**4. Mechanical properties of dissimilar brazed joints of molybdenumstainless steel**

Metallographic and micro X-ray spectral investigations of brazed joints were followed by mechanical shear testing (at room temperature) of overlap flat samples of dissimilar joints of molybdenum-stainless steel (**Figure 10**).

Brazed overlap samples were tested by axial tension. To ensure the conditions of combining the load axis and the plane of the brazed seam to the sections of the samples that are placed in the grips of the testing machine, stainless steel plates were welded. They help to reduce the eccentricity of the sample during mechanical tests.

The mechanical properties of brazed joints depend on the microstructure of the brazed joints [6, 16]. The obtained results of mechanical tests of flat overlap samples of dissimilar joints of molybdenum-stainless steel are in good agreement with previous structural studies. Brazing filler metal of the copper-manganesenickel system containing silicon contributes to the formation of phases enriched in silicon (silicides). They stand out at the interface between the base metalbrazing filler metal [22] and reduce shear strength. The use of brazing filler metal with a solid solution structure without silicon allows obtaining higher values of shear strength.

Performed testing showed that application of brazing filler metal based on Cu-Mn-Ni-Fe-Si system, containing up to 1% silicon, cannot ensure the shear strength above 110 MPa (**Figure 11**).

Lowering of silicon concentration to 0.2% in brazing filler metal №2 ensured an increase of shear strength.

At application of brazing filler metals based on Cu-Mn-Ni-Fe-Si and Cu-Mn-Ni-Si systems, samples fail in the brazing seam. Samples produced using brazing filler metal based on Cu-Mn-Ni system fail in the brazing seam, near-seam zone (mixed nature of fracture), and the base metal—molybdenum (**Figure 12**).

In case of fracture in the seam, the shear strength was on the level of 200–210 (average value of 205 MPa). At fracture through molybdenum, the maximum shear strength was 300 MPa. In some cases, a mixed nature of fracture was observed partially in the seam and partially in the base metal (**Figure 12(c)**).

**115**

with a solid solution structure).

vacuum density gave a positive result.

Cu-Mn-Ni-Fe-Si [23] were manufactured (**Figure 13**).

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

*Shear strength of brazed dissimilar overlap joints of molybdenum-stainless steel.*

When designing telescopic tubular joints from dissimilar materials, molybdenumstainless steel, it is necessary to take into account the difference in thermal expansion coefficients. The inner tube must be made of molybdenum (with a low coefficient of thermal expansion), and the outer one is made of stainless steel. This design provides a high-quality formation of brazed joints and seams. When brazing some lap joints, the main indicator of quality is tightness (vacuum density), and the strength of the joints is ensured by the large length of the overlap (when using brazing filler metal

*Brazed samples after mechanical testing: a fracture in the brazed seam (a), near-seam zone (b), and base* 

The brazed molybdenum-stainless steel telescopic tubular joints with dense seam by high temperature vacuum brazing using system brazing filler metal

Checking the brazed telescopic tubular joint Mo-SS for vacuum density gave a positive result. Testing the brazed telescopic tubular dissimilar joints Mo-SS for

In some cases, nonstandard tubular joints of dissimilar metals Mo-stainless steel are used for mechanical testing. In this case, some parameters influence the stability

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

**Figure 11.**

**Figure 12.**

*metal—molybdenum (c).*

**Figure 10.** *Appearance of brazed samples of molybdenum-stainless steel.*

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

**Figure 11.**

*Welding - Modern Topics*

**stainless steel**

shear strength.

strength above 110 MPa (**Figure 11**).

increase of shear strength.

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,

**4. Mechanical properties of dissimilar brazed joints of molybdenum-**

Metallographic and micro X-ray spectral investigations of brazed joints were followed by mechanical shear testing (at room temperature) of overlap flat samples

Brazed overlap samples were tested by axial tension. To ensure the conditions of combining the load axis and the plane of the brazed seam to the sections of the samples that are placed in the grips of the testing machine, stainless steel plates were welded. They help to reduce the eccentricity of the sample during mechanical tests. The mechanical properties of brazed joints depend on the microstructure of the brazed joints [6, 16]. The obtained results of mechanical tests of flat overlap samples of dissimilar joints of molybdenum-stainless steel are in good agreement with previous structural studies. Brazing filler metal of the copper-manganesenickel system containing silicon contributes to the formation of phases enriched in silicon (silicides). They stand out at the interface between the base metalbrazing filler metal [22] and reduce shear strength. The use of brazing filler metal with a solid solution structure without silicon allows obtaining higher values of

Performed testing showed that application of brazing filler metal based on Cu-Mn-Ni-Fe-Si system, containing up to 1% silicon, cannot ensure the shear

Lowering of silicon concentration to 0.2% in brazing filler metal №2 ensured an

At application of brazing filler metals based on Cu-Mn-Ni-Fe-Si and Cu-Mn-Ni-Si systems, samples fail in the brazing seam. Samples produced using brazing filler metal based on Cu-Mn-Ni system fail in the brazing seam, near-seam zone (mixed nature of fracture), and the base metal—molybdenum (**Figure 12**).

In case of fracture in the seam, the shear strength was on the level of 200–210 (average value of 205 MPa). At fracture through molybdenum, the maximum shear strength was 300 MPa. In some cases, a mixed nature of fracture was observed—

partially in the seam and partially in the base metal (**Figure 12(c)**).

iron concentration is significantly lower and is equal to 6–7%.

of dissimilar joints of molybdenum-stainless steel (**Figure 10**).

**114**

**Figure 10.**

*Appearance of brazed samples of molybdenum-stainless steel.*

*Shear strength of brazed dissimilar overlap joints of molybdenum-stainless steel.*

#### **Figure 12.**

*Brazed samples after mechanical testing: a fracture in the brazed seam (a), near-seam zone (b), and base metal—molybdenum (c).*

When designing telescopic tubular joints from dissimilar materials, molybdenumstainless steel, it is necessary to take into account the difference in thermal expansion coefficients. The inner tube must be made of molybdenum (with a low coefficient of thermal expansion), and the outer one is made of stainless steel. This design provides a high-quality formation of brazed joints and seams. When brazing some lap joints, the main indicator of quality is tightness (vacuum density), and the strength of the joints is ensured by the large length of the overlap (when using brazing filler metal with a solid solution structure).

The brazed molybdenum-stainless steel telescopic tubular joints with dense seam by high temperature vacuum brazing using system brazing filler metal Cu-Mn-Ni-Fe-Si [23] were manufactured (**Figure 13**).

Checking the brazed telescopic tubular joint Mo-SS for vacuum density gave a positive result. Testing the brazed telescopic tubular dissimilar joints Mo-SS for vacuum density gave a positive result.

In some cases, nonstandard tubular joints of dissimilar metals Mo-stainless steel are used for mechanical testing. In this case, some parameters influence the stability

**Figure 13.** *The brazed telescopic tubular joint Mo-SS.*

of the results obtained: assembly accuracy, the size of the gaps, test equipment, and many others. To improve the quality of brazed lapped tubular joints, a threaded fit is used [24].
