Metallographic analysis

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Organizational observation and analysis

The internal structure of metallic materials can only be observed under a microscope. The internal structure seen under the microscope is called microstructure or metallographic structure. The common metallographic structure of steel is ferrite, austenite, cementite, pearlite, etc.

Metallographic analysis is one of the important means in the experimental research of metal materials. Based on the quantitative metallographic principle, the three-dimensional spatial morphology of the alloy structure is determined by the measurement and calculation of the metallographic microstructure of the grinding surface or film of the two-dimensional metallographic sample, so as to establish the quantitative relationship among the composition, structure and performance of the alloy. The application of computer in image processing has the advantages of high precision and high speed, which can greatly improve the work efficiency. The computer image analysis system can easily measure the area percentage, average size, average distance, length width ratio and other parameters of the feature, and then determine the three-dimensional space shape, quantity, size and distribution of the feature according to these parameters, and establish the internal relationship with the mechanical properties of the material, so as to provide reliable materials for more scientific evaluation and rational use Data.

Leica DMR positive mounted wide field metallographic microscope has clear image and high resolution. It is equipped with 10x binocular eyepiece and 5x, 10x, 20x, 50x, 100x objective lens. The magnification adjustment range is between 40 times and 1600 times. It is equipped with 3 million pixel digital imaging system.

Main function: it can observe and shoot the bright field and the dark field.

The resolution of dm4000m Leica metallographic microscope camera is 3 megapixel light source intensity, aperture diaphragm and related parameters, which can be automatically adjusted to the ideal state with the change of objective lens and observation mode.

Main functions: it can mark the position of interest in the observation sample, collect the image in real time and clearly, and carry out various quantitative detection and Analysis on the image, such as the quantity statistics of each phase of metallographic structure, the automatic statistics and rating of inclusions, the measurement and analysis of grain size, the measurement of microhardness and the measurement of the percentage of multiphase area.

Polarization and contrast

To change the ordinary light into polarized light for microscopy, in order to identify whether a substance is single refraction (isotropy) or birefringence (anisotropy). Birefringence is the basic characteristic of crystal. Phase contrast microscope is phase contrast microscope. By using the difference of refractive index and thickness between different structural components of an object, the optical path difference through different parts of the object is transformed into the difference of amplitude (light intensity), and a microscope for observation is realized through a condenser with a circular aperture and a phase contrast objective with a phase piece. It is mainly used to observe living cells or unstained tissue sections, and sometimes it can also be used to observe staining samples lacking contrast.

inclusion

Particles with different shapes and colors were found on the acid immersed test piece. In the process of steelmaking, a small amount of slag, refractories and reaction products may enter the molten steel and form non-metallic inclusions. They will reduce the mechanical properties of steel, especially the plasticity, toughness and fatigue limit. In severe cases, the steel will crack during hot working and heat treatment or break abruptly in use. Nonmetallic inclusions also promote the formation of hot-working fiber structure and banded structure, which makes the material anisotropic. When it is serious, the transverse plasticity is only half of the longitudinal one, and the impact toughness is greatly reduced. Therefore, it is necessary to check the quantity, shape, size and distribution of non-metallic inclusions in steel for important purposes (such as rolling bearing steel, spring steel, etc.). The common internal non-metallic inclusions are as follows: (a) oxides, commonly Al2O3; (b) sulfides, such as FES, MNS, (MNS · FES), etc.; (c) silicates, such as ferrous silicate (2FeO · SiO2), manganese silicate (2mno · SiO2), ferromanganese silicate (MFEO · MnO · SiO2), etc.; (d) nitrides, such as tin, ZrN, etc.; point shaped undeformed inclusions, etc.

Grain size

Grain size refers to the scale of grain size, which can be expressed by the average area or average diameter of grain. Grain size grade is used to express grain size in industrial production. The standard grain size is divided into 8 grades, 1-4 is coarse grain, 5-8 is fine grain. Generally, the larger the grain size is, the finer the better.

According to the austenitizing condition and growth tendency, the grain size of steel can be divided into three kinds: initial grain size, actual grain size and intrinsic grain size. The initial grain size refers to the grain size of the austenite grains initially formed in the process of austenitizing when the steel is heated above the critical temperature, i.e. the grain size when the austenite transformation is just completed and the grain boundary begins to contact is large. J is the initial grain size. The actual grain size refers to the size of the solid grain obtained under a certain actual condition. The intrinsic grain size only represents the growth tendency of austenite under a certain condition. Usually, the standard experimental method is adopted, that is, the steel is heated to (930 + - 10) Celsius, and the austenite grain size is measured after 3-5 hours of heat preservation.