What is the metallographic examination of pressure piping

Metallographic examination is mainly through the use of quantitative metallographic principles, the use of two-dimensional metallographic specimens grinding surface or thin film of the metallographic microstructure measurements and calculations to determine the three-dimensional spatial morphology of the alloy organisation, so as to establish a quantitative relationship between the alloy composition, organisation and properties. Metallographic test analysis not only organisation identification, but also evaluation, both qualitative and quantitative, semi-quantitative testing.
Metallographic examination of the content mainly includes: the material matrix phase of the organisational structure and its defects; microstructure of the orientation and state of non-uniformity, such as banding, uneven distribution, grain size, etc.; the second phase of the type, structure, composition, number, morphology, size and distribution; the study of the crystal structure of the atom according to the distribution of the bonding force and the distribution of electrons according to the energy distribution of atoms, ions structure. As far as microstructure examination is concerned, metallographic examination is divided into 4 levels, i.e. correctly identifying what microstructure is; qualitative microstructure state; quantitative microstructure state; the relationship between microstructure and performance.
Therefore, the method of metallographic analysis to observe and inspect the internal structure of the metal organisation is an important means of industrial production, commonly used metallographic observation and inspection can be divided into the following aspects:
1. pressure piping using raw materials inspection: the metallurgical quality of raw materials such as segregation, non-metallic inclusions distribution type and level check; casting materials casting looseness, porosity, slag organisation uniformity check; forging pieces of surface decarburisation, overheating, overcooking, cracking, deformation and other conditions check.
2. Pressure pipe manufacturing, installation and other quality control in the production process: metallographic analysis can provide the basis for adjusting processes and modifying process parameters to guide production, such as heat treatment quenching heating temperature, holding time, cooling rate, etc., whether appropriate (correct); chemical surface heat treatment process parameter control; forging of the beginning and end of the forging temperature is appropriate, and so on.

3. Pressure pipe manufacturing, installation and other product quality inspection: in addition to the requirements of mechanical properties, physical properties indicators, some also require microstructure parameters, as one of the technical indicators of quality assessment.
4. Pressure pipeline use period of regular inspection to evaluate the material properties: determine the unknown material categories; test the quality of materials and heat treatment status; check the quality of welding, such as whether there is hardened martensite, overheating Weiss organisation; check the effect of heat treatment; test the grain size of the material; testing the material in the microscopic defects, such as inter-crystalline cracks, sparsity, overcooking, etc.; to check the material degradation that may occur in the long-term high-temperature environment, such as pearlite spheroidisation, lithification, and other micro-organisation parameters. For example, pearlitic spheroidisation, graphitization; corrosive environment may produce intergranular corrosion or stress corrosion cracks; high temperature and high pressure pro-hydrogen environment of hydrogen damage, such as decarburisation, hydrogen corrosion cracks.

5. Pressure pipeline failure analysis: in-service pressure pipeline fracture metallographic examination can help determine the type of corrosion or fracture, analysis of the cause of failure. It is very convenient to identify some common malpractices, such as the morphology and distribution characteristics of microcracks; chemical heat treatment defects; abnormal organisation after heat treatment; grain boundary brittle phase precipitation. The results of metallographic analysis are often used as the basis for failure analysis.