rrrans. Indian Inst. Met.

Vobs. No. 4. August 2002, pp.229-234

HNS ' 02TP08

PREDICTING NITROGEN ABSORPTION IN MARTENSITICSTAINLESS STEELS DURING HIGH TEMPERATURE

NITRIDING.

Andre Panlo TschiptschinMetallurgical and Materials Engineering Department, University of Sao Paulo,

Av. Prof. Mello Moraes 2463, CEP 05508-900, Sao Paulo, Brazil.

ABSTRACT

Thermodynamic calculations of the Fe-Cr-N System in the region of the Oas Phase Equilibria have been compared with experimentalresults of maximum nitrogen absorption during nitriding of two Martensitic Stainless Steels (a 6 mm thick sheet of AISI 410S steel andgreen powder compacts of AISI 434L steel) under N2 atmospheres. The calculations have been performed combining the Fe-Cr-NSystem description contained in the SOTE Solid Solution Database and the gas phase for the N System contained in the SOTESubstances Database. Results show a rather good agreement for total nitrogen absorption in the steel and nitrogen solubility in austenitein the range of temperatures between 1273 K and 1473 K and in the range of pressures between 0.1 and 0.36 MPa. Calculations showthat an appropriate choice of heat treatment parameters can lead to optimal nitrogen absorption in the aHoy. It was observed in thecalculations that an increased pressure stabilizes CrN at the expense ofCr2N -type nitrides.

1. INTRODUCTION

Nitrogen improves mechanical properties, wear andcorrosion resistance of martensitic stainless steels1.

High nitrogen martensitic stainless steels can beobtained by means of high- pressure metallurgy.Nitrogen content of martensitic stainless steels can also

be increased by exposure of austenite to high puritynitrogen, under relatively low pressures2. This can bedone with bulk (case hardening) or particulate materials(powder metallurgy)3.

The beneficial effects of nitrogen on mechanical,corrosion and erosion-corrosion properties can only beobtained if nitrogen remains in solid solution after

quenching or is precipitated as coherent and finechromium nitride particles after tempering. If coarsenitride precipitation is not hindered during hightemperature nitriding, chromium and nitrogen depletionof the matrix can impair corrosion and wear resistanceof these steels4.

When stainless steel is exposed to a nitrogen

atmosphere at high temperature, nitrogen may beincorporated in steel through dissolution of nitrogen in

..

the austenitic phase up to its solubility limit, according

to equation (1):

1/2N2(gas) ~[N]y (1)

and through precipitation of

according to equations (2) and (3):

[Cr]y+[N]y~ CrN

2[Cr]y+[N]y ~ Cr2N

chromium nitrides,

(2)

(3)

Each of these reactions has its own equilibrium

constant, for given temperatures and nitrogen gaspressures.

1. r .[%N]KN=.!2!L= ~

,fa: ,fP;;:(4)

where brN]is the Henryan activity of nitrogen in solutionin austenite, and' the nitrogen partial pressure PN2represents the activity of nitrog~n in the atmosphere.The activity coefficient fNrelates the chemical activityof nitrogen with its weight percent concentration [%N].This relationship leads to Sieverts' law under the domain

of validity of Henry's law.

[%N] ~ K" rp-VYN2(5)

229

,TSCHIPTSCHIN: PREDICTING NITROGEN ABSORPTION IN MARTENSITIC SS DURING NITRIDING

THEAMO-CALC (...02.22:15.57) :Fe16Cr 0.15 MPa

1600jr~..L---' ~ j L L +1550JI 3+ gas L

Ij~~- Iz 1500

1

1 n .

::;: 450 1

./-\

y+ gas ., 1 . ~- "~ I i i .

,1400' I \ ,-

~ 1350! IY!~~I"" "-;-;;;N~'--'-~ 1300 '! \ \ ""'" T +CrN+gasW ,/ \ III ' '-..n. 1250 ' ! \ \ '"::!: I1 'fCrN: '" r+Cr~"~ 1200. II r 2 \r+CrtN+CrN\ ~:..

), \ \ \1150 '.'. \ \ ~1100'~~':~'::::::::::'~"'-r--~:_\:::'::~~"::r..lL

<> 5 10 15 20 25 30 35 40E-3 WEIGHT_FRACTIONN

Fig.5: Isopleth of the Fe-16.2% Cr-N phase diagram and 0.15MPa N2 pressure. The nitrogen contents of the samplesare shown as small squares.

THERMO-CALC (...02.22:16.09) :Fe16Cr 0.25MPa

1600-j1.~ .

~~~~~-'~~~§~~~.=-~~~-=-~:-"'-"- -t

.

1550..1, " -I, " .Ii i\ I1500" [; y+ gas

,..

z ".' ...~ 1450~1" . ,W Ii. \ !:I&:: Ii '1 . ,

,1400 I! i ,k"""'-' -- ' ...~w ,'I /.-"" T+CrtN+gas Ia: 1350 J . , ,::> ' / \'- ,-I'- ! , I \""'" I

~ 1300 if \"'" 1-w 1/ \"" "', I!!: 1250 if r,. N \ . \ r+CrN I"" it IT~'2 \r+CrtN+CrN \ . ,~1200 )'

.' \ \!.

, . \ 11150 ' \: \ \ ..)1100 ...~~:~c:"---~"'-c '~'T':::::~~~..1

°E-35 10 15 20 25 30 35 40WEIGHT_FRACTION N

Fig.6: Isopleth of the Fe-16.2% Cr-N phase diagram and 0.25MPa N2 pressure. The nitrogen contents of the samplesare shown as small squares.

squaresonto the isopleths. In Figures 5, 6 and 7 the1373K and 1473 K results lay close to the solubilitylimitof nitrogen in austenite in equilibrium with the gas

phaseat 0.15 MPa. The lowest squares in Figures 5 and6 represent nitrogen contents of Fe- 16.2% Crspecimensthenno-chemically treated at 1273K for 6

hours. These specimens did not reach equilibrium due tolower diffusion coefficients and insufficient time of

exposure to the nitriding atmosphere. Takaki7 reports atotal amount of 4.6 wt % N absorbed by a Fe -22% Cr

alloy during a long tenn nitriding treatment ( t > 100 h )at1273 K, this value being very close to the equilibriumone indicated in the isothermal section of the Fe-Cr-N

system in Figure 8.

THERMO-CALC (**.02.22:23.50) :Fe16CrO.36 MPa

1600 ..tF":T::~:.c:,=...J._;;=:::;,L,:::;=:.::=,-:::::,,::-;::,::=::::,,~::'.<let.1550-1! 1\ t' I i' ;

J I \ LZ 1500 d i \ 1+ gas I'! I ' I

~ 1450 -1/ I \ t~ Ii / ) . _1 (

I 1400.U Y;;;- 1+Cr,N+gas ~.~ 1350 -' .1 i , \;-"""'" y+CrN+gasI- J I , \ ", Icc 1300

I

/ \ \ ", f.

ffi i \ \ ' ,0.. 1250.. ,! \ \ \ ~::E ;/

\ \ y+CrN 'i.

~ 1200 y+Cr2N r+Cr,N+CrN \ !1150 \ \ ""._~-!1100 ---r" -"T ."'.""'"''T---''' ,. , f.

~. 0 5 10 15 20 25 30 35 40~ E-3 WEIGHLFRACTlONN

Fig.7: Isopleth of the Fe-16.2"1o Cr-N phase diagram and0.36 MPa N2 pressure. The nitrogen contents of the

samples are shown as small squares.

THERMO-cALC (**.02.22:15.39) :1273KO.1 MPa

y+.CrN1 , 1 '

=rz 40 I5 35"1r+gas1= 30i

~ 25~'2O.jJ: :(!) :jjj 15-1;t i101

EtL.=.1.¥~::~/:;' i0 0.1 0:2

WEIGHT_FRACTION CR

Fig.8: Isothermal section of the Fe-Cr-N phase diagram at1273K and 0.1 MPa. The experimental results ofTakaki are shown as small squares.

233

TRANS. INDIAN INST. MET., VOL. 55, NO. 4, AUGUST 2002

where Kit = KN / fN.

Nitrogen will remain in solid solution depending on thetemperature of thermo-chemical treatment and nitrogenpressure. Nitrogen loss or nitrogen pickup may occuraccording to Sieverts' law at a given set of temperatureand nitrogenpartial pressure parameters.

The equilibrium of the reaction II2 N2 (gas) <=>[N]ymay be shifted by reactions (2) and (3) relative to CrN

and Cr2Nprecipitation with equilibrium constants givenby equations (6) and (7):

aCrN

-KCrN = h[Cr]"hN

(6)

aCr2N

K Cr2N = h~cr]"hN

(7)

Extensive thermodynamic calculations must be carriedout to know the nitrogen content in solution in austeniteand the amount of precipitated chromium nitrides.Thennocalc@5calculations may also be done leading toa good description of the equilibrium between nitrogengas and steel, provided that convenient databases areselected. Nitrogen contents calculated by this procedurewere compared with measured ones after hightemperaturenitriding of low carbon stainless steels.

2. EXPERIMENTAL

Fe-Cr-N phase diagrams were computed using theThennocalc@ (version L) program for a system inwhich equilibrium between austenite, chromium nitridesand N2 gas' at the metal surface was considered. Thethennodynarnic description of the gaseous phasecontained in the SGTE Substances 1994 Database -SUB94 -was appended to the system's description.

Usually the equilibrium between nitrogen gas and steel

is described by overlaying N2 isobar lines6 on phasediagrams calculated without considering the gas phaseas an equilibrium one, as shown in Figure I-a, for Fe-13

-

%Cr-N alloys. It has the advantage of representingphase diagrams for different pressures, all in one.

The method used in this work considers the equilibrium

between the gas phase and the steel as shown in FigureI-b, calculated for a Fe-13 %Cr-N alloy at 0.15 MPa. It

gives a realistic representation of the phase fieldsbeyond the solid state.

I

~

II"".lee.

L J.--~- 1"".'

lse. ~ L

'--"" \

L L+!\Ds

~17..'!8cl,.....~15..'/6+Y,a....<t<t~'30.t:!129.

5+ go.,

~v"'.e 2 .. 6 ala E-9 Io£IGHT-'''ACTIO!<It

U!8. . . '

'8A,aZ'6a E-9 WEIGHT_rRACTIOI< It

Fig. I: Fe - 13% Cr - N isopleths (a) not considering the gasphase as an equilibrium one, with N2 isobars overlaidand (b) considering the N2 gas phase as an equilibriumone.

AlSI 410S and AlSI 434L stainless steels were high

temperature nitrided in a tubular furnace under highpurity N2 atmosphere at pressures between 0.15 and0.36 MPa and temperatures between 1273 K and 1473K, during 1 to 24 hours.

The AlSI 410S bulk specimens were thermochemicallytreated in order to obtain hardened cases 0.15 to 2 mm

thick. The AISI 434L stainless steel powder wascompacted and simultaneously sintered and nitrided toobtain homogeneous and dense high nitrogen, highhardness martensitic stainless steels. The chemical

compositions of the alloys are shown in Table 1.

Thermo-chernical treatments were performed attemperatures between 1273 K and 1473 K and pressuresbetween 0.15 and 0.36 MPa, for 1 to 24 hours.

Nitrogen absorbed during thermo-chemical treatmentwas evaluated by measuring the increase in weight ofthe specimens and also by fusion under inert gasanalysis to check the results. The nitrogen content at the

.

230

TSCHIPTSCHIN : PREDICTING NITROGEN ABSORPTION IN MARTENSITIC SS DURING NITRIDING

TabletCHEMICAL COMPOSITION OF THE ALLOYS (wt.%)

Steel

410 S bulk434L

powder

C0.070.02

N

0.020

0.018

Cr13.016.2

Mn

0.270.20

Si

0.401.20

Mo

0.8

Table 2NITROGEN ABSORBED, NITROGEN DISSOLVED IN AUSTENITE AND PHASES IN EQUILIBRIUM WITH N2GAS

IN A Fe-13%Cr ALLOY AND AT 0.15 MPa.

Table 3

NITROGEN ABSORBED, NITROGEN QISSOL VED IN AUSTENITE AND PHASES IN EQUILIBRIUM WITH N2 GAS IN A Fe-13%CrALLOY AND AT 0.25 MPa.

surface of the case hardened AISI 410S steel was

determinedby GDOS.

3. RESULTS

3.1The Fe-13% Cr - N system and AISI410SAlloy

Figures 2 and 3 show isop1eths of the Fe-13% Cr-N

system for 0.15 MPa and 0.25 MPa. These diagramsallow predicting that, when exposed to a Nz atmosphere,a Fe-13% Cr alloy absorbs larger amounts of nitrogen,at lower temperatures, provided equilibrium is attained.Nevertheless, the amount of nitrogen in solution inaustenite increases as the temperature is decreased until

the CrzN hexagonal nitride becomes stable. From this

point on, the solubility limit of nitrogen in austenitedecreases with decreasing temperatures. Tables 2 and 3

show for Fe -13% Cr alloys, the total nitrogen content,the nitrogen content dissolved in austenite and the

phases in equilibrium with Nz gas for differenttemperatures and 0.15 and 0.25 MPa pressures,respectively.

It can be seen that thermo-chernical treatments carried

out at 1273K lead to extremely high values of nitrogenabsorbed by the alloy, although the nitrogen contentdissolved in austenite is low and cubic CrN nitride is

present. As the temperature is increased to 1373 K, thetotal amount of nitrogen is decreased but the content of

231

Temperature(K) % Ntotal %Ny Phases in euilibrium

with N2 gas1273 2.30 0.25 y + CrN1373 0.49 0.49 y1473 0.35 0.35 Y

Temper}1ture(K) % Ntotal %Ny Phases in euilibrium

with N2 gas1273 2.6 0.28 y + CrN1373 0.70 0.55 y+ CrzN1473 0.45 0.45 y

TRANS. INDIAN INST. MET., VOL. 55, NO. 4, AUGUST 2002

THERMO-CALC (".02.20:17.21) :Fe13CrO.15 MPa

1600

1550

z 1500:=; 1450w:'::,1400 .w " I .~:a: 1350 .; ,."" . :i= :' .2 1+ Cr,N+w!s Iii 1300" :. "'" ,.w \39 "', "

~ 12501W . "''I',+CrN+gas r\!:!1200 Hk"':rN\ \~ +CN"12; ;(TT'-- 2 '; \ r r \

1150 ~, r+Cr,N<CrN\ ~I '-~ ~8 ' ~ ,151100+~..~.- ,'--- ,._' , ,\ ,

-$ °E 5 10 15 20 25 30 35 401ci. -3 WEIGHT_FRACTIONN

"(+ gas

iif'

Fig. 2: Isopleth of the Fe- 13 % Cr - N system for O. I 5 MPaN2 pressure.

THERMO-CALC (" .01.10:20.57) :

1600, ,-a~jiijL: 1.

1500 j!20~~

~ i \22> ;

~I 1400j Y \...w ;! +--- "a: 1'" ' -'..,~ 300 j' \, -'''. I< ,.iI i 29 \ -"" Y+ erN""". ;a:n . , '-'w i i \ .~0. 1200 .!L Y+Cr N~ \28 ~-"".7

::iE2p 2' -, I~'''' 'W

r.y+{...,."..~ \ ~

'~ ' .1100'-- '; \ ;z~u---~

'~"30 !

Y+ gas

.".. --L -" .

1000-' - ~- ~-- -\ 1,,1\ <> 5 10 15 20 25 30 35 40

~~. E-3 WEIGHT_FRACTIONN

Fig. 3: Isopleth of the Fe- 13% Cr - N system for 0.25 MPaN2pressure.

nitrogen dissolved in austenite increases. Increasingpressure stabilizes the Cr2N phase at expenses of thecubic CrN phase.

Figure 4 shows the Fe 13% Cr - N isopleth for 0.20MPa N2pressure. The small square plotted in the figureindicates the GDOS measured surface nitrogen content(1.2 %) of an AISI 410 S stainless steel after 24 hours of

THERMO-CALC (".02.22:18.00) :Fe13Cr 0,2 MPa

1600+r-' J ' ' L L...1.__L .1.--+-i 15 5+gas !

15501 r2.--!\'c"=",==c=c""=__""3~___=,,§,,-_.,,.,,,,_,,dlz 1500 'I ! \- l" \~ i' i \1

~1450.;~:i I \ y+gas

I " \~

~

f \\:J 1400 ! Y 40.I- ' \394:( i ~ 31 "a: 1350 ' / "w I / "'. 34 Y + Cr,N+ps

a. I ~ / \ ~,<--.-~ 1300 Jj I! :. "

.-'--".,,, ,

W' \' ."I- I. / \19 ' c N

'

J I j Y+Cr N \ y-+{:r,N-+CrN',y+ r ,1250 .' I 2 \ '-.35 ~

!! i \ \!I

Las \ ..,

1200 .I.L, ,--r.., l-,---r-' ..T---'-~T---t .

f~' °E-32 4 6 8 10 12 14 16 18 20:$t:il!.. WEIGHT_FRACTIONN

Fig, 4: Isopleth of the Fe- 13% Cr - N system for 0.20 MPaNz pressure. The small square indicates the GDOSmeasured surface nitrogen content of an AISI 410 Sstainless steel after 24 hours of exposure to a 0.20MPa Nz atmosphere at 1323K.

exposure to a 0.20 MPa N2 atmosphere at 1323K. Thevolume ftaction of precipitated CrN at the surface of thealloy was estimated by quantitative metallography asbeing 3%, leading to a calculated (through massbalance) nitrogen content dissolved in austenite of0.45%. One can see that the agreement between

predicted and measured values is rather good.

3.2 The Fe-16.2% Cr-N System and AISI434L Alloy

Phase diagrams for Fe-16.2 Cr - N alloys werecalculated and Figures 5, 6 and 7 show the isopleths for0.15 MPa, 0.25 MPa and 0.36 MPa N2 pressures,

respectively. One can see that the solubility of nitrogenin austenite increases with increasing temperature untilthe gas phase becomes stable. From this point on, anincrease in temperature leads to a decrease in nitrogensolubility of austenite.

Nitrogen contents of sixteen AISI 434L green compactspecimens, nitrided and sintered at different

temperatures and N2pressures are plotted as small

232

TRANS. INDIAN INST. MET., VOL. 55, NO. 4, AUGUST 2002

4. CONCLUSIONS

I) The calculated phase diagrams were in goodagreement with experimental results on nitrogenabsorption and nitrogen solubility in austeniteobtained in high temperature nitriding experiments.

2) An appropriate choice of heat treatment parameters

can lead to optimal nitrogen absorption in the alloy.Extremely high nitrogen levels can be achieved bynitrogen pickup during lower temperaturetreatments.

3) It was observed in the calculations that increasingpressure stabilises CrN at expenses ofCr2N.

4) The databases SSOL and SUB94 used in the

Thermocalc@program describe thoroughly well thethermodynamics of nitrogen absorption in Fe-Cralloys exposed to N2atmospheres.

ACKNOWLEDGEMENTS >'

The author thanks to FAPpsP-Fundacao de Amparo aPesquisa do Estado de Sao Paulo, processes 98/15758-4,99/07570-8, CNPQ, Consellio Nacional de

Desenvolvimento Cientifico e Tecnologico process n.300286/80-5 and PADCT - Programa de Apoio aoDesenvolvimento Cientifico e Tecnologico process62.0133/98-8.

234

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