Bronze is an alloy based on copper. Auxiliary metals may be nickel, zinc, tin, aluminum and others. In this article we will consider types, technological features, chem. composition of bronze, as well as methods for its manufacture.
Classification
1. The chemical composition of this metal is usually divided into two groups. The first is tin bronzes. In them, tin is the main alloying element. The second is tinless. Below we will talk about this in more detail.
2. According to technological characteristics, bronzes are usually divided into wrought and foundry ones. The former are well processed under pressure. The second are used for shaped castings.
This metal in comparison with brass has much better antifriction, mechanical properties, as well as corrosion resistance. In fact, bronze is an alloy of copper and tin (as the main auxiliary element). Nickel and zinc are not the main alloying elements; for this, components such as aluminum, tin, manganese, silicon, lead, iron, beryllium, chromium, phosphorus, magnesium, zirconium and others are used.
Pewter bronze: foundry
Let's see what constitutes such a metal. Tin bronze (photo below shows cast parts) is an alloy in which the fluidity is lower than other types. However, it has an insignificant volumetric shrinkage, which allows to obtain shaped bronze castings. These properties determine the active use of bronze in casting antifriction parts. Also, the considered alloy is used in the manufacture of fittings intended for operation in an aqueous medium (including sea water) or in water vapor, in oils and under high pressure. There are also the so-called non-standard foundry bronzes for important purposes. They are used in the production of bearings, gears, bushings, pump parts, low-pressure rings. Such parts are designed to operate under high pressure, at high speeds and light loads.
Lead Bronze
This subspecies of cast tin alloys is used in the manufacture of bearings, oil seals and shaped castings. Such bronzes are characterized by low mechanical properties, as a result of which, in the process of manufacturing bearings and bushings, they are simply applied to the steel base in the form of a very thin layer. Alloys with a high tin content have higher mechanical properties. Therefore, they can be used without a steel base.
Pewter Bronze: Deformable
Pressure-treated alloys are usually divided into the following groups: tin-phosphorous, tin-zinc and tin-zinc-lead. They found their application in the pulp and paper industry (of which they make nets) and engineering (the production of springs, bearings and machine parts). In addition, these materials are used in the manufacture of bimetallic products, rods, belts, strips, gears, gears, bushings and gaskets of highly loaded machines, tubes of instrumentation, gauge springs. In electrical engineering, the widespread use of bronze (wrought) is explained by excellent mechanical properties (along with high electrical characteristics). It is used in the manufacture of live springs, plug connectors, contacts. In the chemical industry, spring wire is made from tin bronzes, in the exact mechanics - fittings, in the paper industry - scrapers, in the automotive and automotive tractors - bushings and bearings.
These alloys can be supplied in a particularly hard, hard, semi-solid and soft (annealed) condition. Tin bronzes are usually processed cold (by rolling or drawing). Hot metal is only pressed. Under pressure, bronze is perfectly processed both in cold and in hot form.
Beryllium bronze
This alloy belongs to the group of precipitation hardening metals. It has high mechanical, physical and elastic properties. Beryllium bronze has a high level of heat resistance, corrosion resistance and cyclic strength. It is resistant to low temperature, does not magnetize and does not give sparks upon impact. Quenching of beryllium bronzes is carried out at temperatures of 750-790 degrees Celsius. The addition of cobalt, iron and nickel during heat treatment helps to slow down the rate of phase transformations, this greatly facilitates the technology of aging and hardening. In addition, the addition of nickel contributes to an increase in the temperature of recrystallization, and manganese can replace, although not fully, expensive beryllium. The bronze characteristics indicated above allow this alloy to be used in the manufacture of springs, spring parts, and membranes in the watch industry.
Copper Manganese Alloy
Such bronze is distinguished by special high mechanical qualities. It is processed by pressure, both in cold and in hot condition. This metal is characterized by increased heat resistance, as well as corrosion resistance. Alloy of copper with the addition of manganese is widely used in furnace fittings.
Silicon bronze
This is an alloy composed of nickel, less often manganese. Such a metal is characterized by ultrahigh mechanical, antifriction and elastic properties. At the same time, silicon bronze does not lose its ductility at low temperatures. The alloy is well soldered, processed by pressure at both high and low temperatures. The metal in question does not magnetize, does not spark upon impact. This explains the widespread use of bronze (silicon) in marine shipbuilding in the manufacture of antifriction parts, bearings, springs, gratings, evaporators, nets and guide bushings.
Casting tinless alloys
This type of bronze is characterized by good corrosion, antifriction properties, as well as high strength. They are used for the manufacture of parts that are used in especially harsh conditions. These are, for example, gears, valves, bushings, gears for powerful turbines and cranes, worms that work in tandem with hardened steel parts, bearings operating under high pressure and under shock loads.
How to make bronze?
The manufacture of this metal must be carried out in special furnaces used for the smelting of copper alloys. The bronze mixture can be made up of fresh metals or with the addition of secondary waste. The smelting process is usually carried out under a layer of flux or charcoal.
The process using a charge of fresh metals occurs in a certain sequence. First, the required amount of flux or charcoal is loaded into a very hot furnace. Then copper is placed there. After waiting for its melting, increase the heating temperature to 1170 degrees. After this, the melt must be deoxidized, for which phosphorous copper is added. This process can be carried out in two stages: directly in the furnace, and then in the bucket. In this case, the additive is administered in equal proportions. Then, the necessary alloying elements heated to 120 degrees are added to the melt. Refractory components should be introduced in the form of ligatures. Next, the molten bronze (photo below shows the smelting process) is mixed until all the added substances are completely dissolved and heated to a predetermined temperature. When dispensing the obtained alloy from the furnace, before casting, it must be finally deoxidized with the remainder (50%) of phosphorous copper. This is done to release the bronze from the oxides and increase the fluidity of the melt.
Smelting based on working materials
In order to make bronze using recycled metals and waste, smelting should be performed in the following order. First, copper is melted and deoxidized by phosphorous additives. Then, circulating materials are added to the melt. After this, complete melting of the metals is expected and alloying elements are introduced in the appropriate sequence. In the event that the charge consists of a small amount of pure copper, it is necessary to first melt the working metals, and then add copper and alloying elements. Melting is carried out under a layer of flux or charcoal.
After melting the mixture and heating it to the required temperature, the final deoxidation of the mixture with phosphorous copper is carried out. Next, the melt is covered with calcined charcoal or dried flux. Consumption of the latter is 2-3 percent of the mass of metal. The heated melt is aged 20-30 minutes, is stirred occasionally, and then the allocated slag is removed from its surface. Everything, bronze is ready for casting. For better slag removal, quartz sand can be added to the bucket , which thickens it. To determine whether bronze is ready for casting into molds, a special technological test is carried out. The fracture of such a sample should be uniform and clean.
Aluminum bronze
It is an alloy of copper and aluminum as an alloying element. The melting process of this metal differs significantly from the above, due to the chemical characteristics of the auxiliary component. Consider how to make bronze using aluminum alloy components. In the manufacture of this type of alloy using recycled materials in the charge, the operation for the oxidation of phosphorous components is not used. This is because phosphorus is characterized by a lower affinity for oxygen molecules than aluminum. You should also know that this type of bronze is very sensitive to overheating, so the temperature should not exceed 1200 degrees. In the overheated state, aluminum is oxidized, and the bronze alloy is saturated with gases. In addition, the oxide formed during the smelting of this type of bronze is not reduced by the addition of deoxidizers, and it is very difficult to remove it from the melt. The oxide film has a very high melting point, which significantly reduces the fluidity of the bronze and causes marriage. Melting is carried out very intensively, in the upper limits of the heating temperature. In addition, you should not delay the finished melt in the furnace. When melting aluminum bronze, it is recommended to use a flux, which consists of 50% soda ash and 50% cryolite, as the coating layer.
The finished melt is refined before casting by injection of manganese chloride or zinc chloride (0.2-0.4% of the total mass of the charge). After this procedure, the alloy should be kept for five minutes until gas evolution ceases. After which the mixture is brought to the desired temperature and poured into molds.
In order to prevent segregation in the bronze melt with a high content of lead impurities (50-60%), it is recommended to add 2-2.3% nickel in the form of copper-nickel alloys. Or it is necessary to use alkali metal sulfate as fluxes. Nickel, silver, manganese, if they are part of bronze, should be introduced into the melt before the tin addition procedure. In addition, to improve the quality of the resulting alloy, it is sometimes modified with minor additives based on refractory metals.