High Temperature Low Pressure Carburizing with ... · to keep the distortion level from the high temperature carburizing similar to that for traditional ENDO carburizing, although

Chiang Mai J. Sci. 2013; 40(5) 865

Chiang Mai J. Sci. 2013; 40(5) : 865-873http://it.science.cmu.ac.th/ejournal/Contributed Paper

High Temperature Low Pressure Carburizing withPrenitriding Process −−−−− The Economic Option forVacuum CarburizingPiotr Kula*, Robert Pietrasik, Sylwester Paweta, Konrad Dybowski, Lukasz Kaczmarekand Agnieszka GladkaInstitute of Materials Science & Engineering, Lodz University of Technology, Stefanowskiego St.1/15, 90-924, Lodz, Poland.*Author for correspondence; e-mail: [email protected]

Received: 12 July 2012Accepted: 27 November 2012

ABSTRACTThe original idea of grain growth limitation by preliminary nitriding preceding

low pressure carburizing (LPC) is presented as a useful option for the FineCarb®

technology. This new process called PreNitLPC® enables high temperature carburizingup to 1050oC for a variety of common carburizing steels without any adverse effects onboth microstructural or performance. The shortening of carburizing time may result ina decrease in manufacturing costs as well as energy consumption from 5-50% dependingon the case depth. This has been confirmed on the basis of several pilot industrialinstallations.

The metallurgical background of PreNitLPC® has been discussed at themicrostructural level. The mechanism of nanonitrides precipitation has been found asthe effective way for intensive nucleation of austenite grains and for inhibition of grainboundary movement at significantly high temperatures. The results of comparativeinvestigations of fatigue and impact strength as well as pitting resistance for traditionalendothermic gas carburizing (ENDO), standard LPC and PreNitLPC® are presented.They confirm the efficiency of the pre-nitriding option in grain growth limitation forplain carbon and low alloying steels enabling high mechanical properties to be achievedin machine parts hardened using PreNitLPC®.

The problem of increased carburizing temperature on heat treatment distortion isalso discussed. The results of comparative geometrical measurements show the possibilityto keep the distortion level from the high temperature carburizing similar to that fortraditional ENDO carburizing, although this requires additional optimization of thePreNitLPC® parameters.

Keywords: vacuum carburizing, case hardening, costs assessment, fatigue strength

866 Chiang Mai J. Sci. 2013; 40(5)

1. INTRODUCTIONThermal and thermochemical processing

are significant capital-intensive factors inthe production process of machine partsand tools; therefore, it is important interms of the competitive advantage andprofitability of the metal industry in theglobal market environment to make thisprocessing as economical as possible. Nowthat the competition is fierce, high productquality must be accompanied by lowprices. For a novel technological solutionto be adopted for a commercial application,it must not be worse than previouslyapplied solutions in terms of quality, butit must also provide an opportunity tocreate an economic advantage.

High-temperature vacuum carburizingwith pre-nitriding - PreNitLPC® - is atechnology which meets these requirements.The typical process temperatures intraditional carburizing is 920oC, buttemperatures as high as 1050oC can be onlyapplied in vacuum carburizing furnaces [1].An increase in carburizing processtemperature from 920oC to 1,000oC canreduce the duration of the carburizingprocess, reducing the cost of treatment.However, this is associated with the riskof rapid austenite grain growth. To preventthis, a novel technology of nitriding-supported vacuum carburising - PreNitLPC®

- has been developed. The processing involvesfeeding ammonia into the preliminaryphase of the process - during the pre-carburising heating. This results in anabsence of grain growth in the carburizedlayers even when the process temperatureis higher than traditional processtemperatures. Nitrogen enters the surfacelayers of the steel to form nitrides and/or carbonitrides, which block austenitegrain growth during the carburising phase[2,3].

2. CARBURIZING TRIALTwo low-pressure carburizing processes,

conventional low-pressure (LPC) and low-pressure carburizing with pre-nitriding(PreNitLPC®), have been conducted at920oC and 1000oC in order to compare thestructure and properties of the surfacelayers obtained from these two treatments.Conventional gas carburising (ENDO) at920oC has also been used to provide astandard benchmark for the two lowpressure processes.

The carburising process was conductedin an acetylene - ethylene - hydrogenatmosphere. Nitrogen for the pre-nitridingprocess was obtained by dissociation ofammonia. The dosing parameters of thecarburising atmosphere were selected inaccordance with the relevant patent [4], andthose of ammonia in accordance withanother patent [3]. Ammonia was fedwithin the temperatures range from 400oCto 700oC during the heating stage of thecharge for carburising for a period of 60min. Two kinds of steel were carburised -typical carburising grades 16MnCr5 and17CrNi6-6. The process parameters areshown in table 1. Samples for metallographictests had a diameter of 25 mm and athickness of 10 mm. Samples for mechanicaltesting had shape and dimensionsaccording to the standards used in suchmeasurements.

For process control purposes, carbondistribution in the steel surface layer wasdetermined after carburizing via gradualremoval of surface layers. Carbon contentin the layers was determined byIR (infrared) absorption with a LecoCS200 analyzer. The results are shown inFigure 1.

In the next stage of the research,microstructures obtained at differenttemperatures were compared in regard to

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Table 1. Carburising process parameters.Type of

carburising ENDO LPC PreNit LPC

Processtemperature

Ammonia dosingtemperature - - - 400oC – 700oC

rangeCarburizing time 2h47min 23min 11min 23min 11min

Diffusion time - 1h 52min 43min 1h 52min 43minGas carburizing endothermic gas C2H2+C2H4+H2

Pressure 1,100 hPa Carb:300-800 Pa, Diff.: 10PaLayer thickness

(0.4%C) 0.6 mm

Surfaceconcentration 0.75%C

920oC 920oC 1,000oC 920oC 1,000oC

Figure 1. Carbon profile in the surface layer of 16MnCr5 steel following low-pressurecarburising and ENDO at 920oC and following low-pressure carburising withpre-nitriding at the temperature of 1,000oC.

the austenite grain size (Figure 2). Thesamples were etched by aqueous solutionof picric acid, at 70oC. The grain size wasestimated by a planimetric method,according to ISO 643: 2003.

Figure 3 shows that grains in thecarburized layer are much smaller after thePreNitLPC® processes. Even grainsproduced by carburising at a temperatureof 1,000oC are smaller than those

868 Chiang Mai J. Sci. 2013; 40(5)

Figure 2. Comparison of grain size in the surface layer and core of 16MnCr5 steelfollowing low-pressure carburising, following low-pressure carburising with pre-nitridingand ENDO.

obtained by carburising at 920oC withoutpre-nitriding.

The effect of the process temperatureson the core grain size was examined in theLPC processes conducted at 920oC and inPreNitLPC® processes at 1,000oC. Asexpected, the grains in the core of 16MnCr5steel carburised by the PreNitLPC®

technology at 1,000oC are larger than inthe LPC process at 920oC (Figure 2 and 4).Therefore, it can be concluded that thesmall size of grains in the surface layer iscaused by the presence of nitrogen fedduring the heating stage. The differencesin the grain size at 920oC obtained fordifferent technologies are the result ofmeasurement experimental errors.

In conclusion, raising the temperaturesof low-pressure carburizing treatment cansignificantly reduce the process duration,while at the same time eliminating excessivegrain growth in the steel surface layer(Table 1).

3. EVALUATION OF THE STRENGTHPROPERTIES

The most important feature concerningthe potential application of the PreNitLPC®

technology is the evaluation of themechanical properties.

Comparison of the results of hardnessdistribution in the surface layer of16MnCr5 steel shows that increasing thecarburizing process temperature in thePreNitLPC® technology does not resultin a decrease of the hardness as comparedto low-pressure carburising alone. Thehardness distributions obtained from thetwo processes are similar (Figure 5).

A series of tests were also conductedto determine the fatigue bending strengthusing the resonance method. Themeasurement relies on the implementationof the resonance frequency of vibrationon the sample. Sample fatigue failure isdetected by the change of sample vibrationfrequency. The tests were done accordingto ASTM E 606-04.

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Figure 5. Comparison of 16MnCr5 steel hardness following low-pressure carburisingand ENDO at 920oC and low-pressure carburising with pre-nitriding at 1,000oC.

Figure 3. Microstructure of the surface layer of 16MnCr5 steel following low-pressurecarburising at 920oC (a) and following low-pressure carburising with pre-nitriding at1,000oC (b).

Figure 4. Microstructure of 16MnCr5 steel core following low-pressure carburising at920oC (a) and following low-pressure carburising with pre-nitriding at 1,000oC (b).

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The results, shown in Figure 6, wereused to plot W hler’s curves for limitedand unlimited fatigue strength for samplesfollowing the LPC process at a temperatureof 920oC and following the PreNitLPC®

process conducted at a temperature of1,000oC. A comparison of the values showsthat the fatigue bending strength of16MnCr5 steel is higher following the

PreNitLPC® process.Contact fatigue tests (pitting resistance)

were performed using a modified apparatuswith four-node friction cone-balls under aload of 392.4 N according to PN-76/C-04147. The values of contact fatiguestrength obtained for 16MnCr5 steel ineach variant of carburising (Table 1) arecomparable and close to 1.6 × 106 cycles.

Figure 6. Comparison of unlimited fatigue bending strength for 16MnCr5 steel afterdifferent surface treatments.

4. GENERATION OF CARBURISINGATMOSPHERE

A competitive edge for surface pro-cessing, both on local and global markets,may be created by reducing operating costsfor vacuum machines and equipment andby reducing the duration of processes.However, relatively high costs are stillincurred as a result of having to usecarbonaceous gases, especially ethylene(C2H4). The average price of ethylene(C2H4) on the European market isapproximately six times higher than thatof acetylene and hydrogen. The problemis especially significant in the countrieswhere there are problems with ethylene

supply, e.g. in India. This considerablylimits the possibility of application ofvacuum technologies in less industrialisedcountries and makes them more costly.Therefore, there is a strong need to producea carburizing mixture with set parametersin regard to an additional reduction ofthe cost of the process of low-pressurecarburising.

To this end, a working atmospheregenerator has been developed in whichhydrogen and acetylene (C2H2), as well asa palladium regiospecific catalyst depositedon Al2O3, is used to produce a carburisingmixture with the following composition40%C2H2, 40% C2H4 and 20% H2. Due to

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appropriately selected process parameters,it is possible to hydrogenate acetylene toethylene without formation of ethane,which could change the carbon potentialof the processing atmosphere. Moreover,it is possible to eliminate formation ofoligomeric compounds on the catalystsurface, which effectively limits deactivationof its active sites. This prevents a decreasein the process efficiency, which is close to55%, over time. In the next stage, in orderto obtain a carburizing mixture of thedesired composition, i.e. 40%C2H2, 40%C2H4 and 20%H2, C2H4 has to be diluted

at the appropriate ratio with C2H2 and H2.Then, the carburising gas mixture can besent to the reaction chamber and the low-pressure carburising then be conducted.

It is also important that an on-lineproduction of a mixture of carbonaceousgases is possible in the “Boost” and“Diffusion” system, for the full range ofpossible flow rates and times of each“Boost” stage. It also enables optimisationof the process cost in terms of eliminationof an additional buffer tank where an excessamount of mixture is stored.

Figure 7. An example of change of the carburising mixture, produced: “on line” for thelow-pressure process in the “Boost” and “Diffusion” options.

5. ECONOMICS OF THE PROCESSIn 1999 a research plan was developed

at ASM International, where priorityaspects were identified for the improvementof thermal processes. These includedeconomic aspects, such as: reducing theprocess duration, reducing the production

cost, reducing energy consumption, etc. [5].In general high-temperature low-pressurecarburising with pre-nitriding - PreNitLPC®,meets those objectives.

A breakdown of costs reveals theeconomic advantage of the PreNitLPC®

technology as shown in Table 2 for layers

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Table 2. Results of the cost and profit analysis for real component.Comparison of unit costs and profit generation possibility

Estimated CarburizingDepth (ECD)

[mm]Profit Unit cost Profit Unit cost Profit Unit cost

[%] [%] [%] [%] [%] [%]0.4 100 100 X X 111.3 95.60.6 100 100 X X 113.8 79.80.9 100 100 X X 120.6 78.21.2 100 100 X X 159.2 71.22.0 X X 100 100 106.0 83.23.0 X X 100 100 148.0 84.45.0 X X 100 100 175.0 74.2

ENDO ENDO PreNitLPC®®®®®

920oC 980oC 1000oC

up to 5 mm thick. The low-pressuretechnology is already more cost-effectivefor a 0.4 mm layer. An analysis of theunit cost shows that the PreNitLPC®

technology is cheaper by 4% to 29% whenthe thickest layers were obtained.

Currently, the low-pressure processesaccount for approx. 15% of the carburisingmarket [6]. It is estimated that the level willhave increased to about 35% by 2020 [7].

Table 2 reveals not only lower unitcosts but also the potential for generationof higher profit despite lower pricesresulting from the lower unit cost (assumingthe same 20% margin). Implementation ofthe new technology of high-temperaturecarburising with pre-nitriding makes itpossible to generate from 6% to as muchas 75% higher profit as compared to theconventional technology, depending onthe layer thickness.

6. SUMMARYThe mechanical properties such as

pitting resistance and hardness of layersproduced in the PreNitLPC® technologyare comparable to those achieved in theLPC process. Feeding nitrogen during theheating phase makes it possible to achievehigher fatigue bending strength. Inves-tigation of the grain size shows that raisingthe temperature of the PreNitLPC® processby nearly 100oC still results in smallergrains than in the traditional LPC process.

The PreNitLPC® technology can beapplied at much higher temperaturesas compared to LPC or conventional

technologies, without reduction inproperties. Due to the temperatureincrease, the process duration needed toachieve the desired layer thickness can beconsiderably reduced. This has a positiveeffect on the PreNitLPC® process economyand provides huge application opportunitiesfor mass production.

The possibility of producing a three-component carburising atmospherewithout having to use expensive ethylene,which is not easily available, increases theapplication potential of the technology andimproves its economical aspect.

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REFERENCES[1] Gr fen W., Hornung M., Irretier O.

and Rink M., Applications of low-pressure carburizing with hightemperatures (1,000oC to 1,050oC) inindustrial practice, Haerterei-TechnischeMitteilungen, 2007; 62: 97-102.

[2] Kula P., Pietrasik R., Dybowski K.,Atraszkiewicz R., Wolowiec E.,Korecki M. and Olejnik J., Newtechnological pathways for universalvacuum furnaces, 18th CongressIFHTSE, Rio de Janeiro, 2010.

[3] Kula P., Olejnik J. and Heilman P.,European Pat. No. EP1558781 (2007),United States Patent No. US7550049(2009).

[4] Kula P., Olejnik J. and Heilman P.,European Pat. No. EP1558780 (2007),United States Patent No. US 7513958(2009).

[5] ASM Heat Treating Society’s 1999Research & Development Plan,Available from ASM International,Materials Park, OH44073, USA.

[6] Herring D.H., Pros and cons of atmosphereand vacuum carburizing IndustrialHeating, 2002.

[7] Why vacuum carburizing? Heat TreatAlternative offers advantages overconventional methods. MathewJaster, www.geartechnology.comMarch 2010.

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