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Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Metal Heat TreatmentMetal Heat Treatment
» Processes
» Equipment
» Controls
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
THE HEAT TREATING PROCESSTHE 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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
THE HEAT TREATING PROCESSTHE 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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
THE HEAT TREATING PROCESSTHE HEAT TREATING PROCESS
Requires three basic steps:
1 heating to a specific temperature
2 holding (soaking) at that temperature for the appropriate time
3 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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ANNEALINGANNEALING
heat material for several hours at elevated temperatures (20000F) (1093°C)
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) (677°C)
control cooling
removes stresses resulting from processes such as cold working, casting, forging, or welding
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
NORMALIZINGNORMALIZING
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) (871°C)
rapid quench to develop hardness
this is followed by TEMPERING…
reheat material to a low temperature (4000F) (204°C) to develop specific mechanical properties, to relieve quench stresses and to insure dimensional stability
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
GAS CARBURIZINGGAS CARBURIZING heat material at elevated temperatures (to 18500F) (1010°C)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) (843°C)for varying soak times
rapid quench to develop high case hardness and desired core properties
temper by re-heating to a low temperature (3500F) (177°C)to develop specific case and core properties, to relieve quench stresses and to insure dimensional stability
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
CASE DEPTHCASE 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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
STEEL GRADESSTEEL 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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
CARBURIZINGCARBURIZING
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 1700°F (927°C) and .9%C 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”
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Heat Treating SolutionsHeat Treating Solutions
Simple Boost and Diffuse
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
CARBONITRIDINGCARBONITRIDING Heat material at elevated temperatures (16500F) (899°C) 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) (843°C) for minimal soak time
rapid quench to develop high case hardness and desired core properties
temper by reheating to a low temperature (3000F) (149°C)to develop specific case and core properties, to relieve quench
stresses and to insure dimensional stability
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
HEAT TREATING SOLUTIONSHEAT TREATING SOLUTIONS
CarboNitriding
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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 100’s lbs/hr to ± 10000 lbs/hr
some continuous may have automated, self-contained quenching capability
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
EQUIPMENTEQUIPMENT
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ENDOTHERMIC GENERATORENDOTHERMIC 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 ± 1925°F (1052°C)
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
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ENDOTHERMIC ATMOSPHEREENDOTHERMIC 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 ± 1925°F (1052°C)
Air and Methane in a ratio of 2.75 to 1 is heated to 1900°F (1038°C) to 1950°F (1066°C) and flowed across nickel-impregnated cubes (catalyst) to produce Endothermic gas
Measured and controlled in dew point – typical set point is 40°F
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ENDOTHERMIC ATMOSPHEREENDOTHERMIC ATMOSPHERE
At 1700°F (927°C) with a 20% CO endothermic atmosphere
.40%C = 400F D.P.
.60%C = 280F
.80%C = 200F
1.00%C = 130F
At 15500F (843°C) with a 20% CO endothermic atmosphere
.30%C = 660F D.P.
.50%C = 500F D.P.
.70%C = 400F D.P.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
GENERATOR CONTROL GENERATOR CONTROL
Slide 104.Intro
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
MEASURING CARBON MEASURING CARBON POTENTIALPOTENTIAL
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
SHIM STOCK INSERTION ASSEMBLY
The Low carbon steel shim is cleaned, and weighed to four decimal places and then inserted into the furnace through the assembly.
When equilibrium is met the shim is pulled back into the assembly to cool.
Other devices such as O2 probes and infrared are inferential measurements, which do not measure carbon directly.
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.
Once inserted, the shim reaches equilibrium which is a function of time and temperature.
Shim stock carbon determination is the only true measurement of carbon potential in a heat-treating furnace.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Shim Stock - Verification
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
OXYGEN PROBE OUTPUT
At 1700 degrees the probe output is 1150 mV which represents 1.0% carbon in an atmosphere with 20% CO.
The lower the oxygen concentration the more responsive the probe becomes.
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.
The actual oxygen content in a typical carburizing atmosphere is 1 billionth of 1 billionth of one percent.
The actual carbon value is calculated by solving the carbon equation resident in the microprocessor.
Oxygen (carbon) sensors generate a (EMF) millivolt signal which corresponds to the partial pressure of oxygen in the furnace.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
OXYGEN PROBE OUTPUT
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
OXYGEN PROBE MILLIVOLT OUTPUT
The usable output range of the probe for an endothermic atmosphere is between 1000 and 1250 mV.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ATMOSPHERE COMPOSITION
As illustrated when the CO value decreases in the furnace the CO2 value will increase.
The components of conventional endothermic atmosphere are 40% hydrogen, 40% nitrogen, and 20% carbon monoxide.
This is also similar for the generator where the hydrogen decreases the water vapor (H20) will also increase.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
ATMOSPHERE COMPOSITION
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
Atmosphere at 40oF DP provides different %C at different
temperatures.
Example: 1500oF = .75%C
1600oF = .60%C
1700oF = .40%C
For carbon potential above saturation sooting will occur and non-controlled carburizing will take place resulting poor quality work.
ATMOSPHERE COMPOSITIONATMOSPHERE COMPOSITION
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
PROBE TECHNOLOGY SPRAYED AREA CONTACT ANODE
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.
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.
The sprayed on electrode was designed by Kent instruments in the 70’s and is still being used by some manufacturers today such as MMI and Kent.
This design is expensive to build and inconsistent in quality due to the complexity of manufacturing.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
SPRAYED AREA CONTACT ANODE
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
PROBE TECHNOLOGY SIRO AREA CONTACT ANODE
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.
Over time, cycling of temperature will cause cracks to develop in the cemented junction.
The Csiro probe (known as the siro sensor) is the most widely used probe design in the industry.
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.
When a crack occurs, furnace atmosphere will ingress into the cell and contaminate the reference air, causing inaccuracies in the output of the sensor.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
SIRO AREA CONTACT ANODE
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
PROBE TECHNOLOGY LINE CONTACT ANODE
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 manufacturer’s 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.
The line contact design utilizes slip cast Zirconia with a spring loaded contact to the outer sheath as the external electrode/conductor.
This particular design uses knife-edge contacts which will precipitate cracks in the Zirconia during thermal expansion and contraction typical in batch furnace applications.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
LINE CONTACT ANODE PROBES
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
BARBER-COLMAN AP11 CARBON PROBEBARBER-COLMAN AP11 CARBON PROBE
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
VIRTUAL POINT CONTACT ANODE
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.
This probe takes advantage of the reliability of the gas tight slip cast Zirconia tube design.
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
VIRTUAL POINT CONTACT ANODE
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
OPEN vs. CLOSED SHEATH
Probe Burnoff Required? Probe Burnoff Required
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
TYPICAL PROBE INSTALLATION
Two to Four inch Insertion
Eurotherm Controls Inc / Barber-Colman Company
An Invensys company
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.