Decomposition kinetics of expanded austenite with high nitrogen contents

Thomas Christiansen, Marcel A. J. SomersTechnical University of Denmark, Department of Manufacturing Engineering and Management, Lyngby, Denmark

Decomposition kinetics of expanded austenitewith high nitrogen contents

This paper addresses the decomposition kinetics of synthe-sized homogeneous expanded austenite formed by gaseousnitriding of stainless steel AISI 304L and AISI 316L withnitrogen contents up to 38 at.% nitrogen. Isochronal anneal-ing experiments were carried out in both inert (N2) and re-ducing (H2) atmospheres. Differential thermal analysis(DTA) and thermogravimetry were applied for identifica-tion of the decomposition reactions and X-ray diffractionanalysis was applied for phase analysis. CrN precipitatedupon annealing; the activation energies are 187 kJ/mol and128 kJ/mol for AISI 316L and AISI 304L, respectively. Iso-thermal stability plots for expanded austenite developedfrom AISI 304L and AISI 316 were obtained.

Keywords: Expanded austenite; Nitriding; Austeniticstainless steel; Thermal stability

1. Introduction

Low-temperature nitriding of stainless steel by plasma-based methods has been the subject of several recent publi-cations [e. g. 1 – 3]. The possibility of improving the wearand tribological properties of stainless steel by dissolvingnitrogen in the surface-adjacent region of the steel is thedriving force for this interest. Nitriding at temperatures be-low 723 K (450 °C) brings about a conversion in the sur-face-adjacent region of stainless steel: the development ofexpanded austenite (also called S-phase [1]). In fact ex-panded austenite, containing a significant quantity of nitro-gen in solid solution, is thermodynamically metastable andtends to develop chromium nitride. Expanded austenite(cN) is responsible for the highly favourable properties as-sociated with the low-temperature nitriding treatment: asignificant increase in surface hardness, presumably due tosolid solution hardening, and an associated improvementof the tribological performance, as reflected in lowerfriction coefficients and improved wear resistance [4].Although of crucial importance for designing the processwindow of surface engineering stainless steel and the subse-quent practical/industrial application of low-temperaturenitrided stainless steel, so far little attention has been givento the thermal stability of expanded austenite. Pertinent ex-periments for a quantitative assessment of the thermalstability of homogeneous cN have hitherto been hinderedby the practical difficulties encountered in obtaining homo-geneous cN with a well-defined composition. The publishedinvestigations concern plasma-nitrided samples containingan inhomogeneous surface layer of cN, which has been sub-

jected to various isothermal annealing experiments [5– 7].It is generally agreed upon that thermal exposure of cN

leads to the development of CrN. For a surface layer of cN,decomposition occurs concurrently by the diffusion of ni-trogen from the cN layer into the unnitrided substrate [6].Moreover the development of austenite or ferrite accompa-nying the CrN formation has been reported [6, 8].

In the present work the decomposition kinetics of ex-panded austenite was investigated simultaneously withthermogravimetric analysis (TGA) and differential thermalanalysis (DTA) on homogeneous cN powders, obtainedfrom through-nitriding of thin foils of austenitic stainlesssteel.

2. Kinetic analysis

In the present chapter the theory according to Mittemeijer etal. [9– 11] is adopted. It is assumed that the thermal historyof the investigated samples is fully described by a path vari-able.

2.1. Isothermal and non-isothermal transformations

The fraction transformed, f, is fully described by the pathvariable b:

f ¼ FðbÞ ð1Þ

This path variable can be interpreted as being proportionalto the number of atomic jumps. Since the temperature, T,determines the atomic mobility and time, t, defines theduration of the process considered [9], this implies that bcan be expressed as:

b ¼R

kðTðtÞÞ dt ð2Þ

The rate constant k(T(t)) obeys the Arrhenius expression:

kðTðtÞÞ ¼ k0 exp � QRTðtÞ

� �

ð3Þ

where k0 is the pre-exponential factor, R is the gas constantand Q is the overall effective activation energy [9].

For isothermal annealing kðTÞ is independent of t. Henceit holds (cf. Eq. (2)):

b ¼ kðTÞ t ð4Þ

The assumption of the path variable b as a dependent vari-able for the degree of transformation conforms to the John-son– Mehl – Avrami (JMA) equation for heterogeneous

T. Christiansen, M. A. J. Somers: Decomposition kinetics of expanded austenite with high nitrogen contents

Z. Metallkd. 97 (2006) 1 Carl Hanser Verlag, München 79

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(solid-state) reactions:

f ¼ FðbÞ ¼ 1� expð�bnÞ ð5Þ

where n denotes the JMA exponent [9].The equations applied for describing the degree of trans-

formation for isothermal and non-isothermal annealing areidentical as long as they are expressed in terms of b [10].

2.1.1. Determination of kinetic parameters

In order to evaluate transformation kinetics the degree oftransformation as a function of time/temperature is moni-tored [10]. Describing the progress of the transformation interms of bn makes it possible to determine the kineticparameters n, k0, and Q.

For isothermal annealing it holds [10]:

bn ¼ ðkðTÞ tÞn ð6Þ

The parameters k(T) and n can be determined from one iso-thermal experiment comprising at least two sets of data(t, bn). Determination of Q and k0 requires isothermal ex-periments at (at least) two different temperatures [10].

For non-isothermal isochronal annealing it holds [10]:

bn � k0uRt�2

Qexp � Q

Rut�

� �� n

ð7Þ

The time parameter t* is given by:

t� ¼ t þ T0

uð8Þ

where T0 is the starting temperature and u is the heatingrate. Values of n, Q, and k0 can in principle be determinedfrom a single isochronal annealing experiment comprisingat least three data sets (t*, bn) which gives 3 equations with3 unknowns. However, at least two isochronal annealing ex-periments with different heating rates are recommended inorder to ensure more accurate data [10]. The value for theactivation energy of the decomposition reactions occurringduring isochronal annealing was determined by applying aKissinger-like method based on the equation [9]:

lnT2

f 0

u¼ E

RTf 0þ constant ð9Þ

where u is expressed in K/min and Tf' is the temperature at afixed degree of transformation, f'. In the present work Tf 0 isapproached by the peak maximum in DTA experimentsand the temperature for a fixed degree of transformation inTGA experiments.

3. Experimental

3.1. Sample preparation

Foils of stainless steel AISI 304L and AISI 316L were usedfor investigating the stability of expanded austenite. TheAISI 304L and the AISI 316L foils were provided in thick-nesses 20 lm and 100 lm, respectively. In addition, a5 lm-thick foil of stainless steel AISI 304 (Goodfellows)was applied. The compositions of the three materials are

given in Table 1. The composition for AISI 304L and AISI316L were certified by Sandvik; the composition of theAISI 304 foil was determined with energy-dispersiveX-ray analysis (EDS), taking the certified foils as referencematerials.

AISI 316L was thinned to a thickness of 20 –30 lm byelectrochemical polishing to ensure that saturation with ni-trogen was accomplished within a reasonable nitriding time(to prevent the development of CrN during nitriding; cf.Discussion). For recrystallisation and austenitisation thestainless steel thin foils were heated to 1343 K at a heatingrate of 0.333 K/s and upon reaching this temperature imme-diately cooled in pure H2. During austenitisation the defor-mation-induced martensite, which was introduced uponcold-rolling during the manufacturing of the foils, was to-tally transformed to austenite. After heat treatment, thefoils’ surfaces were activated to enable gaseous nitriding[12].

3.2. Gaseous nitriding

Gaseous nitriding was performed in a Netzsch STA 449Cthermal analyzer, which allows simultaneous thermogravi-metric analysis (TGA) and differential thermal analysis(DTA). Gaseous nitriding was performed in an atmosphereof ammonia and nitrogen until saturation with nitrogenwas obtained in the stainless steel (stationary weight). Aflow of nitrogen gas was led via the measurement compart-ment of the balance, to prevent corrosive attack by ammo-nia, and was mixed with ammonia in the furnace chamberof the equipment. The flows of ammonia and nitrogen wereadjusted with mass flow controllers: 0.833 ml NH3/s and0.0833 ml N2/s. A detailed description of adjusting the ni-trogen content in stainless steel through controlled gaseousnitriding was given in [13]. The AISI 304L and AISI 316Lfoils were nitrided for 28 hours at 703 K; the much thinnerAISI 304 foils were nitrided for 12 hours at 693 K. This dif-ference in experimental conditions was necessary to pre-vent the development of CrN during nitriding. Preliminarywork had shown that CrN develops easier in AISI 304 thanin AISI 316. The thin foils were fully transformed into ex-panded austenite powder.

AISI 304 was also reduced in pure H2 after nitriding inorder to retract “loosely” bound nitrogen [13]. A saturationand reduction cycle was performed within 12 hours at693 K; the remaining nitrogen content after reduction isgiven in Table 2.1 The nitrogen contents vary slightly due


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1 Hydrogen reduction of as-nitrided foils was not possible for thethicker AISI 304L and AISI 316L foils as the time for nitrogensaturation was markedly longer and, hence, the risk for CrN forma-tion during subsequent reduction too high.

Table 1. Composition of the stainless steels in atomic %. Compo-sition of AISI 304L and AISI 316L were certified by SandvikMaterials Technology; the composition of AISI 304 was deter-mined with EDS taking the other foils as a reference.

Alloy Cr Ni Mo Mn Si Fe

AISI 304 19.6 8.7 0 1.7 1.3 balanceAISI 304L 19.45 9.49 0 1.17 0.98 balanceAISI 316L 18.93 13.55 1.69 1.76 0.62 balance

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to the fact that different material thicknesses and thermaltreatments were applied. Hence the foils were not fullyequilibrated with respect to the imposed nitriding potential.The nitrogen contents in AISI 304 and AISI 304L were de-termined directly from the weight gain as recorded withTGA. The nitrogen content in AISI 316L was determinedfrom the lattice parameter obtained by X-ray diffraction[13]. All nitrogen contents determined in the as-nitridedand in the as-reduced materials as well as their thermal his-tory are given in Table 2.

3.3. Thermal analysis

Isochronal annealing of the expanded austenite powderswas performed in the Netzsch STA 449C thermal analyzer,using various heating rates. Both inert (N2) and reducing at-mospheres (N2 + H2) were applied for the investigation ofthe decomposition kinetics of expanded austenite. Constantheating rates within the range 0.0833 K/s to 0.333 K/s wereapplied, thus enabling determination of activation energies.The starting temperature was 303 K and the end tempera-ture was 1173 K for all isochronal annealings. An emptycrucible (alumina) was used as a reference for the DTA ex-periments. The gas flows for the inert atmosphere were0.167 ml/s N2 and for the reducing atmosphere 0.667 ml/sH2 + 0.0833 ml/s N2. The typical sample mass used foreach experiment was within the range 50 –100 mg. ForAISI 304 a much smaller amount of sample was used, be-cause of practical problems associated with obtaining suffi-cient amounts of cN powder from 5 lm thin foils. Conse-quently, the results obtained with this material are at bestsemi-quantitative. A baseline was obtained in order to cor-rect for instrumental effects during heating. This was per-formed in the following way: The thermal analyzer wascooled to the starting temperature (303 K) after the sam-ple-run and a similar heating program was performed –now with the decomposed sample in the sample crucible.All DTA/TGA results presented are baseline corrected andpertain to the sample.

3.4. X-ray diffraction

X-ray diffraction was applied for the identification of thedecomposed material. The decomposed cN powder was(further) crushed in an ultrasonic bath while submerged inethanol, thus avoiding deformation-induced transforma-tions, like martensite formation, from mechanical process-ing, e. g. milling2. The powder-ethanol slurry was smeared

on a glass-plate and, after evaporation of ethanol, analysedwith a Bruker AXS D8 X-ray diffractometer, equipped witha Cr anode and a set of Göbel mirrors in the incident beam.

4. Results

4.1. Annealing in N2

The results obtained with simultaneous TGA and DTA ofAISI 304 with the nitrogen contents corresponding to satu-ration in pure NH3 and after reduction in H2 are depictedin Fig. 1a. The differentiated TGA results for the as-nitridedsample are compared with the DTA results for both samplesin Fig. 1b.

The sample containing 38.1 at.% N begins to looseweight at approximately 750 K, i. e. about 60 K above thenitriding temperature. This is attributed to the release of ni-trogen from the sample powder by association of the atomi-cally dissolved N to N2 molecules, which desorb from thesurface. Apparently, the release of nitrogen proceeds intwo successive steps, as clearly reflected by the two minimain Fig. 1b.

The sample containing 13.9 at.% N does not looseweight; all nitrogen participates in the exothermic reaction


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2 The extreme brittleness of expanded austenite was lost after isochro-nal annealing. Hence, a mixture of flakes and powder of decom-posed expanded austenite was used.

Table 2. Nitriding treatment and nitrogen contents in synthe-sized cN.

Material Duration(h)

Temperature(K)

N : Crratio

yN [N](at.%)

AISI 304 12 693 3.14 0.616 38.1AISI 304 reduced 12 693 0.83 0.162 13.9

AISI 304L 28 703 3.05 0.594 37.3AISI 316L 28 703 2.93 0.554 35.6

Fig. 1a. DTA and TGA curves simultaneously obtained during iso-chronal annealing of AISI 304 with different nitrogen contents corre-sponding to saturation in pure NH3 (38.1 at.% N) and reduction in H2(13.8 at.% N).

Fig. 1b. DTA and differentiated TGA curves during isochronal anneal-ing of AISI 304 with different nitrogen contents corresponding tosaturation in pure NH3 and reduction in H2.

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with a (DTA) maximum at approximately 850 K. Compar-ing the two DTA curves with the differentiated TGA curvein Fig. 1b shows that the positions of the two minima inthe TGA curve for the as-nitrided sample coincide with theexothermic peaks in the DTA curve of the reduced sample.For the DTA curve of the as-nitrided sample no such coinci-dence occurs with the TGA “minima”. Instead the mini-mum at about 950 K coincides with an endothermic reac-tion, whilst an exothermic reaction is observed at about870 K. This discrepancy is attributed to the occurrence ofmore than two reactions in the temperature range investi-gated, implying that the DTA peak at 870 K for the satu-rated sample is the net result of several contributions. A ki-netic analysis of this DTA peak in terms of activationenergy from several heating rates is therefore meaningless;indeed no meaningful values were obtained. The DTA peakfor the saturated sample is shifted to a higher temperature ascompared to the reduced sample; this cannot altogether beascribed to overlap with the desorption reaction. Similarisochronal annealing experiments in an N2 atmospherewere performed for AISI 304L and AISI 316L (Fig. 2). Forthe samples a different nitriding treatment was applied thanfor the samples in Fig. 1: both a higher nitriding tempera-ture and a longer nitriding time (cf. Experimental). Therelatively large quantities of AISI 304L and AISI 316L usedfor the thermal stability experiments allow a quantitativeanalysis of the results.

The same trends as observed for AISI 304 nitrided atlower temperature and shorter time also apply for AISI304L and AISI 316L. Loosely bound nitrogen is released

from the sample at temperatures above approximately750 K. The temperatures at which maximum nitrogen re-lease rates occur, reflected as minima in the differentiatedTGA curves, are identical for AISI 304L and AISI 316L.This indicates that desorption of N2 is not influenced bythe slight difference in composition between these two al-loys. A step-like temperature dependence of N2 desorptionis obvious (Fig. 2). The maximum nitrogen release rates inthe two steps, i. e. the minimum values in the differentiatedTGA curves, are equal for AISI 316L, whereas the secondnitrogen release rate for AISI 304L is significantly slowerthan the first.

In contrast with the results in Fig. 1 the discrepancy be-tween the temperatures at which the first minimum in thedifferentiated TGA curves and the maximum in the DTAcurves occurs is less pronounced and largest for AISI316L, meaning that the temperature at which the exother-mic peak maximum occurs is higher for AISI 316L thanfor AISI 304L for the same heating rate.

The decomposed powders investigated with X-ray dif-fraction had been heated to 1193 K twice, once for the ac-tual decomposition and once for the recording of the base-line. It was verified that the second heating cycle did notinfluence to any noticeable degree the appearance of theX-ray diffraction patterns. X-ray diffraction patterns of as-nitrided and decomposed AISI 304L and AISI 316L aregiven in Fig. 3 for annealing in N2. In AISI 304L the de-composed powder contained mainly CrN and ferrite (α)after annealing; only a very small amount of austenite (c)appears to be present. The X-ray diffraction patterns of de-composed AISI 316L contain mainly CrN and austenite(c); only a very small fraction of ferrite (α) is present.

The above results are explained as follows. Molecular ni-trogen development occurs over the entire temperaturerange beyond about 750 K, which should result in a broadendothermic peak in the DTA curve. In the lower tempera-ture range an exothermic peak associated with the forma-tion of CrN (and possibly ferrite for AISI 304L; but see Sec-tion 4.2 in this respect) in cN is superimposed onto thisendothermic peak. This may explain the shape of the DTAcurve as constituted of two overlapping processes. The con-sumption of Cr during the development of CrN reduces thesolubility of nitrogen in the austenitic matrix. Conse-quently, the driving force for nitrogen release is higher in


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Fig. 2. TGA, differentiated TGA and DTA of (a) AISI 304L and (b)AISI 316L during isochronal annealing at 0.417 K/s. The change inweight (TGA) is expressed as the N : Cr ratio.

Fig. 3. X-ray diffraction patterns of cN and decomposed cN in AISI304L and AISI 316L. The diffraction patterns correspond to a heatingrate of 0.417 K/s and to H2 and N2 atmospheres.

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the region where precipitation of CrN occurs. Thus the firstminimum in the differentiated TGA curve is explained. Pre-suming that all Cr has precipitated, the relatively high nitro-gen content is explained from adsorption of N atoms at theCrN-matrix interface. Upon continued heating the CrN pre-cipitates coarsen, thereby reducing the interfacial area be-tween precipitates and matrix and thus the capacity for Natoms at such interfaces. Coarsening of CrN is not asso-ciated with a distinct DTA peak, but leads to the secondminimum in the differentiated TGA curve.

4.2. Annealing in H2

The purpose of choosing a reducing H2 atmosphere was toinvestigate the thermal stability of expanded austenite with-out the concurrent N2 desorption, which obscures the ki-netic information associated with the actual decompositionof cN. The DTA and (differentiated) TGA curves of bothAISI 304L and AISI 316L are given in Fig. 4 for a heatingrate of 0.417 K/s.

Nitrogen retraction by the development of NH3 occursfrom approximately 525 K (Fig. 4). The maximum nitrogenrelease rate of loosely bound nitrogen occurs for the sametemperature for AISI 304L and AISI 316L. The temperaturefor which the maximum nitrogen release rate occurs coin-cides with the peak position of the DTA curve, implyingthat these exothermic DTA peaks are associated with thedesorption of NH3.

Strong asymmetry of the differentiated TGA curves andthe corresponding DTA peaks is attributed to the geometryof the samples, i. e. flakes and grains of varying dimensions.

Provided that solid state diffusion of nitrogen atoms to-wards the surface, where NH3 forms, plays a determiningrole (as is likely to be the case at these low temperatures),this implies that N atoms do not reach the surface of allparts and pieces at the same time. Consequently, the devel-opment of NH3 spreads over a temperature range. The sec-ond DTA peak is not associated with a change of weightand can therefore be ascribed solely to the decompositionof expanded austenite. As for the reduced AISI 304 sample(Fig. 1) all nitrogen participates in this decomposition reac-tion, since the sample maintains its weight. A significantdifference between the positions of the main exothermicDTA peaks for the two materials can be observed: decom-position is shifted to higher temperatures for AISI 316L.For AISI 304L a small exothermic peak was distinguishedon the high-temperature side of the main peak. This peakwas observed for all 7 heating rates for AISI 304L and didnot occur for AISI 316L.

X-ray diffraction patterns of as-nitrided and decomposedAISI 304L and AISI 316L are given in Fig. 3 for annealingin H2. In AISI 304L the decomposed powder contained onlyCrN and ferrite (α) after annealing; decomposed AISI 316Lcontains only CrN and austenite (c). Therefore, arbitrarily,the main peaks in the 870 K to 920 K range of the DTAcurves in Fig. 4 are ascribed to the development of CrN.The small exothermic peak for AISI 304L at higher tem-perature (at approximately 955 K) is associated with theferrite-austenite conversion (cf. Section 5.2.1).

Interestingly, the nitrogen contents maintained in the sam-ples after full reduction in H2 are higher than correspondingwith a stoichiometric ratio N : Cr = 1 : 1. The higher nitrogencontent in AISI 304L as compared to AISI 316L (cf. Table 2)was maintained after reduction. This is ascribed to the devel-opment of other nitrides than CrN. In this respect Si3N4 is themost likely candidate. In ferrite this hexagonal nitride isknown to nucleate with great difficulty [14] and amorphousSi3N4 develops instead [15]. In an austenitic matrix the nu-cleation barrier is expected to be much lower, because of thepossibility of a favourable orientation relationship betweenthe hexagonal nitride and the fcc matrix.

Comparing the decomposition of the samples in the twoenvironments N2 and H2, a remarkable difference is ob-served. In H2 the temperature at which a maximum appearsin the DTA curve for AISI 316L is shifted to considerablyhigher temperatures (a shift of about 50 K) as compared toits position for annealing in N2.

4.3. Kinetic analysis

4.3.1. Ammonia release

Kinetic analysis was carried out for the two identified reac-tions i. e. retraction of loosely bound nitrogen (NH3 reac-tion) and the decomposition reaction involving the develop-ment of CrN (CrN reaction). Plots of ln T2

m u= Þ�

versus 1/Tfor the NH3 reaction are given in Figs. 5a, b for AISI 304Land AISI 316L. From the slopes of the fitted straight lines(cf. Eq. (9)) the activation energies collected in Table 3were obtained. The values for AISI 304L and AISI 316Lby DTA and for a fixed transformation (TGA) are of com-parable magnitude. The activation energy value obtainedtaking the differentiated TGA minima is systematicallyhigher by about 15 kJ/mol.


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Fig. 4. Isochronal annealing of (a) AISI 304L and (b) AISI 316L cN inH2 at a heating rate of 0.417 K/s.

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4.3.2. Decomposition of expanded austenite

Plots of ln T2m u= Þ

�versus 1/T for the decomposition reaction

are given in Fig. 6 for AISI 304L and AISI 316L. The inten-sity of the DTA peak is strongly influenced by the appliedheating rate: for low heating rates the integrated area of theDTA peak was relatively low (pertinent only for the decom-position reaction). This necessitated the use of a weightingfactor. The integrated peak area per quantity of sample wasused as the weighting factor.

The values obtained for the activation energy for the de-composition of expanded austenite are 128 (± 9.9) kJ/moland 187 (± 17.7) kJ/mol for AISI 304L and AISI 316L, re-spectively.

4.3.3. Coupling of isothermal and non-isothermalannealing

In order to relate the isochronal annealing experiments toisothermal annealing of cN a kinetic analysis was perfor-med. The outline presented in Section 2 was followed.

The DTA peak from the decomposition of cN (CrN reac-tion) in H2 was studied for AISI 304L and AISI 316L. Thedegree of transformation was approximated by the cumula-tive DTA peak area. The applicability of JMA kinetics wasassumed and the state variable bn was introduced. Fittingof Eq. (7) to the data for three different heating rates wasperformed and the average values of the three fits were used(heating rates of 0.333, 0.417 and 0.5 K/s.). The values forthe kinetic parameters are given in Table 4. It is noted thatthe value for the effective overall activation energy Q forAISI 304L is commensurate with the value obtained by thetraditional analysis according to the Kissinger-like method(128 ± 9.9 kJ/mol). However, for AISI 316L the value ob-tained for the effective overall activation energy Q is signif-icantly lower than the value obtained by the Kissinger-likemethod (187 ± 17.7 kJ/mol).

5. General discussion

Heating of expanded austenite in an inert atmosphere doesnot bring about N2 desorption until a temperature of ap-proximately 773 K is reached. Hence, the nitrogen contentsachieved during nitriding and denitriding (693 – 703 K) canbe conceived as a state of (metastable) equilibrium betweengas mixture and solid state, rather than a stationary state be-tween nitriding (i. e. nitrogen dissolution in the solid state)and N2 development (as for the case of ferritic nitriding),because a concurrent development of molecular nitrogencan be neglected at the nitriding temperature [16].


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Fig. 5. Plots of ln T2m u= Þ

�versus 1/T for the NH3 reaction for a) AISI

304L and b) AISI 316L.

Fig. 6. Plots of ln T2m u= Þ

�versus 1/T for the decomposition of cN (CrN

reaction) in AISI 304L and AISI 316L. The vertical bars are the reci-procal of the weighting factor and do not represent the standard devia-tion.

Table 3. Activation energies for the NH3 reaction for AISI 304Land AISI 316L.

AISI 304L AISI 316L

DTA peak 87 kJ/mol (± 3.0) 83 kJ/mol (± 4.0)Differentiated TGA

minimum100 kJ/mol (± 2.9) 98 kJ/mol (± 6.7)

10 % transformation(TGA)

82 kJ/mol (± 6.0) 82 kJ/mol (± 5.3)

50 % transformation(TGA)

84 kJ/mol (± 3.2) 76 kJ/mol (± 6.9)

Table 4. Kinetic values for AISI 304L and AISI 316L.

Kinetic parameter AISI 304L AISI 316L

n 2.64 (± 0.09) 2.20 (± 0.10)Q [kJ/mol] 130 (± 0.6) 139 (± 0.1)

k0 [1/s] 574 � 103 (± 34 � 103) 576 � 103 (± 9 � 103)

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5.1. NH3 desorption

Retraction of nitrogen, by development of NH3, occurs forthe isochronal experiments carried out in a reducing atmos-phere of H2. The development of NH3 (gas) entails trans-port of dissolved nitrogen in cN. Firstly, the morphology ofthe cN material has to be addressed; a mixture of flakesand powder (grains) was used3.

For grains of cN diffusion of N atoms to the surface ofgrains has to occur. Volume diffusion of N may be facili-tated by diffusion along defects in the solid state; this effec-tively lowers the activation energy. For flakes of cN volumediffusion and grain boundary diffusion of N atoms are an-ticipated to be the prevailing mechanisms. Formation ofNH3 at the surface follows a stepwise hydrogenation of thenitrogen atom (and a stepwise dehydrogenation of the am-monia molecule for the reverse process, i. e. nitriding) [17]:

[N], Nads (10)

Nads + Hads, NHads (11)

Nads + NHads, NH2,ads (12)

Nads + NH2,ads, NH3,ads (13)

The adsorbed atomic hydrogen in reactions (11) – (13)stems from the following reaction:

H2(g), 2Hads (14)

The rate limiting step for the stepwise hydrogenation ofatomic nitrogen, cf. (10) – (13), depends on the temperatureand pressure of H2. For high pressures of H2 the rate limit-ing step is (12) [17].

However, for the present case the most likely rate deter-mining step is solid state diffusion. This is substantiated byliterature values for activation energies for nitrogendiffusion, which compare well with the obtained valuesof 80– 100 kJ/mol. For a nitrogen containing c-Fe alloy(9.5 at.% N) the activation energy for nitrogen diffusion is90 kJ/mol [18]. The activation energy for nitrogen diffusionin c0-nitride (i. e. an fcc Fe host lattice with approximately20 at.% N) is 91.4 kJ/mol [19]. For nitrogen diffusion inpure c-Fe the activation energy is 152 kJ/mol [20]. The acti-vation energy depends strongly on the concentration of in-terstitially dissolved atoms; this was established for carbonin c-Fe, where an increase of the carbon content loweredthe activation energy for solid state diffusion (a value of ap-proximately 93 kJ/mol for 9.5 at.% C is stated) [18 and re-ferences therein], obviously as a consequence of a dilationof the lattice with increasing C content. Similarly, for thepresent case N dilates the fcc lattice ([13]) and the diffusioncoefficient of N in cN increases with increasing N content(for cN < 0.45 [21]).

5.2. Decomposition of expanded austenite

5.2.1. Decomposition products

A significant difference between the alloys AISI 304L andAISI 316L was found in the phase constitution after anneal-

ing: ferrite developed in AISI 304L and austenite was main-tained in AISI 316L. Equilibrium diagrams for the investi-gated alloys, taking the nitrogen content equivalent to thechromium content, were calculated (Thermo-Calc) (seeFig. 7).

The change in composition from AISI 304L to AISI316L has a major influence on the temperature dependenceof the equilibrium fractions of α and c. Experimentally, atemperature difference was observed for expanded austen-ite decomposition; decomposition occurs at approximately50 K higher temperature in AISI 316L than in AISI 304L.The DTA peak at 870 K in Fig. 4a, attributed to the devel-opment of CrN lies within the three-phase (α + c + CrN) re-gion. A two phase (c + CrN) region is first expected fortemperatures beyond 948 K (675 °C), which is compatiblewith the temperature where a small peak is observed as ashoulder to the major exothermic DTA peak in Fig. 4a. Thisis interpreted as evidence for the transformation of ferrite toaustenite; the ferrite being formed during (or preceding) thedevelopment of CrN. The development of ferrite or CrN(depending on which of the two nucleates first) is expectedto proceed relatively easily, since the following energeti-cally favourable orientation relationship is expected be-tween CrN and α-Fe: {100}α-Fe//{001}CrN; [100]α-Fe //[110]CrN [22]. During cooling of the samples after thermalanalysis a transformation of austenite into ferrite is ex-pected to occur in AISI 304L.

The DTA peak position associated with the decomposi-tion of expanded austenite for AISI 316L (Fig. 4b) takesplace within or close to the austenite region (cf. Fig. 7).This explains the absence of an exothermic peak on the hightemperature shoulder of the major DTA peak for AISI316L. During cooling a transformation into ferrite shouldoccur at a significantly lower temperature as compared toAISI 304L. Evidently, this transformation does not takeplace, which could be ascribed to the presence of Mo.

Summarizing, for AISI 304L expanded austenite decom-poses according to the reaction cN ! αþ CrN þ cð Þ; forAISI 316L expanded austenite decomposes according tocN ! cþ CrN. It is suggested that in AISI 304L a eutec-toid transformation occurs and in AISI 316L (discontinu-ous) precipitation.


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3 Synthesized expanded austenite is extremely brittle.

Fig. 7. Equilibrium diagram showing the thermodynamically stablephases as a function of temperature for AISI 304L and AISI 316Lwith a nitrogen content equivalent to the chromium content (Ther-mo-Calc). The calculations were performed considering only fcc andbcc phases.

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5.2.2. Kinetics of the decomposition of expandedaustenite

The kinetic values obtained by description in terms of thestate variable b (4.3.2.) allow a prediction of the isothermalannealing behaviour of cN. A stability plot (temperature –time plot) for isothermal annealing of homogeneous ex-panded austenite (in H2) is given in Fig. 8. The stability ofexpanded austenite depends strongly on time and tempera-ture. Furthermore, a significant difference in stability oc-curs for the two different alloys: cN is more stable in AISI316L than in AISI 304L. Expanded austenite in AISI 304Land AISI 316L are stable for many years at a temperatureof, say, 473 K (200 °C). However, for a temperature of800 K (527 °C) it takes 8 and 29 minutes to obtain 50 %transformation into CrN for AISI 304L and AISI 316L, re-spectively. In the nitriding range, say, 693 K (420 °C) ittakes 12 hours to obtain 50 % transformation in AISI 316Lbut only 2.7 hours in AISI 304L.4

The possible steps involved in the decomposition of cN

and the development of CrN are diffusion of N and Cr to nu-cleation sites and diffusion of other atoms away from thenucleation sites. This entails volume diffusion and/or diffu-sion of Cr (and other substitutional alloying elements)along defects. Nucleation of CrN occurs most likely hetero-geneously on dislocations, stacking faults, grain bound-aries, etc. The activation energies obtained by kinetic analy-sis (Kissinger-like method) do not resemble the valuesnormally associated with nitrogen diffusion (as the rate lim-iting step). More likely, the obtained energies should per-tain to the diffusion of the substitutional elements, viz. Cr,as volume diffusion or as short-circuit diffusion (grainboundary and pipe diffusion). Literature values for activa-tion energies for volume and grain boundary diffusion ofCr, Ni, and Fe in stainless steel alloys are listed in Table 5[23, 24].

Values for pipe diffusion in stainless steel are expected tobe markedly lower than for volume diffusion. The activa-

tion energies obtained in the present study are significantlylower than the values for volume diffusion and resemblethose for grain boundary diffusion. The activation energyof 128(� 9.9) kJ/mol, obtained for AISI 304L correspondsto short-circuit diffusion as the rate limiting step. The acti-vation energy of 187(� 17.7) kJ/mol for AISI 316L couldbe ascribed to a rate limiting step consisting of a combina-tion of short-circuit diffusion and volume diffusion. It is an-ticipated that the relatively low activation energy for AISI304L is associated with the simultaneous development ofCrN and ferrite. The interface between ferrite and austeniteis suggested as an easy diffusion path for substitutional ele-ments. For AISI 316L no ferrite develops and the redistribu-tion of substitutional elements has to occur along otherdefects or by volume diffusion. Accordingly, a higher acti-vation energy applies than for AISI 304L.

5.2.3. Influence of the nitrogen content

There appears to be an influence of the nitrogen content onthe decomposition of expanded austenite. Evidently, a high-er nitrogen content postpones the development of CrN(cf. Fig. 1a). Assuming that decomposition is governed bythe mobility of Cr atoms to form CrN and not by nucleation,a high content of interstitially dissolved N may impede theCr mobility and thereby delay the decomposition. Alterna-tively, the postponement of CrN precipitation in high nitro-gen austenite can be explained in terms of coherency be-tween the CrN and the (expanded) austenite lattice. See inthis respect Fig. 3: the (111) reflections of CrN and cN devi-ate only about 2° 2h; those of CrN and c deviate 6– 7° 2h).Possibly, CrN precipitates develop coherent or semi-coher-ent interfaces with (expanded) austenite at higher nitrogencontents and nucleate homogeneously, thus necessitating(slow) volume diffusion of substitutionally dissolved ele-ments. Conversely, for low nitrogen contents the oppositecould hold, i. e. CrN develops heterogeneously and has anincoherent interface with austenite.

5.3. Nitrogen desorption during annealing in N2

The desorption of nitrogen during annealing in N2 occursin, what appears to be, two steps. The thermogravimetriccurves for the annealing of expanded austenite in nitrogengas showed the occurrence of a two-stage desorption(cf. Figs. 1a and 2). In the light of the discussion in Sec-tion 5, these observations can be explained as follows.The first development of N2 starts before the appearanceof CrN precipitates. Therefore, the most likely rate deter-mining step in the development of nitrogen gas is solid


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Fig. 8. Stability plot (temperature-time) of cN in AISI 304L and AISI316L.

4 The values obtained for annealing in H2 do not reflect the maximumallowable time for nitriding. The investigated samples were alreadyexposed to a nitriding treatment for 28 hours at 703 K and the stabil-ity reflects the allowable temperature for exposure of the as-re-ceived stress-free powder.

Table 5. Activation energies for substitutional diffusion.

Element Activation energy for diffusion (kJ/mol)

Fe-18Cr-8Ni Fe-17Cr-12Ni

volume grainboundary

volume grainboundary

Cr 244.5 – 263.8 152.2Ni – – 250.8 131.7Fe 280.5 279.2 177.2

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state diffusion of nitrogen atoms to the surface. Upon thenucleation of CrN the effective cross section for diffusionof nitrogen through austenite is reduced. The nitrogen con-tent in the samples on development of CrN is considerablyhigher than for the samples investigated in a reducing at-mosphere. Therefore, CrN is anticipated to develop(partly) homogeneously and has a (semi-) coherent inter-face with expanded austenite (cf. Section 5.2.3). On con-tinued annealing Ostwald ripening of the CrN precipitatesoccurs or, as in ferritic Fe –Cr alloys [25], discontinuousprecipitation, and the precipitates loose coherency withthe matrix. Consequently, the effective cross-section fordiffusion of nitrogen atoms increases and a new maximumN2 development rate occurs.

Alternatively, this behaviour could be attributed to differ-ent levels of nitrogen bonding in the octahedral intersticesas reflected by nitrogen absorption isotherms for cN �0.36 (cf. Ref. 13). The release of relatively loosely boundnitrogen occurs in the first step followed by release of morestrongly bound nitrogen in the second step.

5.4. Relation to the thermal stability of layers of expandedaustenite on stainless steel

In the present investigations the homogeneous expandedaustenite samples were stress-free foils/powders. The de-composition behaviour of expanded austenite layers on asubstrate (bulk material) may be different from the behav-iour observed here. The scarce data available is included inFig. 8 [5]. Although the same trend is observed, i. e. thetime to decomposition increases with reducing temperature,substantial deviation occurs.

The time for decomposition is longer for a layer/substratesituation as compared to synthesized expanded austenite.However, care should be taken when comparing the data.First of all a strict criterion of 50 % transformation is im-posed for the case of synthesized expanded austenite, whichis not the case for the data for layer/substrate situationwhere each data point – apparently – represents the timefor occurrence of precipitates without further definition ofthe amount. Secondly, the nitrogen contents are not neces-sarily the same and the nitriding conditions prior to anneal-ing are not similar.

Also, the stress situation is bound to have an impact onthe decomposition kinetics; the synthesized expanded aus-tenite in the present study is stress-free, as opposed to alayer/substrate situation where the presence of compressivestress (gradients) most likely affects the decomposition.Qualitatively, comparing the volume of 1 mol expandedaustenite with Cr : N = 1 : 1 with that of 1 mol austenitewherein all Cr and N has reacted to CrN may indicatewhether or not stresses promote decomposition. The trans-formation is associated with a volume decrease of approxi-mately 3 %, which suggests that hydrostatic compressivestresses will promote decomposition.

On the other hand, the density of heterogeneous nuclea-tion sites in layers on a relatively thick substrate is probablyhigher due to large compressive stresses in the layer duringgrowth. For the thin foil samples investigated here, stressrelaxation can occur partly in the substrate and the stresseswill never reach the level as in a thin layer on a thick sub-strate, because overlap of diffusion fields occurs relativelysoon in the process.

Finally, for an expanded austenite layer, inward diffusionof nitrogen from cN into the austenitic parent phase occurs,which is impossible for synthesized homogeneous ex-panded austenite. The associated reduction of the nitrogencontent in cN, lowers the driving force as well as the in-volved strain energy effects for CrN development.

6. Conclusions

The thermal stability of synthesized nitrogen expandedaustenite, cN, in austenitic stainless steel was investigatedfor three different alloy compositions, i. e. AISI 304, AISI304L and AISI 316L. Isochronal annealing experimentswere conducted in both inert and reducing atmosphere.Annealing of cN in inert atmosphere gave rise to nitrogendesorption of loosely bound nitrogen at temperaturesabove approximately 750 K. In the same temperature re-gime precipitation of CrN occurred. Annealing in reducingatmosphere gave rise to nitrogen retraction of looselybound nitrogen as NH3 at lower temperatures (510 –560 K).

The activation energy for the rate limiting step in the de-composition of cN into CrN was found to be higher forAISI 316L (187 kJ/mol) as compared to AISI 304L(128 kJ/mol). The obtained kinetic parameters from theisochronal annealing experiments enabled a prediction ofthe isothermal annealing behaviour of cN (stability plot):cN is significantly more stable in AISI 316L compared toAISI 304L.

The decomposition products of cN in AISI 304L andAISI 316L differ: for AISI 316L cN decomposes into CrNand austenite, whereas for AISI 304L it decomposes intoCrN and ferrite.

Financial support by the Danish Research Agency under grant26-01-0079 is gratefully acknowledged. The authors would like tothank Kristian V. Dahl for providing Thermo-Calc calculations andFinn T. Petersen (Sandvik) for providing stainless steel thin foilmaterial.

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(Received July 1, 2005; accepted September 5, 2005)

Correspondence address

Professor Marcel A. J. SomersTechnical University of Denmark (DTU)Kemitorvet building 204, DK-2800 Kgs. LyngbyTel.: +45 45 25 22 50Fax: +45 45 93 62 13E-mail: [email protected]


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Decomposition kinetics of expanded austenite with high nitrogen contents

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