Development and application of the hottest continu

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Development status and application of continuous gas shielded brazing furnace

1. Preface

in the automotive industry, continuous gas shielded brazing is a very important and widely used method, especially in the welding of automotive radiators, automotive three filters and other parts. Since the 1980s, China has successively introduced various types of gas shielded brazing furnaces from abroad, which has promoted the improvement of product quality and production efficiency. Since the 1990s, China's scientific research institutions have also successively developed nitrogen protection brazing furnaces for aluminum radiators and nitrogen based controllable atmosphere brazing furnaces for other steel and copper parts, which have promoted the development of China's automotive industry and saved a lot of foreign exchange for the country

2. Development status of domestic gas shielded continuous brazing furnace

at present, most of the gas shielded brazing furnaces developed in China are imitation of imported products. Brazing furnaces used for aluminum radiators and other aluminum products are almost the same as imported products, which is the result of noclock process constraints, but some manufacturers require the development unit to simplify in order to reduce investment. For example, a manufacturer in Hubei requires that the brazing flux be sprayed, the air is blown off, and the workpiece is dried and placed outside the furnace for treatment. After treatment, directly enter the brazing furnace for welding. Even so, it is difficult for most users in China to bear the investment in this equipment. As a result, a company in Northeast China developed a single chamber discontinuous brazing furnace to meet this requirement, and its investment was greatly reduced. But the corresponding reduction is also the stability of efficiency and quality. It can be said that there is no way. Brazed aluminum products have high requirements for temperature control accuracy and uniformity of heating zone. Developing this brazing furnace is still very difficult for domestic research units. For this reason, a domestic research unit in Northeast China has adopted the way of cooperation with a Japanese company, but because its technology and key components are from Japan, the cost reduction is not much, and the price advantage is not large. Only on the basis of absorbing foreign advanced technology, can we develop a gas shielded brazing furnace whose performance meets the welding requirements and whose price can be accepted by users. At present, a research institute in Tianjin has adopted this method

compared with imported products, brazing furnaces for steel parts have the following differences:

(1) the protective atmosphere of imported brazing furnaces is mostly liquefied petroleum gas. At present, most of our country still uses ammonia decomposition gas. In fact, in western developed countries, there are basically two kinds of protective atmosphere: a liquefied petroleum gas; B nitrogen based atmosphere. Among them, nitrogen based atmosphere is the most promising. Now the energy crisis threatens not only our country, but also the whole world. We know that the air contains 78% nitrogen, which is inexhaustible. At present, the majority of our country still adopts ammonia decomposition gas, mainly for two reasons: first, the preparation of ammonia decomposition gas is simple; Second, the domestic publicity of nitrogen based atmosphere is not enough, and the majority of users do not know much about it, which affects the promotion

(2) domestic protective atmosphere generating devices are mostly placed outside the furnace. Protective gas is generated outside the furnace and led to the furnace for protection. However, most of the protective atmosphere generating devices of the brazing furnace imported from China are placed in the furnace, which has many advantages, such as saving area, saving energy consumption, simplifying operation, thus saving investment, and so on. This method should be vigorously promoted

(3) heating elements: when visiting and dissecting the brazing furnace imported from abroad, it is found that most of its heating elements adopt two kinds, one is resistance wire, the other is tubular heater. SiC rods are mostly used in China. The author of this paper has been engaged in the research and development of brazing furnace for many years. At present, he is trying to boldly improve the heating element mode in order to compare its advantages and disadvantages

(4) furnace structure: cost brazing furnace imported from abroad, most of which are built with oil heat-resistant and thermal insulation materials, and belong to horse fee structure. In our country, there is almost no such furnace using the horse fee structure made of heat-resistant steel

in addition, Chinese research institutions should make great efforts in the transmission structure. At present, both belt conveyor and boat conveyor have obvious shortcomings. According to the information, a kind of ceramic belt has been developed abroad, which does not change the transmission mode and is still convenient for workers to operate; At the same time, there are no worries about the thermal creep of metal strips and crystal growth. If this kind of belt is successfully developed, the operation cost of brazing furnace will be greatly reduced

in a word, there is still a certain gap between China and foreign countries in terms of gas shielded continuous aluminum brazing furnace, while there is little gap between China and foreign countries in terms of brazing furnace for steel, stainless steel, copper and other materials, and the research intensity is also increasing. Before the late 1980s, there was only one brazing furnace in the country, and so far, according to the author's information, there are 11 in the country. This is a good news for China's auto parts manufacturing industry

3. Introduction to continuous gas shielded brazing

3.1 protective atmosphere

at present, there are four kinds of protective atmospheres for brazing furnaces used in the automotive industry, namely nitrogen decomposition atmosphere, lpg combustion atmosphere, high-purity nitrogen and nitrogen based controllable atmosphere

3.1.1 ammonia decomposition atmosphere

the preparation of ammonia decomposition atmosphere takes liquid ammonia as raw material, and the following reactions occur through the cracking furnace under the action of catalyst:

2nh3 oxidant - → 3h2 ↑ +n2 ↑

the atmosphere generated after cracking contains 75% hydrogen and 25% nitrogen. Because the atmosphere contains 75% nitrogen, its reducibility is strong, and it can be widely used for copper brazing, silver brazing and nickel based brazing of steel parts, such as the welding of automobile three filters, namely air filter, gasoline filter and oil filter, and the welding of automobile carburetor, distributor and vacuum booster. This atmosphere has two major disadvantages:

(1) due to the high hydrogen content, it is dangerous

(2) high energy consumption

liquid ammonia is cracked into a protective atmosphere, which must be heated to a high temperature above 900 ℃ in the cracking furnace. The power consumption is quite large, and the consumption of liquid ammonia raw materials is also large

3.1.2 lpg combustion atmosphere

the preparation principle of this atmosphere is as follows:

the first step C3H8 + 5o22 + 19n2 → 3co2 + 4H2O + 19n2 + Q1

the second step C3H8 + 3co2 + → 6co + 4h-q22

C3H8 + 3ho2 + → 3Co + 7h-q2

the two steps are combined into 2 C3H8 + 3co2 + 11.4 N2 → 6co + 8h2 + 11.4 N2.It can be seen from +q

that on the right side of the formula, except N2 is neutral gas, the rest of CO and H2 are reducing gases, Therefore, the gas after propane combustion has a protective brazing effect

this protective atmosphere is used for copper brazing and silver brazing of steel parts. Such atmosphere can be used for the welding of automobile three filters, carburetors and vacuum boosters

most brazing furnaces burn outside the furnace to produce the above protective atmosphere, which is introduced into the furnace to protect brazing. However, some brazing furnaces imported from China also use gas combustion generators placed in the furnace. When the furnace temperature rises to about 750 ℃, the above reaction occurs. This method is characterized by energy saving, no separate heating power supply, and floor space saving. This type of brazing furnace is mostly imported, and few are developed in China

3.1.3 nitrogen based controllable atmosphere

for this reason, the nitrogen based controllable atmosphere refers to that more than 90% of the atmosphere is nitrogen and less than 10% is active gas, and the proportion of the two gases is adjustable, in which the purity of nitrogen entering the furnace should be more than 5 9

bottled nitrogen or small intermediate standard. The nitrogen purity of clamped nitrogen generator installed according to the vehicle state or similar function is generally 99.5%. In order to make its purity reach 99.9995%, it must be purified, The purification principle is as follows:

ch3oh → CO is specially available for all experimental machines purchased on May 15 (July 15) during this period of time, including preferential delivery and rebate +2h2 ↑

2co+o2 → 2co2 ↑

2h2+o2 → 2h2o

the oxygen reaction in CO, H2 and nitrogen produced by methanol cracking is used to remove the oxygen in nitrogen, so that the purity of nitrogen reaches 99.9995%, The excess CO and H2 (1% - 10% adjustable) enter the furnace as an active atmosphere to protect brazing. Steel parts in auto parts can adopt the above protective atmosphere. Characteristics of the atmosphere:

(1) safety. The atmosphere contains more than 90% nitrogen, and the reducing gas accounts for only 10%. Its safety is obvious

(2) energy saving and convenience. Generally, large factories have nitrogen pipelines, which can be used as long as the nitrogen is connected to the nitrogen purification device on the brazing furnace. If there is no nitrogen pipeline, small nitrogen generator shall be used. The air enters the molecular sieve after being compressed by the gas, and nitrogen can be obtained after being adsorbed by the molecular sieve. This nitrogen production method, namely PSA method, has simple equipment, low cost and good promotion prospect

3.1.4 high purity nitrogen

bottled nitrogen or nitrogen produced by oxygen generator, as described above, generally has a purity of 99.5%, but aluminum products such as automobile aluminum radiator, automobile air conditioning evaporator, condenser, water tank and so on must have a purity of 99.9995% nitrogen to meet the welding requirements, so nitrogen must also be purified. Its treatment method is different from that of nitrogen based controllable atmosphere. It adopts hydrogen free purification method. The details are not described here. This atmosphere is only used for the welding of automotive aluminum products

3.2 types of heating elements

3.2.1 SiC rod

SiC rod is mostly used in the brazing furnace used for the welding of steel parts. Because oxygen free copper brazing is mostly used for the welding of such parts, the brazing temperature is high, generally around 1120~1130 ℃, and the maximum service temperature of SiC rod can reach 1200 ℃. SiC rod heating element is used to protect the atmosphere. If nitrogen is used, muffle is best used in the furnace to avoid the contact between gas and SiC. Because nitrogen and SiC react at high temperature, their service life is reduced

3.2.2 Ni Cr wire

Ni Cr wire is widely used in brazing furnaces imported from abroad, but the protective atmosphere is generally propane combustion atmosphere, and most of the protective atmosphere generating devices are built-in. Because the combustion reaction is an exothermic reaction, it can save energy and reduce the burden of Ni Cr wire, and it can also be used for copper brazing of steel parts

3.2.3 Fe Cr Al wire

this heating element has been successfully developed in China in recent years. It has high service temperature and low cost, and has a good application prospect. However, like SiC rods, when designing the furnace, if the atmosphere is nitrogen, the furnace should be muffle structure, so as to avoid the contact between furnace wire and nitrogen and reduce its service life

3.3 furnace heating area structure

there are two types of furnace heating area structure:

(1) muffle sleeve is built in the furnace made of heat-resistant and thermal insulation materials, and the muffle sleeve is made of heat-resistant alloy steel by molding and welding. This structure saves protective gas due to its good sealing performance. However, there is also a disadvantage. When brazing steel parts, the brazing temperature is about 1120~1130 ℃, and the muff sleeve works at high temperature for a long time, resulting in thermal creep, which eventually leads to burnout. Take the welding of automobile three filters as an example. According to the actual situation of factory use, two 2600mm filters need to be replaced within a year × 240 mm × 190 mm muffle. The cost is 26000 yuan. The annual cost for muffle is 52000 yuan, and the operating cost is too high. The factory has a headache about this. Of course, the welding of aluminum products will not be troublesome due to the low welding temperature

(2) the furnace is completely made of heat-resistant and thermal insulation materials without muffle. The advantage of this structure is that without muffle, its furnace has a long service life. According to factory experience, it can last for 5 years or more

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