In order to identify and study the metallographic microstructure, the metal material to be analyzed needs to be prepared into a sample of a certain size. After grinding, polishing and etching, the microstructure of the metal is observed and analyzed by a metallographic microscope. And its distribution.
The quality of the metallographic sample preparation directly affects the results of the tissue analysis. If the sample preparation does not meet the specific requirements, it may result in a false judgment due to the illusion, so that the entire analysis can not get the correct conclusion. Therefore, in order to obtain a desired metallographic sample, a series of rigorous preparation processes are required.
Sampling is an important step in metallographic microscopic analysis. It should be selected according to the characteristics of the metal materials or parts to be inspected and analyzed, the processing technology, the failure form and different research purposes, and take a representative part.
1. Selection of sampling site and inspection surface
The choice of sampling site and test surface should have the best or better representative.
1) When inspecting and analyzing the cause of damage to the failed part, in addition to sampling at the damaged part, it is necessary to intercept the sample at a location far from the damage for analysis and comparison.
2) When studying the metal forging structure, due to the existence of segregation, it is necessary to simultaneously sample from the surface to the center for observation.
3) For rolling and forging materials, the metallographic specimens in the transverse direction (perpendicular to the rolling direction) and the longitudinal direction (parallel to the rolling direction) should be intercepted simultaneously to analyze and compare the distribution of surface defects and non-metallic inclusions.
4) For forgings that are generally heat treated, since the metallographic structure is relatively uniform, the sample interception can be performed in any section.
5) For welded structures, specimens containing the fusion zone and the superheat zone should normally be taken at the welded joint.
2. Method of sampling
When intercepting the sample, first ensure that the metallographic structure of the test site does not change. The method of intercepting the sample varies depending on the nature of the material: the soft material can be cut by a hand saw or a sawing machine, and the hard material can be cut by a grinding wheel cutter or a wire cutter with cooling water, and a hard and brittle material (such as a white gate iron). ) You can use the hammer to sample.
3. Size of the sample
The size of the sample depends on the specific conditions, and is generally convenient for holding and easy to grind. Usually the square sample has a side length of 12-15 mm and a circular sample of (12-15 cm) x 15 cm. Mounting is required for specimens that are too small in size, irregular in shape, and difficult to grip and grind (such as sheets, filaments, tubules, etc.).
The mounting is mostly by hot pressing and mechanical mounting.
The hot press mounting method is to heat the sample to 10 to 156 ° C in bakelite or plastic pellets and heat-press it on the mounter. Since the hot press mounting method requires a certain temperature and pressure, it is not suitable for materials that undergo low temperature structural transformation (such as quenched martensite) and low melting point metal materials that are more susceptible to plastic deformation.
The mechanical mounting method is to design a special clamp to hold the sample to avoid the lack of hot pressing.