I
HOWARD
H.
ROGERS
Research Laboratories, Allis-Chalmers Manufacturing Co., Milwaukee, Wis.
The Hydrogen-Nitrogen Trifluoride Torch This self-fluxing torch can weld, braze, and cut metals without additional flux. Nitrogen trifluoride also acts as a gaseous flux for welding and brazing
GLOXVISG
wood splints burn more energeticall!- in nitrogen trifluoride than in oxygen. Reducing gases such as ammonia, hydrogen, a n d natural gas burn quietly with nirrogen trifluoride and can be used in combination ivith it as fuels for a torch. Torches using fluorine ( 6 : 7) or chlorine trifluoride ( 7 ) with hydrogen are known. Nitrogen trifluoride, holvever, is much less corrosi\ e and therefore easier to handle, and does not cause combustible substances to ignite spontaneously. T h e heat of reaction for nitrogen trifluoride with hydrogen is relatively high.
+
H? Fz H? +
2
=
:NFB =
+
2HF Hz f
2
2HF
;jClF, = 2HF
+
H?+ 1 2ClF3
(9)
l’;JV.!
+ 110 kcal.
(9)
l/J21?
+ 100 kcal.
(9)
of rod inch in diameter. An attempt to weld aluminum \vas unsuccessful. Substitution of ammonia for hydrogen appeared IO have litlle if any effect on the welding ability of the torch. Very good results \\.ere obtained in welding combination 4. Kanthal .4\cas chosen because its aluminum content makes welding difficult. Acetylene was an unsatisfactory fuel when burned with nitrogen trifluoride because of severe carbon deposition a t the torch tip. T o prevent carbon deposition, the nitrogcn trifluoride was diluted with oxygen and the premixed torch used because it deposited less carbon than the concentric tube torch (Table I ) . \\’hen oxygen was added to the nitro-
=
+ l/?HCI + 86 kcal. :O? = H 2 0 + 58 kcal.
3/2HF
H2 t l,
+ 128 kcal.
torch was built-a brass T-tube with passages 0.04 inch in diameter a n d a conical tip. T h e numerous flashbacks were indicative of high flame velocities. Procedure. Kitrogen trifluoride was prepared by electrolysis of molten ammonium bifluoride (70). employing a nickel anode (5) in a closed cell with a Slonel diaphragm. T h e anode gases \rere condensed in liquid nitrogen, and the liquid product was filtered ( d ) , redisrilled. and condensed into small, cooled steel cylinders. T h e cylinder valves were closed and the cylinders allowed to warm to room temperature. Some combinations were readil! \\elded without a flux (1 to 6). Nos. 1 to 5 were wire pairs. about 0.04 inch in diameter, but No. 6 consisted of tbvo pieces
(9j (9)
‘Temperature of the hydrogen-nitrogen trifluoride flame probably approaches that of tlie hydrogen-fluorine flame. T h e other molecule formed in the flame, nitrogen. is stable a t high temperatures and should not limit the flame temperature because of dissociation.
Experimental
2
.\ concentric tube-type torch similar to Figure 2 of (8)!%’asused for almost all experiments. T h e copper inner tube for nitrogen trifluoride \vas 0.037 inch in inside diameter and 0.094 inch in outside diameter; tlie nickel outer tube \vas 0.188 inch in inside diameter and 0.250 inch in outside diameter. Gas floic was controlled ivith brass needle valves and measured wirh float-type floivmeters. T h e torch was normally operated with i-lo\v rates of 1.5 and 0.4 liter per minute for h) drugen and nitrogen trifluoride, respectively. T o take advantage of the fact that nitrogen trifluoride can be mixed with the fuel gas Jvithout igniting, a premixing
4 Nitrogen trifluoride torch cuts silicon steel (2), cast iron
(4),and stainless steel (6)
VOL. 51, NO. 3
MARCH 1959
309
A hydrogen-nitrogen trifluoride torch satisfactorily brazes stainless steel (larger area) with silver solder About 250X
gen trifluoride supply and the ability to weld Kanthal A tested, the surface layers formed On the were white above and black below about 24% nitrogen trifluoride (Table I).
1. 2. 3. 4.
5. 6.
7. 8. 9. 10. 11. 12. 13.
Material Welded or Brazed Chrome1 P (90 Ni, 10 Cr) to Alumel (95 Ni, balance Si, Mn, Al) Iron to Constantan (60 Cu, 40 Ni) Copper to Constantan Kanthal A (23.4Cr, 6.2 AI, 1.9 Co, 0.06 C,balance Fe) to Kanthal A Nickel to nickel Stellite No. 6 (64 Co, 25.5 Cr, 4.75 Ni, 1.75 Si,2.5 Fe, 1.0 C) to Slellite No.6 Silver solder (45 Ag, 15 Cu, 16 Zn, 24 Cd) on nickel Silver solder on stainless steel (18 Cr, 8 Ni, balance Fe) Brass (70 Cu, 30 Zn) on nickel Brass on stainless steel Brass on molybdenum Brass on tungsten Nichrome (60 Ni, 16 Cr, 24 Be) on tungsten
Table I. Kanthal A Welding with Hydrogen-Nitrogen Trifluoride-Oxygen Mixtures Is Successful over 50% Nitrogen Trifluoride, but AcetyleneNitrogen Trifluoride-Oxygen Mixtures Cause Difficulties NFI in Welda Cc”Min‘ Oxidant, QualHz NF3 OZ Vol. % ity 1450
76 100 123 134
150 191 250 378 0
644b
0 0 214 243
383 374 383 364 340 297 210 0 495 594 584 340 186
0
17 21 24 27 31 39
0
2 3 4 3 5 5 0 1 0
45
100 0 0 0 39 57
3
*
4c CzH2
a 0, nowelding. 5 , excellent bond. Some carbon deposited on flow, cc./min. torch tip.
3 10
or brazing or burned with reducing gases. Probably this self-fluxing action results from formation of fluorides which melt below the melting point of the metal or alloy involved [-AI 660’ C.. Kanthal A ca. 1510’ C.: AlF3 1040’ C . ! .4I2O3 2050” C. (2: 7)j. For example, aluminum cannot be torch-welded without flux; Kanthal A, which contains aluminum, can be welded ivith the nitrogen trifluoride torch but not with an oxygen torch. Priest and Grosse ( 6 ) state that copper fluoride melts at a lo\\.er temperature than copper and therefore acts as a flux. I t is unnecessary to use pure nitrogen trifluoride for rcelding and brazing. Satisfactory welds are obtained \vhen oxygen is added to the nitrogen trifluoride stream (Table I). Cutting of metals requires no additional heat once started, because of nitrogen trifluoride’s highly energetic reactions bcith the metals (3:‘9.
14. Nichrome on graphite
;:: ::~ ~ ! , n u m 17. Ferrosilicon (85 Si, 15 Fe) on graphite
Safety
~~~~~~
18.
19. 20. 21. 22.
Aluminum bronze (Cu 89.2, A1 9.8, Fe 1.0) on nickel Aluminum bronze on stainless steel Copper on nickel Copper on stainless steel Silver-manganese (85 Ag, 15 Mn) on stainless steel
Materials 7 to 17 were successfully brazed. A microphotograph of a sectioned, polished sample of combination 8 shows no evidence of oxides or cracking at the braze boundary. Seirher titanium nor zirconium could be brazed with silver solder using the hydrogen-nitrogen trifluoride torch. T h e experimental procedure \vas altered by preheating the materials to fusion temperatures with a natural gasoxygen torch and momentarily exposing them to nitrogen trifluoride. Fluxing occurred immediately. follolved by wetting
of the fused metals. Cornbinations 1, 4, 7, and 8 gave satisfactory joints. Nos. 18 to 22 were also brazed. heated by the hydrogen-nitrogen trifluoride torch with a reduced hydrogen flow, rapidly became white hot and rvere pierced by the nitrogen trifluoride stream. Cutting action was maintained if the hydrogen was shut off com~letely. Cutting occurred if a metal was preheated by any means and then subjected to a nitrogen trifluoride stream. Metals cut were copper, 3.57, silicon steel, nodular iron (3.670 C, 2.4% Si), cast iron (3.47, C, 2.2% Si), 01‘ J’’’e1ding
mild steel, and stainless steel (18% Cr, 8% h’i). L n d e r equivalent conditions 3.594 silicon iron and stainless steel were not cut by a n oxygen stream.
Discussion Nitrogen trifluoride appears to act as a gaseous flux either used alone in welding
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hydrogen fluoride is a product whenever the torch is used ; therefore, normal precautions (2) for handling it must be taken. T h e physiological characteristics of nitrogen trifluoride have been discussed (17). I t is a relatively stable gas at room temperature and has been safely stored in this laborator>- at pressures up to 600 p.s.i.g. in steel cylinders and at pressures over the critical [672 p.s.i.g. ( f ) ] in glass capillaries. S o corrosion of glass was observed in 2 years. \.alves, hoses, and other equipment recommended for handling oxygen were used for nitrogen trifluoride without apparent effect.
literature Cited (1) Grosse, A . v. (half to J. C . XIorrell), U. S.Patent 2,642,656 (June 23, 1953).
(2) Harshaw Chemical Co., Cleveland, Ohio, “Hydrofluoric .4cid Anhydrous,” pp. 12-20, 1955. (31 Hodgman, C. D., “Handbook of Chemistry and Physics,” Chemical Rubber Publishing c0., Cleveland, Ohio, 1956. (4) Jarry, R. L., Miller, M. C., J . Phys. 60, 1412-13 (1956). jji Chem. Pierce, L,, Pace, E , L,, J , Chem. Phys. 22, 1271 (1954). (6) Priest, H. F., Grosse, A . V., IND.EKG. CHEM.39, 431-3 (1947). (7) Priest, H. F . , Grosse, A. V. (to U.S.A.), LT, S. Patent 2,421,649 (June 3 , 1947). (8) ~ ~ i 1 L, 1 , L,, :;Chemistry and Metaliurgy of Miscellaneous Materials. Thermodynamics,” McGraw-Hill, New York, 1940. p) Rossini, F, D., lvagman, u,D,, E ~ W. M., Levine, S., Jaffe, I., Xatl. Bur. Standards, Circ. 500 (1952). (IO) Ruff, O., 2’. anorg. aligem. Chem 172, 417-25 (1928). (11) Ruff, O.,Straub, L., Zbid., 197, 27386 (1931). RECEIVED for review June 12, 1958 ACCEPTEDOctober 9, 1958
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