfer.
