**1. Introduction**

At present materials which are difficult to join by traditional welding methods are widely applied in different industries. These, primarily, are dissimilar materials, for joining which brazing is extensively used. This method of material joining has been known since ancient times, it is developing continuously, and the area of its application becomes wider. Producing permanent joints of a refractory material—molybdenum with stainless steel by brazing is highly important for many industries, related to structure operation at high temperatures. This is due to high melting temperature, high modulus of elasticity, relatively low density, and excellent specific strength of molybdenum at high temperatures. Recrystallization temperature and mechanical properties of molybdenum depend on many factors and, primarily, on the degree of its purity, method of its production, as it is sensitive to interstitial impurities. Recrystallization temperature of unalloyed molybdenum is in the range of 900–1000°C. It depends on metal purity, temperature, degree of deformation, and

recrystallization duration [1]. Molybdenum becomes brittle after recrystallization, so that the melting temperature range of brazing filler metal is important at its selection.

Joining dissimilar materials is a more complex task than joining similar materials. The complexity of joining dissimilar materials is due to a significant difference in thermal coefficients of linear expansion (TCLE) and low oxidation resistance of molybdenum. So, at a temperature exceeding 500°C, the sublimation of MO3 oxide begins on molybdenum surface. It becomes significant at the temperature of 600°C. At further temperature increase above 800°C, this oxide melts, leading to superactive oxidation of molybdenum in a standard atmosphere [1]. In this connection, it is better to conduct molybdenum brazing in a vacuum. Vacuum brazing has several advantages, compared to the traditional methods of brazing in an air atmosphere. In a vacuum furnace atmosphere, a practically complete absence of any substances is achieved. The most important feature of vacuum brazing is the possibility of conducting the process without the application of fluxes. In addition to eliminating the operation of flux washing, it allows producing joints with high strength, corrosion resistance, and vacuum tightness, which is very important for the fabrication of many structures.

In brazing dissimilar materials, an important task is a correct selection of the chemical composition of brazing filler metal, its solidus, and liquidus temperature. Vacuum brazing of stainless steels is usually performed in the temperature range of 1000–1200°C. This is the temperature range of quenching of most alloy steels, which allows you to combine brazing with heat treatment of the material and thereby achieve high strength brazed joints. The brazing filler metal should readily wet the materials being brazed, it should be sufficiently strong and, at the same time, ductile, should readily deform, and promote relaxation of stresses, arising during brazing and cooling to room temperature.

The authors of [2] presented the problems that arise when brazing dissimilar materials. We focused on the difference in the coefficients of thermal expansion of brazing filler metal and base metals, which can lead to the appearance of internal stresses, as well as to deformations of the base metal.

In various industries, brazing filler metal based on nickel and copper are widely used: VPR1, VPR4, BNi-1, BNi-2, BNi-3, BNi-4, BNi-5, BNi-7, BNi-8, etc. [3–10]. They are used for brazing steels of various grades, heat-resistant nickel alloys, and many other materials. As a rule, these brazing filler metals contain Si and B as depressants (**Table 1**), which provide an acceptable temperature range for melting and good wetting of the brazed metals.

The disadvantages of these brazing filler metal include active diffusion of boron, the formation of fusible boride, and silicide phases that are released in brazing joints (**Figure 1(a)**) and in the base metal during brazing (**Figure 1(b)**).

They relate to brittle intermetallic compounds that adversely affect the performance of brazed joints during prolonged use.

Brazed joints obtained using BNi-2, BNi-3, and BNi-4 brazing filler metal consist of three phases [9]: a nickel-based solid solution adjacent to the base metal and located in the center of the brazed joint of nickel borides and eutectic consisting of nickel silicides and borides.

Boron actively diffuses into stainless steel adjacent to the seam. In the process of brazing at high temperature, it forms intermetallic boride phases along grain boundaries. When these phases are in large numbers, they reduce the fatigue strength and corrosion resistance of steel. The brittle phases determine the brittleness of the joint as a whole, and crack development occurs along these phases. It is possible to increase the strength of brazed joints by forming a structure of a solid solution in the brazed seams, which effectively inhibits the development of cracks. Rabinkin has extensively studied the problems associated with the presence of

**105**

strength—200–230 MPa.

*fillet section, (a) brazed seam (b) [10].*

*Vacuum Brazing of Dissimilar Joints Mo-SS with Cu-Mn-Ni Brazing Filler Metal*

**Chemical composition of the main elements, % (wt.) Cr B Si Fe Mn Cu Ni**

5.0

5.0

3.5

1.5

1.5

0.8

— 1.0–

VPr11\*\* 14.0–16.0 2.0–3.0 4.0–5.0 3.5 — — Base

BNi-3 — 2.75–3.5 4.0–5.0 0.5 — — Base BNi-4 — 1.50–2.20 3.0–4.0 1.5 — — Base BNi-5 18.5–19.5 0.03 9.75–10.50 — — — Base BNi-8 — — 6.0–8.0 — 21.5–24.5 4.0–5.0 Base

— — Base

— — Base

— — Base

— base 27.0–

27.0–30.0 base 28.0–

32–35 — Base

30.0

30.0

borides in brazed joints [7]. He suggests the use of prolonged heat treatment to dissolve borides and silicides, which complicates the process of obtaining joints and

*Microstructure of brazed joints of 1Kh18N9 steel, produced using brazing filler metal Ni-7Cr-4.5Si-3Fe-3.2B:* 

The use of silver brazing filler metals for brazing an alloy of Mo60%-Cu40% provides shear strength in the range of 170–220 MPa [11]. The application of brazing filler metals based on Ni-Cr-Si system containing boron [12] for brazing molybdenum (Mo60%-Cu40%) with stainless steel does allow achieving a high

Promising are brazing filler metals with solid solution structure, which are characterized by acceptable melting temperature and high mechanical properties and act as a damper between two dissimilar metals, which promotes relaxation of

does not always allow you to get rid of them [7, 8] completely.

*DOI: http://dx.doi.org/10.5772/intechopen.92983*

BNi-1 13.0–15.0 2.75–3.5 4.0–5.0 4.0–

BNi-1a 13.0–15.0 2.75–3.5 4.0–5.0 4.0–

BNi-2 6.0–8.0 2.75–3.5 4.0–5.0 2.5–

VPr1 — 0.1–0.3 1.5–2.0 0.1–

VPr7 — 0.07–0.2 0.8–1.2 0.1–

0.25

VPr4\* — 0.15–

*Chemical composition of brazing filler metals.*

*4.0–6.0% Co; 0.1–0.2% P*

**Grade of BFM**

*\**

*\*\*0.1–1.0% Al*

**Table 1.**

**Figure 1.**


*Vacuum Brazing of Dissimilar Joints Mo-SS with Cu-Mn-Ni Brazing Filler Metal DOI: http://dx.doi.org/10.5772/intechopen.92983*

#### **Table 1.**

*Welding - Modern Topics*

during brazing and cooling to room temperature.

stresses, as well as to deformations of the base metal.

and good wetting of the brazed metals.

nickel silicides and borides.

mance of brazed joints during prolonged use.

recrystallization duration [1]. Molybdenum becomes brittle after recrystallization, so that the melting temperature range of brazing filler metal is important at its selection. Joining dissimilar materials is a more complex task than joining similar materials. The complexity of joining dissimilar materials is due to a significant difference in thermal coefficients of linear expansion (TCLE) and low oxidation resistance of molybdenum. So, at a temperature exceeding 500°C, the sublimation of MO3 oxide begins on molybdenum surface. It becomes significant at the temperature of 600°C. At further temperature increase above 800°C, this oxide melts, leading to superactive oxidation of molybdenum in a standard atmosphere [1]. In this connection, it is better to conduct molybdenum brazing in a vacuum. Vacuum brazing has several advantages, compared to the traditional methods of brazing in an air atmosphere. In a vacuum furnace atmosphere, a practically complete absence of any substances is achieved. The most important feature of vacuum brazing is the possibility of conducting the process without the application of fluxes. In addition to eliminating the operation of flux washing, it allows producing joints with high strength, corrosion resistance, and vacuum tightness, which is very important for the fabrication of many structures. In brazing dissimilar materials, an important task is a correct selection of the chemical composition of brazing filler metal, its solidus, and liquidus temperature. Vacuum brazing of stainless steels is usually performed in the temperature range of 1000–1200°C. This is the temperature range of quenching of most alloy steels, which allows you to combine brazing with heat treatment of the material and thereby achieve high strength brazed joints. The brazing filler metal should readily wet the materials being brazed, it should be sufficiently strong and, at the same time, ductile, should readily deform, and promote relaxation of stresses, arising

The authors of [2] presented the problems that arise when brazing dissimilar materials. We focused on the difference in the coefficients of thermal expansion of brazing filler metal and base metals, which can lead to the appearance of internal

In various industries, brazing filler metal based on nickel and copper are widely used: VPR1, VPR4, BNi-1, BNi-2, BNi-3, BNi-4, BNi-5, BNi-7, BNi-8, etc. [3–10]. They are used for brazing steels of various grades, heat-resistant nickel alloys, and many other materials. As a rule, these brazing filler metals contain Si and B as depressants (**Table 1**), which provide an acceptable temperature range for melting

The disadvantages of these brazing filler metal include active diffusion of boron,

They relate to brittle intermetallic compounds that adversely affect the perfor-

Brazed joints obtained using BNi-2, BNi-3, and BNi-4 brazing filler metal consist of three phases [9]: a nickel-based solid solution adjacent to the base metal and located in the center of the brazed joint of nickel borides and eutectic consisting of

Boron actively diffuses into stainless steel adjacent to the seam. In the process of brazing at high temperature, it forms intermetallic boride phases along grain boundaries. When these phases are in large numbers, they reduce the fatigue strength and corrosion resistance of steel. The brittle phases determine the brittleness of the joint as a whole, and crack development occurs along these phases. It is possible to increase the strength of brazed joints by forming a structure of a solid solution in the brazed seams, which effectively inhibits the development of cracks. Rabinkin has extensively studied the problems associated with the presence of

the formation of fusible boride, and silicide phases that are released in brazing joints (**Figure 1(a)**) and in the base metal during brazing (**Figure 1(b)**).

**104**

*Chemical composition of brazing filler metals.*

#### **Figure 1.**

*Microstructure of brazed joints of 1Kh18N9 steel, produced using brazing filler metal Ni-7Cr-4.5Si-3Fe-3.2B: fillet section, (a) brazed seam (b) [10].*

borides in brazed joints [7]. He suggests the use of prolonged heat treatment to dissolve borides and silicides, which complicates the process of obtaining joints and does not always allow you to get rid of them [7, 8] completely.

The use of silver brazing filler metals for brazing an alloy of Mo60%-Cu40% provides shear strength in the range of 170–220 MPa [11]. The application of brazing filler metals based on Ni-Cr-Si system containing boron [12] for brazing molybdenum (Mo60%-Cu40%) with stainless steel does allow achieving a high strength—200–230 MPa.

Promising are brazing filler metals with solid solution structure, which are characterized by acceptable melting temperature and high mechanical properties and act as a damper between two dissimilar metals, which promotes relaxation of stresses in brazed joints. The presence of solid solutions characterized alloys based on copper-nickel and copper-manganese systems [13].

The copper-manganese system has a minimum melting point (821°C) at a manganese concentration of 33.7 atom. %. With decreasing temperature, ordering processes occur in the alloys of this system, and the ordered phases Cu5Mn and Cu3Mn precipitate, increasing the strength of the solid solution. Analysis of nickelmanganese binary system is indicative of complete solubility of manganese in nickel in the liquid state at increased temperature but with temperature lowering precipitation of several phases takes place.

Proceeding from analysis of binary state diagrams of Cu-Ni, Cu-Mn, and Mn-Ni systems [13], Cu-Mn-Ni ternary system was selected as the base one [14, 15]. This system has a wide range of solid solutions. So, in the Cu-Mn-Ni alloy, additional alloying with silicon should improve the spreading of the surface of stainless steel, and alloying with iron should reduce erosion of stainless steel by brazing during the brazing process.

This work aims to study the features of the formation of the microstructure of brazed joints, the relationship between the structure, the initial composition of the brazing filler metal, and the strength of dissimilar brazed joints Mo-stainless steel obtained by vacuum brazing using brazing filler metal of the Cu-Mn-Ni system.
