Metal structure inspection is the qualitative and quantitative measurement of grain size, shape, type, phase composition, relative quantity, and distribution of metal materials by observing their internal structure through various metallographic methods. Metal structure inspection is one of the performance inspection items of steel, as the performance of steel depends on its composition, structure, and organization. After the material composition is fixed, many of its properties will be determined by structure, organization, macroscopic and microscopic defects.
The content and methods of metallographic examination are mainly divided into: low magnification examination, high magnification examination, and electron microscope microstructure examination using Leica microscope.
1. Macroscopic examination refers to the observation of defects on the longitudinal and transverse sections of steel with the naked eye or a magnifying glass, also known as macroscopic examination. The commonly used test methods for macroscopic tissue inspection are as follows:
(1) Acid leaching test: There are two types: hot acid leaching (70-80 ℃) and cold acid leaching. Different acid solutions are used according to different steel grades. The main content of inspection is to expose the porosity, segregation, bubbles, white spots, and non-metallic inclusions in the steel.
(2) Tower turning hairline inspection: Conduct hot acid immersion on tower shaped specimens of the specified size, and show small cracks of varying length and depth along the rolling direction.
(3) Sulfur imprinting test: Cover the inspected sample with a photo paper soaked in 5%~10% sulfuric acid solution, presenting a dark brown imprint, used to test the quantity and distribution of sulfur in steel.
(4) Fracture test: It is one of the important methods for inspecting macroscopic defects in steel, and can be used in conjunction with acid immersion inspection to supplement each other. According to different steel grades and inspection requirements, the sample needs to undergo different heat treatments before inspection, including quenching fracture, annealing fracture, and quenched and tempered fracture, usually using quenching fracture. Defects such as white spots, inclusions, interlayers, and pores can be found on the quenched fracture surface. Annealed fracture surface is used for bearing steel and Tool steel, which can check the uniformity and density of grain to show graphite carbon precipitation, inclusion, shrinkage cavity and other defects. The quenched and tempered fracture surface is only used for a few steels, and the fiber structure appearing on the fracture surface can reflect the mechanical properties of the steel to a certain extent.
2. High magnification microstructure inspection, also known as microstructure inspection, is a method of observing and analyzing the interior of steel using a Leica microscope magnified at 100-2000 times. The inspection content includes non-metallic inclusions, banded structure, non-uniformity of carbides, liquid precipitation of carbonization, depth measurement of decarburization layer, evaluation of spherical structure level, evaluation of network structure level, and evaluation of austenite grain size level.
3. Electron microscopy microstructural examination, also known as fine tissue examination, is the observation and analysis of the interior of steel using an electronic Leica microscope magnified from thousands to hundreds of thousands of times.
In addition, there are also tissue inspection methods such as using X-ray diffraction to determine the crystal structure of various phases inside metals and alloys, and using electron probe analysis to analyze the chemical composition in the microscopic area of the tissue.