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Metal Heat Treatment

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Title: Metal Heat Treatment


1
Metal Heat Treatment
  • Processes
  • Equipment
  • Controls

Retrofits
Service
iTools
Barber-Colman/Eurotherm Thermal Solutions
Sensors
Actuators
Multi-Loop Controllers
Single Loop Controllers
2
THE HEAT TREATING PROCESS
  • Heat treated components are essential to the
    operation of
  • automobile, aircraft, spacecraft, computers,
    heavy equipment of every kind, wood working
    tools, bearings, axles, fasteners, camshafts,
    cutting tools, gears, etc.
  • vast majority of material heat treated is iron
    steel
  • alloys of aluminum, copper, magnesium, nickel,
    and titanium may also be heat treated

3
THE HEAT TREATING
PROCESS
  • metal components are heated cooled under tight
    controls
  • improves properties, performance durability
  • can soften the metal - improve formability
  • can harden the metal - improve strength
  • can put a hard surface on relatively soft metal -
    improve abrasion (wear) resistance
  • can create a corrosion - resistant skin inhibits
    corrosion
  • can toughen brittle products

4
THE HEAT TREATING
PROCESS
  • Requires three basic steps
  • heating to a specific temperature
  • holding (soaking) at that temperature for the
    appropriate time
  • cooling according to a prescribed method
  • heating temperature range to 24000F (1316 C)
  • soaking times vary from a few seconds to 3 to 4
    days
  • cooling may be slowly in the furnace or quickly
    (quenched) into water, brine, oils, polymer
    solutions, molten salts, molten metals or gases
  • 90 of metal parts are quenched in water, oil,
    polymers, or gases

5
ANNEALING
  • heat material for several hours at elevated
    temperatures (20000F) (1093C)
  • control slow cooling
  • facilitates cold working machining, improves
    ductility and some electrical properties and
    promotes dimensional stability
  • STRESS-RELIEF
  • heat material for several hours at an
    intermediate temperature (12500F) (677C)
  • control cooling
  • removes stresses resulting from processes such as
    cold working, casting, forging, or welding

6
NORMALIZING
  • heat material at elevated temperatures for
    varying time soaks
  • control cool in air
  • results in homogeneous microstructure - provides
    better machinability more uniform properties
  • QUENCH HARDENING
  • primary heat treating process for strengthening
    steel components
  • heat material to an elevated temperature
    (16000F) (871C)
  • rapid quench to develop hardness
  • this is followed by TEMPERING
  • reheat material to a low temperature (4000F)
    (204C) to develop specific mechanical
    properties, to relieve quench stresses and to
    insure dimensional stability

7
GAS CARBURIZING
  • heat material at elevated temperatures (to
    18500F) (1010C)for varying soak times (dependent
    on case depth requirement)
  • while soaking, provide a high carbon atmosphere
    source from a hydrocarbon gas such as methane
    (natural gas) or propane
  • resulting chemical combination vs. the gaseous
    carbon and the iron, from the steel components
    provides an iron carbide compound on the surface
  • high carbon surface layer is called case
  • typical case depths vary from .010 to .150
  • furnace cool to stabilizing temperature (15500F)
    (843C)for varying soak times
  • rapid quench to develop high case hardness and
    desired core properties
  • temper by re-heating to a low temperature (3500F)
    (177C)to develop specific case and core
    properties, to relieve quench stresses and to
    insure dimensional stability

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9
CASE DEPTH
  • Case is accomplished by carburizing and quenching
  • The objective is to provide a hard wear resistant
    surface layer and maintain a softer core
  • Case depth is typically measured in thousandths
    of an inch (.037in).
  • Typical range specs are .018 to .025, .032 to
    .038, .038 to .045 etc.
  • Case depth is generally measured as the depth of
    penetration from the surface with a hardness
    above Rc 50 or carbon concentration above .40 C

10
STEEL GRADES
Grades of Steel Typically Heat Treated 1000,
1100, 1200, 1300, 1500 4000, 4100, 4600, 5100,
6100 8600, 9300 series 1040 .37 to .44
carbon content Standard carbon steel 8620
.18 to .23 carbon content Carburizing grade
alloy steel
11
CARBURIZING
  • Provides abundant supply of carbon for absorption
    into surface of steel
  • Provides increased carbon and hardness to surface
    of steel
  • Case depth is time and temperature dependent
  • At 1700F (927C) and .9C settings using a
    conventional 40 Nitrogen , 40
    Hydrogen, and 20 Carbon Monoxide in an
    Endothermic atmosphere
  • After Achieve Effective Case
  • 1 hour .62C .012
  • 4 hours .71C .026
  • 20 hours .86C
    .061

12
Heat Treating Solutions
Simple Boost and Diffuse
13
CARBONITRIDING
  • Heat material at elevated temperatures (16500F)
    (899C) for varying soak times (dependent on case
    depth requirement)
  • while soaking, provide a high carbon atmosphere
    source from a hydrocarbon gas such as methane
    (natural gas) or propane AND a high nitrogen
    containing gas (ammonia)
  • resulting chemical combination vs the gaseous
    carbon, nitrogen, and the iron, from the steel
    components provides an iron carbide/nitride
    compound on the surface
  • high carbon/nitrogen surface layer is called
    case
  • furnace cool to stabilizing temperature (15500F)
    (843C) for minimal soak time
  • rapid quench to develop high case hardness and
    desired core properties
  • temper by reheating to a low temperature (3000F)
    (149C)to develop specific case and core
    properties, to relieve quench stresses and to
    insure dimensional stability

14
HEAT TREATING
SOLUTIONS
CarboNitriding
15
EQUIPMENT
  • most components are heat treated in furnaces
  • most furnaces heated with natural gas fuel - some
    electric
  • two broad categories of furnaces
  • BATCH and CONTINUOUS
  • in batch, material is charged/discharged as a
    single unit or batch
  • in continuous, material is conveyed
    automatically, providing a constant flow

16
EQUIPMENT
  • BATCH
  • consists of insulated chamber with external steel
    shell
  • has a heating system for the processing chamber
  • has one or more doors providing access and
    sealing of the chamber
  • may have internal quench tank or slow cool
    vestibule
  • typically has automated controls

17
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19
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20
EQUIPMENT
  • CONTINUOUS
  • same basic components as Batch, but
  • has means of conveying material through the
    process zones
  • some types are PUSHERS, BELT or chain conveyors,
    ROLLER HEARTH, SHAKER HEARTH, ROTARY HEARTH,
    ROTARY RETORT and WALKING BEAM
  • has a multi-zone heating system for the
    processing chamber(s)
  • has doors or screens providing access and
    isolation
  • typically has multi-loop automated controls
  • size ranges from 100s lbs/hr to 10000 lbs/hr
  • some continuous may have automated,
    self-contained quenching capability

21
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22
EQUIPMENT
  • HEATING METHODS
  • Open-fired furnaces
  • fuel-gas air mixture combustion burns
    directly into heating chamber material may be
    protected from oxidation and/or products of
    combustion by passing through a MUFFLE or RETORT.
  • Radiant tube furnace
  • fuel-gas air mixture combustion circulates
    inside radiant tubes. Heat is radiated through
    the tube walls into the heating chamber. Radiant
    tubes are nickel alloy, ceramic or occasionally
    silicon carbide
  • Electric heating elements, 2 basic methods
  • exposed - open to heating chamber
    environment
  • indirect - isolated from chamber by
    insertion into radiant tube(s)
  • elements are typically Ni - Cr alloy in
    wire, rod, ribbon or sheetmetal form

23
EQUIPMENT
  • QUENCHING METHODS
  • most common quenching media are water, oil, or
    molten salt
  • fastest cooling rate results from spraying
    refrigerated water under pressure
  • quench tanks vary widely in capacity and design
  • many are equipped with variable agitation,
    heating and cooling, filtering and ventilation
  • up-to-date monitoring equipment may include
    computer controls and sensors to monitor
    temperature, chemistry, specific gravity and
    pressure

24
EQUIPMENT
  • NEW FURNACE HARDWARE TECHNOLOGY
  • driven by desire to cut costs, improve
    production, reduce environmental impact
  • computer modeling has promoted improved design
    and construction
  • recuperated combustion systems are more efficient
    and burner design has been optimized
  • insulating brick has been replaced with ceramic
    fiber linings
  • chamber shapes have been modified and improved
  • reliable in-situ sensors provide valuable feed
    back about temperature and atmosphere composition
  • state-of-the-art controls facilitate computer
    integrated management systems

25
ENDOTHERMIC GENERATOR
  • Endo generator creates the atmosphere that
    provides a positive pressure in a heat treating
    furnace, and a platform on which a carburizing
    environment can be formulated by the addition of
    enriching gas or dilution air. An endothermic
    generator consists of
  • Heating Chamber operating temp 1925F
    (1052C)
  • One or more retorts containing
  • Numerous small, porous, ceramic pieces,
    impregnated with nickel as a catalyst for the
    reaction.
  • A cooling heat exchanger to rapidly cool the
    products to a temperature that will not allow the
    reaction to proceed further

26
ENDOTHERMIC ATMOSPHERE
  • A carrier gas provides a positive pressure in a
    heat treating furnace and a mechanism to
    carburize or decarburize by adding oxygen or
    methane
  • 40 Nitrogen 40 Hydrogen 20 Carbon Monoxide
    (CO) and .1 Carbon Dioxide (CO2)
  • Nickel Catalyst 1925F (1052C)
  • Air and Methane in a ratio of 2.75 to 1 is heated
    to 1900F (1038C) to 1950F (1066C) and flowed
    across nickel-impregnated cubes (catalyst) to
    produce Endothermic gas
  • Measured and controlled in dew point typical
    set point is 40F

27
ENDOTHERMIC ATMOSPHERE
  • At 1700F (927C) with a 20 CO endothermic
    atmosphere
  • .40C 400F D.P.
  • .60C 280F
  • .80C 200F
  • 1.00C 130F
  • At 15500F (843C) with a 20 CO endothermic
    atmosphere
  • .30C 660F D.P.
  • .50C 500F D.P.
  • .70C 400F D.P.

28
GENERATOR CONTROL
29
MEASURING CARBON POTENTIAL
30
SHIM STOCK INSERTION ASSEMBLY
  • Shim stock carbon determination is the only true
    measurement of carbon potential in a
    heat-treating furnace.
  • Other devices such as O2 probes and infrared are
    inferential measurements, which do not measure
    carbon directly.
  • The Low carbon steel shim is cleaned, and weighed
    to four decimal places and then inserted into the
    furnace through the assembly.
  • Once inserted, the shim reaches equilibrium which
    is a function of time and temperature.
  • When equilibrium is met the shim is pulled back
    into the assembly to cool.
  • The shim is then removed and weighed to determine
    its carbon content.
  • This method of carbon measurement is not
    practical for control although it provides a true
    measurement of actual carbon which serves as
    verification of the instrument calculation.

31
Shim Stock - Verification
32
OXYGEN PROBE OUTPUT
  • Oxygen (carbon) sensors generate a (EMF)
    millivolt signal which corresponds to the partial
    pressure of oxygen in the furnace.
  • As the temperature increases the probe impedance
    will decrease and around 900oF the probe will
    begin to generate a mV signal which will
    correspond to the O2 partial pressure
    differential.
  • At 1700 degrees the probe output is 1150 mV which
    represents 1.0 carbon in an atmosphere with 20
    CO.
  • The actual carbon value is calculated by solving
    the carbon equation resident in the
    microprocessor.
  • The lower the oxygen concentration the more
    responsive the probe becomes.

The actual oxygen content in a typical
carburizing atmosphere is 1 billionth of 1
billionth of one percent.
33
OXYGEN PROBE OUTPUT
34
OXYGEN PROBE MILLIVOLT OUTPUT
  • The usable output range of the probe for an
    endothermic atmosphere is between 1000 and 1250
    mV.

35
ATMOSPHERE COMPOSITION
  • The components of conventional endothermic
    atmosphere are 40 hydrogen, 40 nitrogen, and
    20 carbon monoxide.
  • As illustrated when the CO value decreases in the
    furnace the CO2 value will increase.
  • This is also similar for the generator where the
    hydrogen decreases the water vapor (H20) will
    also increase.

36
ATMOSPHERE COMPOSITION
37
ATMOSPHERE COMPOSITION
Atmosphere at 40oF DP provides different C at
different temperatures. Example 1500oF
.75C 1600oF .60C 1700oF .40C For carbon
potential above saturation sooting will occur and
non-controlled carburizing will take place
resulting poor quality work.
38
PROBE TECHNOLOGY SPRAYED AREA CONTACT ANODE
  • The sprayed on electrode was designed by Kent
    instruments in the 70s and is still being used
    by some manufacturers today such as MMI and Kent.
  • As indicated the isolated external electrode
    (nickel-chrome alloy) is flame sprayed onto the
    tip of a solid slip cast Zirconia tube. The
    positive electrode is spring loaded internally to
    the Zirconia tube.
  • This design is expensive to build and
    inconsistent in quality due to the complexity of
    manufacturing.
  • The sprayed electrode over time will crack and
    then peel as a result of the thermal cycling and
    different coefficients of expansion of the
    materials in intimate contact.

39
SPRAYED AREA CONTACT ANODE
40
PROBE TECHNOLOGY SIRO AREA CONTACT ANODE
  • The Csiro probe (known as the siro sensor) is the
    most widely used probe design in the industry.
  • The government of Australia currently holds the
    now expired patent for the design of the ceramic
    substrate and is the supplier/licensor to several
    manufacturers.
  • This substrate is constructed with an alumina
    tube into which a Zirconia plug is pressed into
    one end and sealed, using a eutectic ceramic
    cement.
  • Over time, cycling of temperature will cause
    cracks to develop in the cemented junction.
  • When a crack occurs, furnace atmosphere will
    ingress into the cell and contaminate the
    reference air, causing inaccuracies in the output
    of the sensor.

41
SIRO AREA CONTACT ANODE
42
PROBE TECHNOLOGY LINE CONTACT ANODE
  • The line contact design utilizes slip cast
    Zirconia with a spring loaded contact to the
    outer sheath as the external electrode/conductor.
  • Although slightly more expensive to build, the
    gas tight slip cast Zirconia tube has been proven
    to facilitate the most reliable probe
    configuration on the market.
  • This particular design uses knife-edge contacts
    which will precipitate cracks in the Zirconia
    during thermal expansion and contraction typical
    in batch furnace applications.
  • This particular manufacturers design
    incorporates 10 large holes located near the tip
    of the electrode. Due to the location and the
    large number of holes, effective Probe
    conditioning (Burnoff) is virtually impossible.

43
LINE CONTACT ANODE PROBES
44
BARBER-COLMAN AP11 CARBON PROBE
45
VIRTUAL POINT CONTACT ANODE
  • This probe takes advantage of the reliability of
    the gas tight slip cast Zirconia tube design.
  • Virtual point contact reduces the potential of
    stress fractures and also provides a lower
    resistance at the outer electrode interface.
  • sheaths utilizes a cermet coating which minimizes
    the catalytic reaction between methane and nickel
    typically observed with alloy materials exposed
    to reducing furnace atmosphere. As a result less
    soot is deposited, degradation of the sheath is
    reduced, and probe life is extended.

46
VIRTUAL POINT CONTACT ANODE
47
OPEN vs. CLOSED SHEATH
Probe Burnoff Required?
Probe Burnoff Required
48
TYPICAL PROBE INSTALLATION
Two to Four inch Insertion
49
Probe Conditioning The purple and green pen
represent the millivolt probe output on different
scales. The red pen represents the probe
thermocouple. Summary During burnoff the probe
output should drop below 200 mV. As shown by
green pen.
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