diat htt lect 25 to 27

8/11/2019 Diat Htt Lect 25 to 27

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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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6/12

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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8/12


9/12

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


11/12

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


12/12

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

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