Metallography: Grain Size, Precipitates and Inclusions

• Metallography is a powerful tool used to understand the structure and properties of metals and alloys.
• It involves microscopic examination of materials to reveal their internal structure and performance insights.
• Metallographic microscopy is a key technique for analyzing material microstructure, involving sample prep, microscope observation, and result interpretation.

Grain Size and Dimensions

• Grain size refers to the average size of grains in a material and is typically measured in micrometers.
• Grain size is determined from photographs taken at 100x magnification, with multiple photos to account for varied phase sizes within the sample.
• Grain dimensions describe the shape and size of individual grains, providing info on growth and orientation.

Precipitates and Inclusions

• Precipitates are small particles of a different phase that form within a grain structure.
• Precipitates can influence mechanical properties like strength and ductility.
• Inclusions are foreign particles trapped within materials during solidification or processing.
• Inclusions may affect a material's overall integrity and performance.

Steel Microstructures

• Ferrite is a soft, ductile phase commonly found in low-carbon steels with a body-centered cubic crystal structure.
• Austenite is a hard, strong phase with a face-centered cubic crystal structure and is stable at high temperatures.
• Cementite is a brittle and hard phase with a complex crystal structure, often found in pearlite and other steel microstructures.
• Pearlite is a lamellar microstructure of alternating ferrite and cementite layers, known for its strength and toughness.
• Martensite is a hard, brittle phase formed by rapid cooling of Austenite, and has high hardness and strength.
• Ledeburite is a eutectic microstructure of austenite and cementite, found in high-carbon steels and cast iron.

Transformational Analyses of Steel

• Steels undergo phase transformations when heated or cooled, which affects their microstructure and properties.
• Controlled heating and cooling processes/heat treatment are used to manipulate a steel's microstructure to achieve targeted properties.
• By controlling cooling rate and parameters during heat treatment, the final microstructure of steel can be tailored for specific uses.

Thermochemical Treatments of Steel

• Carburizing increases the carbon content on the surface of steel, while accounting for the environment around the metal during heating and cooling.
• Case hardening produces a hard, wear-resistant surface while maintaining a ductile core.
• Carburizing and case hardening are commonly used for gears, bearings, and components that undergo wear and tear.
• Nitriding diffuses nitrogen into steel's surface layer, forming a hard, wear-resistant nitride layer via heating at 400-525°C with a steam (NH3 + N2).
• Cyaniding diffuses both carbon and nitrogen into steel's surface, producing a hard, wear-resistant case and applies baths with carbonate and NaCN at 760-950°C.
• Carbonitriding combines carburizing and nitriding, creating a hard, wear-resistant case with improved fatigue resistance. It involves introducing hydrocarbons (CH 4, C2H6, or C3H8), NH3, and CO at 650-850°C, and it requires quenching and tempering.
• Sulfinizing diffuses sulfur into steel at low temperature (565°C) in a salt bath, producing a hard, wear- and corrosion-resistant surface.

Microscope Observations

• Ocular magnifies the image formed in the lens.
• Objective enlarges the sample image.
• Condenser focuses light through the sample.
• Diaphragm regulates the amount of light.

Differences between Optical and Metallographic Microscopes

• Optical and metallographic microscopes can both magnify up to 1000x.
• Metallographic microscopes have a higher resolution than optical microscopes, and reveal finer details.
• Metallographic microscopes have greater depth of feild, allowing for clearer views of 3D features.
• Metallographic microscopes produce higher contrast than optical microscopes, increasing visibility of microstructural features.
• Metallurgical microscopes are typically inverted.
• Metallurgical microscopes require the sample surface to be in very good focus.

Metallurgical Microscope Features

• A field lens is an added feature that increases the field of view
• A lat glass reflector is an added feature that is a surface capable of reflecting light to form the image.

Metallographic Microscope Mechanism

• Light starts in the illumination box, then passes through glass coated with a reflecting element.
• Light is reflected down into the objective at a 45° angle focused on the sample, then up into the objective, and then travels at a different angle on the reflecting element, which allows it to pass through the glass instead of reflect off of it.
• Light then reaches the eyepieces and is visible to the human eye.

Metallographic Microscopy Applications

• Metallographic Microscopy is a crucial technique to characterize metals and alloys.
• It reveals a materials internal structure, including grain size, phase distribution, and any defects.
• Insights gained from observations directly correlate with a materials mechanical and physical properties.

Monophasic Structure

• Molded copper exhibits a uniform, single-phase microstructure with fine grains before etching.
• Chemical attacks using ammonia persulfate (10%) can be used observe the equiaxed polyhedra grains, with one phase.
• A fine-grained molded copper structure is a result of continuous casting and sand molding solidification processes.

Nodular Cast Iron Structure

• Nodular cast iron shows a ferrite or pearlite matrix, and is hypoeutectoid with the same carbon content as grey cast iron.
• Nodular cast iron is obtained during casting with moderate cooling.
• Chemical attacks are performed with nital.
• The presence of spherical graphite inclusions inside a pearlitic matrix, with cementite and Ledeburite is a characteristic feature.
• Inclusions enhance the ductility and toughness of cast iron.

Hardened Steel

• The hardening process transforms the austenite phase into martensite plus temple.
• Chemical attacks are performed with nital
• The resulting microstructure contains needle-like martensite crystals, to observe the needles, bigger magnification is needed via TEM.
• Transformation significantly increases strength and hardness.

Pure Iron Microstructure

• Pure iron exhibits a relatively simple microstructure with large, well-defined grains.
• Chemical attacks are performed with nital
• Grains primarily consist of ferrite, the body-centered cubic (BCC) phase of iron, featuring polygonal and equiaxed grains.
• The microstructure shows material's inherent softness and ductility.

Normalized Medium Carbon Steel (AISI-SAE-1045)

• The phases are ferrite and pearlite, ferrite is soft and ductile, while pearlite is harder and stronger.
• Chemical attacks are performed with nital
• It contains Mn, Si, P, and S in its composition.
• It is used for construction of mechanical parts
• Normalizing provides a hardened surface

Key Microstructural Characteristics

• Grain size influences a material's mechanical properties.
• Phase distribution, and the presence of different phases impact the material behavior.
• Defects and inclusions like voids can affect strength and ductility.

Practical Applications of Observations

• Understanding microstructure helps engineers choose the right material for specific applications.
• Microscopy ensures manufactured components have quality and consistency.
• Investigating the microstructure of failed components can provide insights into root causes.