Micromechanical model of quenching deformation

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Development of Austempering by Gas Quenching
Technology
T. Calvin Tszeng tszeng_at_iit.edu Smati
Chupatanakul chupsma_at_iit.edu
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Austempering
Austempering is a process in which the heated
steel is quenched from the austenitizing
temperature rapidly enough to avoid formation of
ferrite or pearlite. It is held at a certain
temperature until isothermal transformation from
austenite to bainite is complete and then cooled
to room temperature.
The parts need to be quenched fast enough so that
the transformation to pearlite can be avoided.
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Austenite Decomposition
Bainite has a good combination of ductility,
hardness and strength. Toughness is greater
than martensite
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Advantages of Austempering

• Less distortion and cracking than other competing
  technology.
• No need for final tempering (Less time consuming
  and more energy efficient).
• Improvement of toughness (impact resistance is
  higher than the conventional quench and
  tempering).

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Limitations of Austempering
Austempering can be applied to parts where the
transformation to pearlite can be avoided. This
means that the section must be cooled fast enough
to avoid the formation of pearlite. Thin
sections can be cooled faster than the bulky
sections. Most industrial applications of
austempering have been limited to sections less
than 0.5 in. thick. The thickness can be
increased by the use of alloy steels, but then
the time for completion of transformation to
bainite may become excessive.
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New Technique of Austempering
Austempering by gas quenching

• High pressure vacuum furnace furnaces combine
  heating
• and high pressure gas quenching at pressures up
• to 20 bar.
• Potential advantages
• Environmentally benign
• Uniform quenching
• Higher productivity

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Advantage of Gas Quenching Compared to Liquid
Quenchants

• Gas quenching proceeds more uniformly, minimizing
  residual stresses and distortion.
• Clean, dry components after hardening.
• Can be integrated into a mass production line.
• Spacing saving.
• More product quality, process control, safety and
  economic advantages.
• No washing machine.
• No management and disposal of wash liquor.
• There is little deformation.

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Research Issues and Challenges

• ? It is not clear if the cooling rate in gas
  quenching is high enough to avoid pearlite
  formation for a specified steel part.
• It is difficult to maintain a constant
  temperature to get the bainite transformation.
• It is important to reduce the cycle time due to
  the high cost of vacuum furnaces.
• The heat transfer characteristics is very much
  part dependent. There may not exist a universal
  heating and cooling scheme in vacuum furnace for
  austempering by gas quenching.

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Numerical Experiment
Cylinder of AISI 4140 Diameter (d) 2 in, Height
(h) 0.5 in
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Objective
To examine

• Critical Pressure
• Control of Austempering Temperature

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Approach

• Major computational work is carried out by
  FEM-based package HOTPOINT
• Identify the heat transfer coefficient to obtain
  the critical pressure for the variable diameter.
• Obtain cooling curves at the surface of the 4140.
• Perform inverse calculation and FEM analysis to
  find heat transfer coefficient
• Data analysis and reporting

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Isothermal Transformation Diagram
Bainite
H.E. Boyer and A.G. Gray, Atlas of Isothermal
Transformation and Cooling Transformation
Diagrams, ASM, Ohio, 1977
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Cooling Gas in Gas Quenching

• Hydrogen
• Helium
• Argon
• Nitrogen

He
G. E. Totten and M. A. M. Howes, Streel heat
treatment handbook, Marcel Dekker, New York,1997
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Critical Pressure
AISI 4140, He Gas Pearlite ? 0.5
d Diameter (in) h Height (in) 0.5 in
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Cooling curve AISI 4140, He 10 bar, h 0.69
kW/m2K Diameter 2 in, Height 0.5 in
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Parameter Controlling Cooling
Material Related

• - Heat capacity
• Heat conductivity

Part Related

• - Thickness of the parts
• Surface/volume ratio

Gas Related

• - Gas temperature
• Gas pressure
• Type of gas

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Control of Austempering Temperature by using
HOTPOINT
The assumed method
Quenched the part from 835?C to 370?C
(Austemering temperature)

• After quenched to austempering temperature
  (370?C), turn the gas off and keep the system to
  be close system. To observe how the cooling curve
  is.
• After quenched to austemering temperature
  (370?C), turn the gas off and still have heat
  exchange with environment (heat transfer via
  radiation). To observe how the cooling curve is.

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Control of Austempering Temperature
Cooling curve AISI 4140, He 10 bar, h 0.69
kW/m2K Turn off the gas at 370 C, The close
system.
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Control of Austempering Temperature
Cooling curve AISI 4140, He 10 bar, h 0.69
kW/m2K Turn off the gas at 370 C, Heat transfer
via radiation.
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• To find the heat transfer coefficient to maintain
  the temperature after quenched to austempering
  temperature (370?C) to be constant.
• AISI 4140 (dia. 2 in, h 0.5 in), He gas at 10
  bar.
• Using inverse calculations in Hotpoint.
• Assigned Cooling curve at surface is followed
  this below curve.

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Inverse Calculations Result
To show heat transfer coefficient time curve to
maintain the temperature after quenched to
austempering temperature (370?C) is constant.
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Inverse Calculations Result
To show heat transfer coefficient temperature
curve to maintain the temperature after quenched
to austempering temperature (370?C) is constant.
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Inverse Calculations Result
Actual error (?T) T1 T2 By T1
Calculation Temperature (C) T2 Measure
Temperature (C) The error is minimum.
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Pressure and time curve is come from heat
transfer coefficient and time curve.
P 2.256h2 14.394h 0.561
By P Pressure (bar), h Heat transfer
coefficient (kW/m2K)
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Work Plan

• Find an industrial partner for experimentation in
  vacuum furnace.
• Design experiments
• - Materials
• - Part
• - Instrumentation
• - Data analysis
• 3. Analysis of microstructure - Bainite