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

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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- tion is 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 pre- cipitation 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 con- tent is clearly shown in both the single austenite region and in the austenite/nitride equilibrium re- gion. 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 con- tent 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 . Research Article Transactions ISIJ, Vol. 27, 1987 (651) 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, Research Article ( 652 ) Transactions ISIJ, Vol. 27, 1987 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) equili- brated with 99°0N2-1°0H2 gas at 1 323 K.
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