U 12132
Department of materials engineering
PROJECT I
Material part
Task
For the given part of a gearbox choose a suitable material and elaborate process of heat
treatment. Consider required mechanical properties, constructional, technological and
economical aspects. Design an alternative material and heat treatment or chemical-heat
treatment suitable for toothed parts of the gearbox. See instructions below.
Instructions

For the part of the gearbox (tooth wheel or tooth shaft), which is intended for
Technological part of the Project, design a suitable material and elaborate an appropriate
process of heat treatment.

If the material for this component is given beforehand, elaborate only the process of the
most suitable heat treatment.

Work out a list of fundamental processing steps including a scheme with basic
dimensions.

Take into account constructional, technological and economical aspects of processing.

For the identical part design an alternative material and heat treatment or chemical-heat
treatment. One of the alternatives of heat treatment shall be carburizing.
List of possible treatments suitable for tooth wheels:
1. Classical hardening treatment (quenching and tempering at high temperature).
2. Classical hardening treatment and subsequent nitriding.
3. Classical hardening treatment and subsequent nitrocarburizing.
4. Carbonitriding and subsequent heat treatment.
5. Case-hardening (carburizing) and subsequent heat treatment.
6. Normalizing and face hardening (less stressed wheels).
7. Classical hardening treatment and face hardening (by inductor or burner).

Draw thermograms (temperature vs. time) for both alternatives

Compare both alternatives according to material price (use internet), heat treatment
duration and final properties.
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U 12132
Department of materials engineering
DESIGN PROBLEMS:
Note that all engineering parts must work under complicated mechanical conditions. Accurate
choice of suitable material depends on these mechanical requirements (strength, yield point,
fatigue limit, wear resistance etc.), influence of surrounding (temperature, corrosion
conditions), possibilities of technology (machineability, ductility, weldability) and of course
on overall economy. The material (in natural state) without accurate processing including e.g.
rolling or forging, machining, heat treatment and or surface engineering could not meet
demanded product requirements.
HEAT AND CHEMICAL-HEAT TREATMENT PROBLEMS:
Temperatures and time
consumption during HT
The temperatures of heat treatment
and heating-up rates and cooling
are given by finally achieving
properties which depend on
structure
of
material.
The
recommended temperatures and
cooling rates (cooling media) are
given in material brochures of
each steel.
Recommended thickness of layer using other
technology of surface hardening
Temperatures
Heating rates depends on a various
characteristics. Dimensions and
power of furnace, quantity, size
and arrangement of batch and conditions of
heat transfer which depend of course on
heated material. There are some complicated
mathematical models but in praxis graphs,
tables and coefficients are used. It is
supposed, that the requirement of the Project
is that cooling rate of heating is
≈ 150 °C/hour.
Cooling
Cooling rate of material (velocity) depends on
the type of heat treatment and required final
structure. Continuous Cooling Transformation
diagrams
(CCT)
and
Isothermal
Transformation diagrams (IT) of austenite are
used for prediction of parameters of heat
treatment. Curves of cooling rates of
quenching at given conditions (austenitization
temperature, dimension and shape of piece
and medium) are mapped. This data are
models for real cooling routines, which must
be mostly tested and than the optimal solution
is find.
Demonstration diagram of hardness
dependence vs. depth under surface on
each method of surface hardening
1. nitriding
2. nitrocarburizing
3. carbonitriding
4. case hardening (carburizing)
5. face hardening
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U 12132
Department of materials engineering
Example: for quenching temperature 850°C and diameter of a rod 50 mm the crossection is
cooled with help of steady water (20 °C) in about 150 s (surface is cooled after 5 s), using oil
(20 °C) cooling takes about 20 min (surface is cooled after 60 s) and the cooling by air lasts
about 3 hours (surface is cooled after 20 min).
For some slow cooling processes (after some types of annealing e.g. 100°C/hour) furnaces
with automatically controlled cooling rates are utilized.
Heat treatment
Normalizing
Normalizing is a common heat treatment to optimize the structure after some technological
operations as casting, welding, forging etc. Normalizing is also applied as preliminary
treatment for shape-complicated parts before further heat treatment or chemical-heat
treatment. Note, that normalizing can not reduce the inner tension in treated parts. Heating
must be 30 °C – 50 °C over Ac3 and holding time (usually 1 – 4 hours) must be sufficient for
formation of a homogenous austenite, but not too long to prevent harmful graingrowth.
Warming through rate depends on thickness of treated part. The empiric rule of 30 min
dwelling at annealing tempertaure for each 25 mm of thickness can be used. Afterwards the
air cooling is usually applied.
Spherodizing (soft annealing)
This type of heat treatment is demanded by materials with difficult machinability (mainly
hypereutectoid alloyed steel). The annealing is realized at temperature close to Ac1 or at
oscilating temperature around AC1 for 2 ÷ 4 hours. Then a slow cooling about 10 °C/hour is
applied until the temperature of 600 °C is reached. Under this temperature the cooling rate can
be higher. The spherodised cementite structure is obtained and the steel becomes soft, tough
and well machinable.
Quenching and tempering
Quenching as other heat treatment processes consists of three parts: heating, holding at
austenitizing temperature and cooling. Holding
time at austenitization temperature is specifiend
for each steel by austenitization diagram. Holding
time also depends on characteristic dimensions of
heat treated pieces. The table gives the orientation
holding times at austenitization temperature. The
temperature of tempering is given by final
requirements and it is mostly between 300 °C and
650°C with holding time 1-2 hours. Cooling rate
is given by chemical composition of steel and
must prevent temper embrittlement (see the
material brochure to find recommended media).
Face (shallow) hardening (induction hardening, torch hardening)
Surface hardening employs concentrated sources of heat power (inductors, burners, laser
beam etc.). Thin surface layer of hardened piece is heated to a quenching temperature while a
core remains cool. In case of quick heating, the austenitizing process take place at higher
temperature of 150 °C – 250 °C above Ac3 (see austenitization diagram). The heating rates are
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U 12132
Department of materials engineering
(240 – 420) °C/s and quenching by water or oil spray is applied immediately. Usually
subsequent tempering at 150 °C – 250 °C is utilized.
Hardening using laser is very powerful but technologicaly complicated process which requires
experience and testing of optimum parameters.
Another possiblity is to apply a special volume heat treatment consisting of quenching and
tempering using steels with very low depth of hardenability. This process is relative
complicated, expensive and requires special steels and experiences.
Toothwheals with module up to 5 mm are heated continuously, in case of big module teeth are
heated separately, tooth by tooth (only the sidefaces and bottoms are hardened). The thickness
of hardened layer is (1,5 ÷ 4) mm. Keep in mind that the teeth with modulus  3 mm will bee
undesirably hardened throughout the whole volume (embrittlement of the core)!
Chemical-heat treatment
The most common method to reach higher hardness of tooth surface layer is case hardening
(carburizing) or carbonitriding which makes 95 % of surface hardening processes.
Case-hardening
Case hardening consists of carburizing
(saturation and diffusion period), quenching
and low temperature tempering. Carburizing
is proceed in case of 99 % in atmosphere
using temperature between 900 °C and 950 °C
(higher temperature up to 1100 °C enables
quicker diffusion but requires special fine
grain steels). Main process parameters are
temperature, time and saturation potential of
atmosphere. Carburizing speed for different
saturates is shown in graph. The second figure
shows thermograms of possible heat treatment
after carburizing. Quenching directly from
carburizing temperature (case 1) is cheapest
but gives bad results – internal stress,
peeling. Standard tempering parameters are
(150 ÷ 200) °C / 2 h / air.
Carbonitriding
It is a carburizing process in atmosphere
containing minor addition of nitrogen (e.g. 5
% NH3). Common parameters are:
- temperature of process: 860 – 870°C,
- optimal thickness 0,25 - 0,5 mm,
- time for carbonitriding: 2 - 5 hours,
- subsequential quenching in oil or warm bath
(160°C) and tempering at a low temperature.
Heat treating after carburizing
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U 12132
Department of materials engineering
Nitriding
Nitriding temperature: 500 – 600°C, typical nitriding case depth 0,5 mm.
Time for nitriding: 0,3mm - 20 hours, 0,5 mm – 60 hours.
Hardness of the layer can exceed 1000 HV.
Before nitriding steel is quenchend an tempered at high temperature. However, nitriding
process takes place at lower temperature than tempering.
Steels must contain chemical element making nitrides (Al, Cr, Mo, V).
Nitrocarburizing
Nitriding process in an atmosphere containing minor addition of carbon (≈ 10 % some
hydrocarbon gas like propane).
Temperature of process: 570 – 620°C,
optimum thickness of continuous compound layer 10 - 25 m,
growing rate of nitrocarburized layer: 4 hours  0,05 mm.
Hardness of the layer is close to 1000 HV.
The diffusion layer with hardness of 300 – 400 HV is developed under the surface in depth of
0,3 – 0,5mm. This layer improves ultimate load of surface and fatigue resistance of the piece.
Quenching is not necessary.
Selected wrought steels suitable for application in Material part of the Project
Steel by ČSN
Steel by ČSN EN
Steel by ČSN
Steel by ČSN EN
11500
11600
11700
12010
12020
12040
12050
12051
12060
12061
13141
13241
14120
14220
E 295
E 335
E 360
C10E
C15E
C35E
C45E
C50E
C55E
C60E
28Mn6
37MnSi5
15Cr2
16MnCr5
14221
14240
15241
15260
16120
16121
16220
16231
16250
16420
16430
16440
16520
16720
20MnCr5
36Mn5
42CrV6
50CrV4
15CrNi4
17CrNi4
15CrNi6
19CrNi8
45NiCr8
15NiCr14
26NiCrMo8-5
31NiCr14
14NiCr16-4
18NiCrMo16-6
Choosing other suitable materials is also possible.
The designation according to European standards is requested.
To find corresponding material designation it is possible to use an electronic Lexicon of
technical materials. Lexicon is available only in the study room at Charles Square at the
building of faculty of mechanical engineering (on the first floor, opposite to Copy Centrum).
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