Heat Treatment Technology of Copper Alloy

1. heat treatment method

1. Overview

The heat treatment of copper alloy is mainly heating and annealing for different purposes. Only individual grades of alloys, such as beryllium bronze, can be quenched and tempered. Annealing for different purposes is: softening annealing, finished annealing and blank annealing.

Softening annealing: that is, recrystallization annealing for the purpose of softening between two cold-rolling, also known as intermediate annealing. After cold rolling of the alloy to produce fiber tissue and work hardening, after the alloy is heated to above the recrystallization temperature, insulation for a certain period of time after slow cooling, so that the alloy recrystallization into a refined grain structure, to obtain good plasticity and low deformation resistance, in order to continue cold rolling processing. This annealing is the main heat treatment in copper alloy rolling.

Finished annealing: that is, after cold rolling to the finished size, the post-annealing of different states and properties is obtained by controlling the annealing temperature and holding time. Finished annealing has the requirements of controlling the state and performance, such as obtaining soft (M) state, semi-hard (Y2)-like products, and obtaining better deep-drawing performance products by controlling the grain structure. In addition to annealing above the recrystallization temperature, the finished product annealing also has low-temperature annealing at the recrystallization temperature.

Billet annealing: is a hot-rolled billet, through recrystallization annealing to eliminate the hardening caused by incomplete thermal deformation during hot rolling, and through annealing to make the tissue uniform for the purpose of heat treatment. Quenching-tempering (aging): that is, for some solid solution alloys with dissolution and precipitation and eutectoid transformation, when the temperature is higher than the phase transformation point, the strengthening phase is fully dissolved through heat preservation to form a uniform solid solution, and then the quenching state of supersaturated solid solution is formed in quenching, and then the heat treatment method is used to control the properties of the alloy through precipitation or phase transformation at low temperature or room temperature.

2. Annealing

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

① Ensure uniform heating of the annealed material to ensure uniform organization and performance of the material; ② Ensure that the annealed material is not oxidized and the surface is bright;

③ Save energy, reduce consumption and improve yield. Therefore, the copper annealing process system and the equipment used should be able to meet the above conditions. Such as the furnace design is reasonable, the heating speed is fast, there is a protective atmosphere, the control is precise, and the adjustment is easy. Table 1 lists the annealing process system of some commonly used copper alloys.

Selection of annealing temperature: in addition to alloy properties and hardening degree, annealing purpose should also be considered. For intermediate annealing, the upper limit of annealing temperature should be taken and the annealing time should be appropriately shortened. For finished product annealing, the emphasis is on ensuring uniform product quality and performance. The lower limit of annealing temperature should be taken and the fluctuation of annealing temperature should be strictly controlled. The annealing temperature for thick specifications should be higher than that for thin specifications; the annealing temperature of the large amount of loading is higher than that of the small amount of loading, and the annealing temperature of the plate is higher than that of the strip.

The heating rate of annealing shall be determined according to the properties of the alloy, the loading capacity, the furnace structure, the heat transfer mode, the metal temperature, the temperature difference in the furnace and the requirements of the product. Because rapid heating can improve productivity, fine grain, less oxidation, semi-finished intermediate annealing, mostly using rapid heating.

Heat preservation time: when the furnace temperature is designed, in order to improve the heating speed, the temperature of the heating section is relatively high. When heated to a certain temperature, heat preservation should be carried out. At this time, the furnace temperature is similar to the material temperature. The holding time is based on ensuring uniform thermal penetration of the annealed material.

Cooling method: finished product annealing is mostly air cooling, and intermediate annealing can sometimes use water cooling, which can make oxide skin burst and fall off under rapid cooling for alloy materials with severe oxidation. However, alloys with quenching effect are not allowed to be quenched.

3. Solid solution-aging

The process parameters of solid solution (quenching)-aging (tempering) are mainly determined by the heating temperature, heating speed, holding time, cooling speed, heating medium and the transfer time interval of water cooling. The upper limit of the quenching temperature is lower than the eutectic temperature, the lower limit is higher than the solid solubility line temperature, and the holding time is mainly determined by the dissolution rate of the strengthening phase. In order to maintain a good quenching effect, the shorter the transfer time of quenching, the better, and the water temperature is generally not higher than 25°C. The cooling rate mainly considers two factors. One is that the strengthening particles will be precipitated if the speed is too slow, and the residual stress or crack will be generated if the speed is too fast. During tempering (aging), it is mainly ensured that the reinforced particles are uniformly precipitated and distributed. Table 2 is part of the copper alloy solid solution (quenching)-aging (tempering) process system.

4. Atmosphere control in heat treatment furnace

According to the characteristics of the effect of the medium in the furnace and the surface of the material during heat treatment, the heat treatment is divided into ordinary (I. e. air as the medium) heat treatment, protective atmosphere heat treatment (I. e., commonly known as bright annealing) and vacuum heat treatment.

(1) Atmosphere of flame heating furnace

This is an old-fashioned common heat treatment method, which uses combustion heating such as gas as fuel.

Oxidizing atmosphere, that is, the fuel in the case of excess air combustion, so that there is more oxygen in the furnace.

Reducing atmosphere, combustion process in the furnace air is insufficient, due to the fuel is not fully burned and more CO and CO2.

The neutral atmosphere is an atmosphere controlled between oxidation and reduction in the furnace. Micro oxidation atmosphere, is the furnace contains a trace 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 an oxidizing atmosphere, because not only the surface quality is destroyed, the oxidation loss is large, and the low melting point components in the alloy, such as Sn, Pb, Sn, Zn, Cd, etc., are easily evaporated. Therefore, a reducing or neutral furnace atmosphere is mostly used. For heat treatment is easy to absorb hydrogen to produce "hydrogen embrittlement", sulfur alloy is usually used in micro oxidation atmosphere, such as nickel containing alloy, pure copper, etc. The flame heating furnace will gradually be replaced by an annealing furnace with electric heating and continuous heating of protective gas.

(2) Protective atmosphere

Due to advances in sealing technology and protective gas manufacturing technology, heating in a protective atmosphere is now mostly used. Protective gas composition requirements do not react with the alloy during heating; no erosion of furnace components, thermocouples, resistance devices; stable composition, simple manufacture and convenient supply.

Protective atmosphere heating is mostly electric heating. It is a continuous annealing furnace that uses electric induction heating after the 1990 s. It uses protective atmosphere and forced circulating air type online annealing, which simplifies the process and improves the surface quality and production efficiency. In the past, the protective atmosphere used nitrogen, carbon dioxide, purified gas or water vapor after coal gas combustion. Due to the poor sealing performance of the furnace, the low concentration of the protective gas used and the high cost, satisfactory results have not been achieved. The current protective gas composition is mainly nitrogen hydrogenation. According to different alloys with different nitrogen and hydrogen ratio, hydrogenation protection is particularly effective for brass. Since the thermal conductivity of hydrogen is 1.7 times that of nitrogen, it can greatly increase the heat conduction speed. After hydrogenation, the heating time can be shortened, especially under the action of strong wind, the convective heat transfer effect is better, the annealing material is heated evenly, and the uniformity of the organization and performance of the heat-treated copper material is ensured. Under the action of strong wind, the volatiles of the lubricant are taken away and the surface quality is improved.

Several methods of manufacturing protective gas:

Ammonia decomposition gas: After vaporization, liquid nitrogen enters the cracker filled with nickel catalyst, and at a temperature of 750~850 ℃, it is cracked to generate ammonia and hydrogen, and is sent to the furnace after purifying and removing residual ammonia. This protective gas is suitable for annealing brass. According to the calculation, 1kg of ammonia can produce 1.97 m3 of hydrogen and 0.66 m3 of nitrogen.

Ammonia decomposition gas combustion purification: If you need to reduce the proportion of ammonia, you can burn off part of the hydrogen, in the combustion process to increase nitrogen.

Ammonia decomposition gas into the air nitrogen (or liquid nitrogen vaporization), after purification into the furnace, according to the need to get different proportions of hydrogen and nitrogen protective gas.

When the dosage is less, bottled nitrogen and bottled hydrogen can be used as protective gases.

The gas after coal gas combustion, generally containing 96% of nitrogen and less than 4% of carbon monoxide and hydrogen, need to be purified after use.

⑥ pure nitrogen, the general purity of nitrogen by air separation method can reach 99.9%. The new process of coke molecular sieve pressure swing adsorption air separation nitrogen, adopts an oil-free compressor, the oxygen content in nitrogen is less than 5 × 10-6, and the dew point is -65 ℃. This is the most suitable for the annealing of pure copper and tin phosphor bronze with high oxygen content.

(3) Vacuum heat treatment

During annealing heating, the space of the charge is evacuated, and the annealed material is not in contact with any medium. There are two forms of vacuum annealing: external heating and internal heating. External heating type is the heating element placed outside the furnace liner, using electric heating, external heating type vacuum furnace structure is simple, easy to manufacture, convenient loading and discharging. But the heating is slow, the heat loss is large, and the life of the furnace is low. Internal heating is the heating element in the furnace, heat treatment materials are also placed in the furnace, often using tungsten, molybdenum, graphite and other materials for heating elements. It is characterized by fast heating, high heating temperature and long furnace life. However, the structure of the furnace is complex, the investment is large, and the cooling is slow. When vacuum annealing is used, pure copper is (10-1~10-2)× 133.3 Pa, and most copper alloys are (10-2~10-3)× 133.3 Pa vacuum. During vacuum annealing, the material must be discharged at a temperature below 100°C to break the vacuum and prevent the annealing material from oxidizing. In recent years, the heat treatment of copper alloy does not recommend vacuum annealing, because the vacuum is a poor conductor of heat, it only by thermal radiation heat transfer, especially the surface of bright materials, to radiation heating is more difficult. In addition to low thermal efficiency, it is difficult to obtain a heat-treated product with uniform performance. Therefore, it is gradually replaced by protective atmosphere and air cushion continuous annealing.

5. Heat treatment furnace

The heat treatment equipment is mainly a heat treatment furnace. The selection of heat treatment furnace should consider the following conditions: that is, to meet the requirements of heat treatment process, to ensure quality and performance; select the appropriate heat source to meet the needs of production; furnace structure is simple, accurate temperature control, durable, low investment; high degree of automation, high production efficiency, good working conditions, easy to operate.

(1) Ordinary copper and copper alloy heat treatment furnace

Commonly used annealing furnace for copper alloy: according to the structure, it is divided into box furnace, well furnace, stepping furnace, car bottom furnace, roller hearth furnace, chain water seal furnace, single hearth furnace, double hearth furnace, bell furnace, etc. According to the production mode, there are: monomer batch annealing furnace, air cushion continuous annealing furnace, etc. According to the atmosphere in the furnace, it is divided into annealing furnace with protective atmosphere, annealing furnace with protective atmosphere and vacuum annealing furnace, etc. According to the heat source, it is divided into coal furnace, gas furnace, heavy oil furnace, resistance furnace and induction furnace, etc. There are three types of quenching furnaces: vertical, horizontal and well.

(2) Commonly used copper alloy heat treatment furnace

At present, the main trends in the development of heat treatment furnaces: improving design, looking for new processes, and improving heat utilization efficiency; using low-temperature or high-temperature rapid annealing to reduce oxidation and dezincification; using protective gas annealing and forced circulation ventilation to make rapid heating, uniform temperature, and improve the quality of annealed products; enhancing the sealing effect, simplifying the process and improving the integration and continuity level.

Air cushion furnace is a modern commonly used copper alloy single strip annealing furnace, there are online annealing, there are separate annealing. The current air cushion annealing furnace often combines pickling, washing, drying, surface coating, passivation treatment, etc. Compared with the bell-type annealing furnace, it has high furnace temperature and short annealing time, and can realize high-temperature rapid annealing. For example, for thick strips of 0.05~1.5mm, it only takes a few seconds to heat, and for thick strips, it only takes more than one minute. The heating speed is adjustable, the heating is uniform, the annealing surface quality is good, and the organization and performance are uniform.

The air cushion furnace is composed of uncoiling, welding, degreasing, furnace, pickling, shearing, coiling and rollers, control rollers, and live tower.

Air cushion annealing furnace is a new technology of continuous heat treatment. When the strip passes through the furnace, the upper and lower surfaces are held up and suspended in the heat treatment furnace by uniformly sprayed high-temperature air flow, the upper and lower air jets are 80mm apart, and the strip that is held up and floated reaches no contact. For continuous annealing, two sets of uncoiler and two sets of coiler are provided. In order to improve the surface quality of the strip and remove the rolling oil or emulsion on the surface of the strip, the strip entering the annealing furnace must be degreased, washed and dried. When annealing pure copper or bronze strip, the surface of the strip is not oxidized or pickled. The heating zone and cooling zone shall be filled with a protective gas with a composition of 2% ~ 5% H2 and 95% ~ 98% N2. When annealing the brass strip, the heating zone and the cooling zone are not filled with protective gas, but pickling, water washing and drying are required. For some special purpose copper alloy, coating and drying after annealing. In order to prevent the strip from deviating, a photoelectric centering device and a deviation correcting roller are arranged at the nozzles in the upper and lower rows of the heating zone and the cooling zone. The edge irregularity rate of the strip after coiling is less than 1mm. Some air cushion annealing continuous furnace also with tension leveling device. The use of air cushion continuous furnace greatly improves the surface quality and the uniformity of the tissue properties of the product.

6. Pickling and surface cleaning

01 Pickling

pickling process

In the process of hot rolling and heat treatment of copper alloy, the surface of slab or strip is easy to be oxidized. In order to remove the oxide scale on the surface, pickling is required. The usual pickling procedure is: pickling-cold water wash-hot water wash-drying. For the current use of pickling machine column work process is to go through this procedure.

The pickling process in the production workshop has the following requirements: the surface pickling of materials should be clean, effective acid and acid concentration should be adopted, pickling time should be short, acid utilization efficiency should be high, anti-pollution measures should be taken, and attention should be paid to the protection of human body, and the recycling of waste liquid and products should be considered.

Pickling mainly uses sulfuric acid or aqueous solution mixed with nitric acid, the reaction process is: the oxide scale is dissolved, or the gas generated by the chemical reaction (hydrogen bubbles) will be the mechanical action of the oxide scale and stripping. The chemical reaction of copper and copper alloys with acid is as follows:

　　CuO H2SO4→CuSO4 H2O

Cu2O H2SO4→Cu CuSO4 H2O

Cu 2HNO3→Cu(NO3)2 H2

The surface oxidation of copper and its alloys, the outer layer is cupric oxide CUO, cuprous oxide Cu2O is inside the cupric oxide, cuprous oxide dissolved in sulfuric acid is very slow. In order to clean the surface treatment and accelerate the dissolution of cuprous oxide, an appropriate amount of potassium dichromate K2Cr2O7 or nitric acid HNO3 is added to the sulfuric acid solution. However, this will worsen the working conditions and reduce the life of the acid tank. Its chemical reaction formula 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 that are not easy to clean the surface, as well as copper alloys containing Be, Si and Ni, the interaction with dilute acid solution is slow, and 0.5 ~ 1% potassium dichromate can be added. Some in order to purify the oil and strengthen the pickling effect, add 0.5 to 1% of hydrochloric acid or hydrofluoric acid in the pickling solution.

The pickling time is related to the concentration and temperature of the pickling solution. Generally, the concentration of pickling solution is 5% ~ 20%, the temperature is 30~60 ℃, and the time is 5~30min. It can be adjusted according to the effect of pickling. For example, it is mostly room temperature in summer and steam heating in winter. Pure copper takes the upper limit and brass takes the lower limit. The higher the acid concentration and temperature, the more severe the acid mist, the more serious the deterioration of equipment, environment and labor conditions. In order to reduce the pollution of smoke, a certain amount of buffer is often added to the pickling solution, and low temperature pickling is used as much as possible.

The defects produced during pickling are: over pickling, corrosion spots, residual acid traces, water traces, etc. Over pickling is mainly caused by high acid concentration, high temperature and long time. Over pickling not only produces corrosion spots, resulting in reduced surface quality, but also excessive loss of acid and metal. Conversely, if the acid concentration, temperature is too low and the time is too short, the oxide scale will not be thoroughly cleaned. The residual acid trace and water trace are mainly not cleaned, or dried in time and thoroughly. In order to achieve rapid pickling and improve surface quality, new methods of electrolytic pickling and ultrasonic pickling have emerged.

During the pickling process, the concentration of pickling solution will continue to decrease. When the sulfuric acid content of the acid solution is less than 50~100g/L and the copper content is greater than 8~12g/L, new acid solution should be added or replaced with new acid solution in time. When preparing new acid solution, water must be drained before adding acid to ensure safety. Iron tools are strictly prohibited in the acid tank to prevent spots on the surface of the plate. The replaced waste acid can be neutralized with ammonia to extract copper sulfate, copper powder and make trace element fertilizer. Recycled copper and regenerated acid can also be obtained by electrolysis. Measure the specific gravity of acid with Baume meter when preparing acid solution.

02. SURFACE CLEANING

There are many ways to clean up the surface, and pickling is one of the commonly used chemical methods to clean up. There are also commonly used mechanical cleaning methods, such as surface cleaning and manual repair.

(1) Surface cleaning and manual repair

The purpose is to remove the copper oxide powder and acid traces remaining on the surface of the rolled piece after pickling to improve the surface quality. Commonly used surface cleaning machines are: single-roll cleaning machine and double-roll cleaning machine, as shown in Figure 1.

The single-roller brush cleaner can only clean one surface at a time. Each brush roller has a support roller to play a pressing role. The linear speed of the brush roller is 0.6~6 m/s, the feed speed of the pressure roller is 0.2~0.8 m/s, and the linear speed of the brush roller is 3~10 times larger than the linear speed of the pressure roller. The direction of rotation of the brush roller is the same as or opposite to the feeding direction of the pressure roller. The brush roller can be made of brown, nylon wire, steel wire, tin phosphor bronze wire and other materials, and the diameter of the brush roller is about 200~300mm. For the two-roll brush cleaner, the upper and lower surfaces can be cleaned at the same time. Wet brush can avoid copper oxide powder flying, but brush wire material to avoid corrosion and rust. Dust collector shall be set during dry brushing to recover copper oxide powder.

Local copper oxide powder, discoloration, water stains and spots on the surface of the rolled piece can be removed by wiping with a wire brush or sandpaper. Defects such as pits, cracks, peeling, pressure pits and dust on the surface can be repaired with a scraper.

In order to prevent the surface from being oxidized and discolored, the surface is sometimes treated with an inhibitor. Chromate can be used as an effective anti-rust treatment agent, but the six-valent chromic acid is toxic and pollutes the environment. Now, an organic inhibitor, benzotriazole (BTA,C6H5N3), is used to form a protective film on the surface of the copper alloy by BTA treatment, that is, CuBTA film. The thickness and morphology of the BTA treatment solution vary depending on the temperature, pH value and type of alloy. This method is suitable for treatment in a clean environment, but the effect is not ideal in a corrosive environment. At present, chromic acid treatment is still used to remove the oxide on the copper surface and the substrate contaminated by the surface.

(2) Calendering

For products requiring a high surface finish, calendering or polishing is sometimes used. Roller mill is usually used for calendering, the roller diameter is large, the rolling speed is low, the smoothness of the roller surface is very high, and the total processing rate of the reduction amount during calendering is 3% to 10%. Through multi-pass calendering, the surface of the product can be close to the smoothness of the roll surface. When calendering, the lubricating oil used has high requirements and low viscosity, such as white oil and kerosene.

(3) Polishing

Surface polishing is a roller polishing machine with nylon roller or linen roller, and its structure is similar to that of a double-roller brush cleaner. The nylon rollers are two or three pairs, and the polishing agent and talc powder with a particle size of 3.5 ~ 5μm are injected during polishing. The polishing agent is a mixture of water and Cr2O3, and the ratio is 10:1. (Material Yitong)