The quality of metallographic sample preparation directly affects the results of microstructure analysis. If the sample preparation does not meet the specific requirements, it may be due to the appearance of false judgment, so that the whole analysis can not reach a correct conclusion. Therefore, in order to obtain suitable metallographic samples, it is necessary to go through a series of strict preparation processes.
Sampling is a very important step in metallographic microscopic analysis. It should be selected according to the characteristics, processing technology, failure mode and different research purposes of the metal material or part to be tested and analyzed, and its representative parts should be selected.
1. Selection of sampling site and inspection surface
Sampling sites and inspection surfaces should be selected with the best or better representation.
1) In the inspection and analysis of the failure of the parts of the damage cause, in addition to sampling at the damaged part, but also need to be far away from the damaged part of the sample, for analysis and comparison.
2) When studying the microstructure of metal forgings, it is necessary to take samples from the surface to the center for observation due to the existence of segregation phenomenon.
3) For rolled and forged materials, both transverse (perpendicular to the rolling direction) and longitudinal (parallel to the rolling direction) metallographic samples should be intercepted to analyze and compare the distribution of surface defects and non-metallic inclusions.
4) For general after heat treatment of forgings, due to uniform metallographic structure, sample interception can be carried out at any section.
5) For welded structures, samples containing fusion zone and overheating zone should usually be intercepted at the welding joint.
2. Sampling method
When the sample is cut, the metallographic structure of the test site should be ensured first. Sampling methods vary according to the nature of materials: soft materials can be cut by hand saw or saw machine, hard materials can be cut by grinding wheel cutting machine with cooling water or line cutting machine, hard and brittle materials (such as white door iron) can be sampled by hammer.
3. Sample size
The size of the sample depends on the specific situation and is generally easy to hold and grind. Generally, the side length of the square sample is 12-15mm, and that of the circular sample is (12-15cm) x 15cm. For forgings of too small size, irregular shape, not easy to hold the grinding sample (such as thin section, wire, thin tube, etc.), it is required to insert sample.
4. Sample set
Insert sample mostly adopt hot pressing insert sample method and mechanical insert sample method.
The hot-pressing sample setting method is to heat the sample in bakelite powder or plastic granules to 110-156℃, and hot-pressing on the sample setting machine. Because the hot-pressing method requires certain temperature and pressure, it is not suitable for low temperature microstructure transformation (such as quenching martensite) and low melting point metal materials are easy to produce plastic deformation.
The mechanical sample setting method is to design a special fixture to hold the sample to avoid the shortage of hot pressing sample setting.