Metallographic examination and micrographs

Metallographic examination makes the internal structure of a steel visible. A specimen is cut, mounted, ground, polished and etched; under the microscope, the micrograph shows grain size, type of microstructure and defects that cannot be seen from the outside. Micro-etching reveals grain boundaries and microstructural constituents, while macro-etching gives an overview of grain flow, segregations and welds. RS Randack carries out metallography in its own materials laboratory.

What micrographs are used for on bolts

• Quenched and tempered microstructure: Correctly quenched and tempered steel shows tempered martensite; ferrite or pearlite in the core of large diameters indicates that the steel has not been quenched and tempered through to the core.
• Surface decarburisation: During heating, carbon can burn out of the surface layer, the surface becomes softer and the fatigue strength of the thread decreases. ISO 898-1 limits the permissible decarburisation; the micrograph measures it.
• Grain flow: Macro-etching of a forged head shows whether the grain flow follows the contour of the head or has been cut through.
• Coating thickness: The thickness of a zinc or anti-friction coating can only be measured approximately by non-destructive methods, and reliably on a cross-section. For independent evidence, RS Randack has this cross-section prepared by an accredited laboratory on request.
• Failure analysis: Fracture surfaces and microstructure show whether a bolt failed due to overload, fatigue, hydrogen or a material defect.

When metallographic examination is ordered

For customer specifications that explicitly require microstructure and decarburisation checks, for third-party inspections in nuclear technology, aerospace and pressure equipment, for new combinations of material and heat treatment, and for complaints. Please state the requirement in your enquiry so that the test bar can be planned accordingly.