Metallographic Analysis Methods and Streamlining of Observation and Measurement
In the automotive industry, each company has recently faced increasingly fierce competition in using materials to reduce weight and improve rigidity at the same time. With the adoption of higher performance materials, these companies are now demanding accuracy, convenience, and speed for the observation and measurement of metal structures, operations that are indispensable in the selection and use of optimal metallic materials.
This section introduces the basic knowledge and problems of metallographic analysis and concludes with the latest examples of using KEYENCE’s 4K Digital Microscope.
- Metallic Materials and the Necessity of Metallographic Analysis
- Metal Structures and Changes Due to Temperature
- Metallographic Analysis Methods Using Microscopes
- Latest Examples of Metallographic Analysis
- Metallographic analysis using 4K ultra-high-definition images
- Achieving observation with high levels of detail and contrast thanks to the 4K HDR function
- Large-area images captured through high-speed image stitching
- One-click measurement of metal structures
- Graphite spheroidisation observation and area ratio measurement
- Metallographic Analysis Required by Industry Leaders
Metallic Materials and the Necessity of Metallographic Analysis
Metallic materials are used in a wide range of fields, from automotive and aerospace to infrastructure, electric appliances, and electronic devices. Different metals and alloys are selected and used according to each application. However, even for the exact same metallic and alloy constituents, mechanical properties such as durability and workability differ depending on metal structures. Metallographic analysis allows for evaluation and judgement of metallic material properties and their changes due to processing, heat treating, and other factors, thereby enabling selection and use of suitable materials.
In the automotive and other fields in which breakage of metallic materials can affect product safety and human lives, metal structure observation is particularly important for evaluation and judgement of mechanical properties; evaluation after surface and heat treating; and analysis of defects such as non-metallic inclusions, cracks, and bubbles.
Mechanical properties of metallic materials
Mechanical properties in terms of metallic materials refer to dynamic properties such as strength, rigidity, toughness, fatigue characteristics, and wear resistance. Mechanical properties of metallic materials change as their metal structures change due to heat treating, even if the materials and components exist in the same ratio (the structure is the same). In addition to the durability of materials, these properties are closely related to the efficiency of mechanical processing, such as cutting and plastic processing, and characteristic changes in heat treating.
Metal Structures and Changes Due to Temperature
In general, metal structures consist of crystal structures in which atoms are arranged regularly, but not all atoms are always aligned regularly. Crystal structures in which many grains are arranged regularly are called polycrystalline structures, and boundaries between grains in an area where atoms are aligned irregularly are called grain boundaries.
- Left: Polycrystalline metal composed of many grains
- Right: Grain boundary between grains
In the crystal structures of these metal structures, the grain boundaries change their patterns due to heat treating and other applications of temperature. Different changes are obtained through heat treating even for the same metallic materials, so you can recognise mechanical properties after heat treating by observing changes in the shape, the size, and the distribution of metal structure grains.
As examples, structure changes in stainless steel (SUS material) due to heat treating are described below.
- Austenite (SUS304)
- Although this structure does not exist at room temperature, the structure for alloys of Fe and C becomes stable at a temperature of 723°C or above. Including alloy elements (Ni and Mn) that improve the quenchability also results in a stable structure. The austenite structure that remains unchanged in the steel after quenching is called residual austenite.
- Martensite (SUS410)
- A hard but brittle structure that is formed by rapidly cooling the austenite structure. By annealing martensite at 100 to 200°C, Fe3C is deposited, making this structure slightly tougher but easier to corrode. The martensite in this state is called annealed martensite to distinguish it from the quenched one.
- Ferrite (SUS430)
- A structure that is similar to pure iron and contains up to 0.02% of C in Fe. It is a ferromagnet from room temperature to 780°C and is ductile and the softest of the iron and steel structures.
The structure obtained by slowly cooling steel in an austenite state is called perlite. The interval between layers differs depending on the cooling speed. It is called perlite because extremely thin alternate layers of ferrite and Fe3C provide a pearl shell-like colour.
Metallographic Analysis Methods Using Microscopes
The metallographic analysis process from creating a sample to observing it using a general microscope is described below.
1. Embedment (filling with resin)
Cure the cut out sample with resin. There are various types of curing resins. One commonly used type is one-package visible light curing resin, which is highly transparent and cures quickly. Put the sample in a cylindrical case and slowly pour in curing resin. In this step, attention is required to prevent bubbles from forming.
2. Polishing
Polish the sample, with rough polishing using waterproof abrasive paper and precision polishing using a surface polishing machine. Use SiC papers from 80 to 2400 (from the roughest to the finest) with a wet polishing method. In precision polishing, mirror-finish the sample using synthetic silk sprayed with diamond grains sized 9 to 0.25 micrometres, abrasive, lubricant, and alkaline suspension, and then wash the surface with running water.
3. Etching (corrosion)
Soak the polished surface of the sample in etching solution (corrosion liquid) suited to the sample. Wash the corrosion liquid away with water, soak the sample in ethyl alcohol or the like, and then dry the sample.
4. Structure observation with a microscope
After the above creation process, observe the polished surface of the properly created sample with a microscope. Magnify the structure and adjust the focus properly to observe the changes in the structure due to the application of heat. However, advanced measurements such as non-metal inclusions, graphite spheroidisation, and the ferrite/perlite area ratio need to be carried out after migrating to dedicated software.
Latest Examples of Metallographic Analysis
KEYENCE digital microscopes enable the latest solutions.
KEYENCE’s VHX Series ultra-high-definition 4K Digital Microscope makes it possible to drastically improve the efficiency of various tasks in addition to offering sophisticated observation and evaluation of metal structures using high-definition images.
Metallographic analysis using 4K ultra-high-definition images
A common problem is that creating samples requires a lot of man-hours. Also, if the observation surface on an embedded (resin-filled) sample is not flat, even the slightest height difference will cause the image to go out of focus during high-magnification observation, hence this observation requires delicate adjustment.
The real-time composition interface of the VHX Series 4K Digital Microscope enables quick depth composition through easy operation with no focus adjustment required. This enables you to bring an entire metal structure into focus, even if the embedded (resin-filled) observation surface is not flat, so you can carry out clear structure observation easily using ultra-high-definition 4K images.
Achieving observation with high levels of detail and contrast thanks to the 4K HDR function
The VHX Series 4K Digital Microscope is equipped with an HDR (High Dynamic Range) function that captures multiple images with high colour gradation at varying shutter speeds. This makes it possible to observe detailed structures with images sporting high levels of detail and contrast.
Large-area images captured through high-speed image stitching
With the image stitching function of the VHX Series, just a push of the image stitching button quickly stitches together image data from different fields of view, with no misalignment, to provide an image with up to 50,000 x 50,000 pixels. With the magnification and resolution unchanged, this image can be used as a bird’s-eye view to enable efficient observation.
One-click measurement of metal structures
The VHX Series 4K Digital Microscope is equipped with an automatic area measurement function. With easy operation using a mouse, this single unit quickly completes an entire series of work operations: from magnified observation of metal structures, to quantitative evaluation of area ratios through automatic measurement, to report output.
Graphite spheroidisation observation and area ratio measurement
The VHX Series 4K Digital Microscope is equipped with an automatic area measurement/counting function. With simple operation, this function can measure and count an area ratio within a specified range. Targets that are not required can be excluded, and overlapping targets can be separated. This enables you to view advanced measured values in a table or graph at the same time as you perform observations with a microscope.
Metallographic Analysis Required by Industry Leaders
With automotive manufacturers as forerunners, each company has been working hard to keep their positions as leaders in the areas of new materials and processing technologies so as to increase fuel efficiency through weight reduction and to improve rigidity.
By using the VHX Series high-definition 4K Digital Microscope for metallurgical analysis, you can perform high resolution observation and measurement.
Equipped with many other advanced functions, the VHX Series can be a powerful tool for industries that require both speed and precision in material selection.
For details on the VHX Series, click the button shown below to download the catalogue. For inquiries, click the other button shown below to contact KEYENCE.
