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1、Part 111. Brazing Fumes and Gases Introduction As part of the American Welding Society research program “Improving the Welding Environment ,” Bat- telle-Columbus examined the fumes generated during brazing. The broad objective of this task was to show experimentally the relationship of the fumes giv
2、en off to the brazing filler metal used. Early in the program a literature search was conducted to learn the extent and usefulness of the bibliographic data available. The major conclusions drawn were: (I) The cadmium oxide fume hazard has received by for the most attention, but even this takes the
3、form of “case histories” without significant attention to the basic causes of the problem. Only a few investigators have chosen to examine the effects of process and procedure variables on the magnitude of the hazard. (2) The possibility of harmful fluoride concentration in the brazing environment i
4、s covered briefly in the litera- ture. As in the case of cadmium, there is a dearth of useful information that would permit adequate definition of the hazard under varying conditions of use. (3) Almost no information exists that can be used to define the brazing environment and the safe practices to
5、 be fol1 wed. Bask$ on these conclusions and the experiences of membe of the AWS Research Committee, an experi- mental search program on brazing fumes was agreed benefit or knowledge of any published prior experience by other investigators. Its magnitude and scope were re- strained by the availabili
6、ty of funds assignable for brazing fume studies. upon. z e experimental approach was developed without Scope The research plan was designed to minimize the vari- ables caused by changes in flame shape, character, and position that occur during torch brazing. Thus, proce- dures similar to those used
7、during welding fume studies were suggested. Total fume samples collected in the AWS- Battelle welding fume collection chamber would be used to find the influence of brazing variables on the fume generation rates and quantities. Also the fume sample would be analyzed by atomic absorption methods to d
8、e- termine how brazing variables affect fume compositions. Unfortunately, it increasingly became evident as the pro- gram progressed that the procedures chosen did not pro- vide the precision required for accurate measurement of filler metal fumes. The filler metals chosen were: Ag-15Cu-16Zn-24Cd (B
9、Ag-I) and Ag-22Cu-20Zn-7Cd-ISn. These filler metals have similar melting characteristics and are used exten- sively in industry for torch brazing. Limitat ions The investigation was limited to the BAg-class filler- metals and was directed toward determining the fume gen- eration characteristics of t
10、wo filler metals having widely different cadmium contents. As the data presented in this report show, the quantity of fume developed during brazing with filler metals con- taining cadmium and zinc is quite small. They also show that when a flux is used, it is the major contributor to the fume produc
11、ed. In addition, the results clearly indi- 115 Copyright American Welding Society Provided by IHS under license with AWS Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 04:45:55 MSTNo reproduction or networking permitted without license from IHS -,-,- 116FUMES AND
12、 GASES cate the need for the prior development and validation of precision procedures for use during any experimental study of the fumes produced by brazing filler metals and fluxes. Program The experimental program covered the following tests, conducted in random order: (1) BAg-1 filler metal and I
13、frpe 3A flux, heated to 1200“ F (649“ C), to 1250“ F (677“ C) and to 1350“ F (732“ C); five tests at each temperature. (2) BAg-1 filler metal under COz shielding, heated to 1200“ E to 1350“ E and to 1500“ F (816“ C); five tests at each temperature plus three extra tests at 1500“ E (3) Low cadmium fi
14、ller metal and vpe 3A flux, heated to 1250“ F, to 1350“ E and to 1500“ F; four tests at each temperature plus two extra tests at 1500“ E (4) Low cadmium filler metal under CO2 protection, heated to 1250“ F, to 1350“ E and to 1500“ F four tests at each temperature plus one extra at 1250“ F (of questi
15、on- able accuracy and therefore discarded). Equipment, Materials, Procedu re Crucibles (boats) Ail brazing filler metal melting was carried out in shallow pure alumina boats having these nominal internal dimen- sions: 1.75 x 1.13 x 0.38 in. (44 x 29 x 9.7 mm, approx- imately). The internal dimension
16、s varied because the wall thickness was not uniform from boat to boat. The possible surface area variable was neglected and not recorded during the study because not enough boats were available to permit sorting for uniformity. During analysis of the results, the surface area varia- tion was found t
17、o range between -4 and + 12 percent from the nominal area; thus it was considered a significant possible source of error in the data collected. Filters Initial experiments were run using two types of filters: glass fiber absolute filters (MSA, Cat. No. 75428) when weight determinations were desired
18、and cellulose mem- brane filters (Millipore p p e RA, 1.2pm pore size) for samples to be analyzed. It was found, however, that the absolute filters gave excessive variation in results. A satisfactory explanation of these variations was not found; therefore., all fume data used for study of the resul
19、ts were taken from samples collected on membrane filters. The active filter area was 33 square in. (21 290 mm) in all tests. No prefilter was used. Each filter was dried for 1 hour at 220F (105C) immediately prior to weigh- ing and use. Melting Furnace and Fume Collection Chamber A resistance furnac
20、e capable of heating the brazing filler metal to melting temperature in less than 10 minutes and to 1500“ F (816“ C) in less than 15 minutes was built for the program. The melting furnace was enclosed in the AWS-Battelle fume collection chamber when in use. It is shown in position in Fig. 3.1. Close
21、up views of the fur- nace prepared for melting under a flux and under a CO2 gas blanket are shown in Figs. 3.2 and 3.3 respectively. The CO2 gas outlet was a flare fitting with an opening measuring 0.050 x 1.813 in. (1.27 x 46 mm). A standard temperature control system utilizing a chromel-alumel the
22、rmocouple was used. For tests under CO2, the gas flow was regulated through a flow meter at 8 fi3 per hour (3.8 liters per minute, approximately). Filler Metals The composition of the filler metals chosen for study was given above. The standard BAg-1 filler metal was furnished in 1.50 x 1.25 x 0.38
23、in. (approx. 38 x 32 x 9.7 mm) blocks. These were cut to 1.5 x 1.00 x 0.38 in. (approx. 38 x 25 x 9.7 mm) in order to fit the crucible (boat). The excess pieces were used in check tests near the end of the study. The weight of the cut-down blocks varied between 84.7 and 82.6 grams. Melts made with t
24、he excess pieces varied between 74.7 to 76.6 grams. Atomic absorption analyses of these blocks made at Battelle gave these results 25.4 percent Cd and 16.5 percent Zn. The liquidus 1145“ F (618“ C) was taken as that given in the literature. The low cadmium filler metal was furnished as 1.50 x 1.00 x
25、 0.13-in. (approx. 38 x 25 x 3.2 mm) blocks. Three blocks were used for each melt, the weight of which varied between 85.0 and 85.8 grams. Battelle atomic absorp- tion analysis of these blocks gave these results: 7.02 per- cent Cd, 19.6 percent Zn. The liquidus of this filler metal was determined at
26、 Battelle and found to be 1210“ F (654“ C) . Other Materials The flux used was commercial AWS 5 p e 3A and was weighed to give 3 to 4 grams for each melt. Standard welding grade COz gas having a i50“ F (-46“ C) spec- ified dew point was used. The actual dew point was not checked. Test Procedure The
27、standard procedure used for brazing filler metal melting and fume collection was as follows: (1) Weigh ingot (2) Add 3 to 4 g flux, or start COz gas flow, as (3) Place in cool fuma= in fume collection chamber required Copyright American Welding Society Provided by IHS under license with AWS Licensee
28、=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 04:45:55 MSTNo reproduction or networking permitted without license from IHS -,-,- Part III. Brazing11 17 Filter 1 oca t ion Furnace - F i g . 3.1-Brazing filler metal melting furnace in AWS-J3atteUe welding fume coliection
29、chamber Copyright American Welding Society Provided by IHS under license with AWS Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 04:45:55 MSTNo reproduction or networking permitted without license from IHS -,-,- 1 I 16 of the 60 tests conducted were out- side the
30、 range of 42 to 50 percent. Blank filters run for the length of an average sampling time showed small weight gains of approximately 0.12 percent which might be attributed to moisture pickup. Used filters were sealed in polyethylene bags and stored in a desiccator until trans- fer for analysis. All t
31、ests were conducted at random with respect to test temperature and flux or gas cover. Each filler metal was evaluated in a separate group of tests. Test tempeFa- tures for BAg-1 were 1200, 1350, and 1500“ F (649, 732, and 816“ C). The low cadmium filler metal was tested at 1250,1350, and 1500“ F (67
32、7,732, and 816“ C). Summaries of the laboratory data collected on the fume generation characteristics of the brazing filler metals tested are found in Appendix C. These data were sum- marized and are included in the report where their discus- sion occurs. Section IIIA. Total Fume Generation Characte
33、ristics The data collected on the amount of fume produced by each of the two filler metals studied at these different tem- peratures were averaged and are given in Tables 3.1 and 3.2. The data from these tables are shown graphically in Fig. 3.4 and 3.5. The average loss in weight of the ingots is al
34、so given along with other desirable data.O As mentioned previously, tess made with the glass fiber fiiters and the BAg-1 filler metals were excluded from consideration. This explains the fewer number of tests for BAg-1 than for the low cadmium filler metal. 10. Ingot weight losses are discussed late
35、r in this report. Table 3.1 Brazing fume generation by filler metals covered with AWS Type 3A flux 1200“ F (649“ C) 1250“ F(677“ C) BAg-I filler Low cadmium filler metal, avg. metal, avg. Number of tests 3 4 Time molten, min 6 7 Sampling time, min 25 27 Ingot loss, g 0.0422 0.0058 Fume weight, g 0.0
36、684 0.0550 1350“ F(732“ C) BAg-1 filler Low cadmium filler metal, avg. metal, avg. Number of tests Time molten, min Sampling time, min Ingot loss, g Fume weight, g 3 8 28 0.0527 0.0770 1 500“ F (8 16“ C) 4 8 26 .0.025 2 0.0602 BAg-I filler Low cadmium filler metal, avg. metal, avg. Number of tests 3
37、 Time molten, min 12 Sampling time, min 30 Ingot loss, g O. 1095 Fume weight, g O. 1670 6 16 31 0.1 896 0.0933 Copyright American Welding Society Provided by IHS under license with AWS Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 04:45:55 MSTNo reproduction or
38、networking permitted without license from IHS -,-,- 12O/FUMES AND GASES Temperature, O F F i g . 3.4-Brazing fume generation and metal losses by flux covered filler metai and the molten metal surface, although no fume could be wiped from the collection chamber wails even after sev- eral test runs. A
39、 different sampling system, such as a smaller, completely closed chamber, ,would be required to validate or refute the magnitude of the fume weights produced. Also,.the influence of the presence of a flux and the composition of the fume must be considered. The data show a large increase in weight of
40、 fume pro- Flux Covered Melts Examination of the fume weight data for both filler metals when melted under a flux (lble 3.1) shows the trend that would be expected increased temperature re- suits in increased fume weight. The actual weight of fume produced was smaller than expected. This may be attr
41、ibuted to the relationship between the filter location Copyright American Welding Society Provided by IHS under license with AWS Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 04:45:55 MSTNo reproduction or networking permitted without license from IHS -,-,- Sect
42、ion ZZZA. Total Fume Generation Characteristicsll21 duced at 1500“ F (816“ C) for both filler metals when compared to that produced at lower temperatures. This would be expected because cadmium boils at 1409“ F (765“ C) and zinc has high vapor pressure (approx. 300 mm of Hg) at the same temperature.
43、 The rate of fume formation at the two lower temperatures was nearly the same, but this rate increased significantly at 1500“ F whether or not the rates are calculated on the basis of tirrie molten or total sampling time. For example, using the fume weights produced by BAg-1 filler metal and the ave
44、rage time molten or sampling time, the calculated rates of fume formation are as follows: Temperature, Rate for time molten, Rate for sampling time, “F “C glmin . glmin. 1200 649 0.01 14 0.0027 1350 732 0.0096 0.0028 1500 816 0.0139 0.0056 The low cadmium filler metal appears to produce about one-th
45、ird less fume than the BAg-1 filler metal. Gas Covered Melts The data in Table 3.2 summarize the fume formation attributes of the two brazing filler metals studied where they are “protected“ by a flowing CO, gas blanket rather than a flux. The results parallel those obtained where a flux was used. T
46、hus, the same comments on the general trends apply. A significant difference is noted when the amounts of fume produced by the BAg-1 filler metal are compared to those from the low cadmium filler metal. in all cases more fume was produced by BAg-1 under CO, gas than under a flux; the opposite was tr
47、ue for the low cadmium filler metal. There appear to be two possible explanations for such an event: (a) the higher vapor pressure of cad- mium at all temperatures and (6) an observed more tenacious and impervious dross that formed on the low cadmium filler metal. The observations made while making
48、fume collections on melts protected by a flowing CO, atmosphere indicate Table 3.2 Brazing fume generation by CO2 covered filler metals 1200“ F (649“ C) 1250“ F (677“ C) BAg-1 filler Low cadmium filler metal, avg. metal, avg. Number of tests 3 Time molten, min 6 Sampling time, min 27 Ingot loss, g 0
49、.01 34 Fume weight, g 0.1035 1350“ F (732“ C) 4 7 30 0.0079 0.0435 Number of tests Time molten, min Sampling time, min ingot loss, g Fume weight, g BAg-1 filler Low cadmium filler metal, avg. metal, avg. 3 8 27 0.0446 0.0924 4 9 32 0.03 1 1 0.0357 1500“ F(816“ C) BAg-I filler Low cadmipm filler metal, avg. metal, avg. Number of tests 4 Time molten, min 12 Sampling time, min 33