Understanding and Measuring Decarburization

Understanding and Measuring Decarburization Understanding the forces behind decarburization is the first step toward minimizing its detrimental effects. Decarburization is detrimental to the wear life and fatigue life of steel heat-treated components. This article explores some factors that cause decarburization while concentrating on its measurement. In most production tests, light microscopes are used to scan … Read more

What is Metallography

Metallography is the branch of science dealing with the study of the consititution and structure of metals and alloys, its control through processing, and its influence on properties and behavior. Its original implementation was limited by the resolution of the reflected light microscope used to study specimens. This limitation has been overcome by the development of transmission and scanning electron microscopes (TEM and SEM). The analysis of x-rays generated by the interaction of electron beams with atoms at or near the surface, with wavelength- or energy-dispersive spectrometers (WDS, EDS) with the SEM or the electron microprobe analyzer (EMPA), has added quantitative determination of local compositions, e.g., of intermediate phases, to the deductions based upon observations. Introduction of metrological and stereological methods, and the development of computer-aided image analyzers, permits measurement of microstructural features. Crystallographic data can be obtained using classic x-ray diffraction methods using a diffractometer, or diffraction analysis can be performed with the TEM using selected area or convergent-beam electron diffraction (SAD and CBED) techniques, and more recently with the SEM with the orientation-imaging (EBSD) procedure. There is a wide variety of very sophisticated electron or ion devices that can be utilized to characterize surfaces and interfaces, but these devices are generally restricted in availability due to their high cost.

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Understanding Heat Treatment Results Using Metallography – Version 2

As Hot Rolled Microstructures

  • Microstructure of hot-rolled Fe –0.94% C –0.51% Mn–0.32% Si –1.34% Cr alloy steel revealing a fully pearliticmatrix. Picralrevealed a network of cementite in the prior-austenite grain boundaries (arrows). This is not visible using nital. Originals at 1000X.
  • Microstructure of as-rolled Fe –1.31% C –0.35% Mn–0.25% Si high-carbon water hardenabletool steel. Note the Widmanstättenintragranularcementite that precipitated as pro-eutectoid cementite before the eutectoid reaction. Originals at 1000X.
  • Microstructure of the as-rolled Fe –1.31% C –0.35% Mn–0.25% Si specimen with the intergranular carbide network clearly visible after etching with alkaline sodium picrate, 90 °C –60 s. Original at 500X magnification. Note also some intragranular Widmanstätten cementite.

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Understanding Heat Treatment Results Using Metallography – Version 1

As Hot Rolled Microstructures

  • Microstructure of hot-rolled Fe –0.12% C –0.23% Mn –0.17% Si –0.74% Mo revealing a ferrite-pearlite microstructure with small patches of martensite (arrows). The many small black particles are small patches of pearlite, not inclusions. Originals at 1000X.
  • Microstructure of hot-rolled Fe –0.94% C –0.51% Mn –0.32% Si –1.34% Cr alloy steel revealing a fully pearlitic matrix. Picral revealed a network of cementite in the prior-austenite grain boundaries (arrows). This is not visible using nital. Originals at 1000X.
  • Microstructure of as-rolled Fe –1.31% C –0.35% Mn –0.25% Si high-carbon water hardenable tool steel. Note the Widmanstätten intragranular cementite that precipitated as pro-eutectoid cementite before the eutectoid reaction. Originals at 1000X.

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Thermal Spray Metallography for the 21st Century

Sectioning – Lessond Learned

  • Abrasive Wheels-ultrathin abrasive blades are superior to standard production wheels.
  • Diamond Wafering Blades-are the best universal blades producing the least amount of sectioning damage.

Mounting – A Better Way

  • Compression Mounting-limited to non-friable, very low porosity metallic coatings.
  • Castable Epoxy Mounting-versatile for all types of coatings without introducing damage.
  • Vacuum Impregnation-useful for infiltrating open pores in coatings. Most effective when using castable epoxies with viscosity less than 500 cps.
  • Pressure Impregnation-produces maximum penetration of castable epoxies using pressure of 1500 psi.

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The SEM as a Metallographic Tool

Influence of Preparation on Image Quality

Back-scatter electron images of as-polished 2205 duplex stainless steel shows the improvement in image quality by using the best preparation methods.

Image Contrast Modes

  • EmissiveMode – low-energy secondary electrons (come from the surface to ~10 nm depth)
  • ReflectiveMode – higher-energy backscattered electrons (come from a much greater depth)
  • AbsorptiveMode – detect signal flowing through the specimen to ground (inverse of BSE image)
  • X-RayMode – characteristic x-rays generated by the incident beamto show the distribution of elements at, and slightly below, the surface.

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Solidification Structures

Macrostructure of a VIM (vacuum induction melted) ingot of Fe-52% Ni alloy. A slice was cut transverse to the ingot axis and hot acid etched (50% HCl at 70 °C) revealing the solidification pattern. There is a very thin surface zone of small grains. Below the surface, there is a zone of columnar grains and the central portion is equiaxed but coarse. The scale is in inches.

Transverse Concast Discs, 430 SS

Macrostructure of 5-inch square (127 mm) continuously cast billets of type 430 stainless steel (Fe – 0.03% C – 0.34% Mn – 0.48% Si – 17.78% Cr – 0.26% Ni – 0.05% Mo – 0.07% Cu) taken at three random locations along the strand. The discs were cut transverse to the growth direction and were hot acid etched. Note the thin region of fine grains at the surface, the large columnar zone and the central equiaxed zone (of varying coarseness). Note that there are some fine cracks present.

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Safety in the Metallography Lab

MSDS Sheets

  • Every laboratory should have a file of Material Safety Data Sheets for all chemicals used in the laboratory
  • When working with a chemical especially when it is the first time the MSDS sheet should be read and all safety guidelines mentioned should be followed.
  • All employees who use hazardous chemicals on the job have a legal “right to know” about the hazards they may face and the ways they can protect themselves from those hazards. Lab workers also have this right.
  • Industrial applications generally use large quantities of a few chemicals while labs tend to use a wide variety of chemicals, but in small quantities
  • OSHA defines “laboratory scale” operations as those that use containers “designed to be easily and safely manipulated by one person”

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Preparation of Ceramics, Cermets, Nitrides, Borides and Sintered Carbides

Abstract

Equipment and consumable supplies for preparing very hard metals, oxides, carbides, borides and nitrides has improved substantially over the past forty years. Due to their high hardness, diamond is the chief abrasive used in cutting, grinding and polishing. Preparation procedures have been developed and are straightforward and simple to use. Etching of oxides is still somewhat challenging. Use of illumination modes other than bright field should always be considered when working with these materials.

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