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International Microwave Power Institute 1

ATMOSPHERIC PRESSUREMICROWAVE PLASMA

CARBURIZATIONOF STEELALLOY COMPONENTS

Kuruvilla Cherian, Satyendra Kumar, Devendra Kumar, Mike Dougherty, Sr.,Dominique Tasch, Greg Fett, Dana Combs

Microwave Technologies, Dana Corporation, 2910 Waterview Drive, Rochester Hills, MI 48309, USA

ABSTRACT

A new proprietary atmospheric pressure microwave plasma technology (AtmoPlasTM) has been applied to

carburization of steel alloys. The effect of various processing parameters on surface hardness, surface carbon

concentration, uniformity of case depth, retained austenite percentage and grain size was studied. Judicious selection

of processing parameters could yield uniform case with surface and microstructural properties within the desired

ranges. Similar samples carburized with other existing technologies were also analyzed and the results compared.

INTRODUCTION

Metals and metal components generally cannot be directly heated with microwaves, as they are good reectors of

microwaves unlike some ceramics that are good absorbers. Therefore, direct processing of metal components using

microwaves is a challenging problem. One possible solution is through the employment of microwave susceptor

materials in the vicinity of the part to be processed, their coupling with the microwave eld and heating up to a

satisfactory level, and relying on mostly convective and radiative heat transfer mechanisms to heat and process the

metal component [1]. A more efcient process could be through the use of a plasma, generated and sustained by the

microwave eld, to heat up faster the part to be processed. The need for lower pressures to initiate and sustain such

plasmas had been a constraining factor for this. Such constraints have now been removed with the development of a

new proprietary process, AtmoPlasTM, for the initiation and sustaining of microwave plasma at atmospheric pressure.Preliminary reports of application of this process for various materials processing applications have been presented

at recent conferences [2-5]. This paper deals with a comparative study of carburizing results obtained through

AtmoPlasTM, and conventional gas and vacuum carburization processes.

EXPERIMENTAL PROCEDURES

The AtmoPlasTM set-up used for carburization experiments in the laboratory is capable of delivering up to 6 kW

continuous microwave power at 2.45 GHz. Details of the set up have been reported earlier [2-5].

The processing prole used initially was not complex.. A typical processing cycle would be as follows. The sample

(typically 8620H coupon cut from 1 bar stock) is placed in the ceramic plasma cavity. Plasma is then ignited through

the proprietary AtmoPlasTM process and the temperature of the sample allowed to build up. When this reaches ~ 900C,

acetylene is introduced into the plasma cavity. The temperature is maintained at the required value by careful adjustment

of the microwave power. After the carbon boost and diffuse stages of required duration, the sample is quenched in

oil and then tempered for at ~ 177C for 1 hour. Detailed metallographic analysis is subsequently performed on the

samples to gain an understanding of the various properties. The trials were repeated adopting modications to the

processing parameters, to try to nd the optimum values. The effect of the processing variables on surface hardness,

surface carbon concentrations, uniformity of case depth, retained austenite and grain size was studied.

Similar 8620H coupons were carburized in a conventional gas carburization furnace and also a vacuum carburization

furnace, and analysed as in the case of the AtmoPlasTM carburized samples. Subsequently, 8620 steel side gears were

also carburized by Conventional Gas and AtmoPlasTM processes.

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2 39th Annual Microwave Symposium Proceedings July 13-15, 2005

RESULTS

The results of carburization of 8620H coupons by the three carburization processes are summarised in Table 1, for

easy comparison. On comparing the results, the advantages of the AtmoPlasTM processing route becomes evident.

Table 1: Comparative characteristics of Conventional Gas Carburized, Vacuum Carburized andAtmoPlasTM Carburized, 8620 steel test coupons.

Comparison of Conventional Gas, Vacuum andAtmoPlasTMCarburization

Characteristics /

Properties

Conventional Gas

CarburizedVacuum Carburized

AtmoPlasTM

Carburized

Total Carburization

Time142 min boost + 110 min

diffuse +20 min temp

drop = Total 272 min.

Carbzn zone time

= 205 min

112 min boost + 80 min

diffuse + 20 min temp

drop = Total 212 min

Effective Case Depth ~0.035 ~0.035 ~0.045

Microstructure (Retained Austenite % and Depth)

Corner Microstructure

~15%- ~30% retained

austenite to a depth of

~0.319 mm.

~10%- ~15% retained

austenite to a depth of

~0.172 mm.

~5%- ~20% retained

austenite to a depth of

~0.172 mm.

Surface Microstructure

~10%- ~20% retained

austenite to a depth of

~0.119 mm

~5%- ~15% retained

austenite to a depth of

~0.243 mm

~5%- ~20% retained

austenite to a depth of

~0.148 mm.

ASTM E112-96 Grain Size No.(Comparison Method)

Case8-10 (22.5 -11.2) 8-9 (22.5 - 15.9) 10-12 (11.2 5.6) 10-12 (11.2 5.6)

Core 8-9 (22.5 - 15.9) 9-10 (15.9 11.2) 10-12 (11.2 5.6)

The results of carburization of 8620 gears by the AtmoPlasTM and Conventional Gas carburization processes

are summarized in Table 2. The advantages in processing time and effective case depth offered by the AtmoPlasTM

processing over the conventional route for carburizing real components such as gears, becomes clear.

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International Microwave Power Institute 3

Table 2: Case depth comparison of Conventional Gas Carburized and

AtmoPlasTM Carburized, 8620 steel side gears.

Conventinal Carburized Gear

Total processing time: 142+110+20 = 272 min

AtmoPlasTMCarburized Gear 041130A

Total processing time: 112+80+20 =212 min

CONCLUSION

The atmospheric pressure microwave plasma (AtmoPlasTM) processing route is capable of providing comparable or

better carburizing results, in comparison with other conventional carburization processes. Uniform carburization is

possible through the AtmoPlasTM route; case depths, surface carbon and microstructure can be controlled to advantage,with apparent benets in grain size and processing time as well. This opens up the possibility of developing and

establishing a new and better carburizing technology employing microwave plasma at atmospheric pressure.

ACKNOWLEDGEMENT

The encouragement and support of Dana Management is gratefully acknowledged.

REFERENCES

1. Cherian, K., A. Fliet, S. Ganguly and R. Roy, December 2001. Novel Approaches to Materials Synthesis and Processing

with Microwaves. Proceedings of the Ted White Festschrift Symposium, University of Queensland, Australia2. Dougherty, Mike, Satyendra Kumar,Devendra Kumar, and Kuruvilla Cherian, 2004. Atmospheric Pressure Plasma Microwave

Processing. Presented at the 4th World Congress on Microwave and Radio Frequency Applications, Austin, TX, USA.

3. Cherian, Kuruvilla, Satyendra Kumar, Devendra Kumar, Mike L. Dougherty, Dominique Tasch, David J. Brosky, Dana M.

Combs, 2005. Atmospheric Pressure Microwave Plasma P/M Sintering of Cam Lobes. 2005 SAE World Congress Paper

No. 2005 - 01 0720

4 Kumar, Devendra, Satyendra Kumar, Kuruvilla Cherian, Mike L. Dougherty, Dominique Tasch, 2005. Braze Signature in

Brazing with Atmospheric Pressure Microwave Plasma 2005 SAE World Congress Paper No. 2005 - 01 0899

5 Kumar, Satyendra, Devendra Kumar, Kuruvilla Cherian, Mike Dougherty, Dominique Tasch, Dana Combs, Greg Fett, Bob

Binoniemi and H. Lin, 2005. Microwave Plasma Carburization of Steel Alloys at Atmospheric Pressure. 2005 SAE World

Congress Paper No. 2005 - 01 0989