3.2 Heat Treatments
Refer to Table 4 for features of heat treatments.
Table 4. Features of Heat treatments
Contents

Induction hardening

Flame hardening

Case hardening

Materials

Carbon steel with 0.4-0.6% Carbon
SCM435, SCM440
SMn443, SNC836
SNCM439, etc.

Carbon steel with 0.4-0.6% Carbon
SK5-7, Ductile Cast Iron
SCr435, SCr440
SCM435, SCM440, etc.

Carbon steel with below 0.23%Carbon
SNC415, SNC815
SCM415, SCM420
SNCM420, etc.

①	Low andmean
content ofCarbon steel②	Carbon,Alloy,	Stainless andCast steels

SACM645 and others
For Nitriding process, material should consist of Aluminiumand Chromium.

Put gear into the coil of quenching machine then turn on highpower of eddy current to thecoil. Overheat the surface ofgear and immediately apply
jet cooling water to gear for
instant cooling.. Long items canbe fabricated with quenchingby process of heated coil line
continued by instant cooling inlongitundinal direction.

Economical method of heat
treatment compared with others if the Induction hardening
was expensive (for small volumeand extra large item).
Heating only the part you wishto harden by burner to overheat. When surface becomes
austenite composition, jet-spray
the water for instant cooling. Asa result, only this part hardens.
Method of tempering is processof using low temperature tempering of 150°-200°C.

Gear together with charcoal andCarbonic barium are seald in a
melting pot and heated for 4-8hours at temperature 900°-950°C.
Carbon permeates to gear surface.
Use for producing a variety ofitems in small quantities.

① Soak gear with low and medium
carbon content into the Salt bath(main constituent is NaCN), toproduce film of 0.2mm or below on
the gear surface. Processing temperature will be 750° to 900°C and
it is suitable for small amount ofproduction. This is an economical
method but the salt bath is toxic
and hazardous to health.
② Isonite
Using method of Salt bath Nitriding
NaCNO or Potassium bath to Nitride
by nascent Nitrogen. Treatment
temperature is 500° to 600°C andlast for 24 hours. Effective hardness
depth will be 0.015 to 0.020.

Material is modified to the Sorbite composition by quenching.
After which the gear is put into
the Nitriding furnace. Whenammonia gas is injected into
furnace with temperature 500°
to 900°C, decomposed Nitrogenis absorbed to form a hard layeron the surface of the gear.
Treatment hours will span fromtens of hours to a few hundred
hours depending on the depthof hardness required.

Hardeningdepth

It is difficult to have the bore, core and section to harden. Generally, steel material suitable for hardening is used to perform quick
quenching on the surface. Core area keeps original composition.
Harden surface with little oxidation using instant overheat andinstant cooling. Perform thermal refining with quenching temperature of 30° to 50°C and provide water cooling to allow the Austenite
to diffuse into the gear easily. There is high heat efficiency for directoverheating which causes the hardness of Tooth tip to be higher
than Dedendum area of the gear.

Tolerance of case depth lessthan 0.2mm is difficult for Solidcarburizing. Carburizing depth
below 0.7mm is not suitable. Regardless of the shape of goods,
same layer of hardness can beobtained. Mask the area that
does not require hardening toprevent carburizing.

Isonite is an economical surface
hardening method that saves time
for hardening, self-lubricating andhas low coefficient of friction.

There is less strain from heat
influence during heat treatment
by low temperature. Hardening
layer provides surface with better wear resistance, heat resistance and anti-corrosion. Layerof hardening expands gear by
0.02mm to 0.03mm as Nitrogenis absorbed.

Hardening to limited parts ispossible.
Heat treatment duration only
takes a few seconds.
Simple equipment has inconsistent hardness.

Heat treatment hardens wholebody.
Long heat treatment duration.

Economical cost and short treatment duration

Productivity

Hardening to limited parts ispossible.
Heat treatment duration only
takes a few seconds.
Automated system is possible.
Suitable for mass production.

Heat treatment hardens wholebody.
Very long heat treatment duration.

Hardness

HS55 ～ 75
HRC41 ～ 56

HS55 ～ 75
HRC41 ～ 56

HS70-85
HRC52 ～ 62

HS88-92
HRC64 ～ 66

HS100 以上
HRC68 以上

StrainSmaller strain than quenching
and tempering.

Larger strain than quenching
and tempering.

Larger strain than induction
hardening

Minute strain

Minute strain

CostEconomical cost for mass production

Economical costMore costly than induction
hardening

Economical cost for mass production

CostlyDepth ofhardness

0.8 ～ 7mm (Alloyed steel is over4.0mm)

1 ～ 12mm (Alloyed steel is over4.0mm)

Solid Carbunizing 0.7 ～ 5mm
Gas Carbunizing 0.2 ～ 5mm

0.015 ～ 0.02mm (Specialized steel
is 0.1 to 0.2)

0.1 ～ 0.6mm (Uneconomical touse above 0.4)

No limitation for size and form
Quenching to limited parts ispossible
Quenching equipment is economical cost
Overheating temperature is difficult to control.

Easy to adjust carbon density
Uniform depth of Carbunizing
Easy to adjust depth of Carbunizing

Beware of polluting, as treated salt
is deadly poisonous
Vulnerable to impulse loadFeature

Suitable for mass production insimple form
Electrically controlled automation system is possible
Stable quenching
Quenching to limited parts ispossible
Quenching equipment is expensive

Excellent wear resistant, heat
resistant and anti-corrosion
Heat treatment after Nitriding is
unnecessary
Minute deformation
No occurrence of hardening
crack

Otherapplications

Chain wheelPinCrankshaft
CamshaftShaft, Pin, Cam, Bush for Roller
chainCamshaft

Diesel injection nozzle
Gauge

Heattreatment

Gas carburizing is performed
with easy adjustment for
amount of Carbon carburizing,
Depth of Carburizing has minutescales on the surface of gear. Environmentally friendly and has
consistent quality. Process timeis shorter than Solid carburizing
and suitable for mass production.

Nitrocarburizing

Nitriding

There is also Plasma nitriding, which is not mentioned here that causes minute strain compared to other methods bythermal influence while hardening tooth. It has been omitted from this report. Refer to Table 4 for features of Nitration.

57