Grinding cracks are usually caused by poor quality glazed grinding wheels, in sufficient coolant, more than required feed rate in removing the material.
Grinding cracks occurs on heat treated hardened components. In fact, the cracks are caused by excessive heating of the surface caused by grinding. Grinding cracks appears on the surface at right angle to the wheel rotation direction. That is to say, they are shallow, sharp and clearly identifiable based on the appearance.
Causes for Grinding Cracks
- Using Wrong Grinding wheel
- Poor quality glazed dull wheel
- Inadequate pouring of coolant during grinding
- High feed rate
- Heavy cutting
- Improper heat treatment
Problems associated with Grinding Cracks
- Fatigue cracks due to residual stresses
- Expansion of cracks at the later stages
- May propagate under repeated or cyclic loading
- Severe and intensive than types of cracks commonly found on shafts gears and forgings
Once localized areas are highly stressed, it results in cracking of the surface intensively. Especially materials which are heat treated with hardening and high carbon allow steel are more prone for having such grinding cracks.
During grinding, localized heating added to entrapped stresses that can cause surface opening through cracking.
These cracks are highly harmful for cyclic loaded components such as pistons, pinions, shafts etc. In addition, these cracks are purely on the surface and are easily identified or detecting using Nondestructive testing.
Magnetic particle inspection is the best technique to detect surface cracks in ferro magnetic steels. As these cracks are networked in nature they can be easily detected even by using non-fluorescent MPI testing techniques.
However, Flurescent MPI testing technique can give more visibility and better detectability as these are very fine in nature to find with conventional visual testing.
Conclusion
Though they seems to be light, cracks can easily propagate at faster pace and can destroy the material withing no time. Therefore these cracks have to be machined and removed completely or else you may have to reject the whole material.
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