Grinder Burn Detection using Eddy Current and Preventive Multi-Filter Technology (PMFT)
ibg’s revolutionary new technology for eddy current crack detection, “Preventive Multi-Filter Testing” (PMFT), enables the ibg eddyliner C and eddyvisor C eddy current crack test instruments that utilize the PMFT to robustly detect grinder burn/grinder damage. Detection starts at the level of reduction in compressive stress, continues to include thermal softening and re-hardening (reappearance of austenite & martensite), to appearance of residual tensile stress and on to cracking and pitting. The testing is robust and stable enough for reliable use for 100% testing in production lines as well as for audit testing. The methodology to implement it is the same as for implementation of traditional eddy current crack detection.
What is Grinding?
Three processes run side by side:
- cutting: creates chips which form at the sides of the grits
- rubbing (sliding of the grits against the part): creates heat
- plowing (material is pushed to the sides and the front of the part): creates heat and cold work hardening of material
Grinding produces a lot of heat.
- "Grinder burn" is a word which commonly describes any kind of thermal damage the work piece suffers during the grinding process.
- There are several types of thermal damage:
- some are cosmetic
- some affect the lifetime of the work piece -pitting
- some immediately cause micro cracking
- There are several degrees of thermal damage:
- oxidation burn
- thermal softening
- residual tensile stress
- re-hardening burn
How does thermal damage occur?
Thermal damage arises from a locally limited heat impact to the surface of a workpiece during the grinding process.
How does the unpolished condition look like?
Thermal damage: thermal softening
Thermal damage: residual tensile stress
Thermal damage: re-hardening
Source: RWTH Aachen, Dept. Forschung/Tribologie, Author Dipl.-Ing. Volker Rombach
Preventive Multi-Filter Testing
- cosmetic effects like oxidation not detectable
- thermal softening (annealing) Yes
when sufficient structure change took place
- residual tensile stress Yes
differences in stress are detected
- re-hardening Yes
differences in structure are detected
- cracks Yes
grinding cracks are detected
The actual difficulty of grinder burn detection
The physical effects of grinder burn cause change of stress, alteration of structure and shallow cracks:
The disadvantage of the "old" traditional crack detection method is that:
All these phenomena appear in different filter bands and have different phase angles and amplitudes.
All these phenomena lead to tolerance zone violations in the different filter bands and with all phase angles.
Master parts (for grinder burns) and their use
- The master does serve for verification of test mechanic.
- The master does serve to demonstrate process capability and repeatability of test system during runoff.
- The master should represent a borderline reject component.
- The master must be verified and quantified, e.g. by X-ray and grinding pattern.
- The master should be reproducible.
Refer to: insight NDT Vol 52 no. 6 June 2010 p. 293-295: “laser-processed grinding burn simulation…”
An artificial defect does not exactly look like a natural defect.
The making of an artificial laser defect
(ibg cylinder roller / co-operation with imq-Ingenieurbetrieb GmbH)
Grinding pattern of different laser defects
Laser power 300 - 600 W, feed speed 1m/min, fibre 0.3 mm
Approach to a test task
- minimum extension (length & width)
- surface quality of OK parts must be known
- verified OK parts (at least 10 pieces)
- quantified NG parts (e.g. X-ray)
- nital-etched parts only do not serve effectively as NG samples