eurotherm controls inc / barber-colman company an invensys company metal heat treatment » processes...

Eurotherm Controls Inc / Barber-Colman Company

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Metal Heat TreatmentMetal Heat Treatment

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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


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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


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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


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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


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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


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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


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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


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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


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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”


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Heat Treating SolutionsHeat Treating Solutions

Simple Boost and Diffuse


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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


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HEAT TREATING SOLUTIONSHEAT TREATING SOLUTIONS

CarboNitriding


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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


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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


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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


An Invensys 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


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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


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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


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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


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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


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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.


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GENERATOR CONTROL GENERATOR CONTROL

Intro


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MEASURING CARBON MEASURING CARBON POTENTIALPOTENTIAL


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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.


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Shim Stock - Verification


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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.


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OXYGEN PROBE OUTPUT


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OXYGEN PROBE MILLIVOLT OUTPUT

The usable output range of the probe for an endothermic atmosphere is between 1000 and 1250 mV.


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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.


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ATMOSPHERE COMPOSITION


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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


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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.


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SPRAYED AREA CONTACT ANODE


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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.


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SIRO AREA CONTACT ANODE


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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.


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LINE CONTACT ANODE PROBES


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BARBER-COLMAN AP11 CARBON PROBEBARBER-COLMAN AP11 CARBON PROBE


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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.


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VIRTUAL POINT CONTACT ANODE


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OPEN vs. CLOSED SHEATH

Probe Burnoff Required? Probe Burnoff Required


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TYPICAL PROBE INSTALLATION

Two to Four inch Insertion


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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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