5. INTRODUCTION
BEARING GRADES
Bearing
grade
C% Si% Mn% P% Cr% Mo% Ni%
S%
AISI
52100
USA
0.95-1.1 0.15-0.35 0.5max 0.012max 1.3-1.6 0.08max 0.25max 0.25max
100Cr6
Germany
0.95-1.1 0.15-0.35 0.25-0.45 0.03 max 1.35-1.65 0.1 max - 0.2 max
SUJ2
JAPAN
0.95-1.1 0.15-0.35 0.5 max 0.025 max 1.3-1.6 0.08 max 0.25 max 0.25 max
GCr15
CHINA
0.95-1.05 0.15-0.35 0.25-0.45 0.027 max 1.4-1.65 0.1 max 0.23 max 0.2 max
At ABC we studied and experimented only on Bearing Grade AISI 52100.
11. HEAT TREATMENT PROCESS
SPHEROIDIZED ANNEALING :-
• Spheroidization is achieved by holding the metal for a prolonged period at austenizing temperature and
cooling at a very slow rate by along with appropriate soaking time.
• Spheroidize annealing is done to form spheroids of carbon to improve machinability and prepare the metal
structure for hardening.
• Temperature: up to 860OC
• Cycle time: about 48 to 54 hours with slow furnace cooling along with intervals of socking time.
QUENCHING :-
• Cooling a metal at a rapid rate.
• Used to produce a martensitic transformation.
• Often produce a harder metal in ferrous, softer than normal in non ferrous.
• Metal heated above the upper critical temperature and then quickly cooled.
• Quenchant : Ferro-quench oil.
• Temperature : 90OC
• Cycle time : 8 mins
12. HEAT TREATMENT PROCESS (CONT..)
INDUCTION HARDENING :-
• A surface hardening technique.
• Surface of the metal is heated very quickly, using a no-contact method.
• Martensitic transformation at the surface, leaving the underlying metal unchanged after quenching.
• Creates a very hard, wear resistant surface maintaining toughness in the majority of the object.
FLAME HARDENING :-
• Used to harden only a portion of a metal.
• Only a portion of the metal is heated before quenching.
• Often produces an extremely brittle zone between the heated metal and the unheated metal, as cooling at
the edge of this heat affected zone is extremely rapid.
13. HEAT TREATMENT PROCESS (CONT..)
NITRIDING :-
• Creates a case hardened surface by diffusing nitrogen into the surface of a metal.
• Commonly used on low-carbon, low-alloy steels, medium and high-carbon steels, titanium,
aluminium and molybdenum.
TEMPERING :-
• Usually performed after hardening, reduce some of the excess hardness.
• Heating the metal at temperature below the critical temperature for a certain period of time.
• Air cooling.
• It controls brittleness caused after quenching and retain its strength and toughness.
• Produces martensitic + austenitic structure.
• Temperature : 175OC – 210OC
• Cycle time : 3 hours
14. MECHANICAL TESTING
Hardness Testing :-
• Before Heat Treatment :
25 – 30 BHN
• After Quenching : 64 – 65
BHN
• After Tempering : 61 – 62
BHN
Oval Distortion Testing :-
• Final Turning Ring(FTR) :
10 – 30µ
• Heat Treated Ring(HTR) :
<150µ
• Total material of ring after
removing ovality : >250µ
15. HEAT TREATMENT DEFECTS
• Less Supply Of Endo Gas
• Improper Oil Curtain
• Incoming (Raw Material) Problem
• Power Failure
• Oxygen In F/C. (Disturb Inert Atmosphere)
SURFACE DECARBURIZATION :-
• Thermal gradient in the component
• Over heating
• Non-uniform cooling
• Severe quenching
• Improper tempering
• Design defects
QUENCHING CRACKS :-
23. REMEDIAL MEASURES
Heat Treatment Deformation
Residual stresses that cause shape change when they exceed material yield strength. This occurs on heating
when the strength properties decline.
Stresses caused by differential expansion due to thermal gradients. These stresses increase with the thermal
gradient and cause plastic deformation as the yield strength is exceeded.
Volume changes due to transformational phase change. These volume changes are contained as residual
stress systems until the yield strength is exceeded.
24. REMEDIAL MEASURES
OVALITY
• Ovality is a certain type of circularity deviation. Ovality results from non-symmetrical distribution of internal
tensions before hardening and uneven heating and cooling.
• Devices using very accurate rotational tables or spindles are used to measure circularity. The axis of the
component is the measuring base in this method. The measuring device ensures very high accuracy, often
better than 1 μm.
• Ovality is checked after Quenching. A special diameter gauge (Figure) that included a dial deviation meter
was used to measure ovality.
• Ovality was measured on 10-20 rings so that an ovality deviation was recorded after each rotation. The
following procedure was applied :
• A bearing ring was inserted into the gauge and was rotated manually.
• A value was read from the deviation meter after each rotation.
• If the deviation was higher than 150 μm, the ring being measured was discarded. It did not meet quality
criteria and could not be passed on for subsequent hard machining.
26. REMEDIAL MEASURE (CONT..)
1. Heat Treatment Deformation:
• Preheating of bearing prior to heat treatment.
• Reduce hardening temperature and slow heating.
• Keeping 5 zone in hardening so that it can achieve required temperature at desired cycle time.
• Rate of cooling after hardening should be less.
• Bolting the ring and re-tempering.
27. REMEDIAL MEASURES (CONT..)
2. Surface Decarburization :
• By controlling furnace atmosphere, i.e., by generating “Endothermic Gas(Endo Gas)”
and producing it inside furnace and controlling its carbon potential.
• Carbon potential : 0.94% – 0.97%
• This will allow carbon inside bearing to stay in equilibrium with the atmosphere and
there will be no reduction in carbon % from the bearing.
28. REMEDIAL MEASURES (CONT..)
ENDOTHERMIC GAS
Basic Principles:
To protect the material being processed from surface reaction; i.e., to be chemically inert.
To allow the surface of the material being processed to change; i.e., to be chemically reactive.
Endothermic atmosphere is prepared by reacting hydrocarbon gases like methane or propane
and air, in proportion such that there is enough proportion of oxygen to form hydrogen and
carbon monoxide without excess carbon dioxide and water vapors.
29. ENDOTHERMIC GAS
Temperature : 1050oC
Catalyst : Nickel
2CH4 + O2 + 3.8N2 2CO + 4H2 + 3.8N2
Chemistry of Endothermic Gas:
N2 45.1%
CO 19.6%
CO2 0.4%
H2 34.6%
CH4 0.3%
Dew Point +20/+50
(Air/Gas) ratio 2.6:1
30. REMEDIAL MEASURE (CONT..)
• QUENCHING CRACK :-
• Cracks formed by bearing parts in the Quenching process due to internal stress causes Quenching
Cracks.
• Quenching Crack is cause because Quenching heating temperature is too high or too fast cooling.
• If there’s defect in the surface of machining parts that can also lead to Quenching Cracks. Surface
decarburization carbide segregation, part quenching or tempering, non-metallic inclusions, previous
process (cold punching under stress, sharp edges, forging folding, etc.) are some of the other reasons
behind Quenching Cracks.
• This can cause failure of bearing.
• REMEDIES :-
• Using oil as quenchant and keeping quenching temperature around 90OC.
• Reliving internal stress before Heat Treating.
31. REMEDIAL MEASURES (CONT..)
• SOFT SPOT :-
• When hardness of surface is not enough it causes Soft Spots.
• Soft Spot causes because of insufficient heating and bad cooling.
• This leads to declinations of surface wear resistance and fatigue strength, also bring down hardness.
• REMEDIES :-
• Avoid formation of oxides
• Avoid Decarburization
32. CONCLUSION
With proper analysis and setting suitable process parameters for different
types of bearings, considering their thickness, weight and layer of loading by
controlling hardening temperature and cycle time according to requirement and best
possible results leads to decrease in oval deformation and other defects.
35. REFERENCES (CONT..)
• https://www.passle.net/post/102bwy3/bearing-parts-after-heat-treatment-of-the-common-quality-defects
• www.nmbtc.com/bearings/white-papers/what-is-a-ball-bearing/
• www.oilversteel.com/quench-cracking.htm
• www.intechopen.com/download/pdf/39388 - Deformation Reduction of Bearing Rings by Modification of
Heat Treating by Anton Panda, Jozef Jurko and Iveta Pandová
• www.heat-treat-doctor.com/documents/endothermic%20gas%20generators.pdf
• www.ims.it/ims/ims_spa/cmscontent.nsf/DocumentsByIDWeb/5YWJK7/$File/heat_treatment_defects_TS.pd
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