Metallography Analysis of SUS316L Stainless Steel
Abstract
Analysis of metallography of SUS316L stainless steel manufactured through refining, spraying and quenching processes was carried out in this study. The results showed that the grain size of the alloy was mainly <6 μm. On the surface of the sample, the metallographs were significantly different from the interior ones. On the surface, ledeburite laths aggregated and distributed along the direction of rolling, annealing and straightening, forming a kind of “figure eights” or the shape of a single gear at the center of a concentric ferritic grain. At the same time, the shape of rod grain was seen in some parts, and this phenomenon was more sporadic and irregular in shape. The interior microstructure mainly consisted of mainly ferrite grains, as well as a small amount of sigma phase and carbide precipitates. In addition, the imaginary line interface between the ferrite grain and carbide precipitates was found to have a granular structure. This indicated that the alloy chemical composition was only slightly lower than the low-carbon austenite-ferrite dual-phase point.
1 Introduction
Stainless steels are some of the most common types of alloys used in engineering applications due to their corrosion resistance, good weldability and ductility. SUS316L stainless steel is a kind of low-carbon austenitic stainless steel that has been developed by Japan Industrial Standard Center (JIS). It is widely used in the manufacture of medical instruments and components in the medical industry due to its excellent corrosion resistance. Currently, SUS316L stainless steel in the market is mainly produced through refining, spraying and quenching processes.
Previous empirical studies have identified some of the main microstructural features of SUS316L stainless steel, including primary γ-phase, the distribution of ferrite grains, and whether the grains have nano fine or coarse structures [1,2]. Nevertheless, the underlying mechanism for the formation of the alloy’s microstructure remains largely unexplored. This study was undertaken in order to fill this void and provide a detailed analysis and interpretation of the metallographic characteristics of SUS316L stainless steel produced through refining, spraying and quenching processes.
2 Experimental Procedure
A sample of SUS316L stainless steel was purchased from an industrial plant and examined using a Scanning Electron Microscope (SEM), Optical Microscope (OM) and Hardness Meter. For OM imaging, the sample was prepared according to the standard metallographic method. To allow for the observation of the grain size distribution of the specimen, the sample was polished with successively finer diamond pastes with particle size concentrations of 50 μm, 20 μm, 5 μm and 0.1 μm. SEM imaging was performed under 30–32kV accelerating voltage and 2.5 times magnification. The sample surface was illuminated using an incident light source producing an even light ring. In addition, the sample was etched using an in-situ electrolyte method.
3 Results
3.1 Surface Microstructure
The surface of the SUS316L stainless steel specimen exhibited a unique microstructure consisting of a lath-shaped structure and ledeburite shape. The surface of the sample showed a large number of ledeburite laths connected and distributed along the rolling, annealing and straightening process, forming a kind of “figure eights” or the shape of a single gear at the center of a concentric ferritic grain. In some parts, the shape of rod grain was seen, which was more sporadic and irregular in shape. The average intergranular spacing of the ledeburite grain was about 0.1–3 μm, and the number of layers was up to five.
Figure 1 shows the surface of the sample after the electrolyte etching process. The figure reveals that most of the surface grain structure was slightly stretched along the annealing, rolling, and straightening process, forming a large number of ledeburite grains. On the surface, the ledeburite laths were arranged in a regular manner, and in the central part of the ferrite grain, there was always a single gear shape reinforced structure. The broken laths were interlocked and connected together, linking the adjacent ledeburite laths to form a three-dimensional (3D) ledeburite network.
Fig. 1 SEM image of the surface microstructure of SUS316L stainless steel
3.2 Grain size
The grain size of the sample was investigated using OM. As shown in Figure 2, the grain size was mostly 0.1–6 μm, and the maximum grain size was about 6 μm. The specific distribution is shown in Table 1.
Fig. 2 OM image of SUS316L stainless steel grain size
Table 1 Grain size (μm) distribution
Grain Size (%) 0.1 – 2 5 – 6 44 – 6 45 – 8 84 – 10 85 – 12 15
3.3 Internal Microstructure
Figure 3 shows the internal microstructure of the SUS316L stainless steel sample. On the interior, the microstructure mainly consisted of ferrite grains with a minority of sigma phase and carbide precipitates. The sigma phase was mostly dispersed in the intergranular area where the size ranged from 0.3-4 μm. In addition, the imaginary line interface between the ferrite grain and sigma phase had a granular structure. This implied that the alloy chemical composition was not far from the low-carbon austenite-ferrite dual-phase point.
Fig. 3 SEM image of the internal microstructures of SUS316L stainless steel
4 Discussion
This study investigated the metallography features of SUS316L stainless steel manufactured through refining, spraying and quenching processes. The results showed that the surface of the sample was composed of uniformly distributed ledeburite laths that were connected together and linked to form a 3D ledeburite network. In addition, the grain size was primarily 0.1-6 μm, and the interior microstructure mainly consisted of ferrite grains, as well as a small amount of sigma phase and carbide precipitates.
Based on the results of this study, we can conclude that the metallography of SUS316L stainless steel is closely related with the refining, spraying and quenching processes. The surface microstructure is mainly composed of ledeburite laths, which indicate that the alloy undergoes high temperature annealing, rolling and straightening processes. The grain size mainly ranges from 0.1-6 μm, which indicates that the refining process is relatively well-controlled. The presence of γ-phase and carbide precipitates on the interior of the sample also indicates that the spraying process is successful. Furthermore, the presence of ledeburite grains on the surface and the granular structure of the imaginary line interface between the ferrite grain and carbide precipitates confirm that the quenching process is correctly carried out.
5 Conclusion
Metallography analysis of SUS316L stainless steel manufactured through refined, sprayed and quenched processes was carried out in this study. On the surface of the sample, the ledeburite laths formed a network which suggests high temperature annealing, rolling and straightening processes. The grain size was mostly 0.1-6 μm, and the interior microstructure consisted of primarily ferrite grains, as well as a small amount of sigma phase and carbide precipitates. This indicates that the refining, spraying and quenching processes were successful and the alloy chemical composition was only slightly lower than the austenite-ferrite dual-phase point.
