Low Press Carbg and High Press Qnchg

7/28/2019 Low Press Carbg and High Press Qnchg

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Furnace Atmospheres

No. 6Low pressure carburising and highpressure gas quenching.


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2 Low pressure carburising and high pressure gas quenching.

Page

Low pressure carburising and high pressure gas quenching (Intro) 4

1. Low pressure carburising 6

1.1. History 6

1.1. Dierentiation rom other carburising processes 6

1.2 Carbon transer coecient 8

1.3 State o the art today 8

1.4. Low pressure carburising principles 8

1.4.1 What is low pressure carburising? 8

1.4.2. Benets and drawbacks 10

1.4.3. Pressure range 10

1.4.4. Model o carbon transer 10

1.4.5. Reactions 11

1.5. Low pressure carbonitriding 11

1.5.1. Principles 11

1.5.2. Benets and drawbacks 12

1.6. Gases used or the LPC processes 12

1.6.1. Choice o gas or carburising 12

1.6.2. Purity o gases 13

1.7. Process parameters 13

1.7.1. Process design 13

1.7.2. Carbon mass fow 14

1.7.3. Gas fow rate 14

1.7.4. Gas type 14

1.7.5. Temperature 15

1.7.6. Pressure 161.7.6. Steel grade 16

1.8. Control o process parameters 16

1.8.1. Simulation o low pressure carburising processes 16

1.9. Hardware 17

1.9.1. Furnaces 17

1.9.2. Gas supply 18

1.10. Troubleshooting: common problems 18

1.10.1. Soot and tar problems 18

1.10.2. Near surace eect 18

2. Quenching characteristics or gas and oil 19

2.1. Quenching in oil 192.2. Quenching in gas 20

2.3. Temperature dierences when quenching in oil and gas 20

2.4 Cooling curves and heat transer coecient (HTC) 21

2.4.1. The gases 21

2.4.2. Mixture o gases 22

Content


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3Low pressure carburising and high pressure gas quenching.

Page

2.4.3. Gas recycle 232.4.4. The infuence o gas pressure and gas velocity 23

2.4.5. The eect o the gas temperature 24

2.5. Properties o the gases 24

2.6. Design o the load 26

2.6.1. Cylinder and axle 26

2.6.2. Disk and gear 27

2.6.2.1. Horizontal position 28

2.6.2.2. Vertical position 30

2.7. Distortion 30

2.7.1. Gears 31

2.7.2. Rings 31

2.7.3. Axles 32

2.8. Interrupted gas quenching 32

2.9. Equipment 33

2.9.1. Single-chamber vacuum urnaces 33

2.9.2. Two-chamber vacuum urnace 35

2.9.3. Multi-chamber vacuum urnaces 37

2.9.3.1. Continuous vacuum urnace 37

2.9.3.2. Linked multi-chamber urnaces 37

2.10. Control 39

2.10.1. Flux Sensor 39

2.10.2. QC3-Sensor 39

2.11. Hardness o dierent steel grades ater gas quenching 40

2.11.1. Hardnesses in the literature 40

2.11.2. Calculation o hardnesses 432.12. Environment 45

2.13. Quenching with high velocity gas 47

2.13.1. Principles 47

2.13.2. Heat transer coecient 49

2.13.3. Quenching a part o the component 51

2.13.4. Applications 52

3. Terminology 53

Reerences 54


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Low pressure carburising and high pressuregas quenching.

O the many technologies available today to improve the perormance o engi-

neered suraces, carburising is one o the most common. Carburising is enduringlypopular because it uses a higher temperature than most thermochemical proc-esses so that a deep hard layer can be ormed in a short time. The great majorityo carburising processes take place at atmospheric pressure [1] in an atmosphere

containing large quantities o carbon monoxide. The parts are subsequentlyquenched in oil. Recent developments in vacuum urnaces and steel technologyhave meant that carburising can now be carried out in a more environmentally

riendly way under low pressure [2-4].

Figure 1. Double chamber vacuum carburising urnace(photo courtesy o Seco/Warwick)


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The process is particularly applicable to the automotive industry

because o the cleanliness o the operating environment and o the

nished product. As hydrocarbons are used directly as the source o

carbon, the volume o the exhausted toxic and/or combustible process-

ing gases is vastly reduced and relatively benign. The urnaces are

generally more ecient than the atmospheric pressure alternative.

I the steel allows, processing eciency can be urther improved by

increasing the carburising temperature. The use o gas to replace oil

or quenching ensures that the components emerge very clean, bright

and dry, and less distor ted than oil-quenched components. Improving

technologies or making steel have produced carburising steels that can

be quenched in 10 bar nitrogen (N2) [5]. Even the current lower harden-

ability carburising steels can be quenched with improved helium (He)

quenching technology [6]. Vacuum carburising has also been applied to

parts made by powder metallurgy (P/M) techniques [7]. Analysts predictan upward trend in vacuum carburising rom 1% o the market in 2000

to 13% in 2010 [1, 8].

The single chamber vacuum urnace has largely been replaced by the

double chamber unit (Figure 1) or low pressure vacuum carburising.

This type o unit has the advantage that during quenching the urnace

itsel does not have to be cooled, thus lowering the thermal load.

In addition the quenching chamber design can be optimised or the

quenching task alone, rather than the combination o vacuum carburis-

ing and quenching, and there is less contamination o the quenching

gas. The latter is an impor tant consideration i the helium or quench-

ing is to be recycled. Two-chamber urnaces are well suited or smallerthroughputs and where a very high fexibility is necessary, e.g. at

commercial heat treaters.

The next step is a modular vacuum carburising system such as that

shown in Figure 2. The automotive industry requires a higher through-

put, and two-chamber solutions are ar too expensive. In this case the

combination o several individual treatment chambers with one quench-

ing chamber is very cost eective. Depending on the case depth, a

combination o six to eight treatment chambers with one quenching

chamber is the optimum. Small, individually controlled carbur ising units

give maximum processing fexibility. The system is ully automated with

a heat-treated load leaving it every 20 30 minutes.

Figure 2. A modular vacuum carburising system at BMW (photo courtesy o ALD)


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1.1. History

The possibility o low pressure carburising in vacuum urnaces was

mooted more than 40 years ago [9]. Intensive investigations started

in the 70s, caused by an increase in costs o natural gas and the hope

o dramatic industrial gases savings by this method. At that time low

pressure carburising was unacceptable because o the heavy sooting

produced in the pressure ranges used, between 100 and 600 mbar, and

the diculty o achieving sucient evenness in the carburising results

[10 14].

Reducing the process pressure to values between 3 and 20 mbar so lved

these problems and allowed a denser loading o urnaces [15, 16].

Besides temperature and time, atmosphere composition, number and

length o boost and diusion steps, procedures to homogenise theatmosphere and the infow are o high importance or consistent load

results [17]. In addition to propane, acetylene and ethylene were intro-

duced as carburising gases in low pressure carburising processes

[18, 19].

Plasma carburising can be seen as a variant o low pressure carburising.

The rst hardness proles generated by plasma carburising had been

presented by Edenhoer at Hrterei-Kolloquium, Wiesbaden, 1972 [20].

Methane is the main process gas but the use o propane or mixtures o

these gases with hydrogen or argon is also reported [21 26].

1.1. Dierentiation rom other carburisingprocesses

Gas, plasma and low pressure carburising are compared in Table 1.

1. Low pressure carburising


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Gas carburising Plasma carburising Low pressure carburising

Furnace atmosphere vacuum vacuum

Gases gas mixtures methane propane, acetylene, ethylene,

containing CO, CO2, and other hydrocarbons

CH4, H2 and N2 and their mixture [27]

Gas consumption 3 5 times urnace volume 100 l/(h*m2 load) 100 l/(h*m2 load) *1

(temperature dependent) per hour

Max. temperature 1000C 1300C 1300C

Furnace conditioning necessary not necessary not necessary

Integration in production line? no, unshielded fames, gases yes yes

Process management by gas composition, carburising time length o carburising segments, length o carburising and diusion

pulses segments, and gas fow rate

Process control by carbon potential none No atmosphere CP and its controlSurace eects internal oxidation thermal etching thermal etching

Coverage to hinder carburising cover paste mechanically *2

Carbon transer see table 2

Drillings, blind holes limited up to L/D = 25 up to L/D = 30

*1 Carburising o complex parts requires higher gas fows. Gren [28] states a required gas fow o 400 l/(h*m2) at 2 mbar and 900C to carburise blind holes.*2 The use o average cover pastes in low pressure carburising processes bear the danger that constituents o the pastes are released in the low pressure and condense in hard

layers, which have to be removed ater heat treatment by shot peening.

Table 1: Comparison o the three dierent carburi sing methods


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1.2 Carbon transer coecientThe carbon transer coecient gives a measure o the velocity o carbu-

rising. It is dened as the mass o carbon di used into a square metre

surace o the carburised par t per hour. It can be calculated rom the

weight dierence o small samples or, more accurately, integrated out

o carbon depth proles.

Table 2 shows values or average carbon transer coecients in d ier-

ent atmospheres and or dierent temperatures in comparison to gas

and plasma carburising.

A high carbon transer coecient is not necessarily an advantage,

because steel suraces can absorb only limited amounts o carbon. With

high carbon mass transers the sur ace o components is saturated very

rapidly. Oversaturation leads to carbide ormation, which should beavoided whenever possible. The only way to prevent carbide orming in

processes with high carbon transer coecients is to shorten the carbu-

rising segments. Segments shorter than one minute, however, increase

the inaccuracy o the carburising results.

1.3 State o the art today

Recent work indicates that carbide ormation and dissolution in car-

burising and diusion segments give rise to thermal etching eects.

Thereore low pressure carburising processes are simulated with pro-

grams using carbon transer coecients that depend on time, tempera-

ture and carbon content to obtain optimal times or the carburising anddiusion segments. With this approach, unusually good results were

achieved on low pressure carburised 18CrNiMo7-6 gears, which reached

the endurance limits o shot peened gears in the unpeened state.

1.4. Low pressure carburising principles

1.4.1 What is low pressure carburising?

The process itsel starts with evacuating the urnace, which is then

lled with nitrogen and heated rom 800C by convection to process

temperature. The heating is completed under vacuum. When the proc-

ess temperature is reached, carburising is carried out in a series o

boost and diusion segments.

Figure 3 shows a schematic illustration o a low pressure carburising

process. The carbon donator, e.g. propane or acetylene, adsorbs at the

surace and dissociates catalytically.

carbondonator(gas)

physicaladsorbtion

chemiesorbtion

recombinationof hydrogen

dissolution of carbon

diffusion

carbondonator(adsorbed)

Figure 3. Process scheme o low pressure carbur ising

Table 2: Values o average carbon transer coecient in g/m2 h in the rst 15 minutes or dierent hydrocarbons and temperatures [29] (values or carburising until thesurace is saturated shown in brackets [30])

Temperature Atmosphere Plasma Low Pressure Carburising

Endogas

Methane Propane Methane Propane Ethylene Acetylene900C 25 (25) 100 0 60 (60) 55 55 (50)

950C 35 (70) 150 0 90 (90) 80 79 (110)

1000C 80 (110) 190 0 130 (140) 120 145 (235)

1050C (155) 20 185 (195) 180 200 (335)


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I the carbide ormation limit o the steel is exceeded, carbides can orm

in the outer surace o the components. The maximum length o carbu-

rising (boost) steps is thereore given by the carbide ormation limit o

the steel at process temperature (Figure 4).

The diusion step is carr ied out until the surace carbon content has

been lowered enough to attach another carburising segment o reason-

able duration (


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1.4.2. Benets and drawbacks

The ollowing advantages are oten claimed or low pressure carburis-

ing processes:

1. No internal oxidation

2. Ability to carburise drillings and blind holes

3. The possibility o using high temperatures

4. No urnace conditioning required

5. A reduction in energy and gas consumption [32]

6. Clean and sae working conditions

7. The use o only oxygen-ree gases like propane, hydrogen, argon,

and nitrogen eliminates internal oxidation.

Advantages 3 to 6 are typical eatures o thermochemical treatments in

vacuum urnaces. Furnace conditioning is known to be useul even invacuum heat treatments i there are requent changes in load require-

ments.

A drawback compared to other carburising methods is that problems

can occur with masking parts o components. Compared to gas carbu-

rising, which is controllable by measurement and adjustment o the

Carbon potential, the LPC atmosphere Carbon potential cannot be

controlled.

1.4.3. Pressure range

Low pressure carburising is nowadays carried out in the pressure rangerom 1 to 20 mbar.

1.4.4. Model o carbon transer

The ollowing simplied model describes the transer o carbon at the

steel surace (s) with acetylene as the carburising gas, divided into our

steps [33].

Step 1: Transportation o molecules o acetylene towards the

specimen. Physical adsorption on the specimens surace:

C2H2(gas) 73 C2H2(ad) (1)

Step 2: Chemisorbtion o the atoms o carbon and hydrogen during

dissociation o acetylene on the hot surace o the specimen:

C2H2(ad) + 4s 3 2(C-s)+2(H-s) (2)

Chemisorbed atoms o hydrogen orm molecules o hydrogen which are

released as gas:

2(H-s) 73 H2(gas) + 2s (3)

Step 3: Transer o chemisorbed carbon atoms into a dissolved state:

(C-s) 73 C(diss.) + s (4)

Step 4: Diusion o carbon into the metal lattice


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1.4.5. Reactions

The reactions taking place in low pressure carburising processes are

dependent on the process gas used. The ollowing scheme shows highly

simplied reactions that occur with dierent gases.

The methane produced during pyrolysis is useless or carburising

because o its thermal stability (see above). Furthermore many dier-

ent species o gas exist in parallel. However acetylene is the only spe-

cies applicable to the carburising o steel at low pressure. The amount

o propane that must be applied to provide a homogeneous carburis-

ing depends on a sucient amount o acetylene being produced during

pyrolysis. In consequence the amount o propane has to be signicantly

higher than the amount o acetylene.

1.5. Low pressure carbonitriding

1.5.1. Principles

Unlike carbonitriding processes in gas atmospheres, low pressure car-

bonitriding is carr ied out as an integrated process o low pressure car-burising and nitriding at low pressure. There are two dierent approach-

es to obtaining sucient nitrogen in the sur ace layer. Introducing pure

nitrogen, which will dissociate at temperatures above 1000 C can do

the nitriding. In the majority o cases, however, ammonia is added dur-

ing the last diusion segment.

It is not useul to add ammonia in earlier diusion segments, since

nitrogen will immediately euse rom the surace in low pressure seg-

ments. This ollows Sieverts law, which describes the decreasing solu-

bility o nitrogen in steels as the pressure alls. For that reason the pres-

sure is typically increased up to at least 10 mbar during the adjacent

nitriding segment to provide sucient solubility o nitrogen in steel.

C3H8

CH4

C2H4

C2H6

C 3H6

C2H2

-CH4

-H2

-C2H4

-H2

-C 2H2

-CH4

2x-H2

-H2

+C 2H4/-C3H6

x2

C 6H6 carbonblack

x3 -3 H2

Figure 6. Network o homogeneous propane pyrolysis

methane

ethane

propane

ethene

acetylene

pyrolysis at 4 mbar above 1000 C

above 600 C (mainly)

under 10 mbar above 700 C (mainly)

under 10 mbar above approx. 850 C

above 800 C dissociationbelow 800 C polymerisation

(up to 940 C mainly)

(vinyl acetylene)

4C + 2Hy (at steel surface)

complex reaction

stability increases with T, C-availabity higher at lower T

The pyrolysis o hydrocarbons is always temperaturedependent. The

equations above show that ethane and propane decompose mainly to

orm methane, which will not decompose at 4 mbar at temperatures

below 1000C. At temperatures below 800C acetylene orms long-

chained hydrocarbons and aromatics. Above 800C it dissociates in a

complex reaction, whose last stage needs a s teel surace as a catalyst.

The dissociation o propane is quite complex because a variety o di-

erent components orms during the cracking processes o pyrolysis.

The existence o so many hydrocarbons and radicals leads to a complex

chain o cracking and recombination, described by Gra et al. [34] and

shown in Figure 6.


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1.5.2. Benets and drawbacks

The most signicant benet o a higher nitrogen concentration in steels

is that the austenite exists at lower temperatures, which leads to a bet-

ter hardenability as well as stabilised retained austenite [35]. Nitrogen-

martensite, with signicantly higher amounts o nitrogen, may also be

ormed.

Carbonitrided steel with an accumulated amount o more than 1% car-

bon and nitrogen can provide signicantly higher amounts o stable

retained austenite. This can improve the mechanical properties, espe-

cially wear resistance.

Two drawbacks are the high ammonia consumption o the process and

the technical requirements o the vacuum urnace. Ammonia dissoci-ates very rapidly at temperatures above 850C. Only small amounts o

ammonia are let to dissociate on the load. To provide a homogeneous

carbonitriding process, the quantity o added ammonia should be about

5 10 times greater than the amount o carburising gas [35].

The requirements or urnaces used or low pressure carbonitriding

are related to the special conditions or using ammonia. The gas sup-

ply system and the exhaust system must be ree o non-errous metals.

Vacuum pumps without non-errous metals are not available at present.

The use o ammonia exposes these components to a signicantly higher

risk o corrosion, which may shorten their lives.

1.6. Gases used or the LPC processesLow pressure carburising is carried out in dierent atmospheres:

x In hydrocarbons

x In a mixture o hydrocarbons and noble gases or hydrogen.

For most hydrocarbons a precondition or a low pressure carburising

process is the decomposition o the hydrocarbons at high temperatures.

This is called cracking or pyrolysis. The decomposition reactions depend

on gas type and temperature. A detailed description o possible reac-

tions is given in the doctoral thesis o Gren [28]. He gives an exten-

sive compilation o relevant data or low pressure carburising.

1.6.1. Choice o gas or carburising

Low pressure carburising can be carried out with dierent gases de-

pending on the application, the shape o the parts and the temperature.

Methane is stable up to high temperatures. Measurements by Dorn

using a mass spectrograph at IWT conrmed these results (Figure7).

Methane is generally used in plasma-assisted carburising processes

where local protection rom carburising is needed, as this can easily be

done by masking.

Propane can be used over the whole temperature range, rom low tem-

perature carburising (300 400C, used or carburising o austeniticsteels without loss o corrosion resistance [36]), to high temperature

carburising (


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propane, parameters such as the density o loads and the ratio length/

diameter o blind holes can be increased. At higher temperatures there

is a disadvantage: the dissociation reactions change, making less car-

bon available. At lower temperatures (< 800C [19]) polymerisation

reactions take place.

Acetylene is used at a low pressure level o 2.5 mbar to suppress the

ormation o soot and short-chain hydrocarbons. Otherwise, acetylene

makes a high proportion o carbon available. This is the reason or its

good perormance in treating dense loads or complex shapes.

Propane remains popular but most new systems use acetylene.

1.6.2. Purity o gases

IWT, Bremen has collaborated with AGA AB (member o the Linde

Group) to investigate the infuence o the purity o the gases. Gases o

standard quality were compared with a gas mixture containing signi-

cant amounts o higher-chain hydrocarbons. To nd out how much soot

builds up in the urnace during the carburising processes, a ceramic

plate was xed on the most sensitive ceramic component in the ur-

nace during each process. The Institutet r Metalorskning, Stockholm,

measured the carbon uptake o these ceramic plates using a combus-

tion method. Figure 8 shows that there is hardly any di erence in the

carbon pick-up with the dierent gas qualities. The carbon content o

the ceramic plates increases slightly as the gas quality decreases. In

comparison it could be seen that, while the carbon pick-up increasesby only about a third, the carbon content o the ceramic plates is about

our times as high as in the results o plasma carburising processes. Fig-

ure 9 shows that, while the carbon pick-up decreases only slightly when

an atmosphere containing hydrogen is used, the carbon content o the

ceramic plates almost halves.

1.7. Process parameters

1.7.1. Process design

Figure 10 shows the process design schematically with the actors on

which it depends. Processes have hitherto been designed in a t rial and

error ashion through a reiterative cycle o simulation and result com-

parison.

The infuence and eects o the dierent input parameters are

described in the ollowing sections.

Figure 8. The infuence o gas quality on the average carbon pick-up and carboncontent o the ceramic plates during low pressure carburising

0

20

40

60

80

averageCpick-up[gC/m*h]

0

1

2

3

4

5

C-contentofceramicplate[gC/m]

N2 H2carrier gas

Calculated out of weight difference

Integrated from C-Curve

carbon content of ceramic plates

Low pressure carburizing

Figure 9. The infuence o gas composition (carrier gas) on the average carbonpick-up and carbon content o the ceramic plates during low pressurecarburising

Low pressure carburizing process

pressure

atmosphere

gas flow rate

batch (surface)

gas type

carbon transfer

carbon profile

temperaturesteel grade

simulation

carburizing- and diffusion segments

Figure 10. Schematic o the low pressure carburising process

0

20

40

60

80

averageCpick-up[gC/m*h]

0

1

2

3

4

5

C-contentofceramicplate[gC/m]

3.5 2.5 mixtureGas quality

Calculated out of weight difference

Integrated from C-Curve

carbon content of ceramic plates

Low pressure carburizing


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1.7.2. Carbon mass fow

There are two sets o parameters in operation: the parameters that

infuence the transer o carbon rom the atmosphere to the parts and

those that infuence its subsequent diusion into the part. The combina-

tion results in a carbon mass fow.

1.7.3. Gas fow rate

Figure 11 shows the eect o the process gas fow rate on the average

mass transer. The mass fow o carburising gas cannot be used directly

to control the surace carbon content or carburising depth, since under-

dosing causes a carbon fow under-supply in the load. In consequence

components that interact with the gas supply rst, because o their

position in the load, will be carburised correctly but units urther downthe line will not received enough carbon. In consequence the load will

not have homogeneous case depth and surace carbon content.

On the other hand the aim cannot be to supply as much gas as possible,

because this could result in soot ormation in the atmosphere.

1.7.4. Gas type

Investigations o the infuence o the type o process gas used on the

carbon mass fow were done by Gren [28]. During these investigations

carburising was carried out in a single process with no extra diusion

step, to allow the carburising behaviour to be characterised. No signi-cant dierence was ound between the types o gas investigated, apart

rom methane, which does not dissociate thermally at temperatures

below 1100C. Investigations by Steinbacher [38],however, did show a

dierence between acetylene and propane during carburising. Since

acetylene carburises samples directly while propane needs to decom-

pose by pyrolysis, carburising with acetylene is aster at the beginning

o the process (Figure 12). The acetylene generated by the pyrolysis o

propane is the only source o carbon. This aects how much carbon is

available locally and can cause inhomogeneous sur ace carbon con-

tents in the load.

C-quantityg/m2h

70

60

50

40

30

20

10

0

15 l/h

500 l/h300 l/h200 l/h100 l/h50 l/h

25 l/h

C2H2t = 10 min

p = 2 mbarT = 900 C16 MNCr 5

flowrate

Figure 11. The dependence o carbon mass fow on process gas fow rate [28]

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

time [min]

specificmassgain[g/m

2]

acetylenepropane

Figure 12. Specic mass gain o a 20MnCr5 sample carburised with identical atmos-phere parameters in propane and acetylene


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1.7.5. Temperature

The process temperature during carburising aects the di usion and

solubility o carbon in steels. As Figure13 shows, an increase in tem-

perature reduces the process time, because the carbon diuses at a

higher rate.

In addition to the positive eect on the diusion velocity, carbon also

becomes more soluble in the s teel, as shown in Figure 14.

In consequence more carburising gas must be supplied to provide

enough carbon or the higher carbon transer (Figure 15).

0

5

10

15

20

25

30

900 950 1000 1050

Carburizing temperature C

Carburizingtime

h

0,5 mm

1.0 mm

1.5 mm

2.0 mm

3.0 mm 4.0 mm 18CrNiMo7-6Heating 3 C/minLowering to 840 CHold 840 C 30 min.CAt = 0.28 %

CR = 0.66 %

-35 %

Figure 13. Process time reduction at rising temperature

20MOCr421NiCrMo2

15CrNi616MnCr5

Ck1517CrNiMo6

10Cr4

1000

950

900

850

800

750

C

0.20 0.40 0.60 0.80 1.00 1.20 1.40 %

Mass % Carbon

Temperature

Precipitationof Cementit

G

S

S

EE

Figure 14. Maximum carbon dissolution in austenite depending on temperature

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

time [min]

specificmassgain[g/m2]

Steel grade (DIN-EN):Gas:Flow rate:

Pressure:

20MnCr5propane10 ml/min940 l/m2h5 mbar

Figure 15. Dependence o specic mass increase on carburising temperature


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1.7.6. Pressure

The eect o gas pressure on the results o carburising parts o simple

geometry is negligible. Generally or low pressure carburising a more

complex geometry is linked to areas less accessible to the gas stream,

like bores or blind holes and small gearwheels. Providing these areas

with carbon demands a lower process pressure and a dynamic gas

exchange. This is best provided by short boost and vacuum segment

series. The continuous change o pressure makes it possible or the

achievable process.

Furthermore, as mentioned beore, pressure infuences soot ormation

and polymerisation o reaction products.

1.7.6. Steel grade

The grade o steel has three dierent aspects:

x maximum carbon dissolution

x carbide ormation

x diusion velocity

The maximum amount o dissolved carbon changes with the quantity

o substitution alloy elements (see Figure 14). It is generally considered

that the more alloying elements present, the less carbon can be dis-

solved. In consequence the surace will become saturated and carbides

may be ormed earlier.Other issues related to the alloy content are the amount o carbides to

be precipitated and the type o carbide ormed. Elements like chromi-

um, molybdenum, vanadium and tungsten, which are added to accel-

erate precipitation, will orm carbides which can only be dissolved at

higher temperatures.

Some alloying elements have a strong infuence on the diusion o

interstitial elements like carbon. Most o the alloying elements that

positively infuence carbide ormation also lower the diusion veloc-

ity o interstitial elements. Figure 16 shows an example or chromium

alloyed steels. In consequence most alloying elements reduce the case

depth and accelerate carbide precipitation. To avoid the precipitation o

carbides, short boost segments should be used.

1.8. Control o process parameters

1.8.1. Simulation o low pressure carburisingprocesses

Simulation o gas carburising is well developed, but simulation o low

pressure carburising is more dicult. On one hand, process design uses

experimental data, e.g. or serial processes. On the other, institutes and

urnace constructors use programs they have developed themselves.

The problem with simulation is that there are ew data on carbon t rans-

er during the boost steps. How temperature, gas type, steel grade, sur-

ace carbon content and conditions o carbide ormation and dissolution

all depend on carbon fow are not yet precisely known. These programs

are usually run with experimentally tted data, which contain uncer-

tainties. Todays data provided or simulation are typically obtained rom

the results o carburising experiments rom which the carbon mass fow

is integrated. This carbon mass fow is highly dependent on the lengtho the previous boost step. I there is a signicant dierence between

the timing o the segments, the results may deviate rom reality.

A more convenient way to obtain the eect o the many dierent infu-

ences on mass fow data is to use thermogravimetric measurement. This

method acquires the carbon mass fow by measuring a samples weight

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

time [min]


diffusiondiffusion

C-saturation of surfaceC-saturation of surface

Diffusion and

carbid formation

Diffusion and

carbid formation


Pressure:

20MnCr5propane10 ml/min

940 l/m2h5 mbar

Temperature: 1050 Cmo

Figure 17. Carburi sing o a 20MnCr5 sample: 1050C, 940 l/(m2h), propane, 5 mbar[38]

0

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8

time [min]

specificmassgain[g/m

2]

30Cr6 15Cr12 30Cr12

Figure 16. The dependence o mass gain on the grade o steel


17/56


change during carburising. Most signicant infuences like temperature,

steel grade, gas type and their interrelation can be veried this way.

An example o one such measurement is given in Figure 17, which

shows the specic weight change o a sample o 20MnCr5 dur ing carbu-

rising in propane at about 1050C or eight minutes. Using a tted expo-

nential saturation unction, as shown in Figure 18, and dierentiating

it will give a carbon mass fow unction, which can be used to describe

the rst boost step o a sample in a diusion simulation.

A matter o special importance is the interrelation between the accu-

mulated carbon prole rom the ormer boost steps and the newly

arranged carbon mass fow unction as shown in Figure19. In act there

are two infuences that have to be considered:

x Higher surace carbon content and thereore earlier saturation o

austenite

x Change o diusion speed because o the accumulated prole and

reduced gradients.

Theoretically many experiments would be needed to obtain the exact

relationships. A simpler but quite eective approach is to obtain the

carbon mass fow unction o the rst carburising step and use it as a

basis or the subsequent ones. To account or the aster saturation and

slower diusion, a coecient can be derived, which will result in a

lower mass fow using a time shit that depends on the surace carbon

content.

1.9. Hardware

1.9.1. Furnaces

Conventional vacuum urnaces are used or low pressure carburising.

Furnace construction used to be based on a heating and carburising

chamber made o steel, which is not the ideal choice because o the

large catalytic area o the whole retort during carbur ising. These steel

retort urnaces will need a signicant higher gas supply than a urnace

with an inactive retort made o ceramics or graphite.

Most recent urnaces are equipped with a graphite chamber and graph-

ite heating, which will not interere with the carburising process by cat-

alysing a reaction o the process gas. The only drawback o a graphite

retort is that the oxygen partial pressure has to be kept low.

Todays standard vacuum urnaces can be subdivided into two major

groups:

x Single chamber urnaces

x Multi-chamber urnaces

In a single chamber urnace, carburising and quenching is done in the

same chamber. These urnaces are very oten combined with high pres-

sure gas quenching technology to quench the load. The drawback o

the combined carburising and quenching chamber is that the chamber

has to be quenched as well as the load. The result is a lower coolingrate compared with a two-chamber or multi-chamber system. The multi-

chamber system is able to combine the fexibility o using dierent,

even liquid, quenching media with a signicantly higher heat transer

rate, because the quenching chamber is cold (see section 2.9).

0

3

6

9

12

15

0 1 2 3 4 5 6 7 8

time [min]


fitted function

measured weight gain


Pressure:

16MnCr5acetylene4 ml/min

380 l/m2h5 mbar

Temperature: 940 C

Figure 18. Fitted exponential saturation unction compared with measurement:16MnCr5, 940C, 380 l/(m2h), acetylene, 5 mbar [38]

0

200

400

600

800

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30 35 40 45 50 55 60 65time [min]

0

Surfacecarboncontent[weight%]


Pressure:

20MnCr5acetylene4 ml/min

380 l/m2h5 mbar

Temperature: 1050 C

massflow

[g/m2]

Figure 19. Carbon mass fow and surace carbon distribut ion (carbon in solution)during a simulated multi-step carburising process: 20MnCr5, 1050C,acetylene [38]


18/56


1.9.2. Gas supply

The gas supply equipment o a urnace used or low pressure carburis-

ing should contain no non-errous metal i acetylene is to be used or

carburising. A multi-stream supply system with multiple inlets can be

used to assist uniormity o carburising. I a carbonitriding process is

required, the exhaust system should also be ree rom any non-errous

metal.

1.10. Troubleshooting: common problems

1.10.1. Soot and tar problems

Soot orms i there is too much carbon present in the carbur ising atmos-phere. It can be observed that thermally dissociated ree carbon is

ormed i the gas fow rate is too high respective to the total surace

o the load. In the 1970s, when relatively high process pressures were

used, soot sensors (photo sensors) were used to detect soot by measur-

ing the adsorption o light by the soot og [12]. The process gas fow rate

was controlled as a means o controlling the rate o sooting.

Soot can be minimised by adapting the fow rate to the total surace o

the load and also by limiting the pressure. Tar (long chained hydrocar-

bons) can also orm by complex polymerisation processes, which have

not been suciently investigated up to now. Hydrocarbons and soot are

oten ound in cold parts o plants. Processes that run continuously arereported to produce especially high amounts o tar and soot.

1.10.2. Near surace eects

Internal oxidation is a well known damaging eect in gas carbur ising

processes. It reduces the amount o dissolved alloying elements and

thereore the hardenability in the outer region. The oxides ormed

especially on grain boundaries reduce the atigue strength through

notch eects.

Components carburised at low pressure show no signs o internal oxida-

tion, but other eects are reported, including:

x Eusion o elements, especially manganese

x Formation o carbides on grain boundaries, i the carburising

parameters are less than optimal

x Etching at austenite grain boundaries during the carbur ising step[39], which can be considered as notches.

Recent research work indicates that element eusion and carbide or-

mation and dissolution during the low pressure carburising process

ampliy thermal etch eects. Optimised simulation o the carburising

process led to a marked increase in atigue resistance [40, 41].


19/56


2. Quenching characteristics or gas and oil

Heat treatment operation or hardening steels consists o heating and

quenching processes. Although both are important or the resulting

characteristics, the quenching sequence is generally the more critical. It

must be carried out at a controlled cooling rate in order to achieve the

correct hardness and structure. A cooling rate that is too high generally

results in higher strength and hardness. At the same time a quenching

process with a high cooling rate is associated with greater distortion,

and maybe even cracking.

The rate o cooling must thereore be suited to the type o material and

to the shape and dimensions o the component. Oil is today the most

common quenchant, especially or carburised components, but the use

o high pressure gas quenching, HPGQ, is increasing.

2.1. Quenching in oil

The quenching process [42] in oil can be divided into three phases:

x Vapour phase

x Boiling phase

x Convection phase

Figure 20 shows a schematic representation o the three phases during

quenching. The heat transer in each phase ollows a di erent mecha-

nism and each phase plays its part in the nal result.

In the vapour phase, the surace temperature o the component being

hardened is so high that the liquid quenching media, oil, is rapidly

vaporised and a thin, thermally insulating lm o oil vapour orms over

the entire part. This vapour phase must not last too long i undesirablephase changes to errite and pearlite in the steel are to be avoided. The

actual length o time varies considerably, depending on actors such as

the oil itsel and its latent heat o evaporation, the surace condition o

the component and the packing pattern o the load.

1000

900

800

700

600

500

400

300

200

100

0

1007550250

2015105 5

Cooling rate [C/S]

Time [s]

Temeperature[C]

0

Vapourblanket

Boiling

Convection

Cooling rate

Figure 20. Typical cooling curve or oil quenchants


20/56


During the boiling phase, the oil comes into direct contact with the hot

surace and is immediately boiled away, increasing the rate o heat

removal. In most cases, or ecient cooling o any particular grade o

steel, the rate o cooling must be highest over the same temperature

range as the errite and pearlite nose or the material in the time tem-

perature transormation (TTT) diagram.

The convection phase begins when the surace temperature o the com-

ponent alls below the boiling point o the oil. During this phase, the

fow velocity and the temperature o the oil primarily determine the rate

o cooling. A low boiling point results in a high cooling capacity.

2.2. Quenching in gas

The quenching process in gas consists o only one phase [43], the con-vection phase, which starts at the beginning o the quenching process

(see Figure 21). During this phase, the fow velocity, pressure and tem-

perature o the gas primarily determine the rate o cooling. In theory

heat is also removed through radiation to the surrounding walls and

between the components, but in practice this has very little eect com-

pared with the convection, so it is usually ignored.

Figure 21 shows the dierences between the two quenching media. For

most gases in common use under typical conditions the maximum heat

transer coecient or the oil is two or three times higher than or gas.

It can also be seen that the nal temperature or gas quenching is lower

than that or oil. That is quite natural as the gas temperature at the endo the quenching is about 30 50C while the oil temperature might be

80 120C.

2.3. Temperature dierences when quenching

in oil and gasWhen a component such as a cylinder [44] is quenched in oil, it is rst

surrounded by the vapour phase. The vapour phase collapses ater a

short time, and is replaced by the boiling phase. Ater urther cooling

the convection phase begins. Normally all three phases co-exist or a

period, see Figure 22 at let, until nally only convection remains.

The centre section o Figure 22 shows how the heat transer coecient,

a , is calculated or oil and water. The maximum a-value or oil is almost

5000 W/m2C, which is normal or ast quenching oil.

While the three phases coexist there is a large temperature dierence

between the surace and the core and in the axial direction o the cyl-inder. To the right in Figure 22 the uneven temperature within the cylin-

der can be seen. The maximum temperature is 750 C and the minimum

temperature is less than 200 C at the sur ace. This uneven temperature

causes stresses that deorm the component and urther distortion also

arises because the transormation to martensite starts at a dierent

time.

Gas quenching will have a convection phase only and give a constant

heat transer coecient e.g. 1000 W/m2C along the cylinder, see

Figure 23.

Because the heat transer coecient is constant, the axial temperaturegradient within the cylinder shown to the right in Figure 23 is small.

This type o quenching gives more homogenous stresses and thereore

less distortion. When the gas fow is perpendicular to the cylinder, as

in Figure 23, it results in an uneven heat transer coecient around the

cylinder.

0

100

200

300

400

500

600

700

800

900

0 20 40 60

Time, s

Temperature,

C

0

100

200

300

400

500

600

700

800

900

0 1000 2000 3000 4000 5000

Heat Transfer Coefficient, W/m2 C

Tenmperature,

C

Figure 21. Typical cooling curve or gas (red), in this case helium at 10 bar. Forcomparison the curve or a hot quenching oil is also shown (blue). [43]


21/56


However, during normal quenching the fow direction is parallel to the

cylinder, as usually long thin cylinders are charged into the urnace ver-

tically. Here the heat transer coecient decreases along the cylinder,

see Figure 24.

2.4 Cooling curves and heat transer

coecient (HTC)As Figure 21 shows, the quenching characteristics o oil and gas are

quite dierent, which should be remembered when reading this sec-

tion. The heat transer coecient, HTC, is an important input parameter

or simulation o the hardness, distortion and stresses o a component.

It also characterises the quenching properties o quenchants.

For oil, the three phases are quite distinct . For gas the HTC, ater a

short temperature drop, stabilises at an almost constant value, which

depends on the type o gas or mixture, the pressure and gas velocity as

well as the geometry o the component and its orientation.

2.4.1. The gases

An average value or the HTC can be calculated rom the theory and the

calculation ormulae developed or a heat exchanger when the gas fow

is perpendicular to the tubes [45]. Figure 25 shows the HTC and how it

varies with pressure and gas velocity.

The curves have been calculated or the our most commonly used

gases.

x Argon (Ar) is used or components whose surace would react with

any other gas.

x Nitrogen (N2) is used most oten in this context, because it is cheap

and widely available.x Helium (He) has better cooling properties than N2 but is much more

expensive and is thereore used less.

x Hydrogen (H2) has the best cooling capacity and is cheaper than He.

Because hydrogen in combination with oxygen may orm a combus-

tible mixture, however, all hydrogen installations are subject to spe-

cial saety precautions and are, so ar, very little used.

Figure 23. Heat transer coecient and temperature distribution with perpendicu-lar gas quenching o a cylinder

h1

h2

h3

h1 > h2 > h3

Figure 24. Heat transer and temperature distribution during parallel gas quench-ing o a cylinder (where h is the heat transer coecient).

Figure 22. Heat transer and temperature distribution in oil quenching a cylinder

However, the eects o dierences in the heat transer coecient are

small in comparison to the larger dierences occurring with oil quench-

ing. Thus gas quenching normally causes less distortion. Both axial andperpendicular fows cause di erences in the heat transer coecient

along the component; sometimes it would even be more benecial to

quench in a perpendicular direction in order to get a more even heat

transer (Figure 36).


22/56


The gas speed and pressure are the main parameters aecting the cool-

ing characteristics, as well as the gas composition. Figures 21 and 23

illustrate that quenching is done by convection only. An increase o gas

pressure, and thereore velocity, means that the HTC rises. As Figure 25

shows, this results in aster quenching.

2.4.2. Mixture o gases

A mixture o gases has an optimum composition or heat transer. Figure

26 shows the infuence on the HTC o adding He to an N 2 atmosphere,

compared with the addition H2. For both mixtures the maximum value o

the HTC is reached at about 80%.

1.5

1.4

1.3

1.2

1.1

1

Coe

cent

0 1004020Helium content

8060N

2He

0 1004020Hydrogen content [%]

8060H

2

1200

900

600

300

0Heat-transfer-coefficient[W/m2K]

Figure 26. Heat transer coecient or gas mixtures in a nitrogen atmosphere. For the H2-N2 mixture the points S1 to S7 indicating the dierent positions in the load.

3000

2500

2000

1500

1000

500

0 352015105

Cylinder 12.5 mm

Gas velocity [m/s]

h

[W/m2K]

3025

H2

He

N2

Ar

3000

2500

2000

1500

1000

500

0 352015105

Cylinder 12.5 mm

Pressure [bar]

h

[W/m2K]

3025

H2

He

N2

Ar

20 bar 15 m/s

Figure 25. Relationship between the heat transer coecient at 500 C or pressure and gas velocity

With a mixture o He N2 the HTC will remain the same as or 100% He

[46] even when the He content is 60 70%. This will reduce the cost o

the quenching gas. However, when quenching with pure helium, the

helium recycling system should always be implemented (see Section

2.4.3 or urther details). For the H2 N2 mixture [47] the content o H2

can be 30-40%, which is comparable to 100% H2. In this case it is advis-

able to use a mixture o 70 80% o H2 in order to take advantage o

the higher HTC, about 20 30% compared to 100% H2.


23/56


2.4.3 Gas recycle

Recycling o nitrogen is not economically attractive, but i helium is

used, experience shows that recycling can reduce the cost (Figure 27).

However, careul consideration needs to be given to actors such as the

purity required in the output gas, the pressure and the time available to

carry out the recycle. Because o the high capital cost o the equipment

and the gas losses, the amount o cost saved by the use o a properly

optimised helium-based quenching system depends on the gas quality

required rom the recycling system. A simple helium recycling system

with minimal clean-up, and hence minimal losses, on a typical vacuum

carburising installation with one urnace, can reduce helium costs by at

least hal [48].

2.4.4. The infuence o gas pressure and gasvelocity

The ollowing expression shows the relationship between the HTC, pres-

sure and gas velocity [49].

h = C. pm . vn

h = Heat trans er coecient, W/m2 C

p = Gas pressure, bar

v = Gas velocity, m/s

C, m, n = Constants, depending on the chamber, load conguration and compo-

nents. For a perpendicular gas stream, m and n are equal to 0.6 0.8

In a quenching chamber the gas velocity is controlled by the an speed.

A lower an speed gives slower cooling, (see Figure 28 [50]). Cooling in

He at 20 bar, with the an running at 75% o maximum speed, provides

a cooling capacity equal to that o He at 10 bar and 100% an speed.

Higher gas velocities bring more gas molecules per unit o time into

contact with the surace o the component and remove heat at a aster

rate.

The eect o gas pressure or He and N2 on the cooling rate [51] is

shown in Figure 29. When the quenching changes rom N 2 at 10 bar

to He at 10 bar, the maximum rate increases by about 50 70%. An

increase to 20 bar He gives an increase o 40% in comparison with 10

bar He. A urther increase to 40 bar gives another increase o 40%. 40

bar is however not practical as yet, but is shown here as a possibility.

Figure 27. A helium recycle system (photo courtesy o ALD)

0

900

800

700

600

0 240150120Time [s]

210180

Temperature

[C]

906030

500

400

300

200

100

20 bar He, 25 100 %

10 bar He, 100 %

25 %

50 %100 %

75 %

Figure 28. The eect o an speed on cooling. He at 10 and 20 bar using a 30 mmtest piece o Inconel 600 with the thermocouple in its centre. Coldquenching chamber at IVF. [50]

0

900

800

700

600

0 10050Cooling rate [C/s]

75

Temperature

[C]

25

500

400

300

200

100

Figure 29. Cooling curves or dierent pressures o He and N2. Cold quenchingchamber at IVF.


24/56


2.4.5. The eect o the gas temperature

The gas temperature is one o the parameters needed to calculate

the HTC. Very oten a constant value is assumed, but in an indus-

trial quenching chamber the gas temperature varies over the time o

quenching. The actual gas temperature at any time is needed in order

to get the most accurate value. However, the

temperature o the inlet gas is always lower

than the outlet gas as it heats up passing

through the load. The solution [52] is to use

the average temperature between the inlet

and outlet temperature (see Figure 30).

During the starting phase o quenching the

inlet temperature rises about 40C and theoutlet temperature increases by about 100C.

This means that components at the top and

the bottom o the load are quenched at a slightly dierent temperature.

The calculated HTC is however a mean value or the whole load.

2.5. Properties o the gases

x The rate o heat transer by convection is governed by: the gases

and their thermophysical properties (see Table 3)

x the body geometry

x the average gas velocity, pressure and its turbulence.

In order to calculate the heat transer coecient, h, the well-known key

values such as Nusselt, Reynolds and Prandtl values must be known,

see ormulae below.

Nu = h d /l (1)

Re = U d /u (2)

Pr = ur Cp /l (3)

h = heat transer coecient, W/m2Cd = characteristic measure, e.g. the diameter o an cylinder or axle, ml = gas thermal conductivity, W / m KU = gas velocity, m/su = kinematic viscosity, m2/sr = gas density, kg/m3

Cp = gas specicity heat capacity, kJ/kg K

0

150

125

100

75

0 200140120Time [s]

180160

Temperature

[C]

1008060

50

25

Gas outlet

4020

Mean value of gas temperatures

Gas inlet

Figure 30. Gas temperature during quenching at 10 bar He in a cold chamber.

Table 3: Physical properties o the gases at 15C and 1 bar [53].

Properties Argon Nitrogen Helium Hydrogen

Density, kg/m3 1.67 1.17 0.17 0.08

Spec heat capacity, kJ/kg K 0.52 1.04 5.19 14.30

Thermal conductivity W/m K 0.017 0.026 0.154 0.191

Dynamic viscosity Ns/m2 23 x 10-6 18 x 10-6 20 x 10-6 9 x 10-6

Kinematic viscosity, m2/s 0.14 0.15 x 10-4 1.19 x 10-4 1.06 x 10-4


25/56


These three key values (1 3) have the ollowing correlation:

Nu = C Rem Prn (Pr/Prs )r (4)

Pris the Prandtls value in the gas volume and Prs is the value close to

surace o the component.

C, m, n and rare constants. These constants vary with the design o the

quenching chamber, conguration o the load, position and geometry o

the components and so on. Mostly the value o n is about 0.3 0.4 and

value o r is around 0.25. However, the values o Cand m depend on

the magnitude o the Re value [54] (see Table 4). Similar results have

also been reported by R. Wiberg [55].

From Equation 4 it is possible to calculate the heat transer coecient,

h, using the suggested values or the constants n and rand the m value,

which normally is ~ 0.7.

h = C U0.7

d-0.3

l0.7

u0.4

r0.3

Cp0.3

(5)

The calculated value o h is an average or a component similar to a cyl-

inder or an axle. Its important to keep in mind that the h value varies

over the surace o the component (see 2.6).

Now the heat transer coecient is known, the time, t, it takes to

quench rom one temperature to another can be calculated.

(6)

W = weight o component, kgA = surace area o component, m2

T = average temperature o gasT1 = start temperature or componentT2 = end temperature or component

The physical properties in Table 3 are given or 15 C and 1 bar. When

the temperature and, especially, the pressure are higher, some proper-ties will change. For a more qualied and more exact simulation it can

be valuable to use the corrected values as shown in Figure 31 and 32.

Figure 31 shows the variation in the kinematic viscosity with tempera-

ture at 15 bar or nitrogen. It illustrates that, or example, an increase in

temperature rom 20C to 100C increases kinematic viscosity by 60%.

When this and other temperature-dependent changes are taken into

consideration, the heat transer coecient decreases rom 391 to

355 W/m2C, a reduction o about 10%. In this case the gas velocity is

10 m/s.

Figure 32 shows the change in kinematic viscosity versus pressure at100 C or nitrogen. An increase o pressure rom 10 to 20 bar decreases

the kinematic viscosity by about 50%.

The change in the kinematic viscosity that comes with a pressure

change rom 10 20 bar increases the heat transer coecient rom 355

to 480 W/m2 C.

0

3,0

2,5

2,0

1,5

0 300150100Gas temperature [C]

250200Kinemati

cviscosityx0,0

00001m2/s

[C]

50

1,0

0,5

Figure 31. Kinematic viscosit y versus gas temperature at 15 bar nitrogen.

0

5,0

4,5

4,0

3,5

0 301510Pressure [bar]

2520Kinemati

cviscosityx0,0

00001m2/s

[C]

5

3,0

2,5

2,0

1,5

1,00,5

Figure 32. Kinematic viscosity versus pressure at 100C or nitrogen.

Table 4: Constants and exponents in Equation 4

Re C m

1 to 40 0.750 0.4

40 to 1000 0.510 0.5

1000 to 2 x 105 0.260 0.6

2 x 105 106 0.076 0.7


26/56


2.6. Design o the load

2.6.1. Cylinder and axle

In the previous sections the heat transer coecient used was an aver-

age value around a test probe, a cylinder or a component. However,

during gas quenching o any component the local heat transer is o

major importance to the nal hardness, uniormity and distortion o

the component. Even or a component as simple as a cylinder there is a

variation both along its length and its circumerence, depending on gas

quenching conditions.

Two major parameters are the turbulence and the Reynolds number

[55]. In a test in a wind tunnel, with axial fow along a cylinder, it wasshown that with a turbulence o 0.3%, which is nearly a laminar fow,

the HTC had the highest value at the back part o the cylinder, see Fig-

ure 33. The same gure also shows how the heat transer coecient

(proportional to the Nusselt number) increases with higher Reynolds

numbers. The Reynolds number is proportional to the gas velocity.

(Note: Nu=d/land when Nu is known rom Figure 30, then h can be

calculated; d is the critical dimension, in this case 0.15 m, where the

trials were made or a cylinder with a diameter o 0.15 m [56]. lis the

gass heat transer capacity, in this case 0.026 (see Table 3). The value

o h is then 175 when Nu is 1000. When Re is 300 000 then the gas

velocity is 30 m/s).

However, with increasing turbulence up to and over 6.7% the HTC over

the cylinder has shited so it has it highest value at the ront part along

the cylinder sur ace (see Figure 33). The turbulence was increased by

inserting a disk with a diameter 1/3 o the cylinder diameter, and one

diameter up in the upstream gas.

Figures 33 and 34 illustrate that with increasing Reynolds number, i.e.

a higher fow velocity, the HTC value also increases. With the actual

parameters the HTC value is about twice as high at the ront o the cyl-

inder as it is at end o the cylinder.

In industrial vacuum urnaces there is a high degree o turbulence. The

normal consequence is that the ront part o a cylinder surace has the

highest HTC value along its length. This has also been noted in other

reported results [56, 57].

A more in-depth analysis o axial fow along a cylinder has been made

with dierent congurations [56] (see Figure 35). In conguration A

there is a ree fow o gas along the cylinder axis which gives a higherheat transer at the ront end o the cylinder. Figure 36 shows the heat

transer coecient over the cylinder or all congurations. The HTC is

high at leading edge o the cylinder, between 0 and 40 mm. Beyond

40 mm the heat transer is more uniorm. This is in good agreement with

the result in Figure 34.

In conguration B where the cylinders are supported with holding bars

there is an increase in the heat transer at the supporting points at 50

and 100 mm. The heat transer coecient is increased locally by 25%,

because turbulence is higher.

In congurations C and D the heat transer coecient is more uniorm.The heat transer coecient appears to vary between 0 and 40 mm

rom the leading edge o the cylinder. Beyond 40 mm, the heat trans-

er is almost uniorm. In conguration C the heat transer coecient

is somewhat higher still. This is in good agreement with results rom

experimentally veried tests.

a

d

b

c

Figure 33. The heat transer coecient (Nusselt numbers) along the cylindersurace. The distance a b is the ront end; b c is the cylinder side; andc d is the rear. The turbulence is 0.3%. (The heat transer coecient isproportional to the Nusselt number.)

d

c

b

a

Figure 34. The heat transer coecient (Nusselt number) along the cylinder sur-ace. The turbulence is 6.7%. The turbulence disk is seen in the ront othe cylinder.


27/56


When the fow o gas is perpendicular to a cylinder axis there will be an

uneven HTC around the cylinder (see Figure 37). Also in this case the

level o the Reynolds number has an infuence. With increasing Rey-

nolds number the HTC has a minimum at 0 degrees (see Figure 37), and

will be rising towards the impingent point at an angle o 90 degrees.

At the same time the HTC will also increase.

Industrial tests o perpendicular gas fow against a cylinder or axle

have shown a higher HTC at the cylinder surace turned into the fow in

comparison with the ar side. One test [56] in an industrial urnace with

about 15 m/s gas fow gave 20% higher HTC at the ront side. Another

test [58] with a very high gas velocity gave a dierence between the

ront and back o a cylinder o 4 6 times higher HTC at the ront side.

In a load with more than one layer, the highest HTC is always ound at

the top layer when the gas fows rom top to bottom. The highest values

are ound in the top layer where the cold and ast gas rst impinges on

the components. The heating up o the gas leads to smaller HTC in the

middle and bottom layers. The reduction depends mainly on the design

o the load. Examples in the literature have reported reduction values

rom 50% [47] and 30% [59].

2.6.2. Disk and gear

Common components that industry needs to manuacture are gears or

the automotive industry. With the increasing use o vacuum carburising

combined with gas quenching it is important to optimise the design o a

load in order reduce distortion as much as possible. An extensive study

[60] has been made, simulating gas quenching o disks and gears, bothin horizontal (lying down) and vertical (hanging) positions (see Figure

38). The simulated gears were plain disks without teeth, but with a cen-

tral hole. The gear has an outer diameter o 25 cm and a central hole o

15 cm. The thickness is 5 cm. This gives the gear a square cross-section

with sides o 5 cm.

a) Configuration A b) Configuration B

c) Configuration C c) Configuration C

Figure 35. Four quenching basket congurations are delineated. (a) No basket,the cylinders stand alone in the quenching chamber; (b) basket madeo bars, currently used in industrial gas quenching processes; (c) basketmade o 3 mm thick plates; (d) basket made o tube with 3 mm wallthickness.

0

800

700

600

500

0 1206040Length [mm]

10080

Hea

ttransfercoefficient[W/m2]

20

400

100

300

200

A

B

C

D

Figure 36. Heat transer distribution over the cylinder surace or all congurations.

0

800

700

600

-90 90300Angle [degrees]

60-30-60

500

400

300

200

100

Simulated

Measured

Heattransfercoefficient[Wm2K-1]

Gas flow -90 deg.

0 deg.

0 deg.

90 deg.

Figure 37. An example o a typical pattern or the HTC around a cylinder.

Cross section

Figure 38. Schematic view o the conguration or two types o load


28/56


2.6.2.1. Horizontal position

Figure 39 shows the simulated velocity eld over horizontal stacked

gears (the rst and th) with an axial distance o 5 cm between the

gears. The gas velocity is 20 m/s at a pressure o 20 bar. Starting with

the rst gear shown in F igure 39a, the typical fow conguration is:

1. stagnation fow on the upstream surace, accelerating rom the centre

towards the gear corners

2. separation at the corners and change o direction to axial fow

3. ast fow over the outer and inner diameter o all the gears

4. a much slower type o circulation between the gears indicted by the

dark blue arrows to the right o the shown cross-section.

I the distance between the gears is increased, the velocities between

them also increase.

For the rst gear, the gas velocity at the corners is about 16 18 m/s,

while it is lower, about 6 9 m/s, at the ront side and somewhat less,

4 5 m/s, at the ar side. Along the inside and outside the velocity is

more uniorm at about 15 19 m/s. For the th gear, the velocity at the

ront side is lower than or the rst gear, at about 4 5 m/s.

Figure 39. Velocity eld or (a) rst gear upstream; (b) th gear. The distancebetween the gears is 5 cm, and the gas pressure is 20 bar at 20 m/s. Thegas stream is coming rom the let.

a) b)


29/56


Figure 40 shows the variation o the Nusselt value or the rst and the

th gear. The upstream surace or the rst gear is shown in Figure 40a

or all simulated distances between the gears. Figure 40b shows the

same or the th gear.

Figure 41a shows that the distance between the gears has no infu-

ence on the Nusselt number. The upstream gas is undisturbed when it

impinges on the rst gear, which is the reason the Nusselt number stays

the same. The Nusselt number has a minimum almost in the middle o

the surace and is about six times higher at the corners than in the mid-

dle o the ront surace.

The prole o the Nusselt numbers or the th gear is the same over the

ront surace as or the rst gear. The scatter or the dierent distances

is higher but the relationship between minimum and maximum in this

case is only three times.

A similar pattern as in Figure 40b exists or all the other sur aces. Aver-

age Nusselt numbers have been summarised or each gear and are

shown in Figure 41.

The Nusselt numbers are the highest or the upstream rst gear and

then they drop or gears 2 to 5. However the most even Nusselt num-

bers are obtained with the spacing 5 cm apart rom the rst gear,

although the Nusselt numbers are the lowest in this case.

In gas quenching the dierences in HTC over the surace and between

gears and disks are small compared to the dierences in HTC orquenching in oil, where a typical value is 10 20 times higher. This has

a benecial infuence in reducing distortion.

The divergence between the rst gear and disk can be used in a posi-

tive way by replacing it by a thin dummy disk or spoiler, creating a more

uniorm gas fow downstream. This conclusion has been conrmed in

other work.

2

1,5

1

0,5

00 10,40,2

Distance from inner diameter

Nu

x

10-4

0,80,6

5 cm

10 cm

15 cm

20 cm

2

1,5

1

0,5

00 10,40,2

Distance from inner diameter

Nu

x

10-4

0,80,6

5 cm

10 cm

15 cm

20 cm

Figure 40. The calculated Nusselt number over the ront surace, (a) or the rstgear and (b) or the th gear.

a) b)

10,5

8

7,5

7

6,50 521

Cylinder 1 is the first in the f low and 5 is the last

Average

Nu

percylinder

x

10-4

43

5 cm10 cm

15 cm20 cm

10

9

8,5

9,5

Figure 41. Area averaged Nusselt numbers or all gears and all distances.


30/56


2.6.2.2. Vertical position

For this simulation only three gears were used. Because it includes a

gear located between two others and one that has only one gear next

to it, the results should be adequate to describe the fow and Nusselt

number behaviour or any number o parallel gears in a vertical posi-

tion.

In vertical fow, large dierences in the fow are less likely because o

the direction o the fow and the placing o the gears. Higher Nusselt

numbers are reached in vertical fow, compared to horizontal position

more because o the smaller amount o material, than because o the

hole. This is what makes it easier or the gas to fow through the load,

rather than geometrical reasons. Figure 42 shows the fow or 10 cm

spacing. The eddies inside the cylinder holes, and how the directionchanges and goes upstream inside the hole, can both clearly be seen.

In Figure 43 the area-averaged Nusselt numbers or the three cylinders

are displayed in the same way as in Figure 40. The Nusselt numbers

or the three gears are much the same in comparison with gears in the

horizontal position, and this results in more even hardness and lower

distortions. With the exception o the gear placed in the centre in the

10 cm case, which obtains the highest value, the more separated the

gears, the lower is the cooling, although the dierences are very small.

This is because when as the gears are separated urther apart, the

velocities between two adjacent gears decrease.

2.7. Distortion

Gas quenching is oten claimed to reduce the amount o distortion.

While theoretically valid, this is also true in most practical cases. During

quenching, oil passes through three di erent phases: a vapour phase;

a boiling phase; and a convection phase. All three phases have dier-

ent HTCs (see Figures 20 and 21). The quenched component is thereore

subjected to a dierent HTC at dierent positions on the component at

the same time. This leads to uneven quenching and inhomogeneous

phase transormation in the steel component, which causes uneven

stresses and distortions. In gas quenching, only the convection phase

is present and, consequently, more homogeneous phase transorma-

tions within the quenched component can be assumed, resulting in

more even stresses and less distortion. In practice, however, during

gas quenching the HTC also diers around a component depending onits geometry, its position in a load and the fow conditions dictated by

the perormance and geometry o the quenching chamber. But, com-

pared with oil quenching the di erences in HTC are much smaller when

quenching in gas.

In gas quenching one actor to consider is the distortions, such as ovali-

ty and fatness, that cannot be corrected. These distor tions are very crit-

ical and important to reduce. Distortion in diameters and, sometimes, in

conicity can be adjusted by sot machining beore hardening. However

it is then impor tant to minimise scatter.

Figure 42. Velocity plot or gears with 10 cm distance.

9800

8800

8600

0 32

Number of cylinder. 1 is the left and 3 is the right

A

verage

Nu

percylinder

5 cm10 cm15 cm20 cm

9600

9200

9000

9400

Figure 43. Area-averaged Nusselt number or all gears and all distances.


31/56


2.7.1. Gears

Automotive transmission synchronizer gears were gas quenched, ater

being vacuum carburised, with 20 bar helium [61]. The load consisted

o about 340 gears placed horizontally in nine layers. Compared with oil

quenching, use o high pressure gas quenching provided a signicant

reduction in gear distortion (see Figure 44).

The lower distortion values were consistent and repeatable so the sot

and hard machining tolerances could be decreased. Other transmission

parts such as sun gears, ring gears, pinions, and shats have also been

quenched in 20 bar helium with good results.

Extensive testing with gas- and oil-quenched gears has shown [62]

that gas quenching gives lower distortion. For example, a 30% reduc-tion in out-o-fatness was recorded. Since out-o-fatness is o crucial

importance to the tooth fank deviation, any improvement is particularly

important. Loading the gears in a vertical position also gave a reduction

o 10-20% in distortion compared to the horizontal position. It also gave

slighter higher hardness. This agrees well with the simulation discussed

in Section 2.6. As mentioned previously, the geometry o the gear also

has a noticeable infuence on distortion. With a small reinorcement o

the gear the distortion decreased by a urther 30%. According to Altena

[35] the reinorcement o such components has a large infuence, up to

50%, on the distortion and it must not be neglected.

There are other published results, which also show the comparisonbetween quenching in oil and gas where the gas gives lower distortion

[44, 48].

2.7.2. Rings

The geometric orms o gears and rings are relatively similar. While most

gears are made o carburising steel, most rings are made o ball bearing

steel. Rings o 100Cr6 steel were quenched in gas and two ast-quench-

ing oils [63] respectively (Figure 45). The gas was helium at 10 and

20 bar in a cold chamber. The distortion was determined by comparing

the diameter, ovality and conicity.

Both oils gave a higher distortion and scatter, compared with gas.Quenching at 20 bar gave less scatter, arising rom the act that the

10 bar quenched rings were not ully hardened because the cooling

capacity o the gas was too low.

In other published results, which also show the comparison between

quenching in oil and gas, the gas gives lower distortion [64, 65].

Figure 44. Less distortion is a major advantage o high pressure gas quenchingover oil quenching. These ovality or roundness data are or (a) 50 oil-quenched and (b) 50 gas- quenched synchronizer gears.

0,06

0,01

0

10 barHe

O

valityofOuterDiameter[mm] Average and

Standard Deviation0,05

0,03

0,02

0,04

20 barHe

Oil A Oil B

n = 127

12

6Oil A: Houghton

Quench A

10 bar HeCold Chamber

20 bar He

Oil B: Bellini Fn

}

Figure 45. Change in ovality or the ball bearing steel 100Cr6 with outer diameter70 mm, inner diameter 60 mm and height 15 mm, cooled in gas and oils.


32/56


2.7.3. Axles

Gears and rings can be loaded both horizontally and vertically, but axles

are normally loaded vertically. Pinions in a load o 500 kg were carbu-

rised and quenched in both oil and gas with the same charge pattern.

Figure 46 reveals that the median straightness has decreased rom 100

to 50 m (4/1000 to 2/1000 in), a reduction o 50%.

It is clear that the scatter also is s trongly reduced, which reduces subse-

quent grinding or straightening.

In the same investigation long slender drive shats were oil- and gas-

quenched. The straightness decreased by 60% when quenching in gas.

A shorter and more stable axle decreased in straightness by only 15%

[48].

2.8. Interrupted gas quenching

Temperature gradients during quenching cause stresses and uneven

phase transormations and hence distortion and uneven hardness.

One way to control these deects is to lower temperature gradients by

decreasing the HTC. This can be done either by lowering the pressure

or by lowering the fow rate by reducing the an speed. It is crucial to

know when the interruption starts and by how much the HTC will be

decreased.

Quenching in a salt bath results in very similar cooling characteris-

tics and low distortion. But the possibility o doing the same with gas

quenching avoids the detrimental eect on the environment that salt

entails.

12

8

4

0621

Straightness 1/1000

in

Frec.

43

20

16

5

Gas carburizing & Oil quench

Vacuum Carburizing & Gas quench

Figure 46. Change in straightness or pinions o SAE 8260, quenched in oil and20 bar helium.

Temperature[C]

Time [s]

Fan stop Fan restart

conventional

dynamic

Figure 47. Eect o interrupted quenching, by turning o ventilator or 20 s.

left flank

right flank

Quenchingin oil

Conventionalgas quenching

Dynamic gas quenching(20 sec. fan turn off)

60

0

10

20

30

40

50

m


33/56


As a general rule the interruption in gas fow ought to start at or just

above the martensite temperature, which depends on the carbon

content o the steel. How much and or how long the HTC should be

decreased depends on the hardenability o the steel and the dimen-

sions o the component. The HTC or the urnace and the load congu-

ration must be both known or the calculation. The cooling curves are

calculated rom the HTC to determine the optimal curves that give the

correct hardness at a reduced cooling rate. Figure 47 shows the result

or a gas-quenched carburised SAE 5115 (16MnCr5) gear, compared with

oil quenching.

The quenching was interrupted at about 350C by turning o the an or

20 s and then turning it on again. The pressure remained unchanged.

The scatter o the helix slope distortion was strongly reduced. This, in

turn, reduced the subsequent grinding operation. This technique is com-mon industrial practice used to reduce distortion.

Salt bath quenching is typically used or austempering, but can be used

where low distortion is needed. However, salt baths create environmen-

tal problems.

2.9. Equipment

The gas quenching technique has seen rapid development through-

out the last 25 years. The rst vacuum urnaces were equipped with

nitrogen quenching at 1 2 bar. This combined a brazing process and

hardening o air hardening steel such as high alloy tool steel and highspeed steel [66]. Figure 48 shows how the gas quenching technique has

developed rom 1960 to the present. In about 1981/82 the rst single-

chamber vacuum urnaces with gas quenching at 5 6 bar were devel-

oped. The improved quenching power then allowed hardening o hot

working tool steels, small cold working steel components and the rough

hardening steel components in medium sizes.

The use o lighter gases, such as helium and hydrogen, together with

a gas pressure o 20 bar, gave a large increase in the HTC in the gas

quenching development sequence. A urther increase o the gas pres-

sure to 40 bar has been tested in a small cold chamber. This urther

improved quenching power, but in practice today the limit is 20 bar or

both technical and economical reasons. The 20 bar vacuum urnace has

an HTC o more than 1000 W/m2 C, which corresponds to normal oil

quenching or hot oil quenching. This gives sucient core strength and/

or hardness or carburising steel, through hardening steels in larger

dimensions, or lower alloyed steels. One goal is to develop the cold

chamber and its fow pattern urther to increase HTC without the need

to increase the pressure.

2.9.1. Single-chamber vacuum urnaces

In single-chamber urnaces heating and quenching both take place in

the same chamber. The design o a single-chamber vacuum urnace is

a compromise between the demands on the heating and quenching

systems. During quenching the heating elements and the wall insula-

tion material are cooled together with the load, oten to room tempera-

ture. The urnace can be built so that the fow o the gas can change

direction during the quenching period as well. The drawing to the let in

Figure 49 shows such a urnace where the load is quenched rom below

[67]. By changing the guiding plates the gas fow can be alternatedrom below and rom above. In the drawing to the right in Figure 49, the

guiding plates are adjusted so that the gas fow comes rom the let.

This mode is useul i the layers are tightly packed, making it dicult or

the gas to fow in a vertical direction.

Figure 48. Development o gas quenching technology

1 5 10 20 40

100 350 1000 1600

1960

1970

1980

1990

2000

Steel Grades3high-alloyed

tool steel3high performance

cutting steel3air hardening steel3gas quenching of

case hardening steel3soldering processes

(W/m2K)

+ hot working steel3quenched and tempered

steel (medium cross section)3cold working steel

+ case hardening steel3quenched and tempered

steel (larger cross section)3low alloyed steel

Yearpressure (bar)


34/56


In single-chamber urnaces there is lot o space between the load and

the wall. As a result all the gas will not be orced to pass through the

load, instead some o the gas will pass outside load, see Figure 50 [62].

The distance between the load and the wall ought to be as small as

possible, but in prac tice the gap is usually 30 50 mm. When all other

conditions are the same it normally takes less time to ll a cold cham-

ber with gas because its volume is smaller. The lling time depends on

the buer chamber, pressure and volume, and the piping between the

buer tank and the quenching chamber, but 5 7 s is normal.

The same eect has been ound through simulation, see Figure 51 [57].

The simulation also showed that the heat transer coecients or the

outer plates on the side acing the urnace wall were 30% lower than

or the three plates in the centre.

The same simulation development work has also shown that the inlet

duct should have the same opening area as the load itsel or the

quenching chamber. I the inlet duct is too small the fow pattern will

be less avourable and give a less even heat transer coecient around

the load.

Figure 49. Single-chamber vacuum urnace: let, gas fow rom below; right, gasfow rom let [68].

Figure 50. Dierences in g

Documents

Low Press Carbg and High Press Qnchg