Definitions/Supplementary Information on Metallographic Testing

• Microstructure

  The microstructure analysis of materials examines the structure of a material and can provide indicators on how that material will respond in a given application. Conducted under magnification, Microstructural analysis is a widely used practice throughout the industry to evaluate material samples. The LAB technicians are highly trained in microstructural analysis and can evaluate material degradation and failure mechanisms, as well as analyze grain size, plating/coating thickness, case depth measurements, inclusion rating and more.

• Grain Size

  An accurate assessment of grain size in a given metallic sample is used to determine the effectiveness of a specific manufacturing process, i.e. heat treating or tempering. Technicians prepare the sampling by mounting a cross section, polishing it, and etching it. Etching reveals the grain. Once the sample is prepared, technicians use optical microscopy at a magnification of 100X to determine the precise grain size of the specimen. Typically, a finer grain size can increase hardness and improve the yield strength of a material.

• Grain Flow

  Simply put, grain flow is the directional orientation of metal grains. Grain flow analysis begins with a sample prepared similarly to the grain size metallographic sampling. The LAB technicians take care to properly prepare the sample, making sure the surface is flat, not beveled. Once the etching solution is applied, technicians can see on a microscopic level any particles, nonmetallic inclusions and other imperfections present in the specimen.

• Carburization and Decarburization

  Testing for carburization and decarburization surface conditions in metals will determine the amount of carbon in a specimen. When a test is ordered for one of these conditions, both conditions will be apparent and both will be reported. Too much of either condition may adversely affect the safety and performance of the metal. Testing will also detect changes in the microstructure and hardness. This test method is not intended to address products which are intentionally carburized to achieve specific results. The testing method classifies, measures, and determines the existence of decarburization and carburization in the threaded section of hardened and tempered metals which have been heated to enable fabrication or to alter their mechanical properties.

• Micro Examination

  The LAB performs Micro Examination to analyze the structure of a specimen and check for any abnormalities. Specimens are first cut, mounted, finely polished, and etched in a chemical solution before the technician uses optical microscopy for detailed inspection. Inspection per SAE AS 7477/SAE AS 7478 includes examinations of flow lines, internal defects, microstructure, grain size per ASTM E112, surface hardening and tread defects.

• Micro Indication Verification Exam

  The Micro Indication Verification Examination (MIVE) verifies defects found in NDT methods which cannot be fully measured by the NDT method. The MIVE will provide 3-dimensional measurements of indications such as thread laps and seams and can change the disposition of an NDT test from failure to passing if the indications are within the tolerances of the specification.

• Digital Imaging

  The LAB Materials Testing records digital images of all indications found in NDT and all metallographic tests. Reference images are available upon request. Calibrated images are available for an additional charge.

• Ferrite by Point Count (ASTM E562)

  Ferrite testing uses the point count method to indicate the percentage of delta phase iron inclusions in a given sample. If a specimen contains too much delta ferrite, it may cause harm to the material’s properties. Technicians survey several cross sections of a specimen to provide their final test results. First, a sample is prepared by cross-sectioning, polishing, mounting and etching to reveal the grain structure. Secondly, a photomicrograph is prepared so the technician can examine each intersection of a 10 x 10 grid that has been placed over the image. Each intersection where ferrite is found is then scored by one point or a half point. The technician will then total the scores to reveal the percentage of delta ferrite in the given sample.

• Coating / Plating Evaluation (ASTM B487, ASTM B748)

  The LAB can perform coating and plating evaluations. Each specimen is prepared according to the standards. Proper steps will be taken to protect the integrity of the original plated material when plating is very thin.

• Case Depth

  Metallographic testing for the total case depth is required to verify a carbonitriding or carburizing heat treated lot. Sample preparation begins with sectioning and etching to reveal the microstructure. Then the sample is measured using a filar scale to determine the depth beneath the surface where there is no observable change in microstructure due to heat treatment diffusion. Additional sections are measured for accuracy. Each section will be reported as a separate line item without averaging. After the final measurements are determined, The LAB technicians will further analyze the specimen for any harmful microstructural conditions. As always, The LAB – Materials Testing staff is available to answer any questions about testing processes and standards.

• Inclusion Rating (ASTM E45)

  The LAB Materials Testing conducts the metallographic technique of determining the inclusion rating of non-metallic content of wrought steel. Inclusions are characterized by size, shape, concentration, and distribution rather than chemical composition. Although compositions are not identified, the microscopic method places inclusions into one of several composition-related categories (sulfides, oxides and silicates).

• Macro-etch (ASTM E340)

  Macroetching provides information on variations in structure such as grain size, flow lines, columnar structure, dendrites, etc.. Macroetching also reveals the presence of discontinuities and voids such as seams, laps, porosity, flakes, bursts, extrusion rupture, cracks, etc.

• Vickers Hardness Test

  Microhardness tests a material’s hardness with a microscopic indentation. This test can provide a hardness value for a very precise location on a part, enabling the detection of variations in hardness from one location to another. A controlled force is applied to an indenter for an allotted time. The Vickers hardness value is determined by using the impression measurement and the test load in an appropriate formula. There is a common scale which covers the entire hardness range. This testing method can also measure surface-coating hardness on case-hardened or carburized parts, as well as identifying surface conditions such as decarburization and grinding burns.