Differential Thermal Analysis-Evolved Gas Analysis for the Determination of Carbide and Nitride Phases in Steel W. R. Bandi" and G. Krapf United States Steel Corporation, Research Laboratory, f25 Jamison Lane, Monroeville, Pa, 15 146
Methods for the quantitative chemical separation of (Fe,Cr)3C, (Cr,Fe)&, Cr2C, Cr2(C,N), Cr2N, (CI,F~)~&, and CrN from steel have been studied. It is shown that all but one of these phases can be easily determined in isolated resldues contalnlng mixtures of these compounds by DTA-EGA procedures; CrN can be determined by chemical procedures. The use of DTA-EGA to determine the changes in concentrationof carblde precipitates with heat treatment and composltlonof the steel is demonstrated. The effect of heat treatment on the composition and size of a specific carbfde and on the EGA peak recorded for that carblde is also discussed. Instances are described in which DTA-EGA can be used to dlsclose phases not previously ldentifled In these types of steels by any other analytical technique.
Differential thermal analysis-evolved gas analysis (DTAEGA) procedures were developed more than 10 years ago (1-3) for the determination of second-phase compounds isolated from steels by selective chemical or anodic dissolution of the matrix. Since that time these methods have been used to qualitatively identify and quantitatively determine approximately 35 carbides, carbonitrides, and nitrides, some of which could not be identified by any other method (4-16). These procedures have also been used in studies of chemical methods used in the isolation of pure Fe3C from steel, (15), in studies showing changes in carbonitride composition with heat treatment (12,13),and to show the change in types of carbides and nitrides formed in steel as the heat treatment of the steel is changed (7, 9-12). Such changes are important because the type of precipitate formed is known to have a profound effect on the mechanical properties of the steel. Other investigators ( 1 7-22) have used these techniques in similar studies. The present work concerns the determination of carbide and nitride precipitates in Fe-Cr-C-N alloys by using improved DTA-EGA instrumentation. The most common carbide precipitated in steel is cementite (M3C), where M is a mixture of iron and chromium for these alloys and the ratio of iron to chromium is dependent on the chromium content and the heat treatment of the steel. A steel having 1%chromium, for example, can contain an M3C precipitate with from 2 to 20% chromium depending on the heat treatment and how quickly the phases reach equilibrium (23, 24). An increase in the chromium concentration in the cementite phase increases the chemical stability of the phase and makes it possible to quantitatively isolate a chromium M3C phase (25,26),whereas pure Fe3C is chemically unstable and cannot be quantitatively isolated (15). M7C3 is usually formed from M3C by heat treatment of a steel containing less than 12% chromium (24, 27-29). However, M/C3 can also be precipitated directly from alloys containing 12%chromium when the steels are tempered and aged a t 500 "C (23-27). This carbide can contain as much as 60% iron, although chromium is normally the major constituent. A t the same time that M7C3 precipitates, a phase usually designated M2X, which is impure Cr2C containing a small amount of nitrogen (0.001%)or molybdenum to stabilize the
structure, is also precipitated (23, 27, 30-32). Further tempering of a 12% chromium steel or of an austenitic stainless steel will convert M7C3 to an M23C6 precipitate (24,30). Two nitrides are commonly found in Fe-Cr-N alloys. CrN is precipitated in low-chromium steels and is both chemically and thermally stable (33).In high-chromium steels, CrzN is precipitated (34);it is readily attacked by acid. As noted earlier, before DTA-EGA procedures can be applied, the precipitated phases must be isolated from the steel. The most successful method of isolating M3C has been by an anodic dissolution of the steel matrix in an electrolytic cell. M3C remains in the undissolved residue (26).Many electrolytes (24, 26, 28, 29, 35-37) have been used in attempts to isolate M7C3 from steels. Although most reports state that it cannot be quantitatively recovered from the steel, the authors have successfully isolated M7C3 in 1 2 and aliphatic alcohols (16).Although CrzN is soluble in hydrochloric acid, it can be isolated by electrolytic methods (33), by use of Br2-methyl acetate solutions, or by 12-alcohol solutions (33, 38). 12 and methanol extractions have also been used by the authors to isolate M23C6 ( 6 ) .No references to the isolation of CrzC or Cr2(C,N) could be found in the literature, although Andrews (31)and Hobson (39)isolated (Cr,Mo)2(C,N) both by 12-alcoho1 extractions and by electrolytic methods. Finally, CrN can be isolated by using either a halogen or acid treatment. It can be dissolved by a vigorous attack such as by fuming with a K~SO~-H~SO~-CU solution S O ~ (25,34,40).
EXPERIMENTAL Table I shows the chromium, carbon, and nitrogen contents for nine steels used in this study. Steel samples No. 1through 8 were rolled to 0.95-cm plate or forged into 2.5-cm rounds after melting and casting. Sample No. 9 was a 0.66-cm rod of National Bureau of Standards (NBS)No. 447 chromium-nickel austenitic stainless steel, and was used to provide a source of M23Cs. Each of the plates or rods was heat-treated according to the schedule in Table 11. Reference to the literature indicated that such a treatment would result in the precipitation of the chromium compounds shown in column 4 of Table 11. (Incomplete precipitation of carbon was expected in sample No. 8.)
The first step in isolating the residues from the steels was to shake the samples (2.5-cm diameter by 0.64-cm-thick cylinders or 5.1- by 0.95- by 0.64-cm-thick plates) with a 10%solution of I2 in ethanol. The residues were then filtered through a weighed 10-nm pore-size cellulose-ester membrane. The solution was purged with argon before shaking the samples, and an argon atmosphere was maintained over the solution during filtration and while drying the filter in a desiccator. Details of the filtration apparatus and the weighing of the isolated residue have been previously published (8).Other residues were isolated by using 10% hydrochloric acid, Brz-methyl acetate, or Brzmethanol solutions to dissolve the steel. Details of these isolation procedures have also been previously reported (2,8,15). The residues isolated in iodine solution were analyzed by x-ray diffraction. Some of the residues were also analyzed by chemical methods for chromium, iron, and/or nitrogen to confirm peak assignments and to aid in the interpretation of the DTA-EGA results. From 1to 3 mg of the isolated residue was inserted in the DTA-EGA instrument in the established procedure, details of which have been previously published (2,10,11,15).The modifications completed on an RL Stone Model 12BC2 instrument to limit the gas volume in the EGA system and therefore increase the EGA sensitivity (2, 7) were ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
649
Table I. Composition of Fe-Cr-C-N
a
Alloys Used i n t h e Study
Sample No.
Cr
1 2 3 4 5 6 7 8 9 NBS No. 447)
1.28 12.00 11.61 12.05 11.92 4.30 4.47 1.22 23.72
Composition, % C
Geometry of test specimen before heat treatment
N
0.065 0.16 0.16 0.17 0.007 0.091 0.096 0.14 N.D.a
0.002 0.017 0.003 0.022 0.045 0.008 0.002 0.010 N.D."
Forged into 2.5-cm round Rolled to 0.95-cm plate Rolled to 0.95-cm plate Rolled to 0.95-cm plate Rolled to 0.95-cm plate Forged into 2.5-cm round Forged into 2.5-cm round Rolled to 0.95-cm plate 0.56-cm rod
Not determined.
Table 11. Heat T r e a t m e n t Applied to t h e Samples Annealing temperature and time Hours
Sample No. "C
Tempering temperature and time "C Hours
la lb 2 3 4a 4b 5
1300 1300 1300 1300 1300 1300 1300
0.5 0.5 0.5 0.5 0.5 0.5 0.5
6 7 8a
1300 1300 1000
8b 9
1300
0.5 0.5 5 in argon a t reduced pressure None 0.5
500 64 500 4 500 240 500 240 500 4 500 240 Cool a t 100 "C/h to 700 "C and hold for 100 h 500 4 500 4 Cool at 100 "C/h to 700 "C and hold for 100 h None 700 2
Compounds expected to precipitate M&, M3C M3C, Cr7C3, CrdC,N)3 MBC,Cr7C3 MBC,Cr2(C,N) M&, Cr2(C,N), Cr7C3 Cr2N M3C, Cr2(C,N) M3C MsC, CrN MsC, CrN M23C6, M7C3, M3C, Cr2N
Table 111. Distribution of Carbon a n d Nitrogen i n t h e Samples C Sample No.
a
650
%C in steel
%N in Steel
as M3C
as CrzC
as CrZ(C,N)
as M7C3
as amor. C
as CrN
N as Cr2N
la 1b 2
0.065 0.065 0.16
0.002 0.002 0.017
0.036 0.044 0.017
0.028 0.013 0.052
N.D." N.D." 0.003
N.D." N.D." 0.076
N.D." N.D." N.D."
N.D." N.D." N.D."
N.D." N.D." N.D."
3 4a
0.16 0.17
0.003 0.022
0.011 0.11
0.048 0.021
N.D." 0.003
0.082 N.D."
N.D." 0.025
N.D." 0.0005
N.D." 0.0065
4b 5 6
0.17 0.007 0.091
0.022 0.045 0.008
0.010 0.004 0.030
0.024 N.D." 0.023
0.006 N.D." 0.002
0.043 N.D." N.D."
0.005 N.D." 0.010
0.0008 N.D." N.D."
0.015 0.044 N.D.a
7 8a 8b
0.096 0.14 0.14
0.002 0.100 0.010
0.019
0.021
N.D."
0.029
0.048
N.D." Not analyzed 0.006 Trace
N.D."
0.063
N.D." 0.007 0.006
9
0.10*
Not analyzed
0.003
0.007
0.048
0.004
0.005
N.D. = None detected. b (0.025% C as M23Ctj; 0.010% C as other carbides). ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
N.D." 0.0027 Trace
t 2 u C z
8 J
4 pr
w
I
i
2w
TEMPERATURE, 'C
Figure 8. EGA carbon recording and resolution for sample 6 containing
Cr2C and Cr2(C,N)
A shift of the combustion temperature of M2&6 as the carbide precipitate becomes more massive is also noted if early results (6) are compared with presently shown M2&6 combustion temperatures. The variation of combustion temperature of these two carbides presents problems in the identi-
fication of such compounds in high-alloy steels, and therefore careful work must be done to confirm EGA identification before quantitative results are reported. Figure 7 compares the EGA recording for CO2 evolution from combustion of carbides in residues isolated in iodineethanol (Recording A), in the hydrochloric acid (Recording B), and in Br2-methyl acetate solution (Recording C). Only $$ and ?$o, respectively, of the (Cr,Fe)&s isolated in In-ethanol solution were isolated in Br2-methyl acetate and in the hydrochloric acid solution. Cr2C dissolved in Br2-methyl acetate; the big EGA peak at 400 "C for the residue isolated in this medium is due to cementite and amorphous carbon (15).In this type of analytical work, this instability of the chromium carbides in hydrochloric acid can be used to good advantage. Because stable carbides of titanium, niobium, tantalum, vanadium, and other metals can readily be isolated in hydrochloric acid without Cr2C interference, they can be determined by DTA-EGA even at the parts-per-million level. Although it has only recently been disclosed that CrzC exists in steels, these DTA-EGA data showed from trace to y3 of the carbon in the steel to be present as this compound in every one of the steels except sample No. 5, which was a low-carbon sample prepared for the nitride investigation. This apparently is the first time that Cr2C has been shown to exist in lowchromium steels. These data show the advantages of the DTA-EGA technique over other procedures because quantitative results for Cr2C can be obtained, whereas with other physical procedures it is difficult to identify the compound. (Cr2C was identified in only two of the eight isolated residues examined by x-ray diffraction.) A further demonstration of the usefulness of DTA-EGA is in the identification and determination of Cr2(C,N) in the presence of CrzC and CrZN. The resolution of the C02 peaks shows that both Cr2(C,N) and CrzC are present in sample No. 6 (Figure 8). The temperature range over which nitrogen evolves from the decomposition of Cr2(C,N)is shown in Figure 9. The temperature is the same as that shown in Figure 8 for the evolution of C02 from Cr2(C,N), and this is usually indicative of a carbonitride. Further results showed that the carbonitride isolated from sample No. 4b (Figure 9) and the carbonitride isolated from sample No. 2 (not shown) decomposed over the same temperature range even though their respective formulas were Cr2(Co.~,No.s) and Crz(Co.sN0.l). ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
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Figure 9 also shows that the difference in the temperature for the evolution of nitrogen from Cr2(C,N) and Cr2N is more than 200 OC. It is therefore easy to distinguish these compounds from each other by DTA-EGA, but identification of these two compounds in the same sample by x-ray diffraction or chemical methods is very difficult or impossible. In sample No. 5 (Recording B of Figure 9), all the nitrogen in the steel was found to be present as either Cr2(C,N)or CrzN, which demonstrates that both the carbonitride and the nitride can be completely isolated in 12-ethanolsolution. Conversely, no Cr2(C,N) or Cr2N is isolated in hydrochloric acid, although the CrN is completely isolated. Therefore, it is easy to chemically separate CrN from Cr2N and Cr2(C,N); they cannot, however, be distinguished by x-ray diffraction methods. Also, CrN decomposes a t a temperature greater than 1500 “C and therefore does not interfere with the DTA-EGA determination of Cr2N or Cr2(C,N). In addition, this high decomposition temperature makes it possible to distinguish CrN from other chemically stable nitrides such as TiN and NbN which also are isolated in hydrochloric acid. Table I11 summarizes the DTA-EGA data and shows the distribution of the carbon and nitrogen combined with the chromium compounds. The identifications are for the most part those predicted in Table I1 except for Cr2C and CrdC,N).
ACKNOWLEDGMENT The authors acknowledge the contribution of P. A. Stoll, of our Research Laboratory, who supervised the x-ray diffraction studies needed for this work. LITERATURE CITED (1)W. R. Bandi, H. S. Karp, W. A. Straub, and L. M. Melnick, Talanta, 11, 1327-37 (1964). (2)W. R. Bandi, W. A. Straub, E. G. Buyok, and L. M. Melnick, Anal. Chem., 38, 1336-41 (1966). (3) . . H. S. KarD. W. R. Bandi. and L. M. Melnick. Talanta.. 13.. 1679-67 (1966). (4)H. S.Karp, W. R. Bandi, W. A. Straub, and L. M. Melnick, “The Determination of Carbides and Nitrides in Steel by DTA-EGA”, Society for Anal. Chem. of Pittsburgh, Spring, 1965. (5)W. R. Bandi, W. A. Straub, H. S. Karp, and L. M. Melnick, ASTMSpec. Tech. fubl. 393. ASTM. Philadelahia. Pa.. 1966. (6)H. S.Karp, E-G. Buyok,W:R.-Bandi, andL. M. Melnick, Mater. Res. Bull., 2,311-22(1967). (7)W. R. Bandi, E. G. Buyok, G. Krapf, and L. M. Melnick. “Thermal Analysls” Vol. 2,R. F. Schwenker, Jr., and P. D. Garn, Ed., Academic Press, New York, N.Y., 1969,pp 1363-76. (8)W. R. Bandi, J. L. Lutz, and L. M. Melnick. J. lron Steel lnst., 207,348-52 (1968).
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RECEIVEDfor review November 4,19?6. Accepted January 21, 1977.
