Sampling method and inspection steps for metallographic analysis of stainless steel flanges
Sample selection is the first step in metallographic analysis. The quality of the sample selection directly determines the quality of the experimental results, and its importance is beyond doubt. In general, the selection of the sample is divided into the selection of the sampling part and the inspection surface of the sample, the sample interception process, and the selection of the size of the sample. For samples with special shapes or small sizes that are difficult to hold, inlay or mechanical clamping. The following SSM details the four processes of metallographic sample selection.
Sampling location of sample and selection of inspection surface
The selection of the sampling site and the inspection surface shall be determined according to the characteristics of the analyzed material, such as the quality of the material and the heat treatment process. The sampling location, shape and size of the samples used in routine inspections in lost births have clear regulations.
When studying materials for failure analysis, samples should be taken from the failure parts and the intact parts of the materials according to the reasons for their failures, so as to facilitate comparative analysis.
The research sample is a casting, and its microstructure difference must be observed from the surface to the core, from the top to the bottom to understand the segregation, as well as the effect of shrinkage porosity and cooling rate on the microstructure.
Sampling of rolled profiles or forgings should consider whether there are defects such as decarburization and folding on the surface, as well as the identification of non-metallic inclusions. Therefore, the samples should be intercepted in the transverse and longitudinal directions. The transverse samples mainly study the distribution of surface defects and non-metallic inclusions. For very long profiles, samples should be taken at both ends to compare the segregation of inclusions. Longitudinal samples Mainly study the shape of inclusions; identify the types of inclusions, observe the degree of grain length, and estimate the degree of cold deformation during reverse deformation.
Due to the uniform metallographic structure of the heat-treated parts, any section of the sample can be taken, but the surface chemical heat treatment and the sampling of the coated parts should be perpendicular to the surface, so as to observe the structure and test the thickness.
After the sampling location of the metallographic sample is determined, which surface of the sample should be further determined as the grinding surface.
Generally, the metallographic pictures listed in the research results or inspection reports must indicate the part of the sample intercepted and the direction of the metallographic grinding surface. In some cases, it should be marked with a drawing.
When sampling by the method of sample interception, different methods are adopted according to the hardness of the tested material:
For materials with lower hardness, sawing, turning, planing and other processing methods can be used.
For materials with higher hardness, grinding wheel slicer cutting or EDM cutting can be used.
For hard and brittle materials, the hammering method can be used.
For sampling on large workpieces, methods such as oxygen cutting can be used.
When cutting with grinding wheel or EDM, cooling measures should be taken to avoid tissue changes caused by heating of the sample.
Sample size
The size of the sample for metallographic analysis is convenient for holding and grinding. Usually the microscopic sample is a cylinder with a diameter of 15mm and a height of 15~20mm or a cube with a side length of 15~25mm.
Specimen Mounting
For samples with special shapes or small sizes that are difficult to hold, such as tapes, wires, sheets, and tubes, the preparation of the sample should be inlaid or mechanically clamped.
There are two types of inlays: cold inlay and hot inlay.
Sample preparation:
1.1. The direction, location and quantity of sample interception should be carried out according to the metal type, manufacturing method, technical conditions or the provisions of the agreement between the two parties.
1.2. The size of the sample should be less than 200mm in area and 15~20mm in thickness, generally 16X20mm for trial use.
1.3. The sample can be cut with a hand saw, sawing machine or cutting machine, etc. No matter what method is used for sampling, attention should be paid to the temperature conditions of the sample, and if necessary, it is cooled with water to prevent the formal sample from changing its structure due to overheating.
Grinding of the sample
2.1. The prepared sample should be ground on the coarse grinding wheel first. After the wear marks are uniform, it is moved to the fine grinding wheel for further grinding. The sample must be cooled with water during grinding, so that the metal structure will not change due to heat. .
2.2. The samples that have been ground, washed and blown dry by the grinding wheel are then ground on each grade of sandpaper from coarse to fine in turn, and can be ground on a pre-grinder, from coarse sandpaper to fine sandpaper, and then again. Change the sandpaper once, and the sample must be turned at a 90° angle to the vertical direction of the old one.
2.3. The pre-grinded samples are first rough polished on a polishing machine (the polishing fabric is fine flannel cloth, and the polishing liquid is W2.5 diamond polishing paste), and then fine polishing is performed (the polishing fabric is velvet, and the polishing liquid is W1). .5 diamond polishing paste) is polished until the wear marks on the sample are completely removed and the surface is like a mirror surface, that is, the roughness is below Ra0.04.
Etching of the sample
3.1. The polished sample can be immersed in the etchant contained in the glass dish for etching. During etching, the sample may move slightly from time to time, but the polished surface must not come into contact with the bottom of the dish.
3.2 The etchant is generally 4% nitric acid alcohol solution.
3.3 The etching time depends on the nature of the metal, the purpose of the inspection and the magnification of the microscopic inspection. It is advisable to clearly show the metal structure under the microscope.
3.4 After the sample is etched, it must be quickly washed with water, the surface is dual-purpose, washed with alcohol, and then dried with a hair dryer.
Metallographic microstructure inspection
4.1 The metallographic microscope operation shall be carried out in accordance with the provisions of the instrument manual.
4.2 The metallographic analysis includes the inspection before etching and the inspection after etching. Before etching, the inclusions of steel and the graphite morphology of castings are mainly inspected, and the inspection after etching is the microstructure of the sample. Check according to relevant metallographic standards.
Precautions for using metallographic microscope
5.1 When taking the lens, avoid touching the surface of the lens with fingers, and the lens should be placed in a desiccator properly and effectively.
5.2 When the objective lens is close to the surface of the sample, do not make the objective lens contact the sample during adjustment.
5.3 The microscope should be covered with a dust cover when not in use.
