Metallurgical Analysis Methods

Mechanical properties, such as strength, rigidity, toughness, fatigue characteristics, and wear resistance change due to heat treating and processing, even if the same materials are used and contain the same ratios.

In the crystal structures, the grain boundaries change their patterns due to heat treating. Mechanical properties can be understood by analyzing the change in shape, size, and distribution of grains after heat treating.
As examples, structure changes in stainless steel (SS material) due to heat treating are described below.

• Austenite (SS304)
  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 hardenability results in a stable structure. The austenite structure that remains unchanged in the steel after quenching is called residual austenite.
• Martensite (SS410)
  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 (SS430)
  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 color.

The metal sample is placed in a cylindrical container and resin is slowly poured over it to harden. In this step, close attention is required to prevent bubbles from forming.

The sample is polished using waterproof abrasive paper and a surface polishing machine. Typically, SiC papers ranging from 80 to 2400 are used for wet polishing. In precision polishing, synthetic silk sprayed with diamond grains and lubricant is used to polish the sample until a mirror-like finish is achieved.

The polished surface is soaked in an etching solution, then washed, soaked in alcohol, and dried.

The polished surface is observed with a microscope to view changes in the structure. Advanced measurements, such as non-metal inclusions, spheroidization, and ferrite/pearlite ratios need to be calculated.

Polished surfaces typically are not perfectly flat, requiring the sample to be repositioned and the microscope's focus to be continuously adjusted.
The VHX Series has a built-in depth composition function, enabling the entire sample to be brought into focus and observed, even if the surface is not flat.

Conventional microscopes typically lack the resolution to observe small microstructures or faint-colored structures.
The VHX Series 4K Digital Microscope is equipped with an HDR (High Dynamic Range) function that captures multiple images with high color gradation at varying shutter speeds. This makes it possible to observe detailed structures with images sporting higher levels of detail and contrast, which conventionally could not be observed due to limitations in resolution and contrast.

With a conventional microscope, you cannot quantitatively evaluate the quality of heat-treated ferrite or perlite according to its area ratio. The VHX Series 4K Digital Microscope is equipped with an automatic area measurement function. With just a few clicks, the VHX Series captures a magnified image, automatically quantifies the area ratios and grain sizes of the metal structures, and outputs a report.

Conventionally, advanced measurements such as non-metal inclusions, graphite spheroidization rates, and the ferrite/perlite area ratio have required the use of separate pieces of dedicated software.
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.