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					                                              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.
                                                             Temperatures
                                                             Heating rates depends on a various
      Recommended thickness of layer using other
                                                             characteristics. Dimensions and
      technology of surface hardening
                                                             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
    Demonstration diagram of hardness             treatment. Curves of cooling rates of
    dependence vs. depth under surface on         quenching at given conditions (austenitization
    each method of surface hardening              temperature, dimension and shape of piece
    1. nitriding                                  and medium) are mapped. This data are
    2. nitrocarburizing                           models for real cooling routines, which must
    3. carbonitriding                             be mostly tested and than the optimal solution
    4. case hardening (carburizing)               is find.
    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
Heat treating after carburizing                   (160°C) and tempering at a low temperature.




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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                   E 295                   14221               20MnCr5
     11600                   E 335                   14240                 36Mn5
     11700                   E 360                   15241                42CrV6
     12010                   C10E                    15260                50CrV4
     12020                   C15E                    16120                15CrNi4
     12040                   C35E                    16121                17CrNi4
     12050                   C45E                    16220                15CrNi6
     12051                   C50E                    16231                19CrNi8
     12060                   C55E                    16250                45NiCr8
     12061                   C60E                    16420               15NiCr14
     13141                  28Mn6                    16430             26NiCrMo8-5
     13241                 37MnSi5                   16440               31NiCr14
     14120                  15Cr2                    16520              14NiCr16-4
     14220                 16MnCr5                   16720             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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posted:5/1/2011
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