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Structure of joints in Inconel-718 creep-resisting nickel alloy, produced by high-temperature vacuum brazing.

One of the efficient methods of joining austenitic superalloys based on nickel of the Inconel 718 type (IN-718) is high-temperature vacuum brazing using brazing alloys based on nickel or noble metals. The nickel alloys are used widely for the brazing of corrosion-resisting and creep-resisting steels and alloys. They produce brazed joints with high strength at both room and elevated temperatures and also corrosion resistance.

The most widely used materials and the brazing alloys based on the Ni-Cr system which contain as depressants boron and silicon (1) ensuring efficient wetting of the material to be brazed and also complicating the structural state of the brazed joints and the parent metal.

Brazing of the creep-resisting nickel alloy IN-718 is characterised by the active interaction between the brazing alloy in the parent metal. In the liquid state, boron and nickel are completely soluble in each other (2). In brazing, boron diffuses actively into the material to be brazed (to a distance of up to 200 [micro]m).

Since there is no mutual solubility in the solid condition, solidification is accompanied by the formation of borides of chromium and nickel which precipitates at the grain boundaries of the parent metal, adjacent to the joint (Fig. 1), in the form of a boride network and dispersed inclusions (3).

The microstructure of the adjacent part of the parent metal consists of two zones. The first zone, up to 35 [micro]m wide, contains the dispersed sheets of chromium borides, formed inside the grains of the solid solution. The second zone (up to 200 [micro]m wide) has the formal way boride network precipitates at the grain boundaries of the parent metal. Starting at a distance of more than 200 [micro]m, the parent material is not subjected to any diffusion processes and precipitation of borides at the grain boundaries of the solid solution was not observed.

As regards the second depressant (silicon), according to the Ni-Si equilibrium diagram, the maximum solubility of silicon in nickel is 15.8 at.% at a temperature of 1143[degrees]C (4). Solubility decreases with a reduction of temperature, but the brazed joints (central area is), produced using the BNi-2 (Ni--7Cr--4.5Si--3.1B--3Fe) brazing alloy containing large amounts of silicon which can form nickel silicides (4).

This phenomenon can be explained by the following special features of solidification of the metal of the brazed joints. The cooling of the brazed joint is characterised by the solidification of the solid solution based on equal ([gamma]-phase) in the form of primary grains with a higher melting point. The remaining melt of the liquid metal is enriched with boron, silicon, chromium and is displaced into the central part of the brazed joint where further cooling results in the solidification of the low-melting brittle eutectic phases, consisting of the solid solution and silicides (borides) (3), (5).

These special features of the formation of brazed joints in IN-718 creep-resisting nickel alloys are also typical of other boron-containing nickel brazing alloys, for example, MBF-20: Ni--7Cr--4.5 Si--3Fe--3.2B (6). The tensile strength higher than (784.2[+ or -]10.4) MPa cannot be obtained as a result of the presence of silicon and boron in the brazed metal. The application of pressure in brazing results in partial displacement from the brazed joint of the brittle phases, containing the depressants, and the tensile strength increases to (868.4 [+ or -]12.8) MPa but there is a risk of the loss of the geometrical dimensions of the brazed joints (6).

When using the BNi--5 (Ni--19Cr--10.1Si) brazing alloy with no boron, the resultant brazed joint usually contains at least four microstructural components, including the grains of the solid solution based on nickel, intermetallic compounds, characterised by high hardness and brittleness, binary and ternary eutectics which reduce the strength of the brazed joint (7).

To prevent the formation of intermetallic phases, it has been proposed to increase the brazing time and temperature and the use the width of the brazing gap. These three parameters of the brazing process have the controlling effect on the structure formation in the brazed joints.

Solidification of the brazed joints maybe evaluated using a quarternary diagram (7). With increasing brazing time, the brittle components of the brazed joints are partially dispersed or their size decreases, but it is not possible to remove them completely even in long-term holding. Diffusion of the brazing alloy along the grain boundaries of the parent metal has a negative effect on the strength properties of the brazed joints.

The formation of the brittle components in the brazed joints can be prevented and the high-level of the mechanical properties of the joint in brazing of nickel super alloys can be achieved by using, as brazing alloy, the alloys with the structure of the solid solutions in the initial condition.

In this article, the authors present the results of investigations of structure formation in the brazed joints in IN-718 creep-resisting nickel alloys produced using PZhK 1000 industrial brazing alloy and experimental brazing alloys of the Pd--Ni--Cr--(X)system.

Metallographic investigation were con-ducted and IN-718 creep-resisting deformable this patient-hardened alloy (50...55) Ni--(17...21) Cr--18 Fe--(4.75...5.5) Nb + Ta--(2.8...3.3) Mo--(0.65--1.15) Ti--(0.2...0.8)Al-[less than or equal to]Co...0.06C), and the brazing alloys based on the Pd--Ni--Cr--(X) system (Table 1). The brazing alloy was produced by argon-arc melting with a non-consumable tungsten electrode. The charge materials included 99.99% Pd, 99.99% Ni (double electron beam in melting), 99.9% Cr (vacuum-distilled alloy), 99.99% Si (monocrystalline), Co--K (99.8%), 99.99% Ge) monocrystalline), 99.9% B (powder).
Table 1. Brazing alloys and brazing temperature

Brazing Basic alloying system Recommended brazing
alloy temperature, [degrees]C
No.

1 Ni--Cr--Pd--Si 1250 [8]

2 Ni--Cr--Pd--Co--Si 1230

3 Ni--Cr--Pd--X(Ge) 1230

4 Ni--Cr--Pd--B 1080


To average out the chemical composition, the ingots were melted up to 5 times and this was followed by rolling to produce ductile strips with a thickness of 0.15--0.05 mm. The brazing alloys No. 4 (Table 1) could not be produced in the form of thin foils by rolling and superfast quenching was therefore applied. Specimens were brazed in a vacuum furnaces using radiation heating.

The microstructure of the brazed joints was investigated by optical and scanning electron microscopy after brazing and heat treatment. Hardness HV was determined in M-400 hardness metre (LECO) with a load of 0.5 N, loading time 5 s.

The microstructure of the IN-718 alloy in the as-received condition (after forging and homogenisation at a temperature of 10 50[degrees]C for 1.5 h without etching) is shown in Fig. 1. On the background of the makings of the solid solution that single precipitates of the main secondary phases: complex niobium carbides (spectra 3, 4, Fig. 1, Table 2), in the form of light inclusions of irregular shape and titanium nitrides in the form of dark particles of regular form (spectra 5, 6, Fig. 1, Table 2).

[FIGURE 1 OMITTED]
Table 2. The chemical composition of individual structural components of
IN--718 alloy in the as--received condition, wt.%

No. of C N Al Ti Cr Fe Co Ni Nb
investigated
section

1 -- -- 0.72 0.95 19.58 17.07 0.22 51.78 5.81

2 -- -- 0.42 0.86 19.59 17.37 0.13 51.95 5.94

3 8.90 -- -- 3.25 0.41 0.48 0.06 0.95 84.67

4 9.52 -- 0.10 4.60 0.76 0.45 0.01 1.92 81.32

5 -- 22.60 -- 66.30 0.87 0.46 -- 1.01 8.42

6 -- 18.55 -- 68.09 0.58 0.21 0.10 0.97 11.27

No. of Mo
investigated
section

1 3.87

2 3.75

3 1.29

4 1.31

5 0.34

6 0.12


IN-718 alloy is characterised by the formation of complex carbide compounds containing, in addition to niobium, and the alloying elements of the creep-resisting nickel alloy: titanium, molybdenum, chromium, iron, nickel (in small amounts).

The atoms of titanium and niobium can substitute each other in the compounds (Ti, Nb)C. The mass fraction of carbon in the NbC carbides may reach 8.6--1.5% in accordance with the given homogeneity region (9).

The carbides do not fracture during forging, heat treatment, show very high resistance at high temperatures and do not dissolve when the temperature is increased to 1230[degrees]C, and also have a beneficial effect on the long-term strength at high temperatures (10).

The titanium nitrides contain the components of alloy IN-718 alloy (niobium, molybdenum, chromium, iron, nickel), and also interstitial elements. Single carbonitride precipitates were also found.

The IN-718 creep-resisting alloys belongs in the group of the alloys with the nickel-iron-based image main hardening is not ensured by the [gamma]'-phase [Ni.sub.3] (Al, Ti) with the FCC lattice but by the [gamma]''-phase [Ni.sub.3]Nb with the BCC tetragonal lattice. The presence of niobium increases the dissolution temperature to 950[degrees]C and helps to preserve the hardening effect to higher temperatures (11).

This phase forms after heat treatment, recommended by the developers of the alloy, consisting of homogenising at 1050[degrees]C for 1.5 h, cooling in air and subsequent stepped ageing at 760[degrees]C for 10.5 h, cooling in the furnace to 6 50[degrees]C, holding at this temperature for 8.5 h and cooling in air. Ageing ensures the optimum size and distribution of ultrafine precipitates of the [gamma]''-phase and increases the maximum hardness and strength of the alloy.

The microstructure of IN-718 alloy undergoes changes after brazing at high temperatures. For example, after brazing with the Ni--Cr--Pd--Si industrial alloy at a temperature of 12 50[degrees]C for 15 min, these results show increase of the thickness of the grain boundaries and the formation of globular precipitates in the body of the grains of the matrix, and the grain size increased from number 3 to number 1 (GOST 5639-82, scale 3). When the brazing temperature was reduced to 12 30[degrees]C and with the same holding time, the number and dispersion of the globular particles decrease, the grain boundaries became thinner, the grain size increased but not very rapidly (up to size 2, 1).

In isothermal brazing with the Ni--Cr--Pd--B brazing alloy (10 80[degrees]C, 30 min), the thickness of the grain boundaries of the parent metal away from the joint (more than 200 [micro]m) did not show any significant changes, and the number of the globular precipitates was small. A diffusion zone with phases precipitated at the grain boundaries (Fig. 2, 3), enriched with boron, formed on both sides of the brazed joint in the parent metal as a result of diffusion processes taking place at the liquid brazing alloy--solid substrate interface.

[FIGURE 2 OMITTED]

The parent metal, adjacent of the joint, is rapidly supersaturated with boron as a result of the diffusion activity of boron and its low solubility. This leads to the precipitation of the borides in the brazed metal. The results of x-ray spectrum microanalysis shows that complex phases, containing niobium, boron, carbon, titanium and traces of other elements (Table 3, Fig. 3) form at the grain boundaries. The determination of the mass fraction of boron by the energy-dispersing method in the specimens containing molybdenum is characterised by considerable difficulties due to the fact that the [K.sub.a] -lines of boron and [M.sub.z]-lines of molybdenum coincide.

[FIGURE 3 OMITTED]
Table 3. Chemical composition of the diffusion zone of the parent metal
in brazing with a boron-containing brazing alloy, wt.%*

Spectrum B C O Al Ti Cr Fe Ni Nb Mo
number

1 3.67 6.83 -- -- 7.56 0.75 0.45 1.39 79.35 --

2 4.09 5.89 -- -- 4.93 0.34 -- 0.99 83.76 --

3 -- 6.60 1.15 0.21 0.78 16.60 10.26 28.20 29.50 6.70

4 5.75 0.01 -- -- 0.85 19.19 16.84 30.48 23.14 3.74

5 5.58 0.21 -- -- 0.87 19.77 17.23 47.38 5.14 3.82

6 1.84 -- -- 0.42 0.78 19.07 17.38 51.24 5.48 3.79

* Composition was determined after etching


The majority of the borides are characterised by small deviations from the silicon metric composition, in addition, they often contain carbon, oxygen and other interstitial impurities, and form a low melting eutectic is (11). The presence of these precipitates at the grain boundaries reduces the plasticity properties, in particular, the relative elongation decreases to 2--4%. On the background of the matrix of the parent metal there are hardening interstitial phases, characteristic of the given alloy (titanium nitrides and niobium carbides) (12).

It should be mentioned that in investigating the brazed joints prior to etching it is not possible to obtain complete information on the structure of the parent metal. Etching of the brazed joints provides further information, but in the determination of the chemical composition it is necessary to take into account that the results are relative to a certain extent.

Figure 4a--c shows the microstructures of the brazed joint is (after etching), produced using the experimental brazing alloys No. 3 Ni--Cr--Pd--X (Ge) at Tb = 1230[degrees]C, [tau] = 5 min. The brazed joints are characterised by high structural homogeneity, the absence of carbides, intermetallic inclusions and liquation interlayers (Fig. 4c--g).

[FIGURE 4 OMITTED]

After high--temperature vacuum brazing the volume of the grains of the solid solution of the brazed metal contain precipitates of globular disperse precipitates (Fig. 4a--d). The chemical composition of these precipitates was identical with the composition of the solid solution, but the amount of some elements was higher than in the solid solution (approximately by 1--3% in the case of chromium, 1--2% for iron, 2.5--6.0% for nickel (Table 4, Fig. 5a). The amount of other elements (niobium and molybdenum) decreased approximately 2.5--4.0 times and equalled 1.31...2.04 and 1.02...1.41%.

[FIGURE 5 OMITTED]
Table 4. Chemical composition of the structural components of the
parent metal, wt.%


Condition Spectrum C N Al Ti Cr Fe Ni
of No.
specimens

 1 -- -- 0.67 0.93 20.23 17.33 51.59

 2 -- -- 0.54 0.82 19.93 17.36 51.96

After

brazing 4 10.35 -- -- 6.46 0.96 0.65 1.22

 5 -- 22.45 -- 69.22 0.68 0.43 0.91

 6 9.87 -- -- 5.17 0.87 0.82 2.11

 7 -- -- 0.15 0.88 21.15 17.74 57.75

 8 -- -- 00.07 0.92 21.29 17.77 54.50

 1 -- -- 0.44 0.91 19.17 18.02 52.81

 2 -- -- 0.57 1.04 19.81 17.68 52.31

 3 -- -- 0.52 0.90 19.82 17.96 52.27

After

complete 4 9.51 -- 0.03 5.56 0.83 0.76 1.89

heat 5 9.19 -- 0.09 4.45 1.74 1.53 3.08
treament

cycle 6 -- 21.80 0.05 66.51 1.12 0.67 1.89

 7 -- 18.67 0.02 73.56 0.47 0.68 0.23

 8 -- -- 0.67 0.96 19.97 17.64 52.45

 9 -- -- 0.59 0.84 19.86 18.71 52.16

Condition Spectrum Nb Mo
of No.
specimens

 1 5.44 3.81

 2 5.67 3.12

After

brazing 4 79.31 1.05

 5 6.31 --

 6 80.66 0.50

 7 1.31 1.02

 8 3.04 2.41

 1 5.33 3.23

 2 5.18 3.10

 3 4.93 3.60

After

complete 4 80.14 1.28

heat 5 78.79 1.13
treament

cycle 6 7.84 0.12

 7 6.14 0.23

 8 5.34 2.97

 9 4.95 2.89



The grain boundaries of the parent metal contains single particles of the niobium carbides of irregular shape (Fig. 5a), containing titanium (4.58--6.46%).

According to the results of x-ray spectrum microanalysis, the small amount of the dark precipitates with regular faceting may be regarded as titanium carbonitride: they are enriched with titanium, nitrogen and carbon.

In some sections, the grain boundaries contain single plate-shaped formations in the form of discontinuous filaments with the length of 2--10 [micro]m (Fig. 4a, b). These precipitates are undesirable from the viewpoint of strength. Grain size after brazing (GOST 5639-82, scale 3), increase from 3 to 2-1. Larger grains ensure better long-term strength characteristics (10).

The microhardness HV of the parent metal after brazing (cross-hatched region) slightly decreases (Fig. 6, 1, 2). Since the microhardness is the mechanical characteristic of the material, and the microhardness release correlate with the values of the ultimate strength, it may be assumed that the mechanical properties of the brazed joint are reduced by the effect of heating and slow cooling.

[FIGURE 6 OMITTED]

To improve the mechanical properties of the parent metal, it is necessary to carry out complex heat treatment of the brazed joints. Homogenising (T = 1050[degrees]C, [tau] = 1.5h, air) of the brazed joints, produced using the experimental brazing alloy No. 3, leads to changes in the structure of the parent metal, the diffusion redistribution of the alloying components, reduction of the thickness of the grain boundaries and the parent metal (Fig. 4d--f), and a small increase of microhardness (Fig. 6).

In subsequent two-step ageing (T = 760[degrees]C, [tau] = 10.5 h, cooling in the furnace to 650[degrees]C, [tau] = 8.5 h, air), the globular phase becomes dispersed in comparison with that produced after brazing.

The plate-shaped precipitates are not found at the grain boundaries, there are single precipitates of the niobium carbonitride in the form of particles of regular and irregular shape and the grain boundaries and in the volume of the grain (Fig. 4g, h, Fig. 5b, Table 4) and also single precipitates of the titanium carbonitride.

The oxides and boride can form in the nickel--iron alloys but the amount of these compounds is small and, consequently, they have no effect on the properties not associated with segregation (10). The determination of the chemical composition of the grain boundaries and the matrix showed that these precipitates are identical.

The microhardness of the matrix of the brazed material after the complete heat treatment cycle increased and equals 4210-4500 MPa. The identical tendencies also characteristic of the brazed joint (Fig. 6, 3), and the mean microhardness values increase from 3250 (after brazing) to 4200 MPa (after heat treatment). Thus, as a result of the complete cycle of heat treatment, the values of the hardness of the matrix of the parent metal and the brazed joint are approximately equal. This has a positive effect on the mechanical properties of the brazed joints and this has been confirmed by these investigations. The long-term strength at 550[degrees]C and a load of 785 MPa was 132 h (without fracture of the specimen).

Conclusions

1. The experimental results show that brazing with the brazing alloy of the Ni--Cr--Pd--Si system at a brazing temperature of 1080[degrees]C has no effect on the structure of the parent metal, with the exception of the boundary zone adjacent to the brazed joint characterised by the precipitation of boride compounds at the grain boundaries to a depth of approximately 100...103 [micro]m, has been confirmed by the results of micro x-ray spectrum investigations.

2. It is shown that vacuum brazing of the IN-718 creep resisting alloy with the Ni--Cr--Pd--Si industrial brazing alloy at 1230[degrees]C is characterised by grain growth from grain size number 3 to grain size number 1 or 2 (GOST 5639-82, scale 3).

3. Brazing of the IN-718 creep resisting alloy with the brazing alloy of the Ni--Cr--Pd--X (Ge) system results in preferential solidification of the solid solution in the brazed joint and reduces the microhardness HV of the matrix of the parent metal by approximately MPa. Subsequent heat treatment, consisting of homogenising and two-steg ageing, increases the hardness of not only the matrix of the parent material but also the metal of the brazed joints to 4210-4500 and 4200 MPa, respectively.

References

(1.) Xiaoveri Wu., Journal of Materials Processing Technology, 2000, No. 1-2, 34-43.

(2.) Lyakishev N.P. (editor), Equilibrium diagrams of binary metallic systems, a handbook in three volumes, vol. 1, Mashinostroenie, Moscow, 1999.

(3.) Arafin M.A., et al., Materials science and Engineering, A447, 2007, 125-133.

(4.) Lakishev N.P. (editor), Equilibrium diagrams of binary metallic systems, a handbook in three volumes, vol. 3, book 1, Mashinostroenie, Moscow, 1999.

(5.) Arafin M.A., et al., Thermodynamic modelling and experimental investigation of brazed joints used in aerospace industry, in: Proceedings of the Third International brazing and soldering conference, San Antonio, Texas, USA, 2006, 189-196.

(6.) Yeh M.S. and Chuang T.H., Welding Journal, 1997, volume 76, No. 5, 197-200.

(7.) Grushko b. and Weiss H.Z., Metallurgical and Materials Transactions, A, 1984, No. 4, 609-620.

(8.) Petrunin I.E. (editor), A Handbook of brazing, Mashinostroenie, Moscow, 2003.

(9.) Samsonov G.V. and Vinnitskii I.M., Refractory com-pounds, Mashinostroenie, Moscow, 2003.

(10.) Sims Ch. and Hagel W., Creep resisting alloys, Metal lurgiya, Moscow, 1976.

(11.) Sims Ch.T. (editor), Superalloys 11: Creep resisting alloys for aerospace and industrial power equipment, in two volumes, Metallurgiya, Moscow, 1995.

(12.) Goldschmidt H.G., Interstitial alloys, No. 1, Mir, Moscow, 1971.

S.V. Maksimova and V.F. Khorunov

E.O. Paton Electric Welding Institute, Kiev
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Title Annotation:NEW MATERIALS
Author:Maksimova, S.V.; Khorunov, V.F.
Publication:Advances in Electrometallurgy
Geographic Code:4EXRU
Date:Jul 1, 2010
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