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Vacuum 87 (2013) 26e29

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Vacuum

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Activation of carbon deposit in the process of vacuum carburizing withpreliminary nitriding

Piotr Kula, qukasz Kaczmarek*, Konrad Dybowski, Robert Pietrasik, Micha1 KrasowskiInstitute of Material Science and Engineering, Technical University of Łód�z, ul. Stefanowskiego 1/15, 90-924 Łód�z, Poland

a r t i c l e i n f o

Article history:Received 6 March 2012Received in revised form17 April 2012Accepted 29 June 2012

Keywords:Vacuum carburizingPreliminary nitridingCarbon deposit

* Corresponding author. Tel.: þ48 42 631 22 79; faE-mail addresses: Piotr.kula@p.lodz.pl (P. Kula),

(q. Kaczmarek), Konrad.dybowski@p.lodz.pl (K. Dp.lodz.pl (R. Pietrasik), michal.krasowski@p.lodz.pl (M

0042-207X/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.vacuum.2012.06.018

a b s t r a c t

A variety of vacuum carburizing methods with preliminary nitriding at the stage of heating the charge forcarburizing aim at limiting primary austenite grain growth during the process. In such a case, carburizingmay be conducted at temperatures even above 1000 �C. This article presents a study on the activation ofcarbon deposit (which was formed in the preceding processes of carburizing in the vacuum furnacechamber) under the influence of activation with hydrogen derived from ammonia dissociation. Theoptimum temperature range of ammonia addition to the furnace chamber was determined in order toavoid the activation of the deposit, thus also preventing the introduction of an additional, uncontrolledsource of carbon atoms to the process.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

In the process of vacuum carburizing of steel, carbon atomsquickly saturate the surface layer, even possibly super-saturating it,at the stage when gases are fed to the furnace chamber andcarbides are formed. Subsequently, the carbides dissolve after thediscontinuation of the flow of carburizing gases [1e4]. The aboveprocess aims at taking maximum advantage of the carbon potentialof the atmosphere. A high carbon potential may be reached byusing aliphatic hydrocarbons (i.e. acetylene, ethylene, or propane)as sources of carbon atoms. According to Kula et al. [3] and Baranand Gyulihandanov [5], the potential of those carburizing atmo-spheres for the pyrolytic release of carbon atoms on the steelsurface, in contrast to conventional carburizing, is extremely high.Thermal decomposition of hydrocarbons takes place rapidlythrough different kinds of indirect reactions, whose products formon the steel surface, thus influencing the process of its saturation[3,5e7], and across the entire volume of the operating chamber ofthe furnace. According to Kula et al. [3], these products includemainly carbon, which did not undergo complete absorptionthrough the surface because of its limited solubility in austenite andevolved in the form of graphite, and also hydrocarbons withdifferent levels of saturation, ranging from CHx to C7Hx (where x isa number from 1 to 13) and hydrogen. This deposit is partly

x: þ48 426 366 790.lukasz.kaczmarek@p.lodz.pl

ybowski), Robert.pietrasik@. Krasowski).

All rights reserved.

removed during the evacuation of gases from the furnace chamberduring the stage of holding, but some of it remains.

Kula et al. [8,9] found that in order to limit austenite graingrowth during a high-temperature process of vacuum carbu-rizing, preliminary nitriding may be applied through a tech-nology called PreNitLPC�. That process is based on supplyingammonia to the furnace chamber during the stage of heating ofthe charge for carburizing. However, at that stage, the carbondeposit already produced in the preceding processes and presentin the furnace chamber may be activated. This mechanismprobably results in releasing an additional, uncontrolled sourceof carbon atoms and causes difficulties in predicting the finaldistribution of carbon in the surface layer of the steel. At thesame time, it makes it difficult to control the process. This articlesets out to prove the above-mentioned thesis concerning theactivation of carbon deposit as a result of ammonia addition tothe furnace chamber at the stage of heating the charge forvacuum carburizing with pre-nitriding.

2. Materials and methods

Prepared Armco iron samples, 0.45 mm thick, with thechemical composition presented in Table 1, were exposed to theprocesses of thermochemical treatment according to the Pre-NitLPC� technology, excluding the stage of carburizing. Theseprocesses were based on lowering the pressure in the furnacechamber down to 10 Pa and heating the charge at a constant rateof 5 K/min up to the temperature of 1273 K, which is typical ofcarburizing. During the process of heating, ammonia was added

Table 1Chemical content of Armco iron used in the study.

Chemical content [wt.%]

Armcoiron

C Mn Si Cr Ni Mo Al V Cu Fe0.06 0.168 0.038 0.003 0.013 0.003 0.032 0.004 0.020

Table 2Range of temperatures at which ammonia was suppliedto the furnace chamber.

Process number Temperature range ofammonia saturation

I 723 Ke973 KII 723 Ke1073 KIII 723 Ke1173 K

Fig. 1. Mass spectrum recorded during analysis of the content of exhaust gases fromthe furnace chamber in the course of ammonia addition.

Fig. 3. Microstructure of the surface layer of Armco iron after process 2; a pearliticlayer is visible on the surface, and further on there are ferrite grains as well as pearliteclusters at grain boundaries.

P. Kula et al. / Vacuum 87 (2013) 26e29 27

within a range of different temperatures. Then, the samples werecooled in nitrogen under a pressure of 0.05 MPa. Data concerningthe range of temperatures at which ammonia was added is givenin Table 2. The pressure of ammonia amounted to 2.6 kPa, andthe flow through the furnace chamber was 11 l/min. Theprocesses were conducted in a vacuum furnace VDN-113. Anal-ysis of the atmosphere in the course of ammonia addition at theindicated temperature ranges was conducted using a Prevac RGAmass spectrometer equipped with the control-measurementsoftware TDS/UMS. The analysis results are presented in Fig. 1.The process of ammonia introduction to the chamber was startedat 670 K. Analysis of the atmosphere composition shows that in

Fig. 2. Microstructure of the surface layer of Armco iron before (a)

the temperature range from 670 K to 1070 K the only reaction isammonia dissociation. At above 1070 K, the formation ofcompounds of a molecular mass of 28 was observed. Qualitativeanalysis of the obtained data shows that this peak corresponds toa superposition of nitrogen and ethylene compounds. Ethylene isprobably formed as a result of a reaction of hydrogen and/orammonia with the carbon deposit. A reaction of nitrogen and/orammonia with the graphite from the heating element or othergraphite elements of the furnace is not very likely. Unexpectedformation of ethylene at temperatures below 1070 K leads touncontrolled carburizing of the steel charge.

3. Results

After completing the processes, Armco iron samples were sub-jected to a metallographic study in order to reveal their micro-structure. The results of these observations are presented in thephotographs below (Figs. 2e4). Then, using the CO2 infraredabsorption method according to the standard PN-EN ISO 9556 thecarbon content of the samples was established. The results arepresented in the diagram in Fig. 5.

4. Discussion

Based on interpretation of the Armco iron microstructure(Figs. 2e4) as well as on the measured carbon content (Fig. 5)

and after process 1 (b), respectively; ferrite grains are visible.

Fig. 4. Microstructure of the surface layer of Armco iron after process 3; pearlite grainsare visible across the entire surface layer, including innumerous, single ferrite grains.

Fig. 6. The scheme of probable reactions related to hydrogen activation of carbondeposit during the process of pre-nitriding.

P. Kula et al. / Vacuum 87 (2013) 26e2928

obtained for the studied samples in the processes of thermo-chemical treatment differentiated by the temperature range ofammonia addition, one can easily infer that the carbon depositproduced in the preceding carburizing processes can be activated(within the range of pressure applied) after exceeding a thresholdtemperature value of 973 K by hydrogen derived from ammoniadecomposition. This probablemechanism of activation is presentedin Fig. 6. Atomic hydrogen created as a result of thermal decom-position of ammonia (reaction 1) diffuses into the structure (1a) ofthe deposit previously produced on the elements of the furnacechamber, which may decrease the acceptor properties of theirsurface. Decreasing the acceptor parameter of the deposit mayresult in exceeding the critical value of donoreacceptor interactionsat lower values of the vibration amplitude of the molecules formingthe deposit. In such a situation, the content of compounds withatomic masses ranging from 15 to 80 is found to increase in theprocess atmosphere of the furnace (Fig. 1). Intensified processes ofhydrogenation and/or decomposition of the deposit in the furnaceatmosphere take place at the following stage, according to theproposed mechanism: firstly, bonds with the lowest energy(DH�

CeC ¼ 356 kJ/mol, (DH�C]C ¼ 598 kJ/mol) undergo

cleavage, while intensive processes begin to saturate unsaturated

Fig. 5. Percentage content of carbon in Armco iron foils after different temperaturevariants of ammonia addition during the stage of heating the charge for carburizing.

compounds in reaction with hydrogen produced as a result ofammonia decomposition (reaction 2). A parallel stage of chemicaldecomposition of the deposit involves processes that lead tobreaking CeC bonds in the saturated compounds (reaction 3). Theradicals produced at that time (reaction 4) are attracted as a resultof electrostatic interactions by the metallic surface of the processedelement, which is here the acceptor of the unpaired electron e theradical (reaction 5). Radicals adsorbed in such a way undergosubsequent processes of decompositione CeH bond in reaction (5)with the energy of (DH�

CeH ¼ 415 kJ/mol), until the formation of

Fig. 7. Characteristic physicochemical processes taking place within specifiedtemperature ranges.

P. Kula et al. / Vacuum 87 (2013) 26e29 29

carbon “in statu nascendi” (reaction 6), which is subject to diffusioninto the structure of the material.

Taking the above into consideration, Fig. 7 shows the mostprobable course of the chemical reactions taking place withinspecified temperature ranges during the addition of ammonia intothe operating furnace chamber.

5. Conclusions

Based on the experiments, it appears that the addition ofammonia at the stage of heating the charge for carburizing causesthe activation of carbon deposit previously formed in the furnacechamber, and that the activation begins at a temperature of over973 K at the analyzed pressure of 2.6 kPa. As the amount ofactivated carbon is not regulated, this procedure limits thepossibility to precisely control the vacuum carburizing processconducted according to the PreNitLPC� technology. Therefore,from the point of view of control, a safe range of ammonia addi-tion is between 723 K and 973 K, as long as one can limit the

growth of primary austenite in a high-temperature vacuumcarburizing process.

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