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Vacuum 86 (2011) 448e451

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Vacuum

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The kinetics of low-pressure carburizing of alloy steels

R. Gorockiewicz*

Faculty of Mechanical Engineering, University of Zielona Góra, Podgórna 50, 65-246 Zielona Góra, Poland

a r t i c l e i n f o

Article history:Received 26 March 2011Received in revised form15 September 2011Accepted 16 September 2011

Keywords:Low pressure carburizingKineticsAlloy steelsStructure

* Tel.: þ48 683282547; fax: þ48 683243540.E-mail address: marotti_goro@interia.pl.

0042-207X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.09.006

a b s t r a c t

In the article the author analyzed the kinetics of low-pressure carburizing of alloy steels, based on steel16MnCr5 and CSB 50NIL. It was found that an active radical-carbon layer (carbon deposit) deposits on thesurface of austenite grains or austenite and carbides during boost steps. This layer mediates in movingthe carbon deeper into the austenite grains. During the diffusion steps the layer deposited on the carbidesurfaces transfers into the austenite grains. Both this layer and the one previously deposited on theaustenite grains surface now undergo another catalytic decomposition into atomic carbon and hydrogen,other types of radicals and fine-crystalline graphite. The resulting carbon atoms are absorbed by surfaceaustenite grains and next diffuse deeper into the austenite grains, and, alternatively, into the carbidesurface, making the carbides grow and contributing to the increase in the carburized layer thickness.

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1. Introduction

Low pressure carburizing (LPC) of steel means a diffusive flow ofcarbon mass from the carburizing atmosphere to the surface layerof the items carburized. The process continues under the temper-ature of austenite existence, usually within the range of850e1000 �C and consists of two steps: boost and diffusion. In theboost step the carburizing atmosphere, consisting of aliphatichydrocarbons e acetylene, ethylene, propane additionally dilutedwith hydrogen or nitrogen [1e6], is let into the process chamberunder a lowered pressure (within the range of several or evenseveral hundred Pa). It is thought that the carburizing process of Feand carbon steels in the boost step is a result of a catalytic reactionof the carburizing atmosphere with the charge surface, which leadsto carbon release in the form of atoms, the absorption of it by thesurface and precipitation in the form carbon deposit and diffusivetransport deeper into the material. The carbon deposit is composedof hydrocarbon radicals and fine-crystalline graphite and constituesa source of carbon atoms [8,9]. On the surface of the carburizedparts the limit of carbon solubility in austenite is reached in a veryshort time (according to [5] only after 5 min under the temperatureof 950 �C) which is then followed by precipitation of carbon deposit[7]. The carburized layer after the boost steps contains high level ofcarbon on the surface. Further absorption of carbon atoms by theaustenite from the radical-carbon layer occurs at the beginning ofthe diffusion steps, subsequently followed by their transformationinto fine-crystalline graphite until all radicals are absent [10]. This

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results in additional carbon mass increase in the carburized layer.Simultaneously, as a result of carbon atom diffusion in theaustenite, the carbonmass progresses deeper into the surface layer.The thickness of the carburized layer increases and the concen-tration profile of carbon decreases around the edge area.

The aim of this paper was to analyze the mechanism of low-pressure carburizing of alloy steels based on carburizing of lowalloy steel 16MnCr5 used in the machine industry as well as highalloy steel CSB 50NIL used in heavy-laden engine elements (hassuccessfully been used in the most reliable bearings in aircraftengines [11]). Both steels, while carburized, differ in the diffusionprocess of carbon and alloy components. 16MnCr5 has a loweredlimit of carbon solubility in the austenite than pure iron and carbonsteels. While this steel is being carburized, the carbon diffusionoccurs initially only in the austenite, and when the carbonconcentration exceeds the limit of solubility, a nucleation processtakes place as well as the increase of carbides in the austenite,accompanied by diffusion in austenite-carbide area. In the case ofCSB 50NIL the austenite with carbides is found during the processof annealing at the temperature of 850e1050 �C. Hence, duringcarburizing the diffusion will occur in the austenite-carbide area.

2. Method

Low pressure carburizing was carried out in a vacuum furnacemanufactured by Seco/Warwick by means of FineCarb� method.The FineCarb� method is based on atoms conducting a fixednumber of boost and diffusion steps within a specified period oftime. During the boost step the carburizing atmosphere, consisting

Table 1Nominal chemical composition of the steel grade 16MnCr5 and CSB 50NIL, % of themass.

Steel grade Chemical composition, % of the mass

C Mn Cr Ni Mo Si V

16MnCr5 0.16 1.15 0.95 e e 0.40 e

CSB 50NIL 0.13 0.25 4.20 3.40 4.25 0.20 1.20

R. Gorockiewicz / Vacuum 86 (2011) 448e451 449

of a mixture of acetylene, ethylene and hydrogen, is let intoa process chamber. The boost step was carried out at a temperatureof 950 �C and at a pressure fluctuation of 6e7 h Pa. The chargeconsisted of the ballast e low-carbon steel sheets with a definedsurface and samples of 16MnCr5 and CBS 50NIL steel. Chemicalcomposition of the steels is listed in the Table 1 above. The sampleswere of F 20 � 10 mmwith a polished flat face surface. Carburizingparameters: low-pressure (4Pa) heating up to the temperature of950 �C at a speed of 10 �C/min, heating under low-pressureconditions for 30 min at a temperature of 950 �C, performinga defined number of boost and diffusion steps at this temperatureand then cooling down to the room temperature in nitrogen streamunder a pressure of 0.5 MPa. Seven processes were carried out:process 2 n (2 min of boost), process 5n (5 min of boost), process10n (10 min of boost), process 5n1w (5 min of boost þ 1 min ofdiffusion), process 10n1w (10 min of boost þ 1 min of diffusion),process 10n2w (10 min of boostþ2 min of diffusion).

The carburized surfaces were observed with a scanning electronmicroscope Jeol 5600 JV and the measurements of changes incarbon concentration were taken with the use of the EDS method.The observations and measurements were conducted at an accel-erating voltage of 12 and 15 kV. The findings are presented inFigs. 1e4.

Fig. 1. The photographs of 16MnCr5 steel samples carburized with the low-pressure methodformer austenite grains; b) 5 min - the carbon deposit against former austenite grains and agrains; c) 10 min - the carbon deposit against the matrix and considerably increased contrdeposit.

3. Results and discussion

The surface structure of the sample matrix of 16MnCr5, carbu-rized at various saturation times, visible in Fig. 1, indicates thepresence of carbon deposit on the surface. It is visible both atshorter times than when the limit of solubility of carbon in theaustenite is reached- Fig. 1, and at longer times- Fig. 1 b, c. Hence, asin the case of carburizing iron and carbon steels [6,8], the carbondeposit occurs on the surface of samples at the moment when thecarburizing atmosphere contacts the surface of the steel charge.Further carburizing of this steel, as seen in Fig. 1 b, c, leads tocarbide precipitation on the surface, especially on the edges of theaustenite grains. A delicate coat of carbon deposit can be seen onthe surface of the carbide precipitation. In the surface structure ofCSB 50NIL samples, carburized at similar durations- Fig. 2 a, b, onecan notice a very large contribution of carbide precipitation. Thesurfaces of both thematrix and the carbides are covered in a carbondeposit.

The low-pressure annealing of previously saturated samples ofboth 16MnCr5 steel- Fig. 4, and 50NIL- Fig. 2 c, d, leads to thedecrease in carbon deposit on the matrix surface. This finding wasconfirmed by the results of assessing changes in carbon concen-tration on the surface of 16MnCr5 samples, visible in Fig. 3 (samples5n, 5n1). The surface structure of 16MnCr5 samples after thesaturation steps suggests that when carbon concentration exceedsthe limit of its solubility in the austenite, which depends on thetimes of saturation and low-pressure annealing, a nucleationprocess occurs, accompanied by the increase or dissolution ofcarbides- Fig. 2 aec. In 50NIL samples (Fig. 2) the nucleation andincrease/dissolution processes run simultaneously.

In this case, the active carbon layer deposits mainly on theaustenite grain surface, though it is also present on the carbidesurfaces (most probably in the form of radicals), whichmay be seen

at a temperature of 950 �C at various boost times: a) 2 min - the carbon deposit againstconsiderable amount of fine carbides precipitation on the edges of the former austeniteibution of the carbides on the edges of the former austenite grains with minor carbon

Fig. 2. The photographs of CSB 50NILsteel samples after various steps of low-pressure carburizing at a temperature of 950 �C: a), b) 5 min of boost - almost the whole samplesurface, both the matrix area and the carbides, are covered in carbon deposit; c), d) 5 min of boost þ 1 min of diffusion e the carbon deposit covers the matrix and partly thecarbides, mainly in their central areas. The brighter carbide edges suggest the translocation of the deposit into the matrix.

R. Gorockiewicz / Vacuum 86 (2011) 448e451450

especially on CBS 50NIL steel samples (Fig. 2a, b). Low-pressureannealing of 16MnCr5 samples, in the structure of which a largeamount of precipitated carbides were present after the saturationstep (Fig.1- sample 10n), leads to their dissolution and coalescence-Fig. 4. At sufficiently long times of annealing the carbides dissolvein the austenite completely.

The observations of carbide surfaces of CBS 50NIL samplessaturated for 5min and subsequently annealedwith a low-pressuremethod for 1 min- Fig. 2 c, d- indicate considerable decrease incarbon deposit versus the sample without diffusion annealing.Furthermore, the carbide surface areas adjacent to the matrix show

0

10

20

30

40

50

60

5n 5n_1w 10n 10n_1w 10n_2wThe type LPC step

Th

e carb

on

co

ncetratio

n, %

o

f th

e m

ass

Fig. 3. The change in carbon concentration of 16MnCr5 steel samples depending onthe type of low-pressure carburizing step. Step 5n1w stands for 5 min of boost and1 min of diffusion. The carbon concentration was measured with the EDS method.

less deposit than the central areas. Carbide coalescence is alsovisible.

It may be presumed that during the low-pressure annealing theundissolved radicals progress from the carbide surface to theaustenite, where they undergo catalytic dissociation, and theresulting carbon atoms are absorbed by the surface austenitegrains. Subsequently they diffuse to the carbide surfaces, makingthem grow, and deeper into the austenite grains, contributing tothe increase of the carburized layer thickness.

4. Conclusions

The kinetics of low-pressure carburizing of alloy steels may runas follows: in the boost steps, at the moment when the atmo-sphere contacts the charge surface, themolecules of gases formingthe atmosphere (e.g. actylenee C2H2, ethylene C2H4) are absorbedby active surface centers, and subsequently they dissociate, asa result of catalytic reaction of the surface with simultaneouschemisorption of the dissociation products. The first stage of thedissociation is formation and a deposit process of different typesof radicals on the surface. Influenced by iron atoms, the radicalsundergo another catalytic decomposition into atomic carbon andhydrogen, other kinds of radicals and fine-crystalline graphite. Thecarbon atoms are attracted by the iron atoms and deposit on theaustenite grain surface. With simultaneous dissolution in inter-stitial spaces of the austenite, they diffuse deeper. The hydrogenatoms, in turn, link into molecules and leave the surface. Thisorder of absorption and catalytic dissociation steps hampers theopposite processes, leading to formation of the initial gas mole-cules. The resulting radical-carbon layer, mediates in the carbontransportation from the atmosphere to the austenite. When theconcentration of carbon in the austenite exceeds the limit ofsolubility, the nucleation process occurs accompanied by carbidegrowth. Put simple, these processes are controlled by the level of

Fig. 4. The photographs of 16MnCr5 steel samples carburized with the low-pressure method at a temperature of 950 �C: a) 5 min of boost, 1 min of diffusion e decreased amount ofcarbon deposit against the former austenite grains and mild carbide precipitation of the edges and inside the former austenite; b) 10 min of boost, 1 min of diffusion - thickenedcarbide precipitation and decreased amount of carbon deposit against the former austenite grains; c) 10 min of boost, 2 min of diffusion e thickened and decreased carbidesprecipitation against the matrix, partly covered in carbon deposit.

R. Gorockiewicz / Vacuum 86 (2011) 448e451 451

austenite saturation and the pace of carbon and alloy componentdiffusion from the austenite to the carbides. The formation ofcarbides on the saturated surface changes its crystalline proper-ties. Due to saturation of covalent bonds, the carbide surfaces donot show such catalytic activity as the austenite surface, thereforethey do not cause the dissociation of the atmosphere components(or they cause it to a small extent). The radicals which resultedfrom partial decomposition of the molecules deposit on thecarbide surfaces, and the undecomposed gas molecules undergodesorption and move out of the vacuum chamber. During theinitial stage of the diffusion steps the carbon atoms are furtherabsorbed by the austenite from the radical-carbon layer depositedon it. The layer transforms into fine-crystalline graphite until theradicals are absent. In the case of any precipitated carbidespresent in the austenite, the radicals deposited on their surfaces asa result of surface diffusion progress to the austenite grain areaswhere they undergo dissociation. The resulting carbon atoms areabsorbed by the austenite grain surfaces, diffuse to the austeniteand further to the carbide surfaces, causing the carbides to grow.The gradient of the chemical potential as well as the diffusion ofcarbon and the alloy components in the austenite, as well as thephenomena connected with dissolution/precipitation of carbides,lead to a progression of the carbon mass into the material and

thickening of the carburized layer with a simultaneous decrease inthe carbon concentration profile in the edge area.

Acknowledgments

The research was carried out under the programme R&N ofSeco/Warwick Poland.

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