Metal Working At High Temperatures.

Here we will be dealing with different metal working processes, their parameters, their effect on mechanical properties and more...

First let's see the classification of metal working processes based on the types of forces applied on the workpiece.

1. Direct Compression type: Examples are Forging, Rolling.
2. Indirect Compression type: Examples are Extrusion,Wire drawing, Deep drawing.
3. Tension type: Example is Stretch forming.
4. Bending type.
5. Shear type.

Working Process Parameters:

Working process parameters are mainly flow stress, strain, strain rate, temperature of working.

All these parameters are interdependent i.e. changing one will affect the other.

Now, in this study we are dealing basically with high temperature effects so all the metal working process parameters will affect the flow stress but "strain" will have negligible effect on flow stress , instead in high temperature working strain rate will play an important role....

Let's discuss their effects,

The interdependency of flow stress on strain rate and temperature is given by the equation named as Zener-Hollomon Parameter.

Z=E*exp(Q/RT).

Where,

Z= Flow Stress.

E= Strain Rate.

Q= Activation Energy.

R= Universal Gas Constant.

T= Temperature.

Some Important Points on their interdependency:

1. Higher the strain rate higher the flow stress.
2. Higher the temperature lower the flow stress.

HOT WORKING:

Working on the workpiece at higher temperatures i.e usually at T>0.6Tm.

In hot working flow stress and energy required for deformation are less as compared to cold working, this is because one of the following processes takes place simultaneously along with deformation,

1. Dynamic Recovery.
2. Dynamic Recrystallisation.
3. Static Recovery.
4. Static Recrystallisation.

MERITS Of Hot Working Processes:

1. Lowers flow stress and energy required for plastic deformation.
2. Hot working processes like hot forging, hot rolling, hot extrusion are used as a first step for converting cast product into wrought product.
3. Hot working eliminates porosities and blow holes from casting's this happens by dissolution of trapped gases in hot metal along with the deformation process.
4. During hot working the segregation defect of casting is eliminated because of the diffusion process which takes place at this high temperature.

DEMERITS Of Hot Working:

1. Surface reactions take place at such a high temperature, thus hot working of reactive metals is a problem for ex. Titanium.
2. Dimensional tolerances for hot working workpiece are more than cold working workpiece.
3. Grain coarsening might take place at such high temperatures if precautionary measures are not employed.

TEMPERATURE OF WORKPIECE IN HOT WORKING:

In hot working temperature of workpiece is decided based on:

1. Initial temperature of workpiece and tool.
2. Friction forces that act between the tool and workpiece interface.
3. Heat generated due to plastic deformation.
4. Heat transfer between workpiece, tool, and environment.

Mathematical Analysis,

Average workpiece/material temperature at a particular time t,

Tm=Td + Tf + T.

where,

Td= temperature increase due to frictionless deformation process.

Tf= temperature increase due to friction at material tool interface.

T= average instantaneous temperature at the interface of deforming material.

and,

T_d = U_p/(ρCJ).

Where,

U_{p = the work of plastic deformation per unit volume.}

ρ = density of workpiece material.

C = specific heat of the workpiece.

J = Mechanical Equivalent of heat(4.186 J/cal).

T_f = (μpνAΔt)/(ρCJV).

Where,

μ = hriction coefficient at materia l tool interface.

p = stress normal to interface.

ν = velocity at the material tool interface.

A = surface area at the material tool inetrface.

Δt = time interval of consideration.

V= volume subjected to temperature rise.

(T₀ – T₁)/(T-T₁) = exp(ht/ρCJ).

Where,

h = heat transfer coefficient between the material and the dies.

δ = material thickness between dies.

T_{1 = die temperature.}

T_{0 = initial workpiece temperature.}

_{T = average instantaneous temperature of the deforming material.}

_{Do comment which concepts you want me to explain in details.}

_{Also I will be posting my next blog on Dynamic recovery and Dynamic recrystallisation.}

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