Typical smelting process of pure copper and copper alloys for casting, including pure copper, brass, bronze
1. Smelting of pure copper
(1) First preheat the crucible to dark red, and add a layer of dry charcoal or covering agent (borax 63% + broken glass 37%) with a thickness of about 30-50cm on the bottom of the crucible. Then add leftover materials, waste blocks and bars in sequence, and finally add electrolytic copper.
(2) The additional alloying elements can be placed on the stove for preheating. It is strictly forbidden to add cold materials into the liquid metal. The charge should be moved frequently during the entire melting process to prevent bridging.
(3) Raise the temperature to completely melt the alloy. After the alloy is fully melted and the temperature reaches 1200-1220°C, 0.3%-0.4% of the weight of the alloy liquid is added to deoxidize phosphorus copper. The following reactions occur between phosphorus and cuprous oxide: 5Cu2O+2P=P2O5+10Cu and Cu2O+P2O5= 2CuPO3, the generated phosphorus pentoxide gas escapes from the alloy, the copper phosphate can float on the liquid surface, and the slag is removed, thereby achieving the purpose of deoxidation. Continuous stirring is required during the deoxidation process.
(4) Finally, the slag is removed from the furnace, and the pouring temperature of the alloy liquid is generally 1100-1200°C.
2. Smelting of brass
The copper-based alloy with zinc as the main alloy element is brass, which is divided into two categories: ordinary brass and special brass. Ordinary brass is a binary alloy composed of copper and zinc, mainly used for pressure processing. Special brass is made by adding other alloying elements (such as silicon, aluminum, manganese, lead, iron, nickel, etc.) to ordinary brass. Cast brass is mostly special brass.
(1) Alloy ingredients and metal charge requirements. In the chemical composition of copper alloys, due to the large variation range of the main components, in the process of ingredient calculation, appropriate ingredients should be selected based on their performance requirements. The chemical composition of the alloy should comply with GB1176-87. The ingredients of several commonly used brass smelting ingredients are listed in Table 1.
It is required that the furnace material should be dry and clean, and any dirt or rust should be cleaned by sand blowing.
(2) Charge ratio According to general casting practices, the composition of new materials should account for ≥30% of the total weight of the charge, and the recycled materials should be ≤70%. However, in actual production, we consider that copper alloys have a lot of recycled materials. When the mass percentage of recycled materials is ≥90% in the proportion of the furnace materials, the melting quality is still very good. Chemical spectrum analysis proves that the composition of the castings is qualified. When there is a lot of recycled material, it is necessary to consider whether the impurities in the alloy exceed the standard.
(3) Preparation before smelting ① Preparation of metal charge: The recycled charge is scrap castings, pouring risers and remelted ingots of the same brand, which need to have clear chemical composition. Before entering the furnace, sand blowing is used to remove surface dirt, and the furnace is loaded after preheating (the first batch of cold furnace melts can be preheated with the furnace); electrolytic copper is sand blown to remove dirt, and after preheating at 500-550°C to remove moisture, (The first batch of cold furnace melts can be preheated with the furnace; pure metal elements can be preheated at the furnace before entering the furnace. The maximum size of the metal charge should not exceed 1/3 of the diameter of the crucible, and the length should not be Exceeding 4/5 of the depth of the crucible. ② Preparation of the crucible and smelting equipment and tools: The crucible should be free of cracks and other damage that affects safety before use. The new crucible must be slowly heated at low temperature to prevent cracks; the inner surface of the old crucible Clean the slag; when using a new graphite crucible or changing the type of smelting alloy, the crucible should be melted with the same series of alloys before smelting and washed; the stirring rod made of refractory materials and graphite must be thoroughly cleaned of residual paint and rust, and coated with Apply a layer of refractory material or paint, then dry and set aside for use; the ingot mold must be thoroughly cleaned before use, and preheated to 100-150°C after applying paint for use.
(4) Preparation of covering agent and flux ①Charcoal should be put into a sealed baking oven and baked at no less than 800°C for 4 hours. It should be prevented from absorbing moisture when ready for use. ②The covering agent consists of 63% borax and 37% broken glass. Dry charcoal can also be used as the covering agent. Covering agents require drying and removal of debris.
(5) Alloy smelting process
① Preheat the crucible to dark red first, and add 20-40cm thick charcoal to the bottom.
②Add electrolytic copper, rapidly heat up and melt, then add the intermediate alloy in the order of first with the highest melting point and then with the lowest melting point (if any is added), and finally add back to the furnace charge, and add charcoal at the same time to ensure that the alloy liquid level is not exposed to in the air.
③ Smelting brass generally also requires deoxidation. After all the copper is melted, phosphorus copper (calculated as P accounts for 0.04%-0.06% of the weight of the molten copper) is added when the temperature reaches 1150-1200°C for deoxidation. After practical comparison between deoxidation and non-deoxidation, the surface quality of deoxidized castings is better than that without deoxidation.
④Add alloying elements according to the composition requirements of each alloy grade: add aluminum-copper master alloy at 1100-1120℃, add pure zinc and pure aluminum in batches at 1100-1150℃ during a power outage and stir. When melting silicon brass, silicon should be added first and then zinc; when melting lead brass, zinc should be added first and then lead. The adding temperature of zinc element should be controlled. If the temperature drops after adding zinc, power can be sent intermediately. When the temperature of the alloy liquid is higher than 1200°C, adding zinc is not allowed.
⑤ Remove the slag from the furnace, adjust the alloy liquid to the temperature required by the casting process card, and quickly discharge it from the furnace for pouring. The pouring temperature of the alloy is one of the factors that affects the performance of the casting. The general furnace temperature is ZCuZn38: 1100-1130℃; ZCuZn40Pb2: 1080-1100℃; ZCuZn31Al2: 1120-1140℃; ZCuZn16Si4: 1100-1140℃.
⑥ When two different grades of alloy are smelted and their chemical composition is affected, the furnace should be washed in between. For example, use the crucible and tools that have been used to smelt aluminum bronze to melt tin bronze, and the crucible and tools must contain aluminum. Although aluminum is a qualified component in aluminum bronze, it is the most harmful element in tin bronze. .
Qualified castings can be obtained after general copper alloys are deoxidized. However, aluminum bronze, aluminum brass, silicon bronze, etc. are easily oxidized to form high melting point oxides Al2O3 and SiO2, causing castings to form slag inclusions, which need to be removed by refining. Commonly used refining agents include: sodium chloride with a mass ratio of 60% +40% cryolite or 20% cryolite +20% fluorite +60% sodium fluoride, etc.
3. smelting of bronze
Cast bronze can be divided into tin bronze and tin-free bronze according to its composition. Tin bronze is a copper-based alloy with tin as the main alloy element. It has good wear resistance, corrosion resistance, good strength and plasticity. Tin-free bronze includes aluminum bronze, lead bronze, silicon bronze, etc., which contain different main elements. For example, aluminum bronze is a copper-based alloy with aluminum as the main alloying element.
(1) Alloy ingredients and metal charge requirements. The ingredients for smelting several commonly used bronze alloys are listed in Table 2.
Requirements: It is allowed to add 0.7% ~ 0.9% Ni and 0.3% ~ 0.4% Mn into ZCuAl10Fe3 alloy to improve the mechanical properties of the alloy.
(2) Furnace charge ratio: The proportion of new materials should be ≥30% of the total weight of the furnace charge, and the recycled materials should be ≤70%.
(3) Preparation before smelting The preparation before smelting bronze is the same as that for brass smelting. Charcoal should be put into a sealed baking oven and baked at no less than 800°C for 4 hours. It should be prevented from absorbing moisture when ready for use. Straw ash should be ground into powder, remove moisture, and dry thoroughly. Pay attention to moisture-proofing before use. Covering agents require drying and removal of debris.
(4) Alloy smelting process There are many types of cast bronze. Only a few typical alloys are discussed here. Other copper alloys of the same type can be used as a reference.
Smelting of ZCuSn10Pb1 and ZCuPb10Sn10: ① Preheat the crucible to dark red first, and add 20cm-40cm thick charcoal to the bottom. ②Add electrolytic copper, quickly heat up and melt it, then add the recycled materials, and add charcoal at the same time to ensure that the alloy liquid level is not exposed to the air. ③After the returned charge is melted, add phosphor copper (generally 0.5% of the weight of the charge, and all the phosphor copper used when melting phosphor-tin bronze can be added). ④Add zinc, tin, and lead in sequence (according to ingredients). After the previous charge is completely melted, add the next one and stir the alloy liquid continuously. ⑤Adjust the alloy liquid temperature to between 1100-1150℃. ⑥Break slag out of the furnace, add phosphor copper (generally 0.1% of the charge weight), deoxidize, stir evenly, and sprinkle a layer of straw ash on the surface of the alloy liquid, and adjust the alloy liquid to the temperature required by the casting process card (usually 1130 -1180℃), then quickly pour out of the furnace.
Melting of ZCuAl10Fe3 and ZCuAl10Fe3Mn2: ① Do not use crucibles that have melted other alloys to melt these two alloys. ② Preheat the crucible to dark red and add the prepared flux. (Flux composition: cryolite 20% (mass percentage), sodium fluoride 60%, calcium fluoride 20%). ③Add low-carbon thin steel sheets preheated to about 200℃ and recycled materials at the same time, stir the alloy after melting, and raise the temperature to 1150-1180℃. ④Add 0.3% phosphorus copper by alloy weight for deoxidation, and add flux. ⑤Add pure aluminum and pure manganese (according to ingredients) preheated to 200°C in batches. After each batch is added, press it in with a stirring rod to melt it quickly, and stir continuously to make the ingredients uniform, ** Finally, adjust the temperature of the alloy liquid to between 1120-1220°C. ⑥Cover the slag with straw ash, adjust the alloy temperature according to the casting process card (usually 1160-1200℃), and then quickly adjust the casting process.
4. Safety Precautions
Safety production is a basic requirement in the foundry industry. The smelting of pure copper and copper alloys must:
(1) Operators should wear protective equipment, keep the work site clean, and do not allow accumulation of water or debris.
(2) Before starting the furnace, check whether the equipment used is intact. If there are any unsafe factors, they should be eliminated in time.
(3) Preheating can only be carried out after careful inspection and confirmation that there are no explosive or dangerous substances in the charge.
(4) Smelting and pouring tools, such as stirring rods, iron spoons, slag removal tools, etc., must not come into contact with the alloy liquid without preheating.
(5) The remaining alloy liquid during pouring must be poured into the preheated ingot mold and is not allowed to be poured directly on the ground or poured back into the furnace.
5． Several issues that should be paid attention to during smelting
(1) The melting time should be controlled. The time from the beginning of melting to the end of melting (the alloy comes out of the furnace) is called melting time. The length of melting time will not only affect productivity, but also significantly affect the quality of the cast parts. Because the melting time increases, the element burning rate of the alloy increases and the chance of gas absorption increases. Therefore, the melting work should be completed in the shortest possible time. Where possible, the preheating temperature of the charge should be increased as much as possible, and the operation should be compact and quick.
(2) The stirring rod used for smelting should be a carbon rod. Certain elements in copper alloys, such as iron and lead, exist in the form of mechanical mixtures when melted. There are also some elements that tend to produce specific gravity segregation and stratification due to different densities. Practice has proved that these elements can easily cause unqualified chemical composition and unqualified mechanical properties during the smelting and pouring process. To overcome this phenomenon, we must use stirring, which is an indispensable part of melting and pouring. However, stirring is generally not required during temperature measurement and cooling. The material composition of the stirrer used is generally graphite. This is because if other stirring materials such as an iron rod are used, the iron rod will melt during the stirring process, affecting the chemical composition of the alloy. At the same time, if the iron rod is preheated at a higher temperature in the furnace or the stirring time is longer, the oxides on the iron rod will enter the alloy liquid and become impurities; if the iron rod is preheated at a lower temperature, the alloy will be stirred during stirring. To adhere to the iron rod, this can be observed in production.
(3) Problems with the use of covering agent during smelting. For smelting copper alloys, the amount of covering agent is generally: 0.8%-1.2% of the weight of the charge when using glass and borax, because the thickness of the covering layer must be maintained at 10- 15cm; when using charcoal, the dosage is about 0.5%-0.7% of the weight of the charge, and the thickness of the covering layer should be maintained at 25-35cm. The removal time of the covering agent is generally carried out before pouring. If done too early, it will increase the oxidation and air absorption of the copper alloy. If charcoal is used as the covering agent and the slag-blocking effect is good, the covering agent does not need to be removed, so that it can also play a slag-blocking role during the pouring process, and the effect is more ideal.
