Title: Metal Heat Treatment
1Metal 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
5ANNEALING
- 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
6NORMALIZING
- 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
7GAS 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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9CASE 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
10STEEL 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
11CARBURIZING
- 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
13CARBONITRIDING
- 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
15EQUIPMENT
- 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
16EQUIPMENT
- 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
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20EQUIPMENT
- 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
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22EQUIPMENT
- 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
23EQUIPMENT
- 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
24EQUIPMENT
- 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
25ENDOTHERMIC 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
26ENDOTHERMIC 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
27ENDOTHERMIC 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.
28GENERATOR CONTROL
29MEASURING CARBON POTENTIAL
30SHIM 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.
31Shim 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.
33OXYGEN 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.
36ATMOSPHERE COMPOSITION
37ATMOSPHERE 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.
38PROBE 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.
39SPRAYED 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
42PROBE 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.
43LINE CONTACT ANODE PROBES
44BARBER-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.
46VIRTUAL POINT CONTACT ANODE
47OPEN vs. CLOSED SHEATH
Probe Burnoff Required?
Probe Burnoff Required
48TYPICAL PROBE INSTALLATION
Two to Four inch Insertion
49Probe 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.