Nitrogen Solubility and Austenitic Fe-V Alloys* of Nitride In

Nitrogen Solubility and Precipitation of Nitride in

Austenitic Fe-V Alloys*

By Harue WADA* *

Synopsis same method used in the previous investigation.' The equilibriumnitrogen solubility and nitrideformation in austenitic No increases in AI or Si contents were detected in Fe-V alloyswere measured in thetemperature range from 1273 to 1 523 K. the prepared specimens. Specimens0.5 mm thick were equilibratedwith three differentnitrogen- The experimental apparatus and procedure of argon-1% hydrogengases. The nitrogensolubility obeys Sieverts' law. equilibration were described in previous papers."2) The experimentalresults in 1-phasewere analyzedby the centralatoms Five Fe-V alloy specimens and Fe were equilibrated model,and the Wagnerinteraction coefficient was determinedas a function with three different N2-1 %H2-Ar mixed gases; gas of temperatureas: A(99%N2-1 %H2), gas B(63.55%N2-1 %H2-Ar) and eN= (-323 600±10 860)/T+(165±7.8) gas C(34.0%N2-1 %H2-Ar). All five alloys were The weightpercent interaction parameters, 4 and ey, weredetermined as equilibrated except a few runs in which only selected functionsof temperaturefrom this value. The solubilityof nitrogenat alloys were included to obtain sufficient amounts of PNT2=1is expressedas a function of temperatureas: nitride for analysis of nitrogen in the nitride. log (wt%N)r-re-v = [539+1540(wt%V)]/T 2. Analyses -0 .79(wt%V)-0.20. The nitrogen content in an Fe-V specimen which Theprecipitated nitride was identifiedas vanadiummononitride VN, and contains precipitated nitrides can be divided into the solubilityproduct was determinedas a function of temperatureas: two parts: log (wt%N)(wt%V) _ (-6 777±372)/T+(2.07±0.3). Wt%N)~o6a1~T = (wt%N)r-znaLrix+(wt%N)ui6ridc The equilibriumconstant for the reactionVN(s) =V } N, and the self interactionparameter of vanadiumeV are estimatedfrom these results...... (1) The overalleffect of thesethree parameters on the equilibriumVN forma- To distinguish these two contents an analytical pro- tionis estimated. cedure was established as shown in Fig. 1 after evalu- ~ atingseveralmethodsas described forFe-Ti alloys Key words: vanadium nitride; solubility product; solubility of nitrogen. .' All specimens were closely examined for nitride precipitation by a SEM (Hitachi 5-520). In the I. Introduction specimens in which no nitride is present, (wt%N)totai Vanadium is known to improve the high tempera- _ (wt%N)r -matrix which is determined by, a LECO ture mechanical properties of austenitic alloys by TN 114 nitrogen analyzer. forming a fine vanadium nitride or carbide. Equilib- In the specimens which contain nitrides, the vana- rium between austenite phase and nitride has not dium contents in austenite phase were measured by been established, mainly because of experimental an EPMA (CAMEBAX-MICRO). A calibration difficulties caused by the small size of precipitated line was prepared with several standard Fe-V alloys nitrides. In this study an analytical procedure was as: established to solve this problem by applying instru- mental analyses rather than chemical separation of at%V/a.t%Fe =kvl,O[(I-Io)v/(I-Io)F(] nitrides from metal matrix. The solubility of nitro- where, (I -Io)v, (I -Io),.: measured intensities of gen in austenitic Fe-V alloys and equilibrium be- VKa and FeK, lines tween austenite and nitride were determined in the kvi,0. a constant determined from the temperature range from 1 273 to 1 523 K. standard analyses. The vanadium content in austenite phase, (wt% II. Expeimental Procedure V)r-matrix, was measured four five times for each

1. Materials and Equilibration Table 1. Materials.(wt%) Five Fe-V alloys were melted in the composition range from 0.016 to 0.195 wt% V by the same method described in the previous paper.' The purities of starting materials are shown in Table 1. The 0.5 mm thick specimens were prepared by the

* Presented to the Second International Symposium of Solubility Phenomena, International Union of Pure and Applied Chemistry (IUPAC), August 1986, at New jersey Institute of Technology in Newark, NJ. Manuscript received on January 13, 1987; ac- cepted in the final form on March 13, 1987. © 1987 ISIJ ** Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109, U.S.A.

Research Article (649) (650) Transactions ISIJ, Vol. 27, 1987 specimen then averaged. Results obtained in speci- matrix by HCl solution was not reliable because the mens containing considerable amounts of nitrides solution apparently dissolved smaller nitride particles showed more dissonant values among them in general. resulting in inconsistent nitrogen analyses. These The nitride species was determined on extracted are common difficulties experienced in other Fe- nitrides by electron diffraction or X-ray diffraction; transition metal systems and details have been also several extraction carbon replicas of nitrides described for Fe-Ti alloys.' were analyzed by a TEM (JEOL 100-CX). Direct determination of (wt% N)nitriae was at- III. Experimental Results tempted by separating precipitated nitrides from matrix. The nitrides were extracted from the '- 1. Nitrogen in Y-Fe-V Alloys matrix either by the Beegley method or using 50 % Total nitrogen contents of specimens were sum- HCI, then analyzed for nitrogen by the LECO TN marized in Table 2. The pure iron specimen was 114 nitrogen analyzer or the Kjeldahl method. included as a reference material in equilibration. However, results of both methods were not satisfactory. The solubility of nitrogen in 1-Fe shows good agree- Limited amount of precipitated nitrides in most ment with previous measurements as shown in Fig. 2. specimens made it unrealistic to perform duplicate Figure 3 shows the total nitrogen contents vs. (wt% V) analyses by the TN 114 analyzer, although the results at 1 273, 1 373 and 1 473 K. The open circles re- of TN 114 were more reliable than that by the Kjel- present specimens that contained no nitride, while dahl method. The separation of nitrides from the crossed circles represent specimens that contained nitrides. The effect of vanadium on nitrogen content is clearly shown in both the single austenite region and in the austenite/nitride equilibrium region. The values of (wt% V)Y_matrixin specimens which contained precipitated nitrides are summarized in Table 3. A set of (wt% V)r_matrix and (wt% N)r-matrixwas selected for each temperature and gas as shown in Table 3. An averaged value of the measured (wt% V)r_matrix was selected for each temperature and gas as shown in Table 3 except at 1 373 and 1 423 K. At 1 373 K, the measured value of (wt% V)r_matrix=0.15 in Fe-0.195%V specimen was neglected because this higher value obviously related to the V content of nitrides precipitated in the specimen. At 1 423 K, (wt% V)r_matrix=O.l was selected based on the V content in Fe-0.10% V specimen which showed small amount of nitride precipitates. The values of selected (wt% V)r_matrix are marked by arrows in Fig. 3. The nitrogen content in r-matrix, (wt% N)r_matrix, in equilibrium with nitrides, was then estimated at the arrow on Fig . 1. Analytical procedure. each solubility line.

Fig. 2. Solu bility of nitrogen in Y-Fe .

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Table 2. Experimental results of nitrogen contents.

2. Nitrides the presence of precipitated nitrides, but the analyzed Typical microstructures of specimens with pre- value of (wt% N)total shows only small increase as cipitated nitrides are shown in Fig. 4 for Fe-0.195 %V seen in Table 2. It appears that the temperature alloy. Both specimens were equilibrated with gas was not high enough to melt vanadium nitride during A(99%N2-1 %H2) at (a) 1 423 K and (b) at the nitrogen analyses; the melting point of vanadium 1 323 K. Nitrides precipitated in inner grains and mononitride is reported as 2 450 K. their sizes ranged from 1 }gymto a fraction of 1 tm. The ratios of V/N in nitrides can be determined The analyzed (wt% N)total, however, failed to show from the slopes of (wt% V) res. (wt% N)totai lines in increases expected from the precipitated nitrides the region of 1-nitrides equilibrium shown in Fig. 3. observed by SEM. For instance, the microstructure The slope is approximately 3.6 in Fig. 3(b), which of Fe-0.195%V specimen shown in Fig. 4(a) exhibits is close to the ratio of vanadium mononitride VN,

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Vf N=3.63. However, the values of (wt% N)t°tai not alloyed with Fe. Several electron and X-ray shown on a dotted line are much lower than the values diffractions confirmed that the precipitated nitrides expected from (wt% V)r_matrix by a solid straight are vanadium mononitride VN. line. To determine nitride species, carbon extrac- Specimens equilibrated with gases of lower nitro- tion replicas of nitrides were prepared from selected specimens and analyzed by TEM-EDX as well as by electron diffraction. Figure 5 shows a typical Table 3. Austenite phase analysis by EPMA. energy spectrum of nitride precipitated in Fe-0.195 %V specimen equilibrated at 1 373 K with the gas B, which (wt% N)t°tai is shown in Fig. 3(b). The spectrum show only vanadium and no iron. It implies that the nitride is a V-N compound but

Fig . 3. Total nitrogen contents in 1-Fe--V alloys, (a) at 1 273 K, (b) at 1 373 K and (c) at 1 473 K.

Fig. 4. Microstructures of Fe-0.195%V alloy specimens, (a) equilibrated with 99°N2- 1%FI2 gas at 1 423 K and (b) equilibrated with 99°0N2-1°0H2 gas at 1 323 K.. Transactions ISIJ, Vol. 27, 1987 (653)

Fig. 5. Energy spectrum of vanadium nitride precipitated in Fe-0.195%V specimen equilibrated with 63.55°0N2 -l %H2 Ar gas at 1 273 K.

Fig. 6. SEM micrograph and X-ray images of V and Si of particle precipitated in Fe -0.195%V specimen equilibrated with 34.0%N21 %H2Ar gas at 1 473 K. gen especially the gas C showed departure form the Iv. Discussion iso-activity lines as seen in Fig. 3. Examination of microstructures revealed that these specimens con- 1. AustenitePhase in Fe-V Alloys tained precipitates of relatively large size, generally The r-phase was analyzed using the central atoms > 1 µm. A typical particle precipitated in a Fe- model to correlate all data including the pairs of 0.195 %V specimen equilibrated with the gas C at (wt% N)1_matrixand (wt% V)1_lllatrixdetermined at 1 473 K is shown in Fig. 6. The particle contains phase boundary. The activities of nitrogen are equal Fe, V and Si. Atomic ratios of V, Fe and Si in those in the equilibrated Fe-N and Fe-V-N alloys at the particles are in a range of V/Fe/Si=0.38/0.27/0.35 same temperature and nitrogen partial pressure if to 0.37/0.23/0.40. Sources of Si might be the cement both activities are referred to the same standard used to fix a boat that supported the specimens. state, The nitrogen contents obtained with the gas C were aN(Fe -N) = aN(Fe-V-N) ...... (2) carried less weight on determination of the iso- activity lines. Since the r-phases is a dilute solution of both N and V, the nitrogen solubilities in these two systems are expressed as2-4) :

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{yN(Fe-V-N) /yN(Fe-N) }1/2Z' X e{z'/Z'[YN(Fe-V-N)-yN(Fe-N)IANN} -1 = - YVAVN...... (3) where ANNand AVNare the parameters representing interactions between N-N and N-V , respectively; yN and YV are the lattice concentrations of N and V defined as : yN = XN/(1-2XN) Yv XV/(1 XN) ...... (4) The values of z' and Z' are z'=12 and Z'=6 in the austenite.4~ The interaction between N-N in 1-Fe is reported as zero.' In Fig. 7, the values of (wt% N) are plotted against YN2for three temperatures in Fe-0.016%V alloy which has the widest solubility range. The nitrogen solubility obeys Sieverts' law. This implies that the interaction between N-N in the Fe-V-N system can be neglected, namely ANN= 0. Introducing Fig. 7. Nitrogen solubility vs. these values, Eq. (3) is further simplified as: nitrogen pressure in Fe- 0.016%V alloy. e2[YN(Fe-V-N)-yN(Fe-N)}ANN= 1 The ratio of yN's can be rewritten as :

{YN(Fe-V-N)/yN(Fe-N)}1/12-1 = -YVAVN ...... (5) The parameter 2VNis determined by Eq. (5) for each temperature from the measured values of yN(Fe-V-N) and yN(Fe-N) including estimated values of yN(Fe- V-N) at the phase boundary. Figure 8 shows an example at 1 273 K, in which the open circles represent specimens with no nitrides present while the closed circles represent the pair of YV and yN(Fe- V-N) at the phase boundary of austenite and austenite +nitride. The Wagner interaction coefficients are calculated from the values of 2VNas follows: Fig. 8. Effect of vanadium on the activity of nitrogen in sN= (a In ON/aXV)V-o= 2Z'AVN= 12AVN...... (6) 1-phase of Fe-V-N alloys at 1 273 K. where, ON: the activity coefficient of nitrogen. Table 4. Wagner interaction parameters. The results are summarized in Table 4. The Wagner interaction coefficients are computed as a function of temperature as : ~N= -323 600±l0860)/T+(165±7.8) ...... (7) and N, respectively. Fountain and Chipman5~ measured the solubility The interaction parameters in wt% concentration of nitrogen and nitride formation in 7-Fe-V alloys as units, eN and ey, are calculated from Eq. (7) as well as in the ferrite temperature range by the follows : Sieverts' method. The influence of V on the activity ~N= [a In fN/a(wt%V)]V ~o coefficient of nitrogen was determined as the ratio of the slope of (wt% N)Fe and (wt% N)Y_Fe_VVS. ~ PN2 [EN-(MFe- MV)/MFe{X MFe/230My in the soluble range. They determined log f N/(wt%N) = -1540/ T+ 0.79 ...... (8) (wt%V) as: -0.47 (1223 K), -0.33 (1 323 K) and and because -0 .18 (1 473 K). The corresponding Wagner interaction coefficients are -99, -69 and -38. ev = [Sv-(MFe-MN)/MFe] X MFe/230MN Figure 9 shows the present results compared with = -5 600/TT+2.84 ...... (9) those reported by Fountain and Chipman. The present results show a lower temperature dependence where, f N: the activity coefficients of N than their results. However, both results reasonably Me, MV, MN : the atomic weights of Fe, V agree within the experimental accuracy in spite of

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much higher compared with that of TiN in r-Fe-Ti- N alloys.' The value of (wt%N) (wt%V) is approximately 200 times that of (wt%N)(wt%Ti) at 1 473 K and 1 300 times at 1 273 K. Therefore the precipitation of vanadium nitrides in austenitic steel is expected to be controlled more easily than titanium nitride precipitation. Fountain and Chipman5~ determined the solubility product of VN and proposed two equations, log KA and log KB, depending on the size of precipitated nitrides : KA for the metastable smaller nitrides (N 0.04 µm) and KB for the stable larger nitrides (~0.6 rim). Both log KA and log KB included the Fig . 9. eN as a function of temperature. activity coefficient of nitrogen, and the activity coefficient of V was assumed approximately one. These the limited solubility range and the small differences were expressed as functions of temperature as : of nitrogen solubilities between in Fe and Fe-V alloys. log KA = log (wt%N)(wt%V) x fN Combining Eqs. (8) and (9) with the nitrogen = -6430/T+2.0 solubility in 1-Fe, log (wt%N) = (539± 17)/T- (2.00± and log KB = log (wt%N)(wt%V) XfN 0.01), reported in a previous work,1 the nitrogen = -7 070/TH-2.27 ...... (15) solubility at PN2=1 can be expressed as a function of temperature as : Narita and Koyama6) measured the reaction of nitro- log (wt%N)r_Fe-v= log (wt%N)r_Fe-eN(wt%V) gen and vanadium in r-Fe-V alloys equilibrated with 1 atm N2-3%H2 gas. They reported that the reac- =[539+1 540(wt%V)]/T-0.79(wt%V)-0.20 tion was y1+N=VN and the solubility product was ...... (10) expressed as : 2. Nitride+Austenite Phase in Fe-V-N Alloys log(wt%N)(wt%V) = -8 700/T+3.63 ...... (16) Because the precipitated nitrides are identified as Irvine et al.7~ measured the effect of V on grain VN, equilibrium in the r+VN region can be expressed refining of C-Mn steels (wt% C : 0.05 and 0.15). as: They determined the solubility product of VN as : VN (s) = V+N ...... (11) log K11= log aNav/avN log (wt%N)(wt%V) = -8330/T+3.46 ...... (17) where avN=1. Henrian standard state is used for These results are shown in Fig. 10 and compared both aN and ay referred to 1 wt% N solution in Fe with the present results. Since the KB was for stable and 1 wt% V solution in Fe, respectively, because nitrides, only log KB is shown in the figure. The the r-Fe-V is a dilute solution of both elements. present result is in good agreement with the line The activities of N and V are expressed as : log KB obtained by Fountain and Chipman,5~ but lower than other measurements6,7~ at higher tempera- log aN = log (wt%N) +eN(wt%V) tures. and log av = log (wt%V)+eV(wt%N)+eV(wt%V) The first three terms of Eq. (13) are now obtained as functions of temperature. Then K11 and eV can ...... (12) be estimated as follows : where the interaction of N-N is taken as zero because log K11-eV(wt%V) = log (wt%N)(wt%V) the nitrogen solubility obeys Sieverts' low. Intro- ducing Eq. (12) into Eq. (11), the logarithm of the +eN(wt%V)+ev(wt%N) ...... (18) equilibrium constant is rewritten as: The right hand side of Eq. (18) is plotted against log K11= log (wt%N)(wt%V)+eN(wt%V) (wt% V) in Fig. 11. The values for eV and the log K1, are obtained from the slope and the intersec- +ev(wt%N)+ev(wt%V) ...... (13) tion at each temperature respectively, as : In the right hand side of Eq. (13), the first term is known as "solubility product" of VN. The solu- 4=(1763±21)/T-(0.96±0.02) ...... (19) bility product of VN is computed as a function of log K1, =(-6570±24)/T+(1.91±0.02)...... (20) temperature from the pairs of selected values of The standard free energy of formation of Eq. (11) (wt% N) and (wt% V) summarized in Table 3 as : is obtained as : log (wt%N)(wt%V) 4G° = -R T In K1, = (-6 777±372)/T+(2.07±0.3) ...... (14) =125 900-36.49T(±500) (J/mol VN) The solubility product of VN in r-Fe-V-N alloys is ...... (21)

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Fig. 10. Solu bility product of VN.

Fig. 12. Relationships among interaction parameters.

Fig. 11. Estimation of log log K11and e~ from log (wt%N) (wt%V)+eN(wt%V)+e~(wt%N).

Fig. 13. Relationships among log K11, log (wt%N) (wt%V) and log KB.

Interaction parameters affect both nitrogen solu- difference between these two lines is small: 0.5 and bility and nitride formation as shown in Eqs. (10) 1.4% of log K11 at 1 373 and 1 523 K, respectively. and (13). To show the relative effect of N-V, The effect of non-ideality of the system on the equilib- V-N and V-V interactions, these parameters are rium VN formation causes primarily a change in compared in Fig. 12. Accuracy of these values is ay. However the effect is minimal because of low the highest for eNand the lowest for e~. The overall nitrogen contents at the phase boundary at lower effect of non-ideality on the equilibrium nitride forma- temperatures, while the smaller values of eV restrain tion is shown in Fig. 13, in which log K11 which the effect of increased nitrogen content at the includes all three parameters is compared with log boundary at higher temperatures. The log KB by (wt%N) (wt%V) which includes none of them. The Fountain and Chipman which includes the activity

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coefficient of nitrogen is also shown in Fig. 13. It (5) The self interaction parameter of V is esti- can be concluded that the equilibrium VN formation mated as : in the r-Fe-V-N system is reasonably presented by eV=1760/ 760/T-0.96. the solubility product (wt%N) (wt%V), when the nitrogen partial pressure is 1 atm or lower. (6) The overall effect of three interaction parameters on the equilibrium VN formation is minimal: V. Summary it is less than 0.5 and 1.4 % of log K at 1 373 and The solubility of nitrogen and nitride formation in 1 523 K, respectively. r-Fe-V alloys were investigated in the temperature range from 1 273 to 1 523 K. The size of precipitated Acknowledgements fine nitrides were generally less than 1 µm and The author acknowledges the support of the identified as VN. National Science Foundation under Grant DMR- The following thermodynamic properties were 8206135 (Equipment: TN 144) and EAR-8212764 determined. (Equipment: CAMEBAX-MICRO). Operational (1) The nitrogen solubility in 1-Fe--V alloys help of Mr. C. E. Henderson for the phase analyses obeys Sieverts' law. by CAMEBAX-MICRO is appreciated. (2) The interaction parameters are determined as: REFERENCES eN= -1540/ T+O.79 1) H. Wada and R. D. Pehlke: Metall.Trans. B, 16B(1985), and eV= -5 600/T+2.84. 815. 2) H. Wada: Metall. Trans.A, 16A (1985),1479. (3) The equilibrium solubility product is deter- 3) H. Wada: Metall. Trans.A, 17A (1986),391. mined as: 4) C.H.P. Lupis: ChemicalThermodynamics of Materials, North-Holland,Elsevier, N.Y., (1983). log (wt%N)(wt%V) 5) R. W. Fountain and J. Chipman: Trans. Metall. Soc. =(-6 777+372)/T+(2.07±0.3). RIME, 212 (1958),737. (4) The equilibrium constant for the reaction 6) K. Narita and S. Koyama: Tetsu-to-Hagane,52, (1966), VN = V + N is estimated as: 292. 7) K.J. Irvine, F. B. Pickeringand T. Gladman: J. Iron log K =(-6570±24)/T+(1.91±0.02). SteelInst., 205 (1967),161.

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