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Grinding Cracks (part 2)

August 10, 2012
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The grind crack may not be so obvious to see visually because it will usually occur at right angles to the direction of the grinding pass and at right angles to the surface of the gear tooth. The depth of the crack is not known unless a cross-sectional examination of the gear tooth is made.  Because cross sectioning a gear results in scrapping a part, this is not a practical solution. Also, we don’t know whether the sectioned gear represents the entire load. That gear may or may not be representative of the results of an entire batch.

The depth of the grind burn may only be microns deep from the ground surface where the grind burn is, but the crack can be quite deep into the formed case from the surface. A simple saying is, “From little acorns doth great oak trees grow.”  This means that if a small crack is present only a few microns deep, that crack can propagate into the case to form what might be a catastrophic crack that could lead to a premature failure of that particular tooth or multiples of teeth.

To establish if grind cracks are present, a simple test might be a magnetic particle test.

The writer does not believe that there is any relationship between the depth of the grind burn and the depth of the potential crack that might form from the surface of it. It is most likely that the crack begins to form when the grinding wheel is in direct contact with the carburized and hardened surface when the surface is being “burned” (overheated because of the grinding conditions).

It is also critical (in the writer’s opinion) that the amount of retained austenite is minimized because small cracks might form between the martensite and the austenite.

Please do not interpret from the above that cracks will always form as a result of grind burn. That will be dependent on:
  • The existing metallurgical conditions (retained austenite, etc.)
  • Surface hardness
  • Depth of grind cut
  • Type of grind wheel (hardness of the grind wheel)
  • The grind-face conditions of the grind wheel (Is the grinding wheel regularly dressed?)
  • The absence or lack of grinding fluid to act as both a coolant and a lubricant to also control the developing frictional heat of the grinding conditions and to wash away ground metal fines from the grinding action
  • Traverse speed of the component (or wheel speed traverse)
The next blog (part 3) will continue with the effects of retained austenite and residual cracks that can be caused by gear-face grinding.
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Grinding cracks

Nilesh Gawande
September 3, 2012
I have a question on grinding cracks. On one of our parts we are facing a problem of grinding cracks. The part is made up of steel 16MnCrS5. It is hardened and tempered to 25 - 35 HRC. (Not carburised). Then it under going grinding process on one of the surface. It is possible to see new hardened and tempered zone on the surface by using nital? Or there is any other method for parts having low hardness as mentioned? Or it is not possible to see new hardened and tempered zone on soft components?

Grinding Crack answer

Mr. Nilash Gawande
September 9, 2012
Good day to you Mr Gawande, Thank you for your query to me on the subject of grinding cracks. Nital is an etchant that can be used, (I would tend to us 5% nital and an etching time of 5 to 7 seconds (go to the shorter etching time first) Becuase of the low carbon content, the very best that you can hope for (as quenched would 39Hrc to 45Hrc Max) Because it is generally treated with calcium, and aluminum (small quantities) it will tend to for hard aluminum oxide inclusions. These are considered to be 'non-metallic's) In the as quenched condition (and depending on austenitizing temp and time,you should accomplish a grain size of 5 to 8 As far as grind burn is concerned you would be able to see that show as slight brown surface marks if you swab with nital, That is caused generally by too large a grind cut, which will generate sufficient heat to begin localized surface over heating. Or it can be caused (on a surface grinder for example) by too fast a traverse and too large a grind cut. If you are using coolant, the coolant can act as a quench medium (depending on the temperature generated at the ground steel surface) and transform localized area's into fresh untempered martensite, which can lead to cracking. It all about speeds and feeds. There are other etching methods, however nital is generally an accepted method Best Regards to you David

Grinding Crack

Nilesh Gawande
September 13, 2012
Dear Mr. David, Thank you very much for feedback. It will definitely help me. Regards, Nilesh Gawande

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