Spreading and Aggressive Effects By Nickel-Base Brazing Filler Metals on Stainless Steel Little difference in wettability was observed in the stainless steels tested, but more aggressive penetration was evident in unstabilized AISI-304 than in stabilized AISI-321

BY I. A M A T O , F. B A U D R O C C O , AND M. R A V I Z Z A

ABSTRACT. Nickel-base materials provide a wide variety of brazing filler metals to meet many needs due to their mechani­cal, heat, and corrosion resistance prop­erties. The characteristics of the nickel-base brazing filler metals differ mainly in the alloying elements added to promote spreading and wettability.

The wettability and the aggression of five nickel-base brazing filler metals on stabilized (AISI-321) and unstabilized (AISI-304) stainless steels have been determined.

The brazing filler metals BNi-1 and -2 have shown good spreading and high grain boundary diffusion properties. The BNi-5 and the Ni-Mn-Si-Cu filler metals have shown good wettability but poor solid solution diffusion properties, and the BNi-7 filler metal has shown good spreading and bonding properties. The unstabilized carbon content present in the stainless steel can enhance the agres-sion of the nickel-base brazing filler met­als.

Introduction Brazing is assuming an ever increas­

ing role in the manufacture of hard­ware utilized in different fields (air­craft, marine, military, nuclear, etc.). The severe service conditions have required the development of special nickel-base brazing filler metals1-3

because of the good strength and cor­rosion resistance of these nickel alloys at high temperatures.

The alloying elements (silicon, bo­ron, phosphorus, copper, etc.), added to the nickel in order to lower its melting point and to promote wetta­bility, have also brought increased ag­gression by the brazing filler metal in the form of diffusion into and erosion of the base metal.4 '8

I. AMATO is a Professor in Material Sci­ence, F. BAUDROCCO is Chief of the Metal­lography Laboratory, and M. RAVIZZA is Chief of the Heat Treatment Laboratory, at Fiat Sezione Energia Nucleare, Italy.

Paper to be presented at the Second Inter­national AWS-WRC Brazing Conference in San Francisco, Calif, during April 27-29, 1971.

At the present time, many nickel-base brazing filler metals are avail­able. The determination of their wet­tability and their damaging effects on base metals, as a result of solution or penetration, is very important in choosing commercially available braz­ing filler metals for a given applica­tion. Although the nickel-base brazing filler metals are now widely used by the aircraft industry, where brazing of thin sections is a common practice, little information is available about the physical metallurgy of these braz­ing materials. The investigation re­ported here gives information on the physical properties and metallurgical change induced by five different nick­el-base brazing filler metals on two stainless steels, i.e. AISI-304 and AISI-321.

Materials and Experimental Procedures

The base metals chosen for the investigation are AISI-304 and AISI-321 stainless steel. The chemical com­position of these materials is shown in Table 1. The two stainless steels were chosen in order to investigate the in­fluence of different forms of carbon content (stabilized and unstabilized) on the aggressive effects of the brazing filler metals.

The five different nickel-base braz­ing filler metals were chosen in order to investigate the influence of alloying additions that have different diffusion rates on wettability and aggression. The chemical compositions of the braz­ing filler metals are shown in Table 2.

A fixed amount (by weight) of filler metal was placed on a plate of the base metal. The spreading and aggression determinations were car­ried out at different brazing tempera­tures and times, heating the specimens under a vacuum in the order of IO-4

Torr. The brazing temperatures were chosen in the brazing range of each filler metal; after each heat treatment, the drop enlargement and the penetra­tion of the filler metals were measured at the center of the drop. The sum­marized results are the average value of four determinations.

Results and Discussion The results of the spreading deter­

minations are summarized in Figs. 1 and 2.

For BNi-1, it may be seen that increasing the brazing temperature and the soaking time produces a regu­lar increase in the drop enlargement. Drop enlargements of 20-40% of the initial values show that the filler metal

Table 1—Chemical Compositions of Base Metals, %

AISI 304 AISI 321

C Mn P S Si

0.08 2.0 0.045 0.03 1.0 0.08 2.0 0.045 0.03 1.0

Table 2—Nominal Chemical Compositions of Brazing

Filler Metal

AWS BNi-1 AWS BNi-2 AWS BNi-5 Ni-Mn-Si-Cu AWS BNi-7

C Fe Mn Si Cu

0.70 4.5 — 4.5 — 0.06 2.5 — 4.5 — 0.10 — — 10.0 — — — 23.0 7.0 5.0

0.10 — — — —

Cr

18-12 17-19

Ni

8-12 9-12

Filler Metals, %

Ni

72.8 83.5 70.9 65.0 76.9

Cr

14.0 6.5

19.5 —

13.0

Ti

5 X C min, 0.70 max

P B

— 3.5 — 3.0 — — — —

10.0 —

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Base

has good spreading propert ies for the two stainless steels unde r investiga­tion.

The BNi-2 filler metal has a wider brazing tempera ture range than that of BNi-1 . The spreading results show that maximum drop enlargement is reached at low temperatures in the brazing range. The spreading proper­ties of these filler metals ( 2 0 - 6 0 % of the initial value) are good for brazing applications.

In the BNi-5 filler metal , the pres­ence of silicon as an alloying element instead of boron gives less spreading (i.e., wettabili ty) than the BNi-1 and -2 filler metals. In fact, the drop en­largement for this filler metal in the brazing range is in the order of 10-3 0 % of the initial d rop value. The brazing filler metal Ni-Mn-Si-Cu shows spreading properties similar to BNi-5.

BNi-7 shows the highest drop en­largement among the brazing filler metals under investigation. It is inter­esting to observe that the wettability of this filler metal is enhanced through heat t rea tment at the m a x i m u m braz­ing tempera ture . In this case, a d rop enlargement in the order of 6 0 - 8 0 % of the initial drop value has been obtained. This part icular behavior must be at t r ibuted to the presence of phosphorus as an alloying element.

Concerning the aggressive effects, Figs. 3 and 4 summarize the results obtained. The aggression of the base metal by the brazing filler metals oc­curs through the mechanism of dif-fussion and erosion.7 The diffusion be­gins through the migration of intersti­tial a toms, i.e., a toms of relatively small a tomic radii , present in the filler metals. This migrat ion occurs through the grain boundaries of the base metal and is called grain boundary diffusion.

The elements of relatively large atomic radii diffuse at a slower rate in the lattice of the base metal grains along the entire interface region. This migration, which involves the forma­tion of an interfacial solid solution, is called solid solution diffusion. The dis­solution of the solid base metal by the molten brazing filler metal is called erosion.

Fo r BNi -1 , a large grain boundary diffusion of the alloying element, in this case boron , in the base metal can be observed (Fig. 5 ) . The molten filler metal undergoes an eutectoid decomposi t ion with format ion of in­termetallic compounds and solid solu­tion. It is possible to see that the filler metal-base metal interface shows a great impover ishment of the alloying elements . The grain boundary diffu­sion is enhanced in unstabilized stain­less steel.

F o r BNi-2, a large grain boundary

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70 60 50 40 30 20 10 0

950 975 1000 1025 1050 Temperature °C

Fig. 2—Spreading measurements. Base metal—AISI 321 stainless steel

184-s I A P R I L 1 9 7 1

Brazing Alloy

A W S B N i - 1

A W S B N i - 2

A W S B N i - 5

N i -Mn Si - Cu Alloy

A W S B N i - 7

Tempera-lure °C

JO 70 1130 1190 1010 1050 1090 1130 1170

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Penetration into base metal , M

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Tempera­ture °C

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J 0 9 0 1130 1170 1160 1180 1200

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Penetration into base metal, M

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Fig. 3—Measurements of penetration. Base metal—AISI 304 stainless steel. Soaking time: A (top)—0.5 hr; B (center)—1.5 hr,- C (bottom)—3 hr

W E L D I N G R E S E A R C H S U P P L E M E N T 185-s

Brazing Alloy

A W S B N i - 1

A W S B N i - 2

A W S B N i - 5

N i -Mn S i -Cu Alloy

AWS BNi-7

Tempera­ture °C

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Penetration into base metal , H

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Penetration into base metal, M

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186-s A P R I L 197 1

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Fig. 5—Drop spreading and penetration. Base metal—AISI 304; filler metal—AWS BNi-1; brazing treatment—1130° C, 1.5 hr; vacuum—10-4—Torr: A (top left)—grain boundary diffusion and precipitation; B (top center)—filler metal-base metal interface; C (top right)—brazing alloy after eutectoid decomposition; D (bottom)—semisection of drop spreading

>- M l"~7i~>^- *v •>• "3- t-'-'P

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Fig. 6—Drop spreading and penetration. Base metal—AISI 304; fil ler metal—AWS BNi-5; brazing treatment—1090° C, 0.5 hr; vacuum—IO"4 Torr. A (top left)—grain boundary diffusion and precipitation; B (top center)—-filler metal-base metal interface; C (top right)—brazing alloy after eutectoid decomposition; D (bottom)—semisection of drop spreading

diffusion and erosion can be observed —Fig. 6. The eutectoid decomposition of the brazing filler metal takes the form of rod-like intermetallic com­pounds. The extent of this erosion is too great to adopt this filler metal for brazing thin structures.

For BNi-5, a small solid solution diffusion is the only effect obtained through the entire brazing cycle—Fig. 7. For Ni-Mn-Si-Cu, a small solid solution and erosion are the results of the aggressive effect of the brazing

filler metal—Fig. 8. For BNi-7, the aggressive effects,

grain boundary diffusion and erosion are operative during the brazing cy­cle—Fig. 9. Enrichment of the phos­phorus content at the base metal-molten filler metal interface and diffu­sion in the bulk material through the grain boundary, can be observed.

Conclusions

The following conclusions can be

drawn from the observations carried out during the experiments:

1. BNi-1 and BNi-2 have good wet­tability, but the penetration is too high to use these alloys for brazing thin structures. For BNi-1, the aggression is attributed to grain boundary diffu­sion. For BNi-2, in addition to the grain boundary diffusion, a great ero­sion is present.

2. The BNi-5 and the Ni-Mn-Si-Cu filler metals have acceptable wettabili­ty and low penetration, occurring

W E L D I N G R E S E A R C H S U P P L E M E N T I 187-s

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Fig. 7—Drop spreading and penetration. Base metal—AISI 321; filler metal—AWS BNi-5; brazing treatment—1090° C, 3 hr; vacuum—10~4 Torr. A (top left)—solid solution diffusion; B (top center)—filler metal-base metal interface; C (top right)— brazing alloy after eutectoid decomposition; D (bottom)—semisection of drop spreading

Fig. 8—Drop spreading and penetration. Base metal—AISI 304; f i l ler metal—Ni-Mn-Si-Cu; brazing treatment—1120° C, 3 hr; vacuum—10 4 Torr. A (top left)—grain boundary diffusion and precipitation; B (top center)—filler metal-base metal interface; brazing alloy after eutectoid decomposition; D (bottom)—semisection of drop spreading

mainly by solid solution diffusion. These alloys are recommended for brazing thin structures.

3. BNi-7 shows a high wettability for brazing cycles carried out at high temperatures; and the aggression, which occurs through grain boundary diffusion and erosion, is intermediate between the BNi-1 and BNi-2 filler

metals, and the BNi-5 and Ni-Mn-Si-Cu filler metals.

4. Concerning wettability, no great difference has been observed between the two stainless steel base metals. More aggressive penetration of the base metal by the brazing filler metal is observed in the unstabilized AISI-304 stainless steel than in the stabi­

lized AISI-321 stainless steel.

References 1. Bell, G. R., "Brazing with Nickel-Base

Alloys", paper presented at Spring Meeting of Welding Institute (1967)

2. Peaslee, R. L., "Stainless Brazing: A sophisticated joining method", paper pre­sented at Autumn Meeting of Welding In­stitute (1964)

3. Rhys, D. W., and Betteridge, W., Met­al Industry 6, 13 (1962)

188-s | A P R I L 1971

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Fig. 9—Drop spreading and penetration. Base metal—AISI 301; filler metal—AWS BNi-1; brazing treatment—975° C, 3 hr; vacuum—IO"4 Torr. A (top left)—grain boundary diffusion and precipitation; B (top center)—filler metal-base metal inter­face; C (top right)—brazing alloy after eutectoid decomposition; D (bottom)—semisection of drop spreading

4. Bredzs, N. , and Schwartzbzr t , H. , "Grain Boundary Pene t ra t ion and Base Metal Erosion in High T e m p e r a t u r e Braz­i n g " , WELDING JOURNAL, 41 (3), Research Suppl. . pp . 129-s to 144-s (1962)

5. McDonald, A. S., "Alloys for Brazing Th in Sections of Stainless S tee l" , Ibid. 36,

(3), Research Suppl. , pp. 131-s to 140-s (1957)

6. Feduska , W., " T h e N a t u r e of High T e m p e r a t u r e Brazing Alloy-Base Metal In­terface Reac t ions" , Ibid., 37 (2), Research Suppl. pp. 62-s to 73-s (1958) and " T h e Na­t u r e of the Diffusion of Braz ing Alloy Ele­

ments into Heat-Resis t ing Alloy", Ibid., 40 (2), Research Suppl . pp . 81-s to 89-s (1961)

7. Lamb, S., and Miller, F . M., " T h e Effects of Aggression by Nickel-Base Braz­ing F i l l e r Meta l" , Ibid., 48 (7), Research Suppl. pp. 283-s to 289-s (1969)

WRC Bulletin

No. 159 Feb. 1971

"Welding of Maraging Steels"

by F. H. Lang and N. Kenyon

This report was prepared for the Interpretive Reports Committee of the Welding Research Council. A general description of the metallurgical conditions involved in welding maraging steels is followed by a detailed discussion of the usefulness of a variety of processes. Important parameters are discussed, procedures are recom­mended, and the properties that can be expected from the weld joints are outlined.

Bulletin 159 is $3.00 per copy. Orders for single copies should be sent to the American Welding Society, 345 E. 47th Street, New York, N.Y. 10017. Orders for bulk lots, 10 or more copies, should be sent to the Welding Research Council, 345 E. 47th Street, New York, N.Y. 10017.

W E L D I N G R E S E A R C H S U P P L E M E N T | 189-s