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The copper-zinc (5-45%) alloy is also called brass. Brass with a zinc part from 5 to 20% is called red (or tombak), with a zinc part of 20–36% yellow. Brasses with a zinc part more than 45% are rarely present in the industry.
Zinc is cheaper than copper. That is why its incorporation into an alloy lessens costs, at the same time improving mechanic, technological and antifriction qualities of the product – brass is cheaper than copper. Electric and heat conductivity of brass is lower, than that of copper.
Corrosion resistance of brasses in weather conditions lies between that of the alloy elements, i.e. zinc and copper. A messing with more than 20% zinc is a subject to splitting at maturing in humid weather conditions (especially with traces of ammonia). This phenomenon is often called «seasonal splitting». It is mostly traceable in malformed items due to the fact that corrosion spreads across grain junction lines. To remove the phenomenon, messing undergo baking at 240 - 260°C after its distortion.
Brasses have good technological qualities and are applied in production of many small parts, especially if good processibility and moldability is required. Quality molds are obtained from them, as brasses have good plasticity and small liquation. Brasses are easily plastically yielded and are mainly used for production of tumbled semifinished products - sheets, straps, belts, wires and other shapes.
Usually brasses fall into:
Two-component brasses («Simples»), which consist only of copper, zinc and insignificant impurities.
Multi-component brasses («Specials»), which include supplementary alloy elements in addition to copper and zinc
Two-component brasses.
Phase composition of an alloy is very important for them. Breaking point of zinc solvability in copper at room temperature equals 39%. It falls with increasing temperatures and equals 32% at 905°C. That is why brasses with zinc part lower than 39% have single-phase structure (a-phase) of a solid solution of zinc in copper. They are called à- brasses. If we introduce more zinc into a metal bath, it would not melt in copper completely, and, after indurations, the alloy would reach a second phase – (b-phase). b-phase means being crispy and hard, that is why brasses in the second phase possess higher resistance and lower moldability, as those in the first phase.
With increase of zinc concentration to 30 % resistance and moldability raise simultaneously. Then moldability falls - firstly, because complications in solid solution, then due to crispy b-phase. Resistance increases to zinc part of about 45%, and then falls as abrupt as moldability.
Most brasses are easily processed under pressure. One-phase brasses are especially moldeable. They distort at low and high temperatures. But there exist a crispy range between 300°C and 700°C, and brasses do not distort at these temperatures.
Pressure shaping of brasses features application of the a-messing with a zinc part of 32 % to be processed cold (fine sheets, wire, standardized profiles), as it has possess higher resistance and lower moldability at room temperature. With a temperature increase to 300°C-700°C its moldability falls, and the brass is not processed hot. Either b-brasses with a high zinc part (up to 39 %), able to reach a two-phase condition a+b under heating, are used for that, or (a + b)-brasses.
Brass brand name consists of a «B» letter to pin down an alloy type – brass, and a two-digit number to mark an average copper content. For example, B80 brand name means that this is a brass alloy with 80 % Cu and 20 % Zn.
Lead, iron, antimony, bismuth and phosphorus represent monitored impurities in copper-zinc alloys, in B70 brand name – arsenic (additionally), tin and sulphur. They affect brasses in exactly the same manner as copper. They make brass crispy under heated pressure processing.
Every two-component brass can be easily processed under pressure. They are delivered as pipes and tubes of various section cuts, sheets, stripes, bands, wire and rods of various profiles. Brass products with a high internal stress (for example, hammer-hardened) are subject to splitting. They run longitudinal and cross fractures if kept in the air for a long time. To avoid that, an internal stress should be removed through stress stabilizing annealing at 200-300°C prior to long-term storage.
Multi-component brasses.
There are more multi-component brasses as two-component ones. The name of a „Special“ reflects its composition. For example, if brass is alloyed with iron and manganese, it is called «iron-manganese», if with aluminium – «aluminium» and so on.
The trade name of those brasses is composed as follows: first comes a B character, as in simple brasses, which is followed by a number of letters to indicate which alloyed elements are added to zinc; then come the hyphen-divided numbers – the first one indicates an average of copper in percentage, and other gives he content of alloyed elements in correspondence with the letters in the first part of the brand name. Aluminium, iron, manganese, tin, silicium, nickel are basic alloy components in a multi-component brass. They influence its qualities in a different manner. back |
