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Metallography of Brazed and Vacuum Heat Treated and Quenched Materials All of the materials studied—low aluminum, and —had comparable properties and when conventionally heat treated and quenched and when heat treated in vacuum and quenched by gas

BY JAMES L. McCALL

ABSTRACT. This paper describes the re­ response effects due to thickness vari­ will be limited to a maximum of three sults of metallographic studies that have ations. Also, the effects of different because of the similar been made on several materials included quenching gases were studied. The results obtained from them. in a program directed toward developing a vacuum /, gas quenching gases included , car­ The mechanical properties obtained quenching (VBGQ) process. A previous bon dioxide, helium, and liq­ from tensile tests are shown in Table program1 showed that the process was uid nitrogen. Metallographic studies 1. Three thicknesses were investi­ feasible for joining complex shapes of of the materials are described individ­ gated: 0.125, 0.063 and 0.025 in. 6061 aluminum and indicated that it may ually below. However, since the data from the be useful for other materials as well. various thicknesses were generally the Because of this, a program was initiated 2014 Aluminum same, the data for only one thickness, to develop the process for joining various 2014 aluminum is a , mag­ 0.125 in., are included in the table. aluminum alloys, low alloy , and corrosion resistant steel alloys. This pro­ nesium, alloy normally heat Also included are quenching rates gram included mechanical property eval­ treated by solution treating at 935 measured for the various gases from uations of specimens of these materials ±10F for 40 minutes, water- cooling time curves. that were heat treated by processes in­ quenching and aging 18 hours at Metallographic examinations of the tended to simulate those that would be 320F. Specimens of material in this 2014 material are summarized in Fig. encountered during the VBGQ process. condition were compared to speci­ 1. From the photomicrographs con­ Mechanical properties of these speci­ mens given the same solution treating tained in this figure the structures of mens were compared with those obtained and aging conditions but with the all the gas quenched samples are from conventionally heat treated speci­ intermediate quench accomplished by shown to be nearly identical to that of mens. Also, several of the materials were brazed by the process and the quality of argon, nitrogen or helium. For the water quenched sample, except for the brazed joints were evaluated. Much this material, as well as for all the the helium quenched sample which of this work is described in a previous materials studied, the data presented had more distinct grain contrast. Since paper.2 The present paper primarily is limited to descriptions of the results of rather detailed metallographic studies Table 1—Mechanical Properties of Aluminum Alloys Vacuum Heat Treated that were made of specimens of both and Gas Quenched types. Quenching Ultimate Materials Heat Treated time from tensile Yield Aluminum Thickness Quenching 935 F to RT strength strength Elongation Specimens of three low alloy steels alloy in. medium sec ksi ksi % (4130, 4340 and 18Ni250 maraging 2014 0.125 Water — 73.4 65.8 14.7 steel), three aluminum alloys (2014, 0.125 Argon 17.5 67.2 59.2 9.7 2219 and 6061), and two corrosion 0.125 Liquid 40.0 62.9 52.8 11.7 resistant alloys (type 316 and type nitrogen 321 stainless steel) which had been 0.125 Helium 11.2 74.3 67.9 10.7 heat treated to simulate the conditions — 42.0 13.0 that would be experienced during the 2219 0.125 Water 61.3 0.125 Argon 13.7 57.9 39.0 12.0 process were studied. These were all 0.125 Liquid 45.5 50.4 32.5 11.0 sheet material, but various gauge thick­ nitrogen nesses were included in the studies 0.125 Helium 8.7 59.3 40.6 10.7 to permit an evaluation of thermal 6061 0.125 Water — 45.8 39.1 17.0 0.125 Argon 11.2 45.7 38.3 14.0 0.125 Liquid 65.0 43.1 34.0 15.3 J. L. McCALL is Division Chief ot the nitrogen Structure of Division, Battelle Me­ 0.125 Helium 8.7 45.5 38.1 16.7 morial Institute, Columbus, Ohio.

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• • . If 1H9HHNI Fig. 1—Microstructures of 2014 aluminum alloy solution treated at 935 ± 10F for 40 minutes, water or gas quenched and aged 18 hours at 320F. (a) water quenched; (b) argon gas quenched; (c) liquid nitrogen quenched; and (d) helium gas quenched. Mag: 400X all the samples were aged identically, complished by argon, liquid nitrogen sample, whereas the liquid nitrogen and since the helium was found to be or helium. Three thicknesses were in­ quenched sample had a much greater the most rapid quenching medium vestigated: 0.125, 0.050 and 0.020 in. amount of precipitate. This was con­ (Table 1), the grain contrast must be Table 1 contains mechanical property firmed by electron microscopy. The an effect of the quenching rate. The data for these samples as well as greater amount of precipitate undoubt­ precipitate particles in the grains and quenching time data for the three edly is accounted for by the slower in the grain boundaries are CuAU. gases as determined from cooling time quenching rate of the liquid nitrogen 2219 Aluminum curves. (Table 1). 2219 aluminum is an aluminum- Figure 2 contains photomicrographs 6061 Aluminum of the of these sam­ copper alloy which is normally 6061 aluminum is a , hardened by solution treating at 995 ples. Because of the longer than nor­ silicon, copper, alloy which ±10F for 10 minutes, water quench­ mal aging time (46 hours versus 36 normally is hardened by solution ing and artificially aging at 375F hours), the microstructures of all the treating at 985 ±10F, water quenched for 36 hours. For the present studies, conditions contain a rather heavy pre­ and artificially aged at 320F for however, a 46-hour aging time was cipitate structure and indistinct grain 18 hours. Specimens in this condition used. This condition was compared boundaries. The amount of precipitate were compared with specimens ex­ with specimens exposed to the same in the microstructures of the specimens posed to the same solution treating solution treating and aging conditions quenched using argon and helium and aging conditions but the interme­ but the intermediate quench was ac­ were similar to the water quenched diate quench was accomplished using

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>«*>• ' 'i-»'«"«.,R'..•.;.';'; .. .i,l-....- I D Wfe-^^WXtf&^feYi Fig. 2—Microstructures of 2219 aluminum alloy solution treated at 995 ± 10F for 10 minutes, water or gas quenched and aged 46 hours at 375F. (a) water quenched; (b) argon gas quenched; (c) liquid nitrogen quenched; and (d) helium gas quenched. Mag: 400X argon, liquid nitrogen or helium. 4130 Steel nesses, only the 0.125-in. material is Three thicknesses were investigated: The 4130 alloy is a low considered in this paper. 0.125, 0.063, and 0.032 in. Again, containing 0.27 to 0.33% and To obtain a fully martensitic struc­ since the data did not differ signifi­ having chromium and as ture, 4130 steel must be quenched fast cantly among the thicknesses, results principal alloying elements. It is gen­ enough from the austenitizing temper­ for only the 0.125-in. thickness is in­ erally hardened by austenitizing at ature to below the temperature where cluded in this paper. 1575F for 15 minutes, oil quenched the 'nose' of the transformation curve The mechanical properties of the and tempered. for 1 hour from the T-T-T diagram occurs so various conditions are contained in at 1050F yields a strength of about that no transformation products are Table 1. The properties of the gas 150,000 psi. This condition was com­ formed. This requires cooling to be­ quenched specimens are comparable pared to samples given the same aus­ low about 1000F in less than about to those for the water quenched mate­ tenitizing and tempering treatments 1.5 seconds.3 The time for nitrogen rial. but gas quenched using nitrogen. Only gas to quench 4130 steel to 1000F Figure 3 contains photomicrographs one quenching gas was investigated was found to be 7 seconds which of the microstructures of these sam­ for all the steel materials. means that the resultant istructure ples. The undissolved precipitate in As with the aluminum alloys, three should be a mixture of and the microstructures is Mg2Si. As can thicknesses (0.125, 0.070, and 0.040 . This was the case as shown in be seen, all the microstructures are in.) were evaluated but since the data Fig. 4. However, the microstructure essentially identical. did not differ much among the thick­ of the oil quenched specimen also

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c Fig. 3—Microstructures of 6061 aluminum alloy solution treated at 985 ± 10F, water or gas quenched and aged 18 hours at 320F. (a) water quenched; (b) argon gas quenched; (3) liquid nitrogen quenched; and (d) helium gas quenched. Mag: 400X contained a mixture of martensite and bainite as shown in Fig. 4. Electron Table 2—mechanical Properties of Low Alloy Steels microscopic studies of the structures Ultimate revealed both conditions to be essen­ tensile Yield tially identical. The mechanical prop­ Thickness Quenching strength strength Elongation erties of the samples are shown in Alloy in. medium ksi ksi % Table 2. As can be seen, the mechani­ 4130 steel 0.125 Oil 156.4 146.0 14.0 cal properties of the nitrogen quenched 0.125 Nitrogen 162.0 147.0 13.0 material actually exceed those of the 4340 steel 0.125 Oil 187.0 173.0 9.0 oil quenched material. 0.125 Nitrogen 186.0 i/2.4 8.5 18Ni250 maraging 0.125 Air cooled 268.4 260.7 7.0 4340 Steel steel 0.125 Nitrogen 267.1 255.0 8.0 The 4340 material is a low alloy steel containing 0.38 to 0.43% carbon and principal alloying elements of of about 180,000 psi. This condition 'nose' of the transformation curve to chromium, and molybdenum. It was compared with samples given the obtain a fully martensitic material. is generally hardened by austenitizing same austenitizing and tempering The nose occurs at about 800F and at 1500F for 30 minutes, quenching in treatments but gas quenched with ni­ the allowable time to reach this tem­ oil and tempering. A temper at trogen. As with 4130 steel, 4340 steel perature is about 10 seconds.3 A time 900F for 1 hour produces a strength must be quenched rapidly past the of about 7 seconds was measured for

74-s I FEBRUARY 1972 the nitrogen gas to quench the 4340 balt, 5% molybdenum and smaller this type. Electron microscopic studies samples to 800F so a fully hardened amounts of and . Car­ of the samples revealed that the nitro­ martensitic structure would be expect­ bon content is about 0.03%. The ma­ gen gas quenched sample had a finer ed. This was obtained as shown in terial is heat treated by at acicular structure than the air cooled Fig. 5. A fully martensitic structure 1500F for 30 minutes, air cooling and sample, which apparently reflects the also was obtained in the oil quenched reheating to 900F for 3 hours. This faster quenching rate of the gas. sample. The structure of the marten­ condition was compared with samples site in the nitrogen quenched sample given the same annealing and reheating Type 316 and Type 321 Stainless was found from electron microscopic treatments but cooled by nitrogen gas Type 316 and type 321 stainless studies to be slightly finer than that in quenching. The quenching rate of the steel are austenitic grades which con­ the oil quenched samples, which sug­ nitrogen for this material was found tain chromium and nickel as the prin­ gests that gas quenching is more to be about 25 to 50 seconds from cipal alloying elements. Both alloys rapid. 1500F to room temperature. cannot be hardened by heat treatment Mechanical properties of these ma­ Mechanical properties for these and because of this and other similari­ terials are shown in Table 2. As can material conditions are shown in Ta­ ties only type 321 was evaluated for be seen, similar properties were ob­ ble 2. As can be seen, both conditions its behavior after gas quenching versus tained for both material conditions. have similar properties. conventional water quenching. The microstructures of the materi­ The annealing temperature range Maraging 18NJ250 als are shown in Fig. 6. Both materials for type 321 is approximately 1750 to Maraging 18Ni250 steel is an alloy are quite acicular with some banding. 2050F; however, the furnace used in containing about 18% nickel, 8% co­ This is common for maraging steels of these studies was not capable of these

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Fig. 4— Microstructures of 4130 steel austenitized at 1575F for Fig. 5—Microstructures of 4340 steel austenitized at 1500F 15 minutes, oil or nitrogen gas quenched and tempered at for 30 minutes, oil or nitrogen gas quenched and tempered 1050F for 1 hour. Mag: 500X at 900F for 1 hour. Mag: 500X

WELDING RESEARCH SUPPLEMENT | 75-s tures tor all the samples are essentially Table 3—Mech anical Properties of Type 321 Stainless Steel the same. Vacuum Heat Treated and Gas Quenched Quenching Ultimate Brazing Studies time from tensile Yield Quenching 1500 to 120F strength strength Elongation Several of the samples were brazed Thickness medium sec ksi ksi % into various configurations to deter­ 0.125 Water — 88.8 38.7 48.0 mine, among other things, the quality 0.125 Argon 40 87.4 39.6 48.5 of the joints that could be obtained. 0.125 Nitrogen 35 90.5 39.9 48.0 Several of these are shown in Fig. 8. 0.125 Helium 24 91.7 40.1 51.0 Although several different filler metals were evaluated in the program, only three were used in this part of the program: AMS 4777 was used to braze the low alloy and corrosion • «». »i resistant steels. Braze alloy No. 719 was used for the 2219 aluminum and braze alloy No. 718 was used for the 6061 aluminum. These are described * separately below.

Low Alloy and Corrosion Resistant Steels AMS 4777 brazing alloy was used to braze the low alloy and corrosion resistant steels. This brazing alloy is self-fluxing and has been found to perform well in vacuum. The alloy is nickel base and contains about 7% chromium, 3% , 4.5% silicon, 3% boron and 0.06% maximum car­ bon. For brazing, this alloy was mixed with a lacquer binder to form a slurry which could be applied around joints. (a) Air Cooled Brazing of all the low alloy and corro­ sion resistant alloy steels was done at 1925F for 15 minutes. A vacuum of about 5 X 10-5 torr was achieved during brazing. The samples were quenched directly from the brazing temperature. The corrosion resistant steels were put in an annealed condition by the brazing process and therefore were evaluated in this condition. The low alloy steels were heat treated follow­ ing brazing as follows:

4130 steel—austenitize at 1575F for 15 minutes, nitrogen quench, temper at 1050F for 60 minutes 4340 steel—austenitize at 1525F for 30 minutes, nitrogen quench, temper at 900F for 60 minutes (b) Nitrogen Gas Quenched 18Ni250 steel—anneal at 1500F for 30 minutes, nitrogen Fig. 6—Microstructures of 18Ni250 annealed quench, heat to 900F for 3 at 1500F for 30 minutes, air-cooled or nitrogen gas quenched hours and reheated to 900F for 3 hours. Mag: 500X Figures 9 through 12 are photomi­ crographs of metallographic cross sec­ temperatures. Because of this, the and quenched in water. The mechani­ tions through representative brazed samples were heat treated by heating cal properties of these specimens are joints made using the steel materials. only to a temperature of 1500F and shown in Table 3. As can be seen, the Figure 9 is of a brazed joint be­ holding for 30 minutes. Following properties for the gas quenched speci­ tween 4130 steel sheet material. As heating, the specimens were gas mens are equal to those for the water can be seen, the braze contains only a quenched in argon, nitrogen or helium. quenched one. small amount of porosity and there is Control specimens were heated for the The microstructures of these sam­ a minimum of alloying with the base same time at the same temperature ples are shown in Fig. 7. The struc­ . The shear strength of brazes

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c D Fig. 7—Microstructures of Type 321 stainless steel heated to 1500F for 30 minutes and water or gas quenched, (a) water quenched; (b) argon gas quenched; (c) nitrogen gas quenched; and (d) helium gas quenched. Mag: 500X

made in this material as measured that of the brazes in the 4340 mate­ from shear-lap -specimens was found rial. to be good. Figure 12 contains photomicro­ Figure 10 is of a brazed joint be­ graphs of a brazed joint in type 316 tween 4340 steel sheet material. The stainless steel. Again, the flow and brazes made in this material were filleting of the brazing alloy was good similar to those in the 4130 steel and the amount of porosity was very material in that they contained little small. Some alloying of the brazing porosity and had only a small amount alloy with the base metal was ob­ of alloying with the base metal. The served but in general the brazes were Fig. 8—Samples joined by vacuum braz­ flow of the brazing alloy was observed considered to be of acceptable qual­ ing plus vacuum heat treated and gas to be excellent. ity. quenched Figure 11 shows a brazed joint between sheets of the 18Ni250 marag­ Brazing of Aluminum Alloys Brazing alloy No. 719 was used to ing steel. The brazing alloy showed There is no commercial brazing al­ braze the 2219 aluminum alloys. It is good flow and filleting but in general loy available for use with 2014 alumi­ an aluminum-base alloy containing the brazes in this material contained num because of the low temperature about 10% silicon, 10% , 4% more porosity than in the 4130 or at which this material begins to melt. copper, 0.8% iron, and minor addi­ 4340 steels. The shear strength of the Therefore, no brazing attempts were tions of magnesium, and braze was found to be comparable to made on 2014 aluminum. chromium.

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Fig. 12—Brazed joint 316 stainless steel to 316 stainless steel Fig. 13—2219 aluminum container brazed using No. 719 braz­ using AMS 4777 brazing alloy ing alloy

Brazing was done by wrapping the and therefore it does not appear that The braze was of good quality and work pieces in aluminum foil and vacuum brazing of this alloy can be free of porosity. Shear-lap specimens placing small chips of magnesium in done using this brazing alloy. revealed brazes in this material to the foil. (Magnesium vaporizes and Brazing alloy No. 718 was used to have good strength. encourages flow and filleting by the braze the 6061 aluminum alloy. This brazing alloy.) The brazing was done brazing alloy has an aluminum base Summary at 1060F at a vacuum of about 5 X and contains about 12% silicon, 0.8% IO-5 torr. iron, 0.3% copper and minor amounts All of the materials studied were Since the brazing temperature of of zinc, magnesium and manganese. found to have comparable properties Brazing was done at HOOF in a vacu­ and microstructures when convention­ 1060F is above the initial melting um of about 5 X tO-B torr. This ally heat treated and quenched and temperature of 2219 aluminum, which temperature is below the initial melt­ when heat treated in vacuum and is 1010F, incipient melting of the ing temperature of 6061. After braz­ quenched by gas. In fact, in some grain boundaries of the base metal ing, the samples were vacuum heat materials, improved properties were would be expected. This did occur as treated at 985F for 20 minutes, nitro­ attained by the gas quenching process. shown by the photomicrographs in gen gas quenched and artificially aged Brazed joints of good quality were Fig. 13. Even though sound welds at 320Ffor 18 hours. achieved by the process in all the were achieved, the incipient melting Figure 14 contains photomicro­ materials studied except the 2014 undoubtedly would lower the graphs of a typical brazed joint in 6061 aluminum, for which no suitable braz­ mechanical properties of the material made with the No. 718 brazing alloy. ing alloy with low enough melting

WELDING RESEARCH SUPPLEMENT | 79-s IOOX SOX 1/2 Percent Hydrofluoric Acid Etch 1/2 Percent Hydrofluoric Acid Etch Fig. 14—6061 aluminum container brazed using No. 718 brazing alloy

point is available, the 2219 aluminum VBGQ process holds promise as a 2 Burrows, C., O'Keete, R., Gurtner, F. B., and Kirk, F. T., "Vacuum Brazing- in which incipient melting of grain reliable method of joining certain ma­ Gas Quenching of Materials, Quality Stand­ boundaries occurred, and the 18Ni250 terials. ards, Physical Properties, etc.", Paper presented at National Fall Meeting, Amer­ steel in which brazed joints with good References 1 Schwartz. Gurtner. F. G. and Shutt, P. ican Welding Society, October 1970, Bal­ flow and filleting were achieved but in K., "Vacuum (or Fluxless) Brazing-Gas timore, Md. which quite a bit of porosity was Quenching of 6061 Aluminum Alloy", 3 "Atlas of Isothermal Transformation Edgeicood Arsenal Report EATR 4085, Diagrams", United States Steel, 1951, p. present. The results indicate that the 1967. 101.

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80-s | FEBRUARY 1972