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Vo l . 38 No . 2 August 2005

1. IntroductionAbout 30 years have passed since the first introductionof the vacuum carburizing process, which has attractedattention as a replacement technology for conventionalgas carburizing. This process has the following majorcharacteristics.

(1) High-temperature carburizing for a short time (2) The prevention of grain boundary oxidization(3) Easy control of the case depth(4) Easy carburize of complex shapes such as thin

holes, blind holes or the like(5) Gentle working environment for people and earthMany manufacturers competed to make use of the

vacuum carburizing process, mainly using methane orpropane gas. The major problem with vacuum carburizingwith propane gas was the formation of soot, which ledto big problems. About 7 years have passed sinceacetylene vacuum carburizing process was applied formass production. This solved the problem of sootformation which were affected the conventional vacuumcarburizing using propane gas, and the quality andrepeatability obtained by acetylene carburizing, wasquite satisfactory, and this new process has been runningat various kind of mass productions and applications.

The most important reason why acetylene vacuumcarburizing succeeded in solving the problem of sootformation was that the carburizing gas, acetylene gas,is used under very low pressure (less than 1 kPa). Apressure of 1 kPa is within the pressure range used forgeneral vacuum heat treatment. JH Corporation developeda carburizing process at this normal vacuum pressurerange that is generally used, and was the firstmanufacturer to solve the problem of soot formation,either in Japan or the world. The current state of thequality and economic efficiency obtained from acetylenevacuum carburizing furnaces used in production isdescribed below.(1),(2)

2. The vacuum carburizing basics

2.1 Controlling the case depthIt was discovered that the acetylene vacuum carburizing

process could be used with saturated stage and diffusionstage, in the same way as with propane gas vacuumcarburizing. In this process, carburizing continue duringthe period in which the carburizing gas is backfilled,until a concentration of carbon at which a solid solutionbegins to form is reached in the equilibrium diagram.The concentration of the carbon in the case is thencontrolled using vacuum diffusion. If the carburizingtemperature is fixed, it is possible to obtain a targetcarbon content and case depth for the surface bycontrolling the carburizing time and diffusion time.This is one of the features of vacuum carburizing. Ifthe carburizing temperature is raised, the carburizingtime can be shortened substantially, making carburizingmore economical. Gas carburizing furnaces are thoughtto have a maximum temperature of 960˚C or so. Abovethis, while there should be a reduction in carburizingtime, the furnace is severely damaged, making the totalcost of high-temperature carburizing uneconomical. Incontrast, the structure of a vacuum carburizing furnaceis essentially the same as an ordinary vacuum furnace,so it can withstand temperatures of 1 100˚C or over.This means that the normal carburizing temperature isnot restricted to a level of around 930˚C, but can befreely chosen from a higher temperature range. If adeep case is required, or high grade steel (for example,stainless steel) is being carburized, high temperaturecarburizing can substantially reduce the carburizingtime.2.2 Control the carburizing time and diffusion

timeAs soon as the temperature has reach the temperatureand the all parts of the furnace are exposed to vacuumatmosphere, the carburizing gas (acetylene) is introduced.The furnace reaches the predetermined pressure withinseveral seconds and every part of the work area isuniformly exposed to the decompressed carburizingatmosphere. In a very short time, Fe3C precipitates atthe surface and carburizing begin.(3) After the carburizinggas has been supplied for the prescribed time, the gasis purged to restore the furnace to its initial vacuum

Advanced Acetylene Vacuum Carburizing

IWATA Hitoshi : Director, General Manager, JH Corporation

The vacuum carburizing process was introduced in the 1970’s, and the expected advantages were carburizingtime reduction, high quality, and pollution-free. However, the process with propane gas was not approved in thefield due to heavy sooting. The vacuum carburizing process with acetylene gas was developed seven years ago,and the sooting problem is now solved. The new technology, called the acetylene vacuum carburizing process,is now applied in various fields for mass production. The recent results of quality are reported in this paper.

pressure, and diffusion is performed. The total carburizingtime, carburizing time and diffusion is set based onTable 1. SCM415 was carburized using acetylene gasat a range of temperatures for the predetermined time,and the obtained case was subjected to carbon analysis.The effective case depth was set at 0.3% carbon and0.4% carbon, and the carburizing constant K wasdetermined for each depth and total carburizing time.Table 1 shows the results of this. If vacuum carburizingis used, the carbon content, when carburizing iscompleted, is the saturated value in the equilibriumdiagram (Fig. 1), that is, when the carbon contentbecomes constant. The surface carbon content found wasdirectly proportional to the carburizing time and diffusiontime, giving a value for R. Similar tests were performedon S15C and SNCM420 as well as SCM415, but thevalues of K obtained were almost the same for allmaterials. The alloy elements affect hardenability ratherthan easy to carburizing.2.3 QuenchingAfter carburizing and diffusion have been completed,and when the soaking time for quenching is over, thefurnace is backfilled to about 80 kPa {600 Torr} withnitrogen gas, and oil-quenching is continued. The coolingcharacteristics of the quenching oil can be altered bysetting the pressure to which the furnace is backfilled

to different values, so a feature of this quenching isthat it is possible to obtain the optimum conditions forquenching distortions and also the hardenability.However, if the pressure upon the oil is too low, thequenching oil will not have enough cooling ability,leading to slack quenching. Recently, devices have beenmade to ensure that the optimum hardenability andoptimum distortion for each material is obtained.

High pressure gas quenching from 10 to 30 bar isreported recently. This process places some restrictionson the charge weight and the loading figure, howeverthis process is effective for relatively small works madeof alloy steel with good hardenability. If you have tomake grain size fine, which is coarsened by high-temperature carburizing, gas cooling is recommendedafter diffusion is completed, and the work basket istransferred from heating chamber to quench chamberto heat the soaking temperature, and then oil-quenchingshould be performed.

3. Quality of vacuum carburizing appliedto mass production

3.1 Case uniformityAt present, acetylene vacuum carburizing is applied formany mass production parts. An examination wasperformed to find the variation within lots and thevariation between lots of case depth of automotive parts(using materials including SCM, SCr, and SNCM). Itwas found to be within ±0.05 mm (a width variationof within 0.1 mm) for ordinary carburizing depths of0.5 to 0.8 mm. It was found that a variation of carburizeddepth within a single work of within 0.1 mm can beobtained with the standard carburizing cycle, both forgeneral-purpose parts such as general gears and forcomplex shapes such as diesel nozzles (Table 2). Thevariation within a lot of the surface carbon content ofautomotive gears was investigated. The results of thisinvestigation are shown in Table 3. From this table, itwas verified that a surface carbon content of the setvalue ±0.05% can be obtained very easily, presentingno problems at all for practical use. It was thus foundthat the uniformity within lots and between lots ofacetylene vacuum carburizing is not inferior to gascarburizing. The metallurgical structure obtained is freeof intergranular oxidization layers, and has a normalcarburized, quenched structure.3.2 Brightness of worksIt was found that with acetylene vacuum carburizing,

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Vo l . 38 No . 2 August 2005

Carburizing temperatureSurface carbon

content 0.8%

K

R

Total case depth

Case-hardening steel

Low carbon steel andcase-hardening steel

900℃

0.62

0.44

0.37

1.3

930℃

0.71

0.54

0.46

1.5

980℃

1.14

0.78

0.69

2.2

1 040℃ Applications

1.24

1.04

0.94

3.5

Case depth : Calculated by D = K T (mm) Calculated by Th = Tc + Td，R = Td/Tc

(Note)

Table 1 Carburizing constant K and R value

Effective case depth for 0.3% carbon

Effective case depth for0.4% carbon

Diffusion time Td

Carburizing time Tc

Low carbon steel

Low carbon steel

1 200

1 100

1 000

900

800

700

600

500

0

Tem

pera

ture

(˚C

)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

C (%)

Heat up invacuum

Diffuse at vacuum (Td)

Oil quenching ˙N2 gas cooling

Removal of work basket Loading the work basket

Soaking at carburizingtemperature

Soaking at austenitizingtemperature

Soaking at austenitizingtemperature

Carburize Increase of case depth

γ + Fe3C

α + Fe3C

α + γ

Partial pressure of CmHn ˙ Carburizing (Tc)

Decrease to austenitizingtemperature

Decrease to austenitizingtemperature

γ

Acm

Fig. 1 Equilibrium diagram and carburizing basics

Surface hardness HV

Total case depth (mm)

1

758

0.64

2

768

0.69

26

749

0.72

27

742

0.67

X

757

0.69

R

34

0.08

Cpk

2.3

1.9

—

—

—

(Note) X : Mean value, R : Variation, Cpk : Process capability index,Specification : Total depth of hardened layer, 0.70 ±0.05 mm,Material : SCM420H, Layers of basket : 7 layers, Weight : 30 g/piece, 8 000 pieces

Table 2 Process capability of case depth

Position No.

85

a very bright finish is produced on the surface, and inthe case of some parts, a very beautiful finish is produced.For this reason, the postprocessing plating processrequires little work, and indeed some users ship someof their products with the surface as-is after carburization.It is thought that the reason for this is the increase ofsurface activity due to the low vacuum pressure usedin the acetylene vacuum carburizing process, which isthe same as the pressure used in ordinary vacuum heattreatment.

4. Acetylene vacuum carburizingtechnology applied to mass production

4.1 Vacuum carbonitridingIt was found that if ammonia gas was introduced duringacetylene vacuum carburizing, nitrogen gas dissolvedinto an alpha phase to form a solid solution, producinga nitriding effect. This is widely applied to the productionof automotive parts. A typical acetylene vacuumcarbonitriding process is shown in Fig. 2. Theconcentration of the solid solution nitrogen on thesurface is proportional to the flow rate of ammonia, andso the amount of solid solution nitrogen can be controlledstably by changing the flow rate of the ammonia. This

was found to be useful in improving the basicperformance. The high temperature strength was kept,the fatigue strength improved, and other advancedfunctions of the material obtained using a combinationof heat treatment with supercarburizing, described inthe next section. The details of how to operate thesuper-carbonitriding process and the carbonitridingprocess for case-hardened steel are currently the subjectof investigation. It has been reported that mechanicalproperties can be improved somewhat by adding shotpeening to the carbonitriding treatment, and developmentsin this area are anticipated.(4)

4.2 Vacuum supercarburizingIn supercarburizing, carbides are precipitated very finely,but there were many problems with using this process atconventional gas carburizing. However,supercarburizingis easy for operation with acetylene vacuum carburizing.It has been applied to a wide range of automotive parts,machine tool parts, mold parts, and other mass productionparts, which can be produce with satisfactory qualityfor such as the size and shape of carbides in thecarburized layer and the precipitate layer, greatlycontributing to improving wear durability and fatiguestrength. Typical examples of use are shown in Fig. 3and 4. Improvements in performance have beenconfirmed in machine tool parts, shot blast machine parts,and general machine parts. In some products, the lifeof parts are improved more than twice the value produced

880˚C

780˚C

80˚C

45 75 30 3020 20(45)

Soaking Carburiz-ing

Diffusion & nitride

C2H2

25 l/min8 l/min

NH3

NH3

0.5 N2

N2

NH3 NH3

Oil quenching

46 kPa{350 Torr}

Furn

ace

pres

sure

900

800

700

600

500

400

300

200

Har

dnes

s H

V*1

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Distance from surface (mm)

(Note) Material : SUM24LEffective case depth : 0.3 - 0.5 mmProcess capability index : Cpk 1.58*1 : Load 0.3 kg

(a) Heat treatment cycles for vacuum carbonitriding (b) Hardness distribution

Time (min)

Fig. 2 Typical vacuum carbonitriding process

950˚C

850˚C 850˚C

Carburizing Spheroidizing Quenching

(a) Heat treatment cycles (b) Loading figure

Fig. 3 Typical operation of supercarburizing

4

3

2

1

Mean value

Position of furnace

Numberof

layers

Variation andvariation of width

Work

Case depth

Front side

0.79

0.79

0.79

0.83

Rear side

0.81

0.78

0.78

0.85

0.83

(Note) Units : Carbon content wt% Measurement : Emission spectroscopy

0.78 – 0.85%C, 0.07%C

Gear SCM420H

Effective case depth 0.8 mm

Table 3 Uniformity of surface carbon contents

Vo l . 38 No . 2 August 2005

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Vo l . 38 No . 2 August 2005

by supercarburizing using conventional gas carburizinghas been attained, and the process is expected to beapplied widely to parts for which wear resistance isessential. Investigations into how to performsupercarbonitriding to improve high temperature fatiguestrength and seizure resistance are underway.4.3 Carburizing of stainless steelStainless steels are good at resisting corrosion, andmany attempts have been made to find methods forcarburizing them, to increase their wear resistance andadd advanced functions to them. However, as therewere restrictions on the equipment that could be used,carburizing processes for stainless steels were notpopular. It was found that if acetylene vacuumcarburizing is used, there is no restriction on temperature,and carburizing can even take place at temperatures of1 000˚C or over without problems. The hardnessdistribution, microstructure and loading figure of anautomobile part subjected to acetylene vacuumcarburizing are shown in Fig. 5.

5. Fully-automatic vacuum carburizingfurnaces

An installation in general gas carburizing heat treatmentfactory is shown in Fig. 6 as an example of a vacuum

carburizing furnace installed in a factory, and aninstallation of a furnace as part of a new vacuum heattreatment factory is shown in Fig. 7. In both cases, thecycle of “pre-degrease → carburizing → post-degrease→ tempering” proceeds full automatically undercomputer control, with equipment consisting of vacuumcarburizing furnaces, a tempering furnace and vacuumdegreaser. All the processes up to the tempering stageare fully automatic. The furnaces were originally batch-type furnaces, but it was made possible to operate themfully automatically by controlling the carburizing heattreatment patterns, and many customers now use themin full-automation.

In 2003, to increase the production capacity of thesebatch type furnaces, JH Corporation developed acontinuous vacuum carburizing furnace. This furnace,called the V-MALS furnace, can accommodate up tofour heating chambers which share a oil tank to increasethe throughput of equipment. This continuous vacuumcarburizing furnace can be adapted to the mass productionof both wide ranges of products and narrow ranges ofproducts, and is flexible, allowing added heatingchambers to adapt to production conditions. This hasrecently been used to add a second oil tank instead ofa heating chamber. An add-on convection heating

(a) After gas cooling (carburizing is completed) (b) After quenching and tempering

(Note) Material : SNCM220 Effective case depth : 1.20 mm

Fig. 4 Microstructure of supercarburizing

900

800

700

600

500

400

300

200

100

00.01 0.06 0.15 0.30 0.60 0.90

: No.1 : No.2 : No.3 : No.4 : No.5

Har

dnes

s H

V

300 µ(a) Hardness distribution (b) Microstructure after carburizing (c) Loading figure

(Note) Material : SUS304Carburizing : 1 050 ˚CDistance from surface (mm)

Fig. 5 Carburizing of stainless steel SUS304

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Vo l . 38 No . 2 August 2005

Vacuum carburizing furnace Vacuum carburizing furnace Vacuum carburizing furnace

Stock conveyor beltStock conveyor beltStock conveyor belt

Tempering furnaceTempering furnaceVacuum degreaser No.2

No.3No.2No.1

No.1

Control panel

Controlpanel

Controlpanel

Controlpanel

Control panel

Control panel

Central panel Input conveyor

Output conveyor

Transport cart

Fig. 6 Full automatic vacuum carburizing furnace (3 furnaces)

Output lifter Input lifter

Inputconveyors

Outputconveyors

Inputconveyor

Outputconveyor

Vacuum pumps

Vacuum pumpsCentral panel

Transportcart

Controlpanel

ControlpanelControl

panel

Tempering furnace Vacuum degreaser Vacuum carburizing furnace

HWBV-3V VCB-242648

Stock conveyor belt Hydraulic unitHydraulic unit

Fig. 7 Full automatic vacuum carburizing furnace (2 furnaces)

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Vo l . 38 No . 2 August 2005

chamber can also be installed to shorten treatment time.A picture of the V-MALS type continuous vacuumcarburizing furnace is shown in Fig. 8, and its majorspecifications are shown in Table 4.

6. Conclusion

Seven years have passed since the first use on aproduction line of vacuum carburizing using acetylene,an unsaturated hydrocarbon gas. This method succeededin solving fundamental problems with the conventionalvacuum carburizing using propane (JH Corporation),namely, the various problems related to the formation

of soot. The quality obtained was found to be no lowerthan that of the gas carburizing method. The problemof carbide precipitation in corners has been solved, thesafety of acetylene gas has been verified, and safetyand stability during operation has been considered, frommany different viewpoints. The author intends to solvethe remaining problems with the process and equipment,make improvements where possible, and enhance anddevelop the acetylene vacuum carburizing process tobe an energy-saving carburizing process that is gentleto the environment and people.

— Acknowledgments —

The author thanks Mr. Okumura of Denso Corp. andMr. Matsui of Isuzu Motors Limited for their valuableadvice during the preparation of this article.

REFERENCES

(1) K. Kubota, N. Yamamoto, H. Iwata and K.Ishikawa : High Quality, Vacuum CarburizingProcess with Acetylene, The Japan Society for HeatTreatment, The 43th Lecture Rally OutlineCollection 1996 pp.3-4

(2) M. Sugiyama, K. Ishikawa and H. Iwata : VacuumCarburizing with Acetylene, Proceedings of 18thASM Heat Treating Society Conference (1998.10)pp.29-33

(3) N. Okumura : Vacuum Carburizing Process byAcetylene, Heat Treatment Vol.38 pp.194-197

(4) H. Eto, K. Matsui, Y. Kami and Yann Syou Tyun: Influence of Retained Austenite, ProcessingGeneration Martensite, and Stress by Shot Peeningto Compressive Residual Stress, Series A,Transaction of the Japan Society of MechanicalEngineers No.69-680 pp.37-44

(a) Overall appearance (b) Heating chamber and oil tank

Fig. 8 Typical V-MALS continuous vacuum carburizing furnaceoverall

Numberof

heatingchamber

Powersupply(kW)

1

2

3

4

1

1

1

1

5 200

5 200

5 200

5 200

3.5

1.75

1.17

0.88

185

370

550

740

130

260

370

520

150

260

370

480

Approx.

Approx.

Approx.

Approx.

Oil tank Example of treatment

Numberof

furnaces

Oilcapacities

(l)

Case depth 0.5 mm Case depth 1.0 mm

Loading weightper hour

Loading weightper hour

5.0

2.5

1.67

1.25

Timeper

action

Timeper

action

Table 4 Specification of V-MALS continuous vacuum carburizingfurnace