Cause analysis of molybdenum screen cracking in high temperature vacuum brazing furnace

In high temperature vacuum brazing furnace, the metal honeycomb carrier of automotive three-way catalytic purifier was brazed in vacuum. The molybdenum screen of furnace liner was deformed and cracked in a short time. The morphology and chemical composition of cracked molybdenum screen in high temperature vacuum brazing furnace were compared before and after use. The results show that the reason of accelerated deformation and cracking of molybdenum screen is the deposition of nickel based filler metal on the surface of molybdenum screen. The deposit changes the microstructure and physical properties of molybdenum screen. Based on this, the design points of molybdenum screen for high temperature vacuum brazing furnace are put forward.

 

High temperature vacuum brazing furnace is widely used to braze structural parts which are difficult to connect by common methods, especially for titanium alloy, stainless steel and other metal materials. In the process of welding all kinds of structural parts in high temperature vacuum brazing furnace, the deposition of solder will lead to the deformation and cracking of molybdenum screen in the furnace. Under normal use, the molybdenum screen of high-temperature vacuum brazing furnace will deform and crack due to alternating cold and hot impact deformation and microstructure change of molybdenum screen itself. The time is about 5 years.

 

In the process of using high temperature vacuum brazing furnace to braze the metal honeycomb carrier of three-way catalytic purifier in automobile industry, a large number of nickel based brazing filler metals are volatilized, some of them are pumped out by the vacuum unit of vacuum brazing furnace, and a considerable part of them are deposited on the heat insulation layer of the furnace, mainly the first layer of molybdenum screen facing high temperature is obviously deposited. A large amount of nickel based solder deposits make the molybdenum screen of high temperature vacuum brazing furnace appear deformation, cracking and other failure forms, and the time is short. Generally, the molybdenum screen needs to be replaced in about one year.

In the vacuum brazing process of metal honeycomb carrier of three-way catalytic purifier, the molybdenum screen of the furnace liner will deform and crack in a short time. The morphology comparison and chemical composition analysis of the cracked high-temperature vacuum brazing furnace molybdenum screen were carried out before and after use. The results show that the reason of accelerated deformation and cracking of molybdenum screen is the deposition of nickel based filler metal on the surface of molybdenum screen. The deposit changes the microstructure and physical properties of molybdenum screen.

 

Based on this, the design points of molybdenum screen for high temperature vacuum brazing furnace are put forward. In the high-temperature vacuum brazing furnace, once the molybdenum screen used for thermal insulation appears deformation, cracking and other failure forms, it will cause the furnace lining of the high-temperature vacuum brazing furnace to leak heat and heat preservation failure, and the final result may be that the temperature index of the high-temperature vacuum brazing furnace can not reach; more seriously, the warped molybdenum screen will be short-circuit with the heating belt, resulting in heating live short-circuit and damage the vacuum furnace 。 Figure 1 shows the cracking of molybdenum screen in high temperature vacuum brazing furnace used for brazing metal honeycomb carrier of three-way catalytic purifier: as shown in the figure, the heating band is basically not deformed, and the molybdenum screen has warping and cracking.

Cause analysis of molybdenum screen cracking in high temperature vacuum brazing furnace

Fig. 1 photo of molybdenum screen cracking

 

1. Test method

Through visual observation on the appearance and crack morphology of cracked molybdenum screen in high temperature vacuum brazing furnace, the composition of molybdenum screen with solder deposit after use was tested, analyzed and compared, and on this basis, the causes of molybdenum screen cracking were analyzed. The components of the samples were analyzed by the universal multi matrix spark direct reading spectrometer of Germany OBLF QSN750.

 

2. Test results and analysis

 

2.1. Sample appearance inspection

The two molybdenum screen samples submitted for inspection are taken from the same molybdenum screen block which has been used for two years. One of them is without sediment (the black box indicated by the arrow in Figure 2 is the interception position), and the surface of the other piece is covered with solder deposit. Figure 2 shows a partial picture of the front molybdenum screen. Carefully check the cracked molybdenum screen before cutting, the solder deposit is only covered on the side facing the furnace temperature zone, and there is no solder deposit at the overlap of molybdenum screen (the arrow indicates the black box). Using vernier caliper, the thickness of molybdenum screen coated with solder deposit is 0.1 mm higher than that without solder deposit. Through visual observation of the cracked molybdenum screen, it can be seen that the solder deposits are layered and the surface roughness of the two parts is obviously different.

Cause analysis of molybdenum screen cracking in high temperature vacuum brazing furnace

2.2 sample composition analysis

The two molybdenum screen samples submitted for inspection were taken from the same molybdenum screen block which has been used for two years. There was no solder coating on sample 1 and solder coating on sample 2. The chemical composition analysis results of samples are shown in Table 1. From table 1 It can be seen that the change of impurity content in sample composition: in sample 1 (no solder coated molybdenum sheet), the molybdenum content is significantly higher, the impurity content is less, and some metal impurity components such as manganese and nickel content are 0; in sample 2 (molybdenum sheet with filler metal coating), the mass percentage of molybdenum content is reduced, the impurity content is doubled, and the metal impurities such as manganese and nickel content that are not contained in the original molybdenum screen are also found The increase is obvious. According to the analysis of impurity sources, liquid nickel based brazing filler metal which is widely used in vacuum brazing process of metal honeycomb carrier of three-way catalytic purifier is the main source.

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