**3.3 Microstructure of brazed joints of molybdenum with stainless steel at the application of brazing filler metal of Cu-Mn-Ni system**

Brazing filler metal of Cu-Mn-Ni system, not containing silicon [18], was used, to prevent cracking in brazed joints. In brazing with this brazing filler metal of dissimilar joints of stainless steel-molybdenum (Tb = 1100°C, τ =3 min), the structure of direct fillet differs from that of the reverse one by its morphology and chemical composition (**Figure 7(a)** and **(b)**).

This is due to the features of the sample assembly before brazing. The cast brazing filler metal is placed at the gap on the plane of the plate to be brazed. During brazing, the brazing filler metal melts, and the liquid phase flows into the capillary gap, wets the solid surface being brazed, and is saturated by elements of base metal (steel). Brazed joint forms as a result of thermal and physicochemical interaction of the brazing filler metal and base metal [19]. The interaction of liquid copper brazing filler metal and solid base metal at brazing temperature results in the dispersion of the latter (stainless steel).

Micro X-ray spectral analysis showed that the direct fillet consists of copper matrix-solid solution, containing 4.31 wt. % iron (**Figure 7(a)** and **(b)**). In the copper matrix of reverse fillet, the weight fraction of iron practically does not change (4.19 wt. %), but a considerable number of dispersed particles based on iron (67.83–67.89 wt. %, **Figure 7 (c)**, **Table 4**) appears. They also contain nickel (8.80–9.31%), chromium (17.81%), and a small amount of the other component elements of the brazing filler metal and base metal (**Figure 7 (c)**, **Table 4**).

These results confirm the identity of the chemical composition of stainless steel and dispersed particles located in the solid solution.

From the side of the reverse fillet, brazed seam forms similarly. Round particles of different size based on iron (44.15%) are located in the brazed seam matrix—in the solid solution, and take up a large area of the braze seam [19]. Their composition includes all the elements of brazed metals and brazing filler metal: chromium, nickel, manganese, copper, silicon, and molybdenum (**Figure 8** (**a**), **Table 5**).

**111**

**Figure 7.**

**Table 4.**

*reverse fillet region (c).*

one is based on iron.

*Chemical composition of the fillet area.*

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

Such stainless steel elements, as iron and chromium, are found in the copper-based solid solution, but in much smaller amounts of 3.27–3.45 and 0.62–0.64%, respectively. Two reaction layers in the form of continuous bands (about 2.4 μm width) are also observed along the brazed seam from molybdenum side. One is based on molybdenum and contains an increased concentration (wt. %) of iron, 22.27%; chromium, 7.29%; and silicon, 0.84% (**Figure 8(a)** and **(b)**; **Table 5**). The second

*The appearance of the brazed sample (a), the microstructure of direct fillet regions (b), and studied areas of* 

 0.22 0.10 5.73 3.30 22.47 11.92 55.00 1.24 0.15 0.00 0.84 5.30 4.19 8.59 80.51 0.44 0.63 0.46 17.81 1.88 67.83 9.31 1.80 0.27 0.67 0.26 17.81 1.93 67.89 8.80 2.25 0.39 0.10 0.00 0.00 0.00 0.00 0.00 0.00 99.90 0.70 0.39 18.44 1.94 68.69 8.92 0.91 0.00

**Si Ti Cr Mn Fe Ni Cu Mo**

**Spectrum No. Chemical elements wt. %**

Obtained data of X-ray spectral analysis show, that similar formation of brazed joints proceeds at the application of brazing filler metals of Cu-Mn-Ni-Fe-1Si and

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

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

#### **Figure 7.**

*Welding - Modern Topics*

**Figure 6.**

However, with temperature lowering, these regions are quickly reduced, and at room temperature, the mutual solubility is practically absent. Some intermetallic

*Microstructure (a) and studied regions, in which chemical composition (b) was determined in the brazed joint* 

**3.3 Microstructure of brazed joints of molybdenum with stainless steel at the** 

Brazing filler metal of Cu-Mn-Ni system, not containing silicon [18], was used, to prevent cracking in brazed joints. In brazing with this brazing filler metal of dissimilar joints of stainless steel-molybdenum (Tb = 1100°C, τ =3 min), the structure of direct fillet differs from that of the reverse one by its morphology and chemical

This is due to the features of the sample assembly before brazing. The cast brazing filler metal is placed at the gap on the plane of the plate to be brazed. During brazing, the brazing filler metal melts, and the liquid phase flows into the capillary gap, wets the solid surface being brazed, and is saturated by elements of base metal (steel). Brazed joint forms as a result of thermal and physicochemical interaction of the brazing filler metal and base metal [19]. The interaction of liquid copper brazing filler metal and solid base metal at brazing temperature results in the dispersion

Micro X-ray spectral analysis showed that the direct fillet consists of copper matrix-solid solution, containing 4.31 wt. % iron (**Figure 7(a)** and **(b)**). In the copper matrix of reverse fillet, the weight fraction of iron practically does not change (4.19 wt. %), but a considerable number of dispersed particles based on iron (67.83–67.89 wt. %, **Figure 7 (c)**, **Table 4**) appears. They also contain nickel (8.80–9.31%), chromium (17.81%), and a small amount of the other component elements of the brazing filler metal and base metal (**Figure 7 (c)**, **Table 4**).

These results confirm the identity of the chemical composition of stainless steel

From the side of the reverse fillet, brazed seam forms similarly. Round particles of different size based on iron (44.15%) are located in the brazed seam matrix—in the solid solution, and take up a large area of the braze seam [19]. Their composition includes all the elements of brazed metals and brazing filler metal: chromium, nickel, manganese, copper, silicon, and molybdenum

**application of brazing filler metal of Cu-Mn-Ni system**

phases form between the considered areas [13].

*of molybdenum-stainless steel (Cu-Mn-Ni-0.2Si brazing filler metal).*

composition (**Figure 7(a)** and **(b)**).

of the latter (stainless steel).

(**Figure 8** (**a**), **Table 5**).

and dispersed particles located in the solid solution.

**110**

*The appearance of the brazed sample (a), the microstructure of direct fillet regions (b), and studied areas of reverse fillet region (c).*


#### **Table 4.**

*Chemical composition of the fillet area.*

Such stainless steel elements, as iron and chromium, are found in the copper-based solid solution, but in much smaller amounts of 3.27–3.45 and 0.62–0.64%, respectively.

Two reaction layers in the form of continuous bands (about 2.4 μm width) are also observed along the brazed seam from molybdenum side. One is based on molybdenum and contains an increased concentration (wt. %) of iron, 22.27%; chromium, 7.29%; and silicon, 0.84% (**Figure 8(a)** and **(b)**; **Table 5**). The second one is based on iron.

Obtained data of X-ray spectral analysis show, that similar formation of brazed joints proceeds at the application of brazing filler metals of Cu-Mn-Ni-Fe-1Si and

### **Figure 8.**

*The microstructure of brazed seam of Mo-SS joint produced at 1100°C brazing temperature.*


#### **Table 5.**

*Composition of brazed seam.*

Cu-Mn-Ni systems. In both the variants, reaction layers form on molybdenum-brazing filler metal interface. In the first case (Cu-Mn-Ni-Fe-1Si), silicon and iron are present in the brazing filler metal. In the second variant (Cu-Mn-Ni), these elements diffuse from the base metal into brazed seam metal, leading to its saturation with component elements of stainless steel and formation of reaction layers on the interface. The difference between the chemical compositions of these layers consists in that silicon concentration is significantly lower in the second variant, compared to the first one.

The results of the conducted studies show that lowering of the temperature of brazing the dissimilar joints to 1084°C allows avoiding base metal dispersion and ensures the formation of tight homogeneous brazed seams (**Figure 9(a)** and **(b)**).

In individual areas of the seams, the brazing filler metal penetrates along the boundaries of stainless steel grains to the depth of 15–20 μm.

It should be noted that the reaction layers at the interface of molybdenum-brazing filler metal form in a similar way (**Figure 9 (c)**, **Table 6**), as in brazing other samples, described above. However, their width decreases: from 2.4 μm to 1.7 for the molybdenum-based layer and to 1.8 μm—for the iron-based layer.

Results of local micro X-ray spectral analysis show that the maximum concentration of silicon in the layer near molybdenum does not exceed 0.78%. Brazed seam matrix, similar to the previous samples, is represented by copper-based solid solution (**Table 6**—Spectrum 4; **Figure 9(c)**) with inclusions of dispersed particles. It contains

**113**

**Table 6.**

**Figure 9.**

*1084°c brazing temperature.*

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

the same concentration of component chemical elements, as does the solid solution at brazing temperature of 1100°C [19]. A small area of the microsection field of view (about 1%) is made up by particles (of 2–10 μm size) enriched in iron (35.45–39.54%)

*The appearance (a), microstructure (b), and studied areas (c) of brazed seam of* Mo*-SS joint, produced at* 

In keeping with state diagrams of binary metal systems, iron and copper have limited solubility at elevated temperature, but with temperature lowering their solubility decreases, and at 20°C it is practically absent. The iron-nickel binary system is characterized by the existence of a continuous series of solid solutions between γ-iron and nickel at elevated temperature. Temperature lowering leads to the formation of several intermediate ordered phases (Fe3Ni, FeNi, FeNi3). Coppernickel system is characterized by the formation of a continuous series of solid solutions [13, 20]. Thus, it can be assumed that individual particles of intermediate

 0.78 7.17 1.09 26.15 9.81 2.65 52.34 0.23 7.40 3.72 36.66 28.62 18.86 4.52 0.26 7.36 4.00 35.45 29.33 18.18 5.42 0.07 1.25 6.17 6.86 12.60 72.67 0.38 — — — — — — 100.00

**Si Cr Mn Fe Ni Cu Mo**

and other component elements of stainless steel (**Table 6**, **Figure 9(c)**).

**Spectrum No. Chemical elements, wt. %**

*Composition of brazed joint at 1084°C brazing temperature.*

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

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

#### **Figure 9.**

*Welding - Modern Topics*

**Figure 8.**

**Table 5.**

*Composition of brazed seam.*

Cu-Mn-Ni systems. In both the variants, reaction layers form on molybdenum-brazing filler metal interface. In the first case (Cu-Mn-Ni-Fe-1Si), silicon and iron are present in the brazing filler metal. In the second variant (Cu-Mn-Ni), these elements diffuse from the base metal into brazed seam metal, leading to its saturation with component elements of stainless steel and formation of reaction layers on the interface. The difference between the chemical compositions of these layers consists in that silicon concentration is significantly lower in the second variant, compared to the first one. The results of the conducted studies show that lowering of the temperature of brazing the dissimilar joints to 1084°C allows avoiding base metal dispersion and ensures the formation of tight homogeneous brazed seams (**Figure 9(a)** and **(b)**). In individual areas of the seams, the brazing filler metal penetrates along the

 0.84 0.00 7.29 0.99 22.27 4.11 0.71 63.78 0.29 0.09 6.77 3.38 24.82 8.25 48.08 8.32 0.46 0.10 11.25 3.55 44.15 16.77 21.57 2.15 0.00 0.08 0.64 5.29 3.27 3.79 86.66 0.27 0.08 0.00 0.62 5.14 3.45 3.61 86.86 0.24 0.84 0.68 18.33 1.95 68.13 8.59 1.14 0.34 0.29 0.16 8.34 2.98 30.10 7.84 49.10 1.19

**Si Ti Cr Mn Fe Ni Cu Mo**

*The microstructure of brazed seam of Mo-SS joint produced at 1100°C brazing temperature.*

**Spectrum No. Chemical elements, wt. %**

It should be noted that the reaction layers at the interface of molybdenum-brazing filler metal form in a similar way (**Figure 9 (c)**, **Table 6**), as in brazing other samples, described above. However, their width decreases: from 2.4 μm to 1.7 for the

Results of local micro X-ray spectral analysis show that the maximum concentration of silicon in the layer near molybdenum does not exceed 0.78%. Brazed seam matrix, similar to the previous samples, is represented by copper-based solid solution (**Table 6**—Spectrum 4; **Figure 9(c)**) with inclusions of dispersed particles. It contains

boundaries of stainless steel grains to the depth of 15–20 μm.

molybdenum-based layer and to 1.8 μm—for the iron-based layer.

**112**

*The appearance (a), microstructure (b), and studied areas (c) of brazed seam of* Mo*-SS joint, produced at 1084°c brazing temperature.*

the same concentration of component chemical elements, as does the solid solution at brazing temperature of 1100°C [19]. A small area of the microsection field of view (about 1%) is made up by particles (of 2–10 μm size) enriched in iron (35.45–39.54%) and other component elements of stainless steel (**Table 6**, **Figure 9(c)**).

In keeping with state diagrams of binary metal systems, iron and copper have limited solubility at elevated temperature, but with temperature lowering their solubility decreases, and at 20°C it is practically absent. The iron-nickel binary system is characterized by the existence of a continuous series of solid solutions between γ-iron and nickel at elevated temperature. Temperature lowering leads to the formation of several intermediate ordered phases (Fe3Ni, FeNi, FeNi3). Coppernickel system is characterized by the formation of a continuous series of solid solutions [13, 20]. Thus, it can be assumed that individual particles of intermediate


#### **Table 6.**

*Composition of brazed joint at 1084°C brazing temperature.*

phases form against the background of copper-based solid solution at brazed seam solidification. At the same time, it should be noted that at brazing, the brazed seam metal solidification proceeds under nonequilibrium conditions (in the capillary gap) in the presence of a concentration gradient on the interface, leading to saturation of brazed seam metal with the component elements of the brazed metal [21]. Diffusion processes in the brazed seam result in the formation of particles based on the iron-nickel-copper system. In the brazed seam—copper-based solid solution, iron concentration is significantly lower and is equal to 6–7%.
