Metallographic Examination

Metallographic examination is the preparation of specimens for microscopic examination and the study of microstructures in relation to the physical and mechanical properties of a particular material. An etching process would reveal the microstructure of the metal. Through optical microscopic observation of metal surfaces, this can reveal valuable information about the material such as grain size, segregation, shape and size. Besides that, distribution of the phases and inclusions that are present also can be reveal. While other aspects such as mechanical deformation and thermal treatments may also be able to be determined.

NUSATEK is an accredited metallurgical lab, staffed with knowledgeable engineers and technicians who are well trained and ready to evaluate your materials.

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Our Metallographic services and capabilities as follows:

  • Raw Metal Examination
  • Weld Metal Examination
  • Phase Count as per ASTM E562
  • Inclusion Count as per ASTM E45
  • Grain Size Determination as per ASTM E112
  • Decarburization Measurement as per ASTM E1077
  • Case depth
  • Coating/Plating Measurement
  • Fractography Examination 
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    Preparation Of Test Specimen

    The micro-specimen is inspected by examining the etched surface through a microscope with a magnification greater than 50x to reveal the microstructure. Nusatek uses high optical microscopes to perform this test in the range of 50x magnification to 1000x magnification.

    The cold mounted micro-specimen will undergo surface grinding to ensure that the specimen is flat in surface. Then the micro-specimen is polished by using grit P80 until P1200 and fine polishing is done with Diamond polishing paste. It is crucial for this to be performed in order to remove all scratches and produce a mirror-like surface finish. A metallographic etching solution is applied to the surface. The etchant solution used depends on the material of the specimen.

    The etched surface is then viewed through a microscope at high magnification, thus a picture of the microstructure is produced. The metallurgical structure of the base metal, weld metal and Heat Affected Zone (HAZ) are interpreted and analyzed for characteristics such as grain size, phase content etc.

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    Raw Metal Examination

    The structure of a metal can be revealed by an optical microscope. Metals and alloys consist of a large number of irregularly shaped crystals (grains), which are usually indiscernible to the naked eye. The grains are rounded or elongated; they may be large or small and are arranged in a regular order or randomly. The shape, size, arrangement, and orientation of the grains depend on the conditions of their formation.

    The microstructure analysis for raw metal checks the presence of cracks, any abnormal structure and deleterious / intermetallic phase in the material.

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    Weld Examination

    Metallographic weld evaluations are normally taken during developing welding procedure and / or monitoring production of welding through a production test sample. 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.

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    Phase Count

    Many metallographic structures are dual or multi-phase. Examples are stainless steel, duplex stainless steels, etc. and it is often important to quantitatively establish the phase balance as it can affect the mechanical and corrosion properties of the material. Two methods are mainly used, the preferred being manual point counting (ASTM E562). This method requires a square grid to be superimposed on a metallurgical microscope image at a suitable magnification and counts taken of underlying structures at the grid intersections. For a 50/50 structure, 30 random fields on a 100 point grid are counted to give a good statistical population.

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    Inclusion Count

    Most metallic materials have some form of inclusion content. In steels, for example, non-metallic inclusions exist in the form of compounds such as manganese sulphide. Inclusion counts are performed to assess their type, shape, quantity and distribution (ASTM E45). Cleanliness of materials can be an important factor in many applications and current high quality steel-making processes ensure that inclusion content is kept to a minimum.

    The presence of inclusions in a material can be used to determine the rolling direction of a plate. Three mutually perpendicular axes are polished and examined. Inclusion stringer direction can be used to assess the direction of rolling.

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    Grain Size Determination

    In order to establish a scale for grain size, ASTM E112 shows charts with outline grain structures of various dimensions. This has led to a universally accepted standard by which grain sizes range from 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 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 x100 or x200.

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    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.

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    Case Depth

    Case hardening may be defined as a process for hardening 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 can be measured through metallographic examination and should be selected on the basis of specific requirements.

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    Coating  Or  Plating Thickness Determination

    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 and it can be measured as per standard guide ASTM B487 and ASTM B748. 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. 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.

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    Fractography Examination

    This involves the study of the fracture faces of broken components and can help determine the cause of failure. In most cases, the appearance of a failed surface illustrates certain features or patterns which offer clues leading to a reason for failure. Sequential striations initiating at a change in section, for example, may suggest a fatigue mechanism to have been in force. Distortion of a pin at the fracture surface may suggest ductile overload, etc. Fractographic examination can be performed on a macro scale by visual or low magnification stereoscope examination or at much higher magnifications with a Scanning Electron Microscope (SEM).