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Surface Hardening:Non-chemical Treatment
Dr. Santosh S. Hosmani
DEPT. METALLURGY & MATERIALS SCIENCE,
COLLEGE OF ENGINEERING, PUNE 411 005
ur ace ar en ng
Thermochemicaltreatment:
Changeinchemical
Phase
transformation
by
rapid
heating
and
cooling:
compositionofsurfaceby
diffusionofotherelements.
Nochangeinchemical
compositionofsurface.
Examples:
Nitriding,
Examples:
Flamehardening,
,
Cabonitriding,
Boronizing,etc
,
Laserhardening,etc
InductionHardening
Nochangeinchemicalcompositionofsurface.
Rapidheatingthesurfacetoaustenitetemperatures,andthenquenchingittomartensite.
Materialbelowthehardenedsurfaceremainatlowertemperature.
Heatforhardeningasteelorcastironpartsisgeneratedbyelectromagneticinduction.
Induction
Heating:
Analternatingcurrentthroughtheinductor,orworkcoil;
Magneticfieldthusestablishedinducesanelectricpotentialintheparttobeheated;
Inducedvoltagecausestheflowofcurrent(eddycurrent);
Resistance(R)oftheparttotheflowofcurrentcausesheatingbyI Rlosses.
Rateofheatingdependsthestrengthofthemagneticfieldtowhichpartisexposed.
Maximumheating(i.e.hightemperature)atthesurfaceanddecreasesrapidlybelowit.
Highfrequencycurrent(I)isusedwhenshallowheating(thincasedepth)isdesired.
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InductionHardening
ep o ar en ng 0
Degree of flow of current on the outer surface of a component depends on the frequency,
.
For a given material, last two factors depend on temperature. The depth to which the current
penetrates (din mm) and raises the temperature is given by:
At 800 C, d800 = 500/ f (where, f is frequency in Hz)
n a on o e rec ea ng o e sur ace s n y n uce curren , ere s aso some
heating of the core due to conduction of heat. Hence, overall hardening-depth is greater than
d800:
(d0)800= (500/ f ) + (0.2*t) (where, t is heating-time in seconds)mm
As frequency increases, hardening-depth decreases.
InductionHardening
theshapeofinductioncoilproducingmagneticfield,
thenumberofturnsinthecoil,
eopera ng requency,an
thea.c.powerinput.
Fi ure: Patterns of ma netic
field and induced current
produced by various
induction coils.
OD:outerdiameter
ID: innerdiameter
InductionHardening
Induction hardening is generally done at frequencies of 1000 cycles/second or higher.
Type of high frequency equipments used for induction heating:
-
, .
Spark-Gap Unit: 20,000 6000,000 cycles/second (or Hz).
Vacuum Table Unit: > 200,000 cycles/second (or Hz).
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InductionHardening
Table: Guidelines for frequency and input-power selection during induction hardening.
kilocycles/second(or kHz)
Low kw can be used
when generator
For best
metallurgical
For higher production
when generator
. . .
InductionHardening
e ec on o o es gn:
Coils are usually made from coppertubes (or solid bus bar) and are water cooled
.
The success of induction heating applications is related to selection of the proper
work-coil (inductor) design.
The design of coil is influenced by:
- dimensions and configuration of the part to be heated,
- the heat-pattern desired: whether the part is heated throughout its length at the same time or progressively,
- ,
- the amount of power available.
InductionHardening
e ec on o o es gn:
Basic work-coil designs for use with high frequency units and the heat-patterns
developed by each:
a For external heatin :
(b)For internal heating of bores
in a narrow band for scanning applications
InductionHardening
e ec on o o es gn:
A number of basic work-coil designs for use with high frequency units and the heat-
patterns developed by each:
d A sin le turn coil for scannin a rotatin surface
(e)For spot heating
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InductionHardening
e ec on o o es gn:
Irregular shapes:
- ,
remembered that the portion of the workpiece closest to the inductor will be instrongest magnetic field and will heat more rapidly.
- Sometime it is necessar to increase air- a around the section havin least
mass to reduce the heating rate.
Figure:Influence of air-gap
on hardness pattern in
irregular shapes.
InductionHardening
ar en ng or wear res s ance:
Based on the depth (thickness) of hardened layer, induction hardening can be
c ass e n o wo ypes
For shallow hardening:
- -. .
- Good wear resistance in light/moderate loading;
- Frequency range: 10*103 2000*103 cycles/second (Hz);
- e.g.roc er-arm s a s, suc er-ro coup ngs, an pump s a s.
For deep hardening:
- Depth: 0.06 - 0.25 inch.- Good for the parts under heavy or impact-type loading;
- Frequency range: 1*103 - 10*103 cycles/second (Hz);
- e.g.Gears, truck pins, heavy crankshaft bearings, camshafts, and bearing
races.
InductionHardening
Improving fatigue strength
De endin u on t e of a lication there can be:
(i) Torsional fatigue, (ii) Bending fatigue, or (iii) Torsional+Bending fatigue.
Induction case hardening of bars and shafts to depths of 0.12 0.50 inch has
resulted in improved torsional and bending fatigue strength.
Long bars and shafts are passed through inductor coil; and they are rotated to
obtain more uniform results in processing.
e.g. Truck, tractor and automobile axle shafts and hydraulic piston rods.
Recommended current frequency: 1 10 kilocycles/second (kHz).
Selective hardening:
- Critically stressed areas of steering knuckles, flanged axle shafts areselectivel case-hardened to im rove torsional and bendin fati ue ro erties.
- Current frequency: 3 450 kilocycles/second (kHz).
InductionHardening
Improving fatigue strength
Com ressive residual surface stress develo ed b induction hardenin hel s inimproving fatigue properties:
Figure: Effect of induction
hardening on bending fatigue
strength of medium-carbon
steel tractor axles (2.7 in OD).
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InductionHardening
Quenching methods
Heat in coil; manually lift
part out of coil; submerge
part in tank of agitated
Heat and quench in one
position; quench by means
of integral quench chamber
-
Heat in coil with part
stationary; quench ring
moves in place. Single-shot
.
for limited production.
.
method
method
InductionHardening
Quenching methods
Part is hydraulically lowered into Vertical or horizontal Vertical or horizontal
quench tank after single-shotheating. Quench media is
agitated by submerged spray
ring or propeller.
scanning with integralspray quench. Single-
turn inductor. Used for
shallow hardening.
scanning with multiturncoil and separate
multirow quench ring.
Used for deep-case or
through hardening.
InductionHardening
Quenching methods
Coil scans and heats
workpiece; self-quench** or
compressed air quench. Used
in special applications with
Horizontal cam-fed parts pushed through coil; dropped
onto submerged quench conveyor.
g - ar ena y s ees.
** A special quenching technique
some mes empoye on par s wsufficient mass is referred to as selfquenching, because most of the heat atthe surface is rapidly absorbed by the
unheated mass of metal below the surface.
InductionHardening
Quenching methods
S.V. F.V.
Vertical scanning with single-
turn inductor in combination
with integral dual quench: one
quench ring for scan
Vertical scanning with
single-turn inductor with
integral spray quench and
Split inductor and integral
split quenching. Used for
crankshaft bearingar en ng; e secon or
stationary quenching when
the scanning travel stops.Used for parts having a diameter of
submerged quench in tank. surfaces.
through the inductor, wherein it is
desired to harden up to the
shoulder or flange.
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FlameHardening
I. Stat ionary f lame hardening: In this method, both burner and workpiece
are stationary. This method requires that the specified area be
heated. Then the part is taken to quench or quench is brought to the part.This method is particularly well-suited for shaft ends, special steel
casting configurations and large parts.
FlameHardening
II. Progressive f lame hardening: This is carried out by using a burner
combined with a water-spray. In this method, the burner moves over the
large stationary workpiece (sometimes called "scanning). This is followedby quenching.
This method is particularly well-suited for ways, knives and flats.
FlameHardening
III. Spin flame hardening: In this method, workpiece is rotated, while burner
remains stationary. After heating, flame is removed and quenching is
carried our by a water jet.
This method is particularly well-suited for gears, wheels and sprockets.
sprocket
FlameHardening
IV. Progressive-Spin (combination) flame hardening: In this method, the
burner moves over a rotating workpiece and at the same time quench head
also moves along the length of rotating workpiece.
This method is ideal for hardening shafts.
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FlameHardening
As in induction hardening, flame hardening also involves short heating times
with high intensity heating source (high heat-rate), and therefore, prior
manner as in induction hardening.
Flame hardening is generally used for one or more of the following reasons:
i. Workpieces are so large that conventional furnace heating and quenching is impractical
. , ,
rolls, etc.
ii. Only a small segment or section of the part requires hardening.
.
quenching, due to distortion.
FlameHardeningversusInductionHardening
ElectronBeamHardening
e wor p ece s ep n vacuum a . m ar pressure. acuum env ronmen
protects the emitter (source of electron) from oxidizing and avoids scattering of theelectron-beams by air.
ec ron eam s e ocuse on e wor p ece o ea e sur ace. n e
beginning, energy input is kept high. With time, power input is reduced as the
component gets heated up. This is done to avoid melting.
o separa e quenc ng me a s requre s nce quenc s e ec e y e mass o
the surrounding unheated portion (self quenching). In this regard, mass of the
treating workpiece should be sufficient.
- , , , .
Achievable case-depth: 0.75 mm
The surface can be hardened ver recisel both in de th and in location.
Electron beam processing is the most efficient for hardening steels, Ti-, Al-based
alloys, etc.
van ages o e ec ron eam ar en ng es n e poss y o rea ng po n s,
lines or areas of surfaces without metallurgically affecting other adjacent areas ofthe workpiece.
ElectronBeamHardening
Electron-
beam
generating
unit
Electromagnetic
coils
Interaction of electron-beam with workpiece
workpiece
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LaserBeamHardening
LASERis an anachronism for Li ht
Amplification by Stimulated EmissionofRadiation.
For certain specific applications,
laser hardening offers a sensible
alternative to induction hardening.
In laser hardening process, less time
is required than in induction and flame
hardening process, and the effect of
heat on the surrounding surface is
, .
1 laser beam
2 workpiece surface
Procedural principle:
Heat: Short heating phase which varies
4 tempered-zone
.
Hold: The temperature is held for a short
time in order to diffuse the heat to the
.
Cool:The high temperature gradient into the
workpiece results in self-quenching.
LaserBeamHardening
Case-depth can be given by:
Benefits of laser hardening:
LaserBeamHardening
Minimal distortion due to low thermal load (partial energy input).
Complex, bulky components can be hardened with extreme simplicity, e.g. inside cavities.
High degree of flexibility.
Precision hardening, high degree of hardening, finely dispersed, fine-grained but relatively
tough martinsite.
Minimized reworking, shortened process chain. No need for external quenching, e.g. using water etc.
Possibility for integration in processing systems.
Optimum process control due to integrated temperature guidance.
Bodywork toolInjection nozzle holderCutting tool
Hardening of cutt ing edges and bendingedges.
Hardened face surface without boreholedistortion.
Hardening of cutting edges.
Gears:
le:te
elsfor
Examp
rd
ening
rough-H
Th
Ref.: Book by Joseph R. Davis on Gear Materials, Properties, and Manufacture
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The throu h-hardenin rocess is enerall used for ears that do not re uire
What is Through Hardening Process?
high surface hardness.
Typical gear tooth hardness following through hardening ranges from 32 to 48
.
Most steels used for through-hardened gears have medium carbon content (0.3
to 0.6% and a relativel low allo content u to 3% . The ur ose of allo in is
to increase hardenability. The higher the hardenability, the deeper is the through
hardening of gear teeth.
nce s reng ncreases rec y w ar ness, g ar ena y s essen a
for through hardening steels. High hardenability, again, has some adverse effect
on material ductility and impact resistance.
The other drawback of through-hardened gears is lower allowable contactstresses than those of surface-hardened gears. This tends to increase the size ofthrough-hardened gears for the same torque capacity compared with those with
ar ene sur aces.
Ref.: Gear Materials, Properties, and Manufacture (ASM International), Sep 2005, Pages: 155-162
Example:Through-Hardening Steels for Gears:
Ref.: Book by Joseph R. Davis on Gear Materials, Properties, and Manufacture, ASM International
Example:
Through-Hardening Steels for Gears:
Ref.: Book by Joseph R. Davis on Gear Materials, Properties, and Manufacture, ASM International Ref.: Book by Joseph R. Davis on Gear Materials, Properties, and Manufacture, ASM International