no averaging of data from successive analyses on any sample. Under these conditions, the accuracy of the specific heat functions determined abovc is estimated to be within &l.O%. LITERATURE CITED
(1) Dauphinee, T. M., MacDonald, D. K. C., Preston-Thomas, O., Proc. Roy. SOC.(London) 221, 267 (1954). (2) Evans, W. H., NBS Rept. 6928 (1960).
(3) Furukawa, G. T., Douglas, T. B.,
McCoskey, R. E., Ginnings, D. C., J . Res. Natl. BUT.Sld. 57, 67 (1956). (4) Ginnings, D. C., Corruccini, R. J.,
Ibzd., 38, 593 (1947). (5)Kelley, K. K., U . S. BUT. Mines Bull. 584 (1960). (6) Maier, C. G., Kelley, K. K., J . Am. Chem. SOC.54, 3243 (1932). (7) O’Neill, M. J., ANAL. CHEM. 36, 1238 (1964). (8) O’Reilly, 3. M., Karasz, F. E., Bair, H. E., J . Polymer Sci., Part C, Polymer Symp., No. 6 (1963).
(9) Stull, D. R., JANAF ThermochemI Data (1961). Victor, A. C., J . Chem. Phys. 36, )3 (1962). Watson, E. S., O’Neill, M. J., Jumitj!h^J.!.Bre?ner, N., ANAL. CHEM. 36, l Y 6 6 ( I Y t j 4 ) . (12) Westrum, E. F., Jr., Hatcher, J. B., Phys. 21, 419 Osborne. D. W.. .I. Chem. (1953). ’ I
-
RECEIVED for review March 29, 1966. Accepted June 22, 1966.
Differential Thermal Analysis-Effluent Gas Analysis Method for Identification and Determination of Nitrides in Steel W. R. BANDI, W. A. STRAUB, E. G. BUYOK, and L.
M. MELNICK
United States Steel Corp., Applied Research Laboratory, Monroeville, Pa.
b A DTA-EGA technique has been developed for the determination of specific nitrides in residues extracted from steel. The method can be used to identify nitride phases in steel which are not identifiable by any other analytical procedure. The procedure can be used to detect as little as 5 pg. of nitrogen. Other cornpounds present in the extracted residue did not interfere with the nitrogen determination. Above 10 pg. an accuracy within 10% of the amount present appears to be possible.
M
literature shows that more than 30 nitrogenbearing compounds have been identified or postulated to exist in steels that are produced today. From consideration of the composition of these steels and reference to the thermodynamics for the formation of nitrides, it appears possible that as many as 75 simple and complex nitrogen compounds may exist in various steel alloys now manufactured. These compounds are known to affect the physical and mechanical properties of steel. Although the effects of nitrides are at least partially known for simple steelalloy systems, they have not been extensively studied in complex ferrous alloys mainly because of the lack of a good analytical method for determining which nitrides are present. At least 25 nitride-forming elements are used in various types of steels and eight or 10 may be used in a single type. The problem is further complicated by the fact that metal carbides, sulfides, or oxides of the same elements may also be present. Thermodynamics can be used ETALLURGICAL
1336
0
ANALYTICAL CHEMISTRY
to predict which compounds might occur, but without knowledge of kinetics these predictions are unreliable, and compounds are found in steel that would appear to be thermodynamically impossible. Optical and electron microscopy, x-ray and electron diffraction, the electron microprobe, chemical analysis, and combinations of these techniques have been used to gain analytical information about compounds in steel. All of these are useful, but they have limitations in terms of applicability. For instance, the resolution of optical microscopy is limited, and poor diffraction patterns are obtained when the particles are fine. (Particles of carbides and nitrides as small as 100 A. have been found in steel.) Furthermore, diffraction patterns of phases may not be identified if the constituent is present as less than about 5% of a residue extracted from the steel. Also, some carbides and nitrides have similar diffraction patterns. Beeghly’s ester-halogen extraction method (4) and methods for the determination of acid-soluble (6, 7) and/or heat-treatable nitrogen (9-12) have been used in many past metallurgical investigations of nitrides, but as the alloy system increases in complexity, the information gained by these methods is less specific. Therefore, for the past several years we have been investigating the possibility of using differential thermal analysis-effluent gas analysis (DTA-EGA) to analyze for compounds that can be extracted from steel. Our preliminary work with extracted residues has been reported previously (1-3, 13). Also, preliminary experiments with an EGA analytical train
were reported by Garn (8) in 1964. This work discloses the design and test’ing of a DTA-EGA system that will detect 10 pg. or more of nitrogen in milligram quantities of residue extracted from steel. EXPERIMENTAL
DTA Apparatus. R. L. Stone Model 12BC2. The DTA assembly was modified for these experiments as follows : Copper tubing (l/s-inch 0.d.) was soldered to the pressure regulator of the oxygen tank and a needle valve capable of controlling the dynamic oxygen flow a t 3 ml. per minute was substituted for the gas controls in the furnace platform. A modified sample holder assembly (Figure 1) was fabricated from Inconel. Salient features of the design include a small effluent gas volume to attain the desired EGA sensitivity, gas seals away from the heated area to ensure a minimum gas leakage, and a stacked thermocouple arrangement to hold platinum dishes containing the sample and reference material. The stacked thermocouple is part of a bridge circuit containing the thermocouple beads. To facilitate use of the sample holder assembly, a hole was drilled through the top of the DTA furnace. An asbestos washer was placed on top of the furnace and around the sample holder assembly. To protect the gas seals from heat a split water jacket made of copper was clamped around the tubing between the asbestos washer and the gas seal. EGA Train. The assembly for the determination of nitrogen in the effluent gas is shown in Figure 2. To maintain a small gas volume, minimum lengths of l/&nch 0.d. copper
b Figure 1.
Sample holder assembly
DTA thermocouple leads Cement seal (epoxy) 3. Gas outlet to EGA train 4. Exit head assembly (Incanel) 5. Weld bead 6. Thermocouple Insulator (ceramic) 7. Knurled screw cap (stainless) 8. Seal washer ( r u b b i d 9. Split water jacket (copper) 10. Asbestos washer 11. DTA furnace 12. Sample block (Inconel) 13. Bridged DTA thermocouple and sample holder wires (platlnel) 14. Block temperature thermocouple (Pt-PtRh) 15. Porous disk (ceramic) 16. Weld bead 17. Thermocouple insulator (ceramic) 18. Heat shields 19. Water-cooled heat shield 20. Gas inlet tube (Inconel) 21. Inner gas outlet tube (Inconel) 2 2. Flange (Inconel) 23. Outer gas outlet tube (Inconel) 1. 2.
tubing were used to connect the components. Connections were made with standard gas chromatographic equipment. TRAPS. A Leco Model 507-000 catalyst furnace was used to convert any carbon monoxide in the effluent gas to carbon dioxide. The carbon dioxide and sulfur dioxide were trapped in a 3-inch section of Tygon tubing filled with asbestos impregnated with sodium hydroxide; a similar section of Tygon tubing filled with MgCIOl was used to trap water. CHROMATOGRAPHIC COLUMN. Copper tubing (8.5 feet of 3/la-inch 0.d.) was filled with Linde Molecular '3'ieve No. 5A (30- to 60-mesh). Helium flowing a t 60 ml. per minute was used as a carrier gas for the column. DETECTOR. The R. L. Stone Model EG/C hot tungsten wire detector was used with the R. L. Stone Model EG/A power supply. Helium was used as both the reference and carrier gas. RECORDER.A Model 6708 Esterline Angus 1-mv. recorder with a chart speed of 0.1 inch per minute was used.
EIGHT-PORT VALVE. To monitor continuously the concentration of nitrogen in the effluent gas, a Loe Engineering Co. eight-port valve was connected so that a second sample could be collected for analysis while the first sample was being analyzed. Details of these connections are shown in Figure 3. To automate the 3-minute analytical cycle, a microswitch was actuated by the cam of a synchronous motor which operated a pneumatic valve and moved
the eight-port valve from the filling to the sweeping position (Figure 3). FLOWMETER ASD NEEDLE VALVE. Dynamic gas flow was measured by means of a soap-bubble buret and regulated by the needle valve a t 3 ml. per minute. This flow was used to obtain adequate sensitivity for the nitrogen determination and, at the same time, maintain enough oxygen for the combustion of sulfides and carbides in the sample.
b Figure 2. EGA train for determination of nitrogen 1. 2.
3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Oxygen Regulator Needle valve DTA furnace Catalyst tube COz and SO2 absorber H20 absorber &Port valve Solenoid-operated pneumatic valve Cam Microswltch Helium by-pass loop Sample loop Chromatographic column Thermal conductlvlty detector Recorder Detector power supply Helium needle valve Vent to soap-bubble flowmeter
i.
16
VOL. 38, NO. 10, SEPTEMBER 1966
1337
SAMPLE LOOP
FILLING 170 seconds
CHROMATOGRA,PHIC
SWEEPING, 10 seconds
HELI UM G:AS FROM FURNACE
Figure 3.
Gas paths of 8-port valve
Ester - Halogen Extraction Apparatus. For the bromine-methyl acetate extractions, a 200-ml. tallform jar connected to a reflux condenser with an S T 55/35 joint was used (available from Scientific Glass Apparatus Co., No. JE-2100). Filtration Apparatus. Because 100-A. particles of nitrides are known to exist in steels, special filtering techniques are needed. For these experiments, a Millipore Model XX1002500 filtering apparatus was used. Organic membranes with a pore size of 100 and lo00 A. were used for the isolation of residues from acid media, but a 1.5-micron Mipore filter was used when residues were isolated from bromine-methyl acetate media. Reagents. Special grade methyl acetate (low in water content) was used for the ester-halogen extractions. Other chemicals were reagent grade. DTA-EGA Procedure. To test and standardize the assembled instrument, milligram quantities of several metal nitrides containing a known amount of nitrogen were weighed on a 1/4-inch diameter platinum dish by using a Cahn electrobalance. The dish and sample were transferred to the upper loop of the stacked thermocouples, while a similar dish containing A1203 was transferred to the lower loop. The upper half of the sample holder assembly was then inserted int9 the cell 1338
ANALYTICAL CHEMISTRY
block (lower half) and the knurled nuts were tightened to seal the system. Adequate time was allowed for the EGA power supply to stabilize (the power supply can be left on overnight with a low helium flow passing through
400
500
Figure 4.
600
the detectors). Cooling water waa then turned on and the helium flow was adjusted to 60 ml. per minute. The power supply bridge current was set at 150 ma. and the EGA sensitivity was set (usually 50% of maximum). The EGA recorder was turned on and the synchronous motor was started. After purging the gas train with a fast flow of oxygen for 10 minutes, the oxygen flow was adjusted to 3 ml./ minute. When a stable base line and a low reproducible nitrogen blank were obtained the programmer of the DTA unit was started. A heating rate of 10" C. per minute was used, and the sensitivity of the d.c. amplifier on the DTA unit was set (usually at 150 lv). Acid Extraction Procedure. Up to 4 grams of steel millings were transferred to a 600-ml. beaker and 300 ml. of 1 to 1 HCI were added. The sample was placed in a water bath a t 40" C. until all metal was dissolved. Two filter membranes (individually preweighed) were placed in the filter apparatus and the residue remaining after solution of the matrix was collected on the upper membrane. After washing the residue and filter membranes alternately six times each with dilute acid and water, and finally three times with isopropyl alcohol, the membranes were separated, dried in a desiccator and weighed. The weight of the top membrane containing the residue was adjusted for the weight loss found from the lower membrane, and a net residue weight was obtained. An aliquot weight of this residue was transferred to the platinum dish and the same DTA-EGA procedure that was used in the standardization was applied to the sample. Bromine-Methyl Acetate Extraction Procedure. Up to 2 grams of steel were transferred to the esterhalogen apparatus, 3 ml. of bromine per gram of sample were added
700 800 T E Y PERATURE:~.
900
IO00
Recording from decomposition of 2 mg. of TIN
In w
z 0
n In w
a
W z
0
---
,
2 1
400
600
800
TEMPERATURE,.^. Figure 5.
DTA-EGA response for VN
through the condenser, and 2-ml. increments of methyl acetate were added until 16 ml. of methyl acetate per gram of sample had been used. Heat was applied and the mixture was refluxed for hour or until testing with a magnet indicated the complete dissolution of the metallic portion. Two 1.5-micron Mipore filters (individually preweighed) were placed in the filter apparatus and the residue from the sample was collected on the top membrane. The residue and filters were washed with methyl acetate until clear washings were obtained, dried in a desiccator, and weighed. A net residue weight was obtained by correcting the weight of the top filter for the weight gain found for the bottom filter. An aliquot weight of the residue was transferred to the platinum dish and the same DTA-EGA procedure that was used in the standardization of the instrument w a s applied to the sample.
50% of the maximum sensitivity and the heating rate is 10' C. per minute. This figure shows the actual recording of 31 separate nitrogen determinations on the effluent gas as the decomposition of titanium nitride occurs. The oxygen response and other responses caused by pressure surges during
the valve switching are also shown clearly. By drawing a curve through the nitrogen peaks shown in Figure 4, an area related to the nitrogen evolved from the sample is obtained. By plotting this area os. the total nitrogen content for several commercially prepared nitrides of known composition, a linear calibration curve was obtained for 0.05 to 1 mg. of nitrogen evolved from the sample. The response is 15 pg. per square inch at the full sensitivity of the detector. An average deviation of about 10% was found. Most of the scatter was caused by the instability of the 3 ml. per minute gas flow. Although the deviation was greater than anticipated, the sensitivity and detection limit were much better than expected. If the full sensitivity of the detector is utilized, and if the decomposition requires no longer than 20 minutes, then 5 pg. of nitrogen can be detected. This represents 0.000130/0 nitrogen, and could be, for example, either 0.0005% TiN or O . O O l ~ o NbN in 4 grams of steel. Next, some residues from the acid extraction of steel were examined for nitride content. Figure 5 is the DTAEGA response for a residue from a vanadium-bearing steel that contains vanadium nitride. Here it may be noted how closely the characteristics of the DTA and EGA curves match. The displacement of the EGA curve represents the time delay between the detection of the decomposition of the nitride by DTA and the appearance of nitrogen in the EGA thermal conductivity detector. The DTA endotherm at 675°C represents t8hemelting of V20s, the standard value of which is 690' C. The melting endotherm is
I
Analytical Procedure for Chemical Determination of Nitrogen. The solu-
ble nitrogen in an extracted residue ( A N ) and the insoluble nitrogen (TIN TiC,N, VN . . .) were determined by a chemical procedure similar to that published by Beeghly (4, 4.
+
+
RESULTS AND
+
DISCUSSION
Figure 4 shows the EGA response for the nitrogen evolved from 2 mg. of powdered commercial titanium nitride when the EGP. sensitivity setting is
Figure 6. EGA recording of nitrogen evolved from extracted residue containing three nitrides VOL. 38, NO. 10, SEPTEMBER 1966
1339
not always present when vanadium is in the residue, but when it does appear on the thermogram it is reliable evidence of the presence of vanadium compounds. After observing the DTA responses of a number of compounds, it is possible to find characteristics in the DTA curves that are helpful in identification. Further characterization of the compounds is obtained by identification of the type of effluent gas and chemical analysis of the residue for metals. In Figure 5 the match of the DTA with the EGA is ideal because no carbides or sulfides were present in the extracted residue. If such compounds had been present in the residue, other peaks would have appeared. Figure 6 shows the EGA nitrogen response for the extract from a steel that contained aluminum, titanium, and boron as well as other alloying elements and demonstrates the advantage of this method over other analytical procedures. The steel actually contained four nitrides, but the aluminum nitride was dissolved by acid extraction. Besides the nitrides, the extracted residue contained several carbides, a sulfide, several oxides, and amorphous carbon. No other analytical method or combination of analytical methods could be used to identify and quantitatively determine the three nitrides shown in this figure in a single sample. The unstable base line shown in Figure 7, curve I, is caused by the interference of carbon monoxide in the EGA nitrogen determination. For comparison a normal base line is shown in Figure 7, curve 11, where a copper oxide catalyst tube was used to convert any carbon monoxide to carbon dioxide. Carbon monoxide interference commonly occurs when the bromine-
Table 1.
I--
Table 11.
0,009 0.002 0.002
...
z
z
W K
1340
I
CURVE II W I T H CuO CATALYST
I
600
400
Figure 7. EGA recording for decomposition of AIN extracted from steel with bromine-methyl acetate
methyl acetate extraction procedure is used. I n this extraction procedure Fe& present in the steel is decomposed and large quantities of elemental carbon are collected with the residue. An oxygen flow of 3 ml. per minute is not sufficient to convert this amount of amorphous carbon to carbon dioxide and therefore a catalyst tube is necessary. Curve I1 shows another advantage as well as one disadvantage of this analytical approach. The nitrogen response shown in the figure is caused solely by aluminum nitride. The wide range of decomposition indicates either a huge difference in particle size or two different crystal structures for aluminum nitride. Such information
Nitrogen (EGA), % 0.007 0.006 0.006 0.007 0.013 0.006 0.013 0.009 0,001
Nitrogen (chemical), % 0.006 0.005 0.005 0.006 0.010 0.006
0.014 0.011 0.002
