1.Heat treatment method

The heat treatment of copper alloys is mainly heating and annealing for different purposes. Only some alloys, such as beryllium bronze, can be quenched and tempered. There are three kinds of annealing for different purposes: softening annealing, finished annealing and billet annealing.

Softening annealing: that is, between two cold rolling for the purpose of softening recrystallization annealing, also known as intermediate annealing. After cold rolling, the alloy produces the fiber structure and the processing hardening takes place. After heating the alloy above the recrystallization temperature, the alloy is cooled slowly after holding the heat for a certain time, and the alloy is recrystallized into the refined grain structure, obtain good plasticity and low deformation resistance to continue cold rolling. This annealing is the main heat treatment in the rolling of copper alloys. Final annealing: that is, after cold rolling to the size of the finished product, the final annealing of different states and properties can be obtained by controlling the annealing temperature and holding time. The finished product annealing has control state and performance requirements, such as obtaining soft (m) state, semi-hard (y 2) products and controlling grain structure to get better deep-drawing property products. In addition to annealing above recrystallization temperature, there is also annealing at low temperature at recrystallization temperature. Billet annealing: a method of removing the hardening caused by incomplete thermal deformation during hot rolling by recrystallization annealing of the hot rolled billet, and a method of heat treatment for homogenizing the microstructure by annealing. Quenching and tempering (aging) : that is to say, for some solid solution alloys with solubility and precipitation and eutectoid transformation, when the temperature is higher than the transformation point, the strengthened phase is fully dissolved by heat preservation, and then the supersaturated solid solution is formed in rapid cooling after the homogeneous solid solution.

2. annealing

The annealing process is determined according to the properties of the alloy, the degree of work hardening and the technical conditions of the product. The main technological parameters of annealing are annealing temperature, holding time, heating speed and cooling mode. The determination of the annealing process system should meet the following three requirements:

 (1) ensuring the uniform heating of the annealed material to ensure the uniform structure and properties of the material; 

(2) ensuring that the annealed material is not oxidized and has a bright surface;

 (3) saving energy and reducing consumption, increase yield. Therefore, the copper annealing process system and the equipment used should be able to meet the above conditions. If the furnace design is reasonable, heating speed is fast, there is a protective atmosphere, accurate control, easy adjustment and so on. The annealing process of some commonly used copper alloys is shown in Table 1. The choice of annealing temperature: In addition to the properties and degree of hardening of the alloy, the aim of annealing should be taken into account, for example, the upper limit of annealing temperature should be taken for intermediate annealing and the annealing time should be shortened appropriately, and the annealing of finished products should be focused on ensuring uniform quality and performance of products, the lower limit of annealing temperature should be taken and the fluctuation of annealing temperature should be strictly controlled. The annealing temperature of thick gauge should be higher than that of thin gauge The annealing temperature of the sheet is higher than that of the Strip. The heating rate of annealing should be determined according to the properties of Alloy, charging capacity, furnace structure, heat transfer mode, metal temperature, temperature difference in furnace and product requirements. Because the rapid heating can improve productivity, fine grain, less oxidation, the intermediate annealing of semi-finished products, most of the use of rapid heating;. Holding time: In order to improve the heating speed, the temperature of the heating section is relatively high. When the heating reaches a certain temperature, the temperature of the furnace is close to the material temperature. Heat preservation time is to ensure that the annealing material uniform heat through prevail. COOLING MODE: Most of the finished products are air-cooled, sometimes the intermediate annealing can be water-cooled, which has a serious oxidation of the alloy, can be in the rapid cooling of the oxide scale cracking off. However, quenching is not allowed for alloys with quenching effect.

3. The process parameters of solid solution-aging (quenching)-aging (tempering) mainly depend on heating temperature, heating speed, holding time, cooling speed, transfer time interval of heating medium and water cooling. The upper limit of quenching temperature should be lower than the eutectic temperature, and the lower limit should be higher than the solid solubility line temperature. In order to maintain a good quenching effect, the shorter the transfer time, the better, the water temperature is generally not higher than 25 °C. Cooling rate is mainly considered two factors, one is too slow to strengthen the particles will be precipitated, too fast will produce residual stress or crack. Tempering (aging) is mainly to ensure that the enhanced particle precipitation uniformity and distribution, etc. . Table 2 shows the process schedule of solid solution (quenching)-aging (tempering) for some copper alloys.

4. According to the characteristics of the interaction between medium and material surface in heat treatment furnace, heat treatment is divided into ordinary heat treatment (i. e. air as medium) , protective atmosphere heat treatment (i. e. Bright Annealing) and vacuum heat treatment. 

(1) flame furnace atmosphere this is an old-fashioned way of general heat treatment, is to use such as gas for fuel combustion heating. Oxidizing atmosphere, that is, the fuel in the case of excess air combustion, so that there is more oxygen furnace. Reducing atmosphere, the combustion process in the furnace air is insufficient, due to inadequate combustion of fuel and more CO and CO2. Neutral atmosphere is the atmosphere controlled in the furnace between oxidation and reduction. Micro-oxidation atmosphere, is contained in the furnace a small amount of oxygen, there is a certain oxidation. The atmosphere in the furnace is controlled according to the properties and technical requirements of the alloy. Copper and copper alloys generally do not use oxidizing atmosphere, because not only destroy the surface quality, oxidation damage, and alloy low melting point components, such as Sn, PB, Sn, Zn, CD, etc. . Therefore, most of the reduction or neutral furnace atmosphere. For Heat treatment easy to absorb hydrogen to produce "hydrogen embrittlement" , sulphurized alloy usually used micro-oxidation atmosphere, such as nickel-containing Alloy, pure copper and so on. The flame heating furnace will gradually be replaced by electric heating, protective gas continuous heating annealing furnace.

(2) protective atmospheres due to advances in sealing and protective gas manufacturing technologies, most of the heating is now in protective atmospheres. The composition of protective gas should not react with alloy during heating, and should not corrode furnace components, thermocouples and resistors. The composition is stable, the manufacture is simple and the supply is convenient. The protective atmosphere is heated mostly by electric heating. After 1990s, the continuous annealing furnace adopted electric induction heating, which used protective atmosphere and forced circulation air type on-line annealing, simplified working procedure, improved surface quality and production efficiency. In the past, the protective atmosphere used was nitrogen, carbon dioxide, purified gas or water vapor after gas combustion. Due to the poor sealing of the furnace, low concentration of protective gas and high cost, no satisfactory results were obtained. Now the protective gas component is mainly nitrogen hydrogenation. Hydrogenation protection is especially effective for brass, depending on the nitrogen-hydrogen ratio of the alloy. Since the thermal conductivity of hydrogen is 1.7 times that of nitrogen, the heat transfer rate can be greatly increased. The heating time can be shortened after hydrogenation, especially under the action of strong wind, the convection heat transfer effect is better, the annealing material can be heated uniformly, and the microstructure and properties of heat treated copper can be ensured. Under the action of the strong wind, the volatiles of the lubricant are carried away and the surface quality is improved. Several methods of making protective gas are as follows: (1) ammonia decomposition gas: liquid nitrogen is vaporized into a cracker filled with nickel catalyst, and is cracked to produce ammonia and hydrogen at 750 ~ 850 °C. This protective gas is suitable for the annealing of brass. Lkg Ammonia is calculated to produce 1.97 M3 of hydrogen and 0.66 M3 of nitrogen. 2 AMMONIA DECOMPOSITION GAS COMBUSTION PURIFICATION: If you need to reduce the proportion of ammonia, can burn part of the hydrogen, in the combustion process to increase nitrogen. Ammonia decomposition gas added air separation nitrogen (or liquid nitrogen vaporization) , after purification into the furnace, according to the needs of different proportions of hydrogen and nitrogen protection gas. 4 when used in small amounts, bottled nitrogen and bottled hydrogen can be used as a protective gas. 5 Gas after burning gas, generally 96% of nitrogen and less than 4% of carbon monoxide and hydrogen, to be used after purification. 6 pure nitrogen, the general purity of nitrogen can reach 99.9% by air separation. An oil-free compressor was used in the new process of air separation of nitrogen from coke molecular sieve by pressure swing adsorption. This is the most suitable for the annealing of pure copper and tin phosphorus bronze with high oxygen content. (3) vacuum heat treatment in annealing heating, the charging space will be drawn into a vacuum, annealing material does not contact with any medium of an annealing method. There are two forms of vacuum annealing: external heating and internal heating. External Heating Element is placed outside the furnace, electric heating, external heating vacuum furnace structure is simple, easy to manufacture, loading and discharging convenient. But the temperature rises slowly, the heat loss is big, the furnace liner service life is low. Internal heating is heating elements in the furnace, heat treatment materials are also placed in the furnace, often using tungsten, molybdenum, graphite and other materials as heating elements. The utility model is characterized in that the heating temperature is fast, the heating temperature is high, and the furnace liner has long service life. But the furnace structure is complex, large investment, slow cooling. Pure copper was annealed in vacuum with (10-1 ~ 10-2)133.3 PA, most copper alloys use a vacuum of (10-2 ~ 10-3)133.3 PA. In vacuum annealing, when the material is discharged from the furnace, the temperature should be below 100 °C to destroy the vacuum and prevent the oxidation of the annealed material. In recent years, vacuum annealing is not recommended for the heat treatment of copper alloys, because vacuum is a poor conductor of heat. In addition to the low thermal efficiency, it is difficult to obtain heat-treated products with uniform properties. Therefore, it is gradually replaced by protective atmosphere and air cushion continuous annealing.

5. Heat treatment furnace heat treatment equipment is mainly heat treatment furnace. The following conditions should be considered when choosing heat treatment furnace: to meet the requirements of heat treatment process, to ensure quality and performance; to select suitable heat source to meet the needs of production; High degree of automation, high production efficiency, good working conditions, easy operation. (1) common annealing furnaces for heat treatment of copper and copper alloys: Box Furnace, well furnace, walking-beam furnace, car bottom furnace, roller bottom furnace, chain water seal furnace, single hearth furnace, double hearth furnace, cover furnace, etc. . According to the mode of production there are: Single Batch annealing furnace, air cushion continuous annealing furnace. According to the furnace atmosphere is divided into non-protective atmosphere annealing furnace, protective atmosphere annealing furnace and vacuum annealing furnace, according to the heat source is divided into: coal furnace, gas furnace, heavy oil furnace, resistance furnace and induction furnace. Quenching furnace has vertical, horizontal and well-type Three. (2) at present, the main developing trend of heat treatment furnace of common copper alloy is to improve the design, find new technology, improve heat utilization efficiency, adopt low or high temperature rapid annealing to reduce oxidation and dezincification; The protective gas annealing and forced circulation ventilation are used to make the products heated rapidly and the temperature uniform, to improve the quality of the annealed products, to enhance the sealing effect, to simplify the working procedure and to improve the integration and the continuous level. Air Cushion furnace is a modern common annealing furnace for single strip of copper alloy, either on-line annealing or individual annealing. Now, the air cushion annealing furnace, often pickling, washing, drying, surface coating, passivation treatment together. Compared with bell-type annealing furnace, the furnace temperature is higher and the annealing time is shorter, so it can realize high temperature rapid annealing, for example, the heating time of 0.05 ~ 1.5 mm thick strip is only a few seconds, and the heating time of thick strip is only more than one minute. Adjustable heating speed, uniform heating, annealing surface quality, uniform structure and performance. The ratio of maximum thickness to minimum thickness is 10:15, the ratio of maximum width to minimum width is 2, and the ratio of thickness to width of the same strip is less than 1/250. At present, the annealing of 0.05ー1.5 mm thick and 250ー1100 mm wide strip can be realized, the annealing speed is (4ー100) M/min, the production capacity is 5 t/h, the thermal efficiency is 85% , and the heat source can be heated by electricity or gas. The air cushion furnace is composed of uncoiling, welding, degreasing, furnace, pickling, shearing, coiling and roller, control roller, looper tower, etc. . Air Cushion annealing furnace is a new technology of continuous heat treatment. When the Strip is passed through the furnace, the upper and lower surfaces are suspended in the heat treatment furnace by a high-temperature air stream which is evenly sprayed. The distance between the upper and lower air jets is 80mm, and the floated strip achieves no contact. For continuous annealing, there are two uncoiler and two coiler. In order to improve the surface quality of the Strip and remove the rolling oil or emulsion from the Strip surface, the strip must be degreased, washed and dried before entering the annealing furnace. When annealed pure copper or bronze strip, the surface of the strip is not oxidized and pickling is not carried out. The heating zone and cooling zone are filled with 2% ー5% H2, containing 95% ー98% N2 protective gas. The heating and cooling zones are not filled with protective gases when annealed brass strips, but acid pickling, water washing and drying are required. Coating and drying of copper alloys for certain special purposes after annealing. In order to prevent the Strip from running off, photoelectric centering device and rectifying roll are installed at the nozzles of the heating and cooling zones. Some air cushion continuous annealing furnace also with tension leveling device. The adoption of air cushion continuous furnace greatly improves the uniformity of the surface quality and the microstructure and properties of the products.

6. Pickling and surface cleaning in the process of hot rolling and heat treatment of copper alloy, the surface of Slab or strip is easy to be oxidized. The usual pickling process is: PICKLING-COLD WATER WASHING-HOT WATER WASHING-DRYING. For the current use of pickling line of work is to go through this process. The pickling process in the production workshop has the following requirements: pickling the surface of the material should be clean, the effective acid and acid concentration should be adopted, the pickling time should be short, the use efficiency of acid is high, measures should be taken to prevent pollution, and attention should be paid to the protection of human body, consider the recovery and reuse of waste liquid and products. ACID pickling mainly uses sulfuric acid or nitric acid mixed with water solution, the reaction process is: Oxide scale is dissolved, or by chemical reaction of gas (hydrogen bubble) will oxide scale mechanical action and stripping. 

Cuo + H2SO4→ CuSO4 + H2OCu2O + H2SO4→ Cu + CuSO4 + H2OCu + 2HNO3→ Cu (NO3)2 + H2 CuO

 and its alloys are oxidized on the surface of Cuo, Cuo is the outermost layer and CuO is the Cuprous oxide Cu2O in the Cuo, the dissolution of Cuprous oxide in sulfuric acid is very slow. In order to clean the surface and speed up the dissolution of the Cuprous oxide, an appropriate amount of oxidant potassium dichromate K2CR2O7, or HNO3, is added to the sulfuric acid solution. But this will worsen the working conditions and reduce the life of the acid bath. The chemical equation is:

 K2CR2O7 + 2H2SO4 + 3Cu2O → CuSO4 + CU5(Cro? 4)2 + 2H2O + K2SO44HNO3 + H2SO4 + Cuo → CuSO4 + Cu (NO3)2 + 2NO2 + 3N2O 

for pure copper, bronze, zinc white copper and other alloys with hard surface to wash, copper alloys containing Be, SI and NI react slowly with dilute acid solution and can Be added with 0.5% ~ 1% potassium dichromate. Some in order to purify the oil and enhance the effect of pickling, pickling solution in addition to add 0.5% ~ 1% hydrochloric acid or hydrofluoric acid. Pickling time is related to pickling solution concentration and temperature. The concentration of general pickling solution is 5% ~ 20% , the temperature is 30 ~ 60 °C, the time is 5 ~ 30 min. Specific can be adjusted according to the effect of pickling, such as summer for more room temperature, winter with steam heating, pure copper take the upper limit, brass take the lower limit. The higher the concentration and temperature of acid solution, the more severe the acid fog will be, and the worse the equipment, environment and working conditions will be. In order to reduce smoke pollution, often add a certain amount of buffer in pickling solution, and try to use low-temperature pickling. The defects produced during pickling are: over pickling, corrosion spots, residual acid traces, water traces and so on. Over-pickling is mainly caused by high concentration of acid solution, high temperature and long time. over-pickling not only produces corrosion spots and reduces the surface quality, but also causes excessive loss of acid and metal. Conversely, if the acid concentration, temperature is too low and time is too short, oxide scale will not be cleaned thoroughly. Residual acid trace, water trace is mainly not clean, or dry not timely, not thorough. In order to achieve rapid pickling and improve surface quality, a new method of electrolytic pickling and ultrasonic pickling has been developed. In the process of pickling, the concentration of pickling solution will decrease continuously. When the sulfuric acid content of pickling solution is less than 50 ~ 100 G/L and the copper content is more than 8 ~ 12 G/L, the new acid solution should be added or replaced in time. In the preparation of new acid, water must be added after the acid to ensure safety. Iron tools are strictly prohibited in the acid bath to prevent spots on the surface of the strip. The replaced waste acid can be neutralized with ammonia to extract copper sulfate, copper powder and make micronutrient fertilizer. The regenerated copper and regenerated acid can also be obtained by electrolysis. The specific gravity of acid was measured by baume meter in preparing acid solution.

02. Surface cleaning surface cleaning methods are many, acid cleaning is one of the commonly used chemical cleaning methods. Also commonly used are mechanical cleaning methods, such as surface cleaning machine cleaning and manual repair. 

(1) the purpose of surface cleaning machine cleaning and manual repair is to remove the residual copper oxide powder and acid traces on the surface of rolled parts after pickling, so as to improve the surface quality. Common surface cleaning machines are: single-roll cleaning machine and double-roll cleaning machine, as shown in figure 1. The single-roll cleaning machine can only clean one surface at a time. Each brush roller has a supporting roller, which acts as a pressing roller. The linear speed of the brush roller is 0.6 ~ 6 m/s, and the feed speed of the pressing roller is 0.2 ~ 0.8 m/s, the LINEAR VELOCITY OF BRUSH ROLLER IS 3 ~ 10 times larger than that of compact roller. The rotation direction of the brush roller is the same or opposite to the feed direction of the pressing roller. Brush roller can be made of Brown, nylon, steel wire, tin-phosphor bronze wire, etc. . The diameter of brush roller is about 200 ~ 300mm. For the two-roller cleaning machine can be cleaned at the same time both upper and lower surfaces. Wet Brush can avoid copper oxide flying, but brush wire material to avoid corrosion and rust. Dry Brush should be set when the dust collector, recovery of copper oxide powder. Copper oxide powder, discoloration, water spots and spots on the surface of rolled parts can be removed by cleaning with steel wire brush or sandpaper. In order to prevent the surface oxidation discoloration, sometimes the inhibitor is used for surface treatment. Chromate can be used as an effective antirust agent, but + 6 chromic acid is toxic and will pollute the environment. Now, an organic inhibitor, Benzotriazole (BTA, C6H5N3) , is used to form a protective skin film on the surface of copper alloy, namely Cutta skin film. The thickness and morphology of BTA solution vary with the temperature, Ph value and alloy type. This method is suitable for cleaning environment, and the effect is not ideal in corrosive environment. At present, chromic acid is still used to remove the oxide and the polluted Matrix on the surface of copper. 

(2) calendering for products requiring high Surface finish, sometimes a calendering or polishing process is used. The calender is usually used with large diameter, low rolling speed and high surface finish. The total processing rate of calender is 3% ~ 10% . By multi-pass calendering, the surface of the product can be close to the finish of the roll surface. Calendering, the use of lubricating oil requirements are very high, viscosity is very small, such as white oil, kerosene and so on. 

(3) polishing surface polishing is a kind of roller polishing machine with nylon roller or flax roller. Its structure is similar to that of a two-roller cleaning machine. Two or three pairs of nylon rollers are sprayed with polishing agent and TALC powder with particle size of 3.5 ~ 5 M. The polishing agent is a mixture of water and Cr2O3 with the ratio of 10:1. 

Source: Caitong

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