research article effect of process parameters on gas...

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2013 Article ID 217848 6 pageshttpdxdoiorg1011552013217848

Research ArticleEffect of Process Parameters on Gas Nitriding of Grey Cast Iron

Nana Wang and Jinxiang Liu

School of Mechanical Engineering Beijing Institute of Technology Beijing 100081 China

Correspondence should be addressed to Jinxiang Liu liujxbiteducn

Received 2 August 2013 Revised 22 October 2013 Accepted 18 November 2013

Academic Editor Martha Guerrero

Copyright copy 2013 N Wang and J Liu This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Gas nitriding process parameters have significant effects on the nitriding layer of material In the present work a series of gasnitriding experiments on pearlitic grey cast iron specimens were carried out at different temperatures To study the influence ofnitriding process parameters on the nitriding layer the numerical simulations of nitriding processing are performed which takeinto account the threshold value of nitriding potentialThe results show that the numerical simulations incorporating the nitridingpotentialrsquos threshold value can accurately predict the case depth and nitrogen concentration profile The nitriding layer increaseswith increasing nitriding time and temperature whereas the nitrogen concentration on the surface decreases with increasingtemperature Besides the results also reveal that the nitriding potential nearly has no effect on the case depth whereas it has greatinfluence on nitrogen concentration on the surface and chemical composition of the compound layer

1 Introduction

Gas nitriding is a typical thermochemical surface treatmentwithin the eutectoid temperature in which the nitrogen istransferred from the ammonia gas into theworkpiece surfacegenerating the gamma phase (1205741015840-Fe

4N) and epsilon phase (120576-

Fe3N) [1 2] After nitriding the so-called compound layer

and diffusion zone can be formed which are capable ofimproving the wear and corrosion resistance of materialsand enhancing the fatigue endurance [3 4] respectively Thenitriding process parameters such as nitriding time nitridingtemperature and nitriding potential have great influence onthe forming of nitriding layer The nitriding process param-eters are mainly determined by experience in practical engi-neering applications Their effects on nitriding layer are notvery clear from a mathematical view Therefore consideringthe sensitivity of nitriding layer to the process parametersthere is an urgent demand to investigate the influence of theprocess parameters mathematically

In recent years a significant amount of experimentalwork has been conducted aiming to investigate the nitridingprocess parametersrsquo effect on nitriding process Neverthe-less due to high time-consuming and economic costs theexperimental method is not very effective and feasible inpractical applications especially when the parameters vary in

a wide range Under the circumstances numerical analysiscan be a good choice to study the processing parametersrsquoinfluence Arif et al [5] investigated the effect of nitridingtime nitriding temperature and nitriding potential on thenitrogen concentration and hardness depth profile by usingfinite element code In their research the influence of thecompound layer on the nitrogen concentration distributionwas not considered so the calculated case depth was largerthan the experimental data To avoid the aforementionedshortcomings Yang et al [6 7] proposed a compound layergrowth model for an alloy steel However the differencebetween the critical value and threshold value of nitridingpotential was ignored in Yangrsquos research Besides Hu et al [89] simulated the nitriding process under multistage nitridingpotential controlling In their study a nitriding layer withoutthe compound layer was obtained by controlling nitridingprocess parameters Furthermore most of the researchespublished in the literature were focused on the nitridingprocess of pure iron or steels For grey cast iron a widely usedmaterial related studies are lacking

In the present work a series of gas nitriding experimentson grey cast iron specimens were conducted at 550 570 and590∘C Afterwards the numerical simulations were per-formed which combined the influence of nitriding potentialrsquosthreshold value on nitriding layer and Fickrsquos second law The

2 Advances in Materials Science and Engineering

calculated thickness of diffusion zone was compared withthe tested data to verify the numerical simulations Finallythe effect of nitriding process parameters including nitridingtemperature nitriding potential and nitriding time was fur-ther investigated

2 Experimental Work

21 Material Thematerial tested in the study is pearlitic greycast iron The chemical composition of the material is shownin Table 1 Figure 1 depicts the microstructure of the grey castiron The graphite within the material is in a sheet form Dueto the large amount of graphite the grey cast iron has goodheat transfer ability self-lubricating performance and goodcasting property It is a very suitable material for componentsoperating under high temperature and friction condition

22 Nitriding Process Sample Preparation and EquipmentGas nitriding experiments were carried out on grey cast ironspecimens The specimens were divided into three groupswith two specimens in each group The tested temperatureswere set as 550 570 and 590∘C for each group with the sametotal time of 24 hours and nitriding potential of 15 atmminus12

After nitriding the specimens were prepared by a stan-dard grinding and polishing procedure and microstructureobservation was performed using JSM-5610LV SEM Spec-imens were etched by 5 Nital in order to observe thenitriding layer byOLYMPUSPMG3Hardness of the sampleswas also tested by applying a load of 1 N with holding time of10 s Besides the hardness tests were conducted in 6 points ofone sample and the average value was taken to represent thefinal hardness

In order to obtain the composition of the nitriding layerXRD analysis of the surface layer was performed using D8ADVANCE X-ray diffractometer with CuK at 40 kV and40mAThe scanned area was in the range of 15ndash90∘

3 Numerical Model of Gas Nitriding Process

31 Theoretical Background In gas nitriding the atomicnitrogen is dissociated from ammonia (NH

3) gas and then

diffuses into the workpiece surface of component The disso-ciation obeys the following rule

NH3=3

2H2+ [N] (1)

where [N] represents N atom dissolved in the workpiece sur-face

The above equation presumes that equilibrium has beenestablished The activated atomic nitrogen is dissolved in theiron crystal lattices which not only promotes the nitridingof 120572-Fe but also accelerates the formation and growth ofcompound layer The solution of atomic nitrogen in pureiron is relatively fast which does not affect the nitriding rate[10] When building numerical model for convenience it isassumed that the graphite has no influence on the nitridingrate although graphite is widely distributed in the ironmatrix[11] The resulting effect of this assumption will be discussedin detail in Section 51

Figure 1 SEM analysis of the grey cast iron used in an engine cylin-der liner

The transformation of atomic nitrogen within grey castiron is determined by the diffusion velocity which is de-scribed by Fickrsquos second law

120597119888

120597119905= 1198631205972119888

1205971199092 (2)

where 119888 represents the nitrogen concentration within thematerial and119863 is the diffusion coefficient of atomic nitrogen

32 Numerical Model and Boundary Conditions Nitridingpotential is the measurement of nitriding ability of NH

3

gas The phases and nitrogen concentration on the nitridingsurface are closely related to the nitriding potential after localequilibrium is established At the earliest the well-knownLehrer diagram of pure iron which was constructed byLehrer through investigating the equilibrium conditions ofNH3-H2and Fe-N [12] was used to determine the rela-

tionship of the nitrided phases nitrogen concentration andnitriding potential

The critical value of nitriding potential is defined as theturning point when the nucleation of 1205741015840 phase occurs atcertain temperatureThis value can be directly obtained fromthe Lehrer diagram However in engineering applicationsthe nitriding potential for compound layer nucleation is notcompletely equal to the critical value since a minimal valuethe so-called threshold value has to be exceeded for certainnitriding time The theoretical equation [13] for thresholdvalue of nitriding potential is

119870nt =119870nc

1 minus exp (1205732119905119863) erfc (120573radic119905radic119863) (3)

where 119870nt is the threshold value of nitriding potential fornitriding time 119905119870nc is the critical value of nitriding potential120573 is the mass transfer coefficient in gas-solid reaction anderfc represents the complementary error function

At a certain nitriding temperature the time 119905119888 repre-

senting the nucleation starting time of compound layer canbe obtained by substituting the critical value of nitridingpotential into (3) For accuracy a two-stage process is defined

Advances in Materials Science and Engineering 3

Table 1 Chemical composition (wt) of the studied grey cast iron

C S Si P Mn Cr B Cu Ni Ti V Mo Nb Fe298 0032 1646 0061 0469 0051 0003 0348 1278 0045 0042 0874 0066 Bal

in the current study In the first stage that is 119905 le 119905119888 the

compound layer is not formed In the second stage that is119905 gt 119905119888 the compound layer starts to nucleate and grow The

initial condition for stage I can be described as

119905 = 0 119888 = 1198880 (4)

where 1198880is the initial nitrogen concentration within grey cast

ironThe boundary condition for stage I is

119909 = infin 119888 = 0 119909 = 0

minus119863119889

120597119888

120597119909= 120573 (119888

119892minus 119888119904)

(5)

where119863119889represents the diffusion coefficient of atomic nitro-

gen in the diffusion zone 119909 is the depth from surface 119888119892is the

nitrogen concentration in the gas phase and 119888119904is the nitrogen

concentration on the surface of grey cast ironAt the end of stage I the compound layer begins to

be formed and the nitrogen concentration in the interfacebetween compound layer and diffusion zone is in equilib-rium In the diffusing process the nitrogen concentrationin the interface is set as unchanged Therefore the obtainednitrogen concentration on the surface of cast iron can beconsidered as the initial boundary condition for stage IISince the thickness of compound layer is much less than thatof the diffusion zone [7] the influence of the interfacemovingduring the nitriding process is ignored in the study Based onthe hypothesis the boundary condition for stage II is of thefollowing form

119909 = infin 119888 = 0

119909 = 0 119909 = 119888119889

(6)

where 119888119889is the nitrogen concentration on the workpiece sur-

face after stage IThe parameters needed to be calibrated are the diffusion

coefficient119863119889and mass transfer coefficient 120573 For parameter

119863119889 it is mainly influenced by the alloying element which is

capable of reacting with the atomic nitrogen For grey castiron the governing alloying element is Cr whereas it has littleeffect on the diffusion coefficient when its content is lowerthan 5 [14 15] The Cr element in the analyzed grey castiron is 0051 so the influence can be ignored The diffusioncoefficient is determined by the following equation [16]

119863119889= 066 exp(minus77900

119877119879) (7)

where 119877 is the gas constant with a value of 8314 JmolsdotK and119879 is the nitriding temperature

Table 2The tested hardness of the compound layer at 550 570 and590∘C

Distance from the surface (120583m) Hardness value (Hv)550 570 590

100 440 501 474130 403 455 447

For parameter 120573 it is determined by the following equa-tion [8]

120573 = 00621 exp (minus6267119879) (8)

In practice since the compound layer is a multiphasecomposition the diffusion coefficient of atomic nitrogen inthis area is hard to be determined and the thickness ofcompound layer can only be determined by experimentaldataTherefore the study is aimed at predicting the thicknessof diffusion zone and analyzing the influence of nitriding pro-cess parameters

4 Experimental Assessment

41 Optical Microscope The cross-sectional observationthrough optical microscopes was made Figure 2 shows thecase depths after nitriding at 550 570 and 590∘C As shownin the figure the white layer is formed on the surface which isknown as the compound layer Beneath the compound layerthere is a diffusion zone which is not so clear to be observedIt can be found that the compound layer increases with theincrease in nitriding temperature and the average thicknessvalues at 550 570 and 590∘C are 450 905 and 129 120583mrespectively The relationship between the compound layerthickness (ℎ) and nitriding temperature is plotted in Figure 3which exhibits a linear characteristic that is ℎ = minus1109 +02119879

42 Hardness Measurement The tested hardness values inthe depth of 100 and 130 120583m are listed in Table 2 With theincrease in distance from the surface the hardness decreasesto some degree For both 100 and 130 120583m the hardness getsits maximum value at 570∘C

The case depth is determined by testing the microhard-ness of the nitrided samples The depth is considered to bethe distance between the workpiece surface and the locationwhere the hardness is the same with the matrix material Forthe analyzed grey cast iron the obtained average case depthsafter nitriding are 79 139 and 155 120583m at 550 570 and 590∘Crespectively

43 XRD Analysis Figure 4 presents the XRD diffractogramof the samplersquos surfaceThe horizontal and longitudinal coor-dinate represents the diffractogram angle and diffractogram


Compound layer10120583m

(a)

Compound layer 10120583m

(b)


(c)

Figure 2 Cross-sectional scanning electron microscopy observa-tion of the nitrided samples at (a) 550 (b) 570 and (c) 590∘C

strength respectively Due to the relative thinness of thecompound layer the reflection intensity of the 1205741015840-Fe

4N phase

is not distinct enough Nevertheless it can be found that afternitriding the major phases on the surface are 1205741015840-Fe

4N and 120576-

Fe3N which confirms the presence of superficial compound

layer

5 Numerical Results and Discussions

51 Numerical Results The gas nitriding process in the studywas simulated by using the model described in Section 3Thecalculated nitrogen concentration of grey cast iron is depictedin Figure 5 It can be seen that with the increase in nitridingtemperature the thickness of diffusion zone increases For aclear presentation the experimental and calculated thicknessvalues are listed in Table 3 and the calculation errors are alsopresented It needs to be noted that the tested thickness of dif-fusion zone is determined by removing the compound layerfrom the tested case depth The calculated thickness valuesare in good agreement with the experimental data with anabsolute maximal error value of 68 which demonstratesthe accuracy and feasibility of the numerical simulations

Com

poun

d lay

er th

ickn

ess (120583

m)

Temperature (∘C)

15

12

9

6

3

540 550 560 570 580 590 600

Tested valueFitted line

Figure 3 Experimental compound layer thickness as a function ofnitriding temperature

200

150

100

50

0

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Inte

nsity

(CPS

)

2120579 (∘)

FeFe3NFe4N

(203

)(301

)

( 111

)

( 110

)(200

)

(200

)

(220

)

(331

)(211

)Figure 4 XRD diffractogram of the samplersquos surface for the ana-lyzed grey cast iron after nitriding

Table 3 The calculated and experimental case depths of the greycast iron at 550 570 and 590∘C

T (∘C) Tested (120583m) Calculated (120583m) Error ()550 74 79 68570 129 125 minus31

590 142 141 minus07

For the presence of calculation error it can be attributedto the following reasons Firstly during calculation theinfluence of the interfacersquos movement was not consideredwhich was a disadvantage of the present model Secondlythe critical value of nitrogen potential which was directlyobtained from the Lehrer diagram of pure iron may not beaccurate for grey cast iron Furthermore for the analyzedgrey cast iron the largely distributed graphite may also havesome uncertain effects on the nitriding potential value


10

08

06

04

02

00

0 50 100 150 200

Distance from the interface (120583m)

550∘C

570∘C

590∘C

N (w

t)

Figure 5The calculated nitrogen concentration distribution at 550570 and 590∘C

Nevertheless the accuracy of the numerical simulations isconsidered to be acceptable which is a guarantee of thefollowing effect analysis of nitriding process parameters

52 Effect of Nitriding Temperature on the Threshold Value ofNitriding Potential Nitriding temperature is a crucial processparameter which greatly affects the gas nitriding process Inthe study a threshold value of nitrogen potential is adoptedinstead of the traditional critical value which is a significantadvantage of the present model in comparison with theexisting ones [5ndash7] The relationship between the thresholdvalue and temperature is given in Figure 6 As temperatureincreases the threshold value decreases significantly Underthe circumstances the time needed to form the compoundlayer decreases at higher nitriding temperature

53 Effect of Nitriding Potential Nitrogen potential whichrepresents the nitrogen activity on the workpiece surface isused to control the compound layerrsquos thickness and phasesIn order to investigate the influence of nitrogen potentialnumerical simulations were conducted with respect to dif-ferent nitriding potential values that is 032 075 15 and35 atmminus12 Figure 7 presents the calculated nitrogen con-centration profile for different nitriding potentials It can beobserved that the nitrogen potential has no effect on the casedepthHowever the nitrogen concentration on theworkpiecesurface is of great difference which exhibits the highest valuefor nitriding potential of 35 atmminus12 The analysis resultsdirectly demonstrate that the nitriding potential has greateffect on the phase composition of compound layer since itstrongly influences the nitrogen concentration on the surface

54 Effect of Nitriding Time To investigate the influence ofnitriding time on the case depth numerical calculations wereperformed with different nitriding time of 7 11 24 and 40hours Figure 8 shows the calculated nitrogen concentration

5

4

3

2

1

0

0 5 10 15 20 25 30

tc (h)

Kn t

(atm

minus12

)

550∘C

570∘C

590∘C

Figure 6The threshold value of nitrogen potential at different tem-peratures

032 atmminus12

075 atmminus12

15 atmminus12

35 atmminus12

10

08

06

04

02

00

0 50 100 150 200


N (w

t)

Figure 7 The nitrogen concentration distributions with differentnitriding potentials

profile for different nitriding time Because the thresholdvalues of nitriding potential at the nitriding time of 7 and 11hours are higher than those of the present nitriding potentialno compound layer is formed on the surface When nitridingtime is 24 and 40 hours the compound layer is formed andthe nitrogen concentration on the workpiece surface turnedinto the same value At the same time the case depth increasesas nitriding time increases from 24 to 40 hours

6 Conclusions

Based on the gas nitriding experiments and numerical simu-lations the following conclusions can be drawn

(1) The numerical simulations taking into account theinfluence of threshold value of nitriding potential can


7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)

Figure 8 The nitrogen concentration distributions with differentnitriding time

precisely predict the thickness of diffusion zone andnitrogen concentration profile

(2) With the increase in nitriding temperature and timethe thickness of case depth increases to some degreewhereas the nitrogen concentration on the surfacedecreases as nitriding temperature increases

(3) For the analyzed grey cast iron the nitriding potentialhas little effect on the thickness of case depth buthas a relatively great influence on the nitrogen con-centration of workpiece surface and composition ofnitriding layer The proposed numerical simulationsfor nitriding layer prediction and analysis can be areference for practical engineering applications

References

[1] M A J Somers ldquoThermodynamics kinetics and microstruc-tural evolution of the compound layer A comparison of thestates of knowledge of nitriding and imitrocarburisingrdquo HeatTreatment of Metals vol 27 no 4 pp 92ndash102 2000

[2] D Pye Practical Nitriding and Ferritic Nitrocarburizing Mate-rials Park Ohio Ohio USA 2003

[3] M A J Somers and E J Mittemeijer ldquoLayer-growth kineticson gaseous nitriding of pure iron evaluation of diffusion coeffi-cients for nitrogen in iron nitridesrdquoMetallurgical and MaterialsTransactions A vol 26 no 1 pp 57ndash74 1995

[4] H Du M A J Somers and J Agren ldquoMicrostructural andcompositional evolution of compound layers during gaseousnitrocarburizingrdquo Metallurgical and Materials Transactions Avol 31 no 1 pp 195ndash211 2000

[5] A F M Arif S S Akhtar and B S Yilbas ldquoEffect of processvariables on gas nitriding of H13 tool steel with controllednitriding potentialrdquo International Journal of Surface Science andEngineering vol 4 no 4ndash6 pp 396ndash415 2010

[6] M Yang and R D J Sisson ldquoModeling the nitriding process ofsteelsrdquo Advanced Materials and Processes vol 170 no 7 pp 33ndash36 2012

[7] M Yang C Zimmerman D Donahue and J R D SissonldquoModeling the gas nitriding process of low alloy steelsrdquo Journal

of Materials Engineering and Performance vol 22 no 7 pp1892ndash1898 2013

[8] M J Hu J S Pan and Y J Li ldquoMathematical modelling andcomputer simulation of nitridingrdquo Materials Science and Tech-nology vol 15 no 6 pp 547ndash550 2000

[9] M J Hu and J S Pan Fundamentals of ThermochemicalSurface Treatment of Steel Shanghai Jiao Tong University PressShanghai China 1996

[10] W D Jentzsch F Esser and S Boehmer ldquoMathematical modelfor the nitriding of soft iron by ammonia-hydrogen mixturesrdquoNeue Hutte vol 26 no 1 pp 19ndash23 1981

[11] Z X Zhu and K F Yao Engineering Materials TsinghuaUniversity Press Beijing China 2011

[12] L S Darken and R W Gurry Physical Chemistry of MetalsIstedition London McGraw-Hiu 1953

[13] T Bell ldquoControlled nitriding in ammoniahydrogen mixtursrdquoHeat Treatment vol 73 pp 51ndash57 1975

[14] Y Sun and T Bell ldquoA numerical model of plasma nitriding oflow alloy steelsrdquo Materials Science and Engineering A vol 224no 1-2 pp 33ndash47 1997

[15] T Spalvins ldquoTribological andmicrostructural characteristics ofion-nitrided steelsrdquoThin Solid Films vol 108 no 2 pp 157ndash1631983

[16] L Torchane P Bilger J Dulcy and M Gantois ldquoControl ofiron nitride layers growth kinetics in the binary Fe-N systemrdquoMetallurgical and Materials Transactions A vol 27 no 7 pp1823ndash1835 1996

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


calculated thickness of diffusion zone was compared withthe tested data to verify the numerical simulations Finallythe effect of nitriding process parameters including nitridingtemperature nitriding potential and nitriding time was fur-ther investigated

2 Experimental Work

21 Material Thematerial tested in the study is pearlitic greycast iron The chemical composition of the material is shownin Table 1 Figure 1 depicts the microstructure of the grey castiron The graphite within the material is in a sheet form Dueto the large amount of graphite the grey cast iron has goodheat transfer ability self-lubricating performance and goodcasting property It is a very suitable material for componentsoperating under high temperature and friction condition

22 Nitriding Process Sample Preparation and EquipmentGas nitriding experiments were carried out on grey cast ironspecimens The specimens were divided into three groupswith two specimens in each group The tested temperatureswere set as 550 570 and 590∘C for each group with the sametotal time of 24 hours and nitriding potential of 15 atmminus12

After nitriding the specimens were prepared by a stan-dard grinding and polishing procedure and microstructureobservation was performed using JSM-5610LV SEM Spec-imens were etched by 5 Nital in order to observe thenitriding layer byOLYMPUSPMG3Hardness of the sampleswas also tested by applying a load of 1 N with holding time of10 s Besides the hardness tests were conducted in 6 points ofone sample and the average value was taken to represent thefinal hardness

In order to obtain the composition of the nitriding layerXRD analysis of the surface layer was performed using D8ADVANCE X-ray diffractometer with CuK at 40 kV and40mAThe scanned area was in the range of 15ndash90∘

3 Numerical Model of Gas Nitriding Process

31 Theoretical Background In gas nitriding the atomicnitrogen is dissociated from ammonia (NH

3) gas and then

diffuses into the workpiece surface of component The disso-ciation obeys the following rule

NH3=3

2H2+ [N] (1)

where [N] represents N atom dissolved in the workpiece sur-face

The above equation presumes that equilibrium has beenestablished The activated atomic nitrogen is dissolved in theiron crystal lattices which not only promotes the nitridingof 120572-Fe but also accelerates the formation and growth ofcompound layer The solution of atomic nitrogen in pureiron is relatively fast which does not affect the nitriding rate[10] When building numerical model for convenience it isassumed that the graphite has no influence on the nitridingrate although graphite is widely distributed in the ironmatrix[11] The resulting effect of this assumption will be discussedin detail in Section 51

Figure 1 SEM analysis of the grey cast iron used in an engine cylin-der liner

The transformation of atomic nitrogen within grey castiron is determined by the diffusion velocity which is de-scribed by Fickrsquos second law

120597119888

120597119905= 1198631205972119888

1205971199092 (2)

where 119888 represents the nitrogen concentration within thematerial and119863 is the diffusion coefficient of atomic nitrogen

32 Numerical Model and Boundary Conditions Nitridingpotential is the measurement of nitriding ability of NH

3

gas The phases and nitrogen concentration on the nitridingsurface are closely related to the nitriding potential after localequilibrium is established At the earliest the well-knownLehrer diagram of pure iron which was constructed byLehrer through investigating the equilibrium conditions ofNH3-H2and Fe-N [12] was used to determine the rela-

tionship of the nitrided phases nitrogen concentration andnitriding potential

The critical value of nitriding potential is defined as theturning point when the nucleation of 1205741015840 phase occurs atcertain temperatureThis value can be directly obtained fromthe Lehrer diagram However in engineering applicationsthe nitriding potential for compound layer nucleation is notcompletely equal to the critical value since a minimal valuethe so-called threshold value has to be exceeded for certainnitriding time The theoretical equation [13] for thresholdvalue of nitriding potential is

119870nt =119870nc

1 minus exp (1205732119905119863) erfc (120573radic119905radic119863) (3)

where 119870nt is the threshold value of nitriding potential fornitriding time 119905119870nc is the critical value of nitriding potential120573 is the mass transfer coefficient in gas-solid reaction anderfc represents the complementary error function

At a certain nitriding temperature the time 119905119888 repre-

senting the nucleation starting time of compound layer canbe obtained by substituting the critical value of nitridingpotential into (3) For accuracy a two-stage process is defined







119905 = 0 119888 = 1198880 (4)



119909 = infin 119888 = 0 119909 = 0

minus119863119889

120597119888

120597119909= 120573 (119888

119892minus 119888119904)

(5)






119909 = infin 119888 = 0

119909 = 0 119909 = 119888119889

(6)






119863119889= 066 exp(minus77900

119877119879) (7)




100 440 501 474130 403 455 447


120573 = 00621 exp (minus6267119879) (8)









(a)


(b)


(c)



4N phase


4N and 120576-


layer



Com

poun

d lay

er th

ickn

ess (120583

m)

Temperature (∘C)

15

12

9

6

3

540 550 560 570 580 590 600



200

150

100

50

0

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Inte

nsity

(CPS

)

2120579 (∘)

FeFe3NFe4N

(203

)(301

)

( 111

)

( 110

)(200

)

(200

)

(220

)

(331

)(211




590 142 141 minus07



10

08

06

04

02

00

0 50 100 150 200


550∘C

570∘C

590∘C

N (w

t)






5

4

3

2

1

0

0 5 10 15 20 25 30

tc (h)

Kn t

(atm

minus12

)

550∘C

570∘C

590∘C


032 atmminus12

075 atmminus12

15 atmminus12

35 atmminus12

10

08

06

04

02

00

0 50 100 150 200


N (w

t)



6 Conclusions




7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)





References

























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









119905 = 0 119888 = 1198880 (4)



119909 = infin 119888 = 0 119909 = 0

minus119863119889

120597119888

120597119909= 120573 (119888

119892minus 119888119904)

(5)






119909 = infin 119888 = 0

119909 = 0 119909 = 119888119889

(6)






119863119889= 066 exp(minus77900

119877119879) (7)




100 440 501 474130 403 455 447


120573 = 00621 exp (minus6267119879) (8)









(a)


(b)


(c)



4N phase


4N and 120576-


layer



Com

poun

d lay

er th

ickn

ess (120583

m)

Temperature (∘C)

15

12

9

6

3

540 550 560 570 580 590 600



200

150

100

50

0

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Inte

nsity

(CPS

)

2120579 (∘)

FeFe3NFe4N

(203

)(301

)

( 111

)

( 110

)(200

)

(200

)

(220

)

(331

)(211




590 142 141 minus07



10

08

06

04

02

00

0 50 100 150 200


550∘C

570∘C

590∘C

N (w

t)






5

4

3

2

1

0

0 5 10 15 20 25 30

tc (h)

Kn t

(atm

minus12

)

550∘C

570∘C

590∘C


032 atmminus12

075 atmminus12

15 atmminus12

35 atmminus12

10

08

06

04

02

00

0 50 100 150 200


N (w

t)



6 Conclusions




7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)





References

























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





(a)


(b)


(c)



4N phase


4N and 120576-


layer



Com

poun

d lay

er th

ickn

ess (120583

m)

Temperature (∘C)

15

12

9

6

3

540 550 560 570 580 590 600



200

150

100

50

0

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Inte

nsity

(CPS

)

2120579 (∘)

FeFe3NFe4N

(203

)(301

)

( 111

)

( 110

)(200

)

(200

)

(220

)

(331

)(211




590 142 141 minus07



10

08

06

04

02

00

0 50 100 150 200


550∘C

570∘C

590∘C

N (w

t)






5

4

3

2

1

0

0 5 10 15 20 25 30

tc (h)

Kn t

(atm

minus12

)

550∘C

570∘C

590∘C


032 atmminus12

075 atmminus12

15 atmminus12

35 atmminus12

10

08

06

04

02

00

0 50 100 150 200


N (w

t)



6 Conclusions




7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)





References

























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10

08

06

04

02

00

0 50 100 150 200


550∘C

570∘C

590∘C

N (w

t)






5

4

3

2

1

0

0 5 10 15 20 25 30

tc (h)

Kn t

(atm

minus12

)

550∘C

570∘C

590∘C


032 atmminus12

075 atmminus12

15 atmminus12

35 atmminus12

10

08

06

04

02

00

0 50 100 150 200


N (w

t)



6 Conclusions




7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)





References

























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




7h11h

24h40h

06

04

02

00

0 50 100 150 200


N (w

t)





References

























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article effect of process parameters on gas...

Documents