Preparing Metallographic Stainless Steel Specimens By yaang.com

DataTime:[2015-5-7]

The surface of a metallographic stainless steel tubes are prepared by polishing, grinding,and etching. After preparation, it is often analyzed using optical or electron microscopy. Using only metallographic techniques, a skilled technician can identify stainless steel material properties.
Mechanical preparation is the most common preparation method. In a series of steps, successively finer abrasive particles are used to remove material from the sample surface until the desired surface quality is achieved. Many different machines are available for doing this grinding and polishing, able to meet different demands for quality, capacity, and reproducibility.
A systematic preparation method is easiest way to achieve the true structure. Sample preparation must therefore pursue rules which are suitable for most materials. Different materials with similar properties (hardness and ductility) will respond alike and thus require the same consumables during preparation.

Metallographic stainless steels are typically "mounted" using a hot compression thermosetting resin. In the past, phenolic thermosetting resins have been used, but modern epoxy is becoming more popular because reduced shrinkage during curing results in a better mount with superior edge retention.

A typical mounting cycle will compress the specimen and mounting media to 4,000 psi (28 MPa) and heat to a temperature of 350 °F (177 °C). When specimens are very sensitive to temperature, "cold mounts" may be made with a two-part epoxy resin. Mounting a specimen provides a safe, standardized, and ergonomic way by which to hold a sample during the grinding and polishing operations.

macro etched copper discard, After mounting, the specimen is wet ground to reveal the surface of the metal. The specimen is successively ground with finer and finer abrasive media. Silicon carbide abrasive paper was the first method of grinding and is still used today. Many metallographers, however, prefer to use a diamond grit suspension which is dosed onto a reusable fabric pad throughout the polishing process. Diamond grit in suspension might start at 9 micrometres and finish at one micrometre. Generally, polishing with diamond suspension gives finer results than using silicon carbide papers (SiC papers), especially with revealing porosity, which silicon carbide paper sometimes "smear" over. After grinding the specimen, polishing is performed. Typically, a specimen is polished with a slurry of alumina, silica, or diamond on a napless cloth to produce a scratch-free mirror finish, free from smear, drag, or pull-outs and with minimal deformation remaining from the preparation process.

After polishing, certain microstructural constituents can be seen with the microscope, e.g., inclusions and nitrides. If the crystal structure is non-cubic (e.g., a metal with a hexagonal-closed packed crystal structure, such as Ti or Zr) the microstructure can be revealed without etching using crossed polarized light (light microscopy). Otherwise, the microstructural constituents of the specimen are revealed by using a suitable chemical or electrolytic etchant. A great many etchants have been developed to reveal the structure of metals and alloys, ceramics, carbides, nitrides, and so forth. While a number of etchants may work for a given metal or alloy, they generally produce different results, in that some etchants may reveal the general structure, while others may be selective to certain phases or constituents.

Macro Examination
Macroetching is the procedure in which a specimen is etched and evaluated macro structurally at low magnifications. It is a frequently used technique for evaluating steel products such as stainless steel tubes, billets, bars, flanges, and pipe fittings. There are several procedures for rating a steel specimen by a graded series of photographs showing the incidence of certain conditions and is applicable to carbon and low alloy steels. A number of different etching reagents may be used depending upon the type of examination to be made. Steels react differently to etching reagents because of variations in chemical composition, method of manufacturing, heat treatment and many other variables.
Macro-Examinations are also performed on a polished and etched cross-section of a welded material. During the examination, a number of features can be determined including weld run sequence, important for weld procedure qualifications tests. As well as this, any defects on the sample will be assessed for compliance with relevant specifications. Slag, porosity, lack of weld penetration, lack of sidewall fusion and poor weld profile are among the features observed in such examinations. It is normal to look for such defects either by standard visual examination or at magnifications of up to 50X. It is also routine to photograph the section to provide a permanent record. This is known as a photomacrograph.

Micro Examination

This is performed on samples either cut to size or mounted in a resin mold. The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope. Micro-examination is performed for a number of purposes, the most obvious of which is to assess the structure of the material. It is also common to examine for metallurgical anomalies such as third phase precipitates, excessive grain growth, etc. Many routine tests such as phase counting or grain size determinations are performed in conjunction with micro-examinations.

Weld Examination

Metallographic weld evaluations can take many forms. In its most simple form, a weld deposit can be visually examined for large scale defects such as porosity or lack of fusion defects. On a micro scale, the examination can take the form of phase balance assessments from weld cap to weld root or a check for non-metallic or third phase precipitates. Examination of weld growth patterns is also used to determine reasons for poor mechanical test results. For example, an extensive central columnar grain pattern can cause a plane of weakness giving poor charpy results.

Case Depth

Case hardening may be defined as a process for hardening a ferrous materials in such a manner that the surface layer (known as the case), is substantially harder than the remaining materials (known as the core). This process is controlled through carburizing, nitriding, carbonitriding, cyaniding, induction and flame hardening. The chemical composition, mechanical properties, or both, are effected by these practices. Methods for determining case depth are either chemical, mechanical or visual and should be selected on the basis of specific requirements.

Decarburization Measurement

This method is designed to detect changes in the microstructure, hardness, or carbon content at the surface of the steel sections due to carburization. The depth is determined as the depth where a uniform microstructure, hardness, or carbon content, typical of the interior of the specimen is observed. This method will detect surface losses in carbon content due to heating at elevated temperatures, as in hot working or heat treatment.

Coating / Plating Evaluation (ASTM B487, ASTM B748)

A coating or plating application is used primarily for protection of the substrate. The thickness is an important factor in the performance of the coating or plating. A portion of the specimen is cut, mounted transversely, a prepared in accordance with acceptable or suitable techniques. The thickness of the cross section is measured with an optical microscope. When the coating or plating is thinner than .00020", the measurement should be taken with the aid of the scanning electron microscope. Cross-sectioned metallographic examinations of substrates with platings, surface evaluations, thickness measurements, weight per volume, and even salt spray testing can aid in the evaluation of platings.

Surface Evaluation

Surface inspection includes the detection of surface flaws and he measurement of surface defects and roughness. One method includes the use of a laser light. When the scattered light is reflected off the surface of a sample, it can be analyzed and measure. Another method is the use of a motorized stylus (profilometer). The stylus is placed on the surface and the texture of the material is measured in micro-inches or millimeters.

Grain Size Determination

In order to establish a scale for grain size, ASTM E112 shows charts with outline grain structures at various dimensions. This has led to a universally accepted standard by which grain sized range form 1 (very coarse) to 10 (very fine). A material's grain size is important as it affects its mechanical properties. In most materials, a refined grain structure gives enhanced toughness properties and alloying elements are deliberately added during the stainless steel making process to assist in grain refinement. Grain size is determined from a polished and etched sample using optical microscopy at a magnification of 100X.

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