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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.

Artificial Defects - Grinder Burn - Manufacture & Characterization

Applications - Races & Rollers.

 

What is Grinding?

Grinding is machining with irregular cutting geometry (grinding grits)

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.

What is Grinder Burn?

  • "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
    • crack

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

a) Reduction of compressive stress in the surface
b) Formation of annealing zones

Thermal damage: residual tensile stress

Thermal damage: re-hardening

Cracks

What are grinder burn‘s effects?

Grinder burn reduces the lifetime of the component pitting.
Fatigue of a roller bearing

Source: RWTH Aachen, Dept. Forschung/Tribologie, Author Dipl.-Ing. Volker Rombach

Is grinder burn detectable by eddy current?

Historical approaches:
a) structure test (magnetic induction testing)
large areas only
very limited scanning
b) crack detection (traditional / without PMFT)
calibration must be with specific master crack – Thus setting modification to also detect grinding burn compromises setting for crack detection. So there is no chance to reliably detect both cracks and grinding burn!
The new kind of grinder burn detection by eddy current is:
Differential probes in combination with
Preventive Multi-Filter Testing
- differential probe
gives absolutely stable, no drift test
- Preventive Multi-Filter Testing
easy to set
calibration with OK parts
- Now grinding burn and cracks are detected
Now we have the working solution!

How effective is the differential probe?

  • 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

Grinding cracks

“Grinding cracks may exhibit characteristic short, parallel cracks or they may exhibit a ‘chicken-wire’ pattern and are typically between 0.076 and 0.13 mm (0.003 and 0.005”) deep.”
Source: Failure Analysis of Heat Treated Steel Components Editors: Canale, Mesquita, Totten

Does the differential probe detect tensile stress?

Alternation of tensile stress to compressive stress creates a clear signal.

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.

That is no problem at all for Preventive Multi-Filter Testing!

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

Thanks to PMFT, the test instrument is not set up using master defects.
  • 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.
  • Laser defect

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

Source: imq for ibg
Source: imq for ibg
Source: imq for ibg
Source: imq for ibg

Approach to a test task

1. Determination of a specification:
  • degree
  • direction
  • minimum extension (length & width)
one or more master defects must be agreed on, which represent that specification, can be artificial or natural
  • surface quality of OK parts must be known
2. Feasibility study
3. Determination of suitable sensors and definition of borderline defects.

Summary

1.) Not every thermal damage is grinder burn.
2.) It has to be defined what is and what is not a reject part.
3.) The new ibg crack detection technique with PMFT has proven to be a practical test method for grinder burn detection in production.
4.) A feasibility study should be done in any case to guarantee that your application can be solved.
5.) A feasibility study requires:
  • 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
ibg NDT Systems - Designers and manufacturers of state-of-the-art NDT eddy current testers, instruments, and systems for automotive, bearing and other metal components including fastener, forged and cast parts, hard metals, precision stamped parts, etc.
ibg NDT Systems Corporation, 20793 Farmington Road, Farmington Hills, MI 48336
Phone: 248.478.9490, Fax: 248.478.9491
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