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Overview

Microscopy
Microscopy

Material inspection comprises especially: 

  • Chemical analysis
  • Hardness measurement (HRC, HRB, HV etc.)
  • Micro hardness measurement
  • Metallography

 


Metallography

Conspicuous near surface structure of a failed bearing
Conspicuous near surface structure of a failed bearing

Metallography allows not only for detection of conspicuities in microstructure but also for quantitaive assessment. In case of materials for gears or bearings, this applies especially to:

  • Grain size determination
  • Measurement of layer thickess, both of coatings and near surface layers produced by heat treatment
  • Evaluation of impurities according to various standards
  • Assessment of homogeneousity, especially carbide distribtion in case of chromium steels

Apart from that, metallography supports failure analysis by displaying and evaluation of crack networks. Furthermore, it allows for identification of conspicuities in microstructure or large inclusions close to cracks.

Grain size determination

Former austenite grain boundaries in bainite structure
Former austenite grain boundaries in bainite structure

Grain size is especially relevant for gear marials. DIN 3990 requires grain size numbers according to different material quality levels. Typically, grain size is determined on raw material prior to heat treatment where on ferrite or austenite structure. Later assessment of heat treated parts can be possible too on bainite structure, at least with some restictrions.

Evaluation of impurities

Non-metallic inclusions
Non-metallic inclusions

Non-metallic inclusions in steel materials can be assessed according to various standards. For gears, the K-factor method according to DIN 50602 is very common, for bearings, Jernkontored standard series and ISO 4967 are most relevant.

Assessment of carbide distribution

Carbide segregation
Carbide segregation

Chromium steels typically form chromium carbide which should appear as fine globes. A homogeneous distribution leads to high material strength while local accumulations of carbides might be signifcant weak points. Therefore, it is essential to restrict admissible non-uniformity and to assess it, e. g. following ISO 5949.

 


Hardness measurement

Rockwell-C
Rockwell-C

For many machine elements such as gears and bearings, material hardness is highly important for endurance in operation. For hardness measurement, various methods are available. Methods according to Rockwell can be used on parts with paralell faces where no specimen preparation is necessary. In case of embedded and polished specimens, however, especially Vickers method is used.


Micro-hardness measurment

Hardness distribution of a gear
Hardness distribution of a gear

Micro-hardness measurement is very useful for inspection of near-surface hardened parts like e. g. case-hardened gears. Then, hardness is measured at very small testing loads which allows for measuring at many points with a high resolution leading to quantification of hardness distribution by which especially hardening depth can be determined.


Chemical analysis

Chemichal composition of a steel specimen
Chemichal composition of a steel specimen

Chemical composition of material specimens is measured by arc-spectrometry. By such testing results, it can be verified whether specimens fulfill the expections according to technical specifications. Especially in case of failure analysis, one frequently has to clarifiy whether e. g. substandard material might have been used instead of required highly alloyed steel.