Microstructural Examination

Mechanical Testing | Metal Technology Engineering

MICROSTRUCTURAL ANALYSIS

The microstructure of a material can have a significant influence on its physical properties including corrosion resistance, strength, toughness, ductility, and hardness. These properties help determine how the material will perform in a given application.

Various steel microstructures are shown on the left 

Microstructural analysis is used extensively in failure investigations and supplements material performance tests such as environmental degradation studies and welding qualification testing. We routinely section and examine materials to understand their building blocks, from simple grain size to complex structures of a broad spectrum of metals. This understanding helps identify potential problems and gives insight into the effects of the various processes that we carry out at MTE on a material’s microstructure. Conventional metallographic techniques are used to prepare specimens for microstructural analysis using a research-grade optical microscope.

FRACTOGRAPHY

Fracture surfaces can reveal specific information about the conditions and cause of a fracture. An examination can determine information such as the location and nature of flaws including material or manufacturing defects, the crack initiation site and direction of propagation, and the type of stress and load direction that were applied. This information is fundamental to determining the cause of the fracture as part of a failure analysis

METALLOGRAPHY

In support of failure analysis and materials evaluation, metallographic examination of the constitution and structure of metals and alloys can detect material microstructures, flaws, and abnormalities, which may be critical to determining metal failure and preventing future failures. Often a critical tool for characterizing the extent of pits and cracks in metal material, metallographic examination can also detect changes to metals due to welding, machining, heat treating, chemical exposure, environmental effects, etc.

PERFORMING A MICROSTRUCTURAL EXAMINATION

Examination of the microstructure of a material provides information used to determine if the structural parameters are within certain specifications. The analysis results are used as a criterion for acceptance or rejection.

Microstructural examination is generally performed using optical or scanning electron microscopes to magnify features of the material under analysis. The amount or size of these features can be measured and quantified and compared to acceptance criteria. These examinations are often used in failure analysis to help identify the type of material in question and determine if the material received the proper processing treatments.

Metallurgical examinations may evaluate:

  • Extent of decarburization and carburization, grain size, intergranular attack or corrosion
  • Depth of case in carburizing alloys
  • Percent spheroidization
  • Inclusion ratings
  • Volume fraction of various phases or second phase particles in metals
Mechanical Testing | Metal Technology Engineering

SAMPLE PREPARATION

In order to identify and evaluate the microstructure of material, it is important to prepare the test sample carefully and properly. The various steps in sample preparation for microstructural examination include:

  • Selecting a representative sample of the materials
  • Sectioning the sample to avoid altering or destroying the microstructure.
  • Mounting the section without damage to the test sample
  • Grinding to achieve a flat sample with a minimum amount of damage to the sample surface.
  • Polishing the mounted and ground sample to a mirror finish
  • Etching in the proper etchant to reveal the microstructure.

THE PROCESS

In support of failure analysis and materials evaluation, metallographic examination of the constitution and structure of metals and alloys can detect material microstructures, flaws, and abnormalities, which may be critical to determining metal failure and preventing future failures. Often a critical tool for characterizing the extent of pits and cracks in metal material, metallographic examination can also detect changes to metals due to welding, machining, heat treating, chemical exposure, environmental effects, etc.

SELECTION

Selecting a representative test sample to properly characterize the microstructure or the feature of interest is a very important first step. For example, grain size measurements are performed on transverse sections, whereas general microstructure evaluations are performed on longitudinal sections. Therefore, it is important to provide the laboratory with information about the orientation or the rolling direction of the test specimen.

Mechanical Testing | Metal Technology Engineering

SECTIONING

Test samples are carefully sectioned to avoid altering or destroying the structure of the materials. If an abrasive saw is used, it is important to keep the sample cool with a suitable coolant or lubricant, so it doesn’t burn or overheat. However, no matter how carefully abrasive sawing is performed, a small amount of deformation occurs on the sample surface. This deformation must be removed during subsequent preparation steps.

MOUNTING

After the test sample is sectioned to a convenient size, it is mounted in a plastic or epoxy resin to facilitate handling and the grinding and polishing steps. Mounting media must be compatible with the sample with respect to hardness and abrasion resistance. Typical mounting materials are thermosetting phenolics such as Bakelite, and thermoplastic materials such as methyl methacrylate (Lucite). Mounting involves putting the sample in a mold and surrounding it with the appropriate powder. When the mold is heated and pressurized at predefined levels, setting, or curing of the media occurs. The mounted sample is removed from the mold. If the use of heat or pressure might alter the structure of the sample of interest, then castable cold mounting materials such as epoxies are employed.

Mechanical Testing | Metal Technology Engineering
Mechanical Testing | Metal Technology Engineering

GRINDING

Grinding follows mounting to remove the surface damage that occurred during the sectioning step and to provide a flat surface. Grinding generally involves the use of water lubricated abrasive wheels and the use of a series of progressively finer abrasive grits. This procedure provides a flat surface that is nearly free of the disturbed or deformed metal that has been introduced by the previous sample preparation steps.

POLISHING

The polishing step removes the last thin layer of the deformed metal. It leaves a properly prepared sample, ready for examination of the unetched characteristics such as inclusion content or any porosity that may exist.

Mechanical Testing | Metal Technology Engineering
Mechanical Testing | Metal Technology Engineering

ETCHING

The final step that might be used is etching to clearly show the microstructure of the test sample. This step reveals features such as grain boundaries, and the various phases in the material that cannot be distinguished in the unetched condition. The most common etchant for carbon steels is Nital, however, there are a lot of other potential etchants which can be used for specialized etching to enhance certain structures and phases within the material. Most etchants used for steel consist of an acid mixed with ethanol or methanol.