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Metallography is the characterization of the structure and substructure of metals and is often considered a science and art. The science portion of metallography is the evaluation of microstructures for grain size, porosity, inclusions, decarburization layer, case depth, crystalline phases, defects, coating thickness and other details. The art of metallography is preparing the sample to allow for optimal visual imaging of the structures through polishing and etching samples. Q.C. Metallurgical are experts in metallographic evaluation and sample preparation. Q.C. Metallurgical Laboratory conducts metallographic examination primarily through the use of an Olympus BX51 M Optical Microscope with Paxit digital camera and quantitative imaging software that can magnify up to 1000X. Scanning electron microscopy (SEM) can also be used in metallographic evaluations. As manufacturing and materials have evolved, metallography has expanded to incorporate materials ranging from circuit boards to composite materials. By analyzing a materials microstructure, its reliability and performance can be better understood. This is a key component for material development, production/manufacturing controls, quality inspections and failure investigations.

Mag 100X
Primarily Type A graphite flakes in random orientation.

Mag 200X Electroetch Oxalic Acid
This shows a cross section of a corrosion pit.

Metallographic Examinations

  • Microstructure
  • Grain size
  • Porosity and voids
  • Decarburization/Carburization
  • Effective Case Depth
  • Coating Thickness & Integrity
  • Nitriding Thickness
  • Dendritic growth
  • Cracks and other defects
  • Corrosion analysis
  • Intergranular attack (IGA)
  • Inclusion size, shape and distribution
  • Weld and heat-affected zones (HAZ)
  • Solder joints
  • Graphite nodularity
  • Recast
  • Intergranular fracturing
  • HAZ Sensitization
  • Flow-line Stress

Metallographic samples are prepared in accordance with accredited test specification ASTM E 3 “Standard Practice for the Preparation of Metallographic Specimens”. Our metallurgical engineers have the knowledge and experience to evaluate your metallographic samples and provide you with in-depth evaluations and recommendations for your products.

Mag 100X Nital Etch
This shows weld metal, HAZ and base metal. The HAZ has enlarged grains with grain boundary films and cracks (arrow) extending into the base metal.

Mag 500X Oxalic Acid Electroetch
The structure is tempered martensite with precipitated chromium carbides (arrow).

Standard grain size analysis

Microstructure of Welds

The metallurgy of welds is very unique and differs from other metal forming process, but there are similarities between weld pool formation and casting processes. Parameters that control the solidification of castings also control the solidification and microstructure of welds. There are various physical processes that occur due to the interaction of the heat source with the metal during welding that adds a new dimension to the understanding of the weld pool solidification. Welding filler metals are designed to create strong and tough welds, they contain fine oxide particles that permit the nucleation of fine grains. When a weld solidifies, its grains grow from the course Heat Affected Zone (HAZ) grain to the finer grains in the Fusion Zone (FZ).

Chromium Carbide Precipitation

Stainless steels typically stain less than regular carbon steels because the chromium content in the 304-stainless (about 18% of the stainless’ total chemistry) reacts very readily with oxygen to form a tenacious and an even covering of corrosion-resistant chromium-oxides on its surface. Chromium carbide precipitation can occur in metals with high chromium and significant amounts of carbon, about 0.03%, such as 304 SS. The chromium bonds with the carbides and are pulled into the grain boundaries which reduces the chromium oxides on the surface of the metal therefore reducing its corrosion resistance. This can happen during welding and annealing of stainless steel.

Grain Size Analysis

The manufacturing process relies upon an accurate evaluation of the grain size. Proper assessment of grain size is critical to the understanding of its effect on the mechanical and physical properties of parts & products during processing and in service of materials. Service life of a component would be adversely affected if the grain size is operating outside its design parameters.

Q.C. Metallurgical Laboratory can conduct standard grain size analysis on unquenched stainless and carbon steels. For more complex grain size analysis such as McQuad-Ehn (carburizing) grain size, the samples are surrounded by a carburizing compound, heat treated and slowly furnace cooled. This method is utilized mainly for quenched and tempered carbon/alloy steels with low carbon content.

Chrome Plating
Knoop Microhardness tests on
chrome plating and case depth of base metal

Microhardness testing of weld, Heat Affected Zone (HAZ), Fusion Zone and Base Metal.

Mag 500X Nital Etch
This shows the Al-Si T1 40 coating with the Fe-Al-Si intermetallic layer.

Mag 500X
Area of heavy oxidation. Chemical analysis by SEM-EDS indicated potential corrodents were sulfur and chlorine from the environment.

Case Depth

Case depth is the depth of the hardened case from the outer surface. Materials are case hardened to obtain Surface Wear Resistance, at the same time retaining the ductility of the core. Q.C. Metallurgical conducts case depth measurements in accordance with test methods ASTM E384 ASTM E1077, SAEJ423, SAE J121.

To harden or case harden a part requires it to be heat treated. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where they are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Microhardness Weld Evaluation

The microstructure of the base metal is always altered by the fusion of its substance. Microhardness testing is a way to measure a specimens ability to resist plastic deformation. Microhardness testing on welds evaluates the hardness of the fusion zone or weld bead, the heat affected zone (HAZ) and the base material. Hardness readings are taken inline across the base metal, HAZ and Fusion zones.

Plating Evaluation

Coating and plating’s can be evaluated for hardness and dimensional thickness measurements. The sample is cross sectioned and meticulously polished to prevent smearing of the coating. Hardness is conducted with a microhardness testing in either Knoop Vickers scales. Dimensional measurements are conducted with a digital optical microscope utilizing PAX-IT quantitative imaging software in accordance with ASTM B487. Coating thickness can be measured to down to microns.

Surface contaminants and corrosion products on coatings and surfaces can be evaluated using scanning electron microscopy electron dispersive spectroscopy (SEM-EDS). These products can be analyzed down to minute particles. This can be helpful in determining the cause of stains and pits on coating surfaces. It’s routinely performed as part of a root cause failure analysis.

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