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Metals -- Brass

Brass

Brasses are Copper-zinc alloys whose zinc content ranges up to 40%. If the copper crystal structure is face-centered cubic, there will be up to 36% of zinc present. This solid solution, known as the alpha phase or alpha brass, has good mechanical properties, combining strength with ductility. Corrosion resistance is very good, but electric conductivity is considerably lower than in copper. When above 30 to 36% of the alloy is zinc, a body-centered-cubic crystal structure is formed, known as the beta phase, or beta brass. This phase is relatively brittle and high in hardness compared to the alpha phase. However, ductility increases at elevated temperatures, thus providing good hot-working properties. Gamma brass, with the zinc above 45%, is not easily worked, either hot or cold.

The mechanical properties of brasses vary widely. Strength and hardness depend on alloying and/or cold work. Tensile strengths of annealed grades are as low as 30,000 lb/in² (206 MPa), although some hard tempers approach 90,000 lb/in² (620 MPa). Although brasses are generally high in corrosion resistance, two special problems must be noted. With alloys containing a high percentage of zinc, dezincification can occur. The corrosion product is porous and weak. To prevent dezincification, special inhibitors-- antimony, phosphorus, or arsenic-- in amounts of 0.02 to 0.05% can be added to the alloy. The other problem is stress corrosion, or season cracking, which occurs when moisture condenses on the metal and accelerates corrosion.

Simple copper-zinc brasses are made in standard degrees of temper, or hardness. This hardness is obtained by cold-rolling after the first annealing, and the degree of hardness depends upon the percentage of cold reduction. When the thickness is reduced one number of the Brown & Sharpe gage, or about 10.9%, the resulting sheet is known as 1/4 hard. The other grades are ½ hard, hard, extra hard, spring, and finally extra spring, which is a reduction of 10 numbers on the Brown & Sharpe gage, or about 68.7% without intermediate annealing. Degrees of softness in annealed brass are measured by the grain size, and annealed brass is furnished in grain sizes from 0.010 to 0.150 mm. The ASTM standard grain sizes are:

• 0.015 to 0.025 mm for light anneal
• 0.035 mm for drawing or rod anneal
• 0.050 mm for intermediate anneal
• 0.70 mm for soft anneal, and
• 0.120 for dead-soft anneal.

Brasses with smaller grain sizes are not as ductile as with larger grain sizes, but they have smoother surfaces and require less polishing.

Even slight additions of other elements to brass alter the characteristics drastically:

• Slight additions of tin change the structure, increasing the hardness but reducing the ductility.
• Iron hardens the alloy and reduces the grain size, making it more suitable for forging, but making it difficult to machine.
• Manganese increases the strength, increases the solubility of iron in the alloy, and promotes the stabilization of aluminum, but makes the brass extremely hard.
• Slight additions of silicon increase the strength of brass, but large amounts promote brittleness, loss of strength, and danger of oxide inclusion.
• Nickel increases strength and toughness, but when any silicon is present, the brass becomes extremely hard and more a bronze than a brass.

There are hundreds of brasses, but most can be grouped into a few major classes:

• The straight brasses constitute by far the largest and most widely used group. They are binary copper-zinc alloys with zinc content ranging from 5% up to about 40%. As the zinc content increases in these alloys, the melting point, density, electric and thermal conductivity, and modulus of elasticity decrease while the coefficient of expansion, strength, and hardness increase. Work hardening also increases with zinc content.
• The leaded brasses have essentially the same range of zinc content as the straight brasses. Lead is present, ranging from less than I to 3.25%, to improve machinability and related operations. Lead also improves antifriction and bearing properties. Common leaded brasses include leaded commercial bronze (0.5% lead), medium-leaded brass (1% lead), high-leaded brass (2% lead), free-cutting brass (3.25% lead), and hardware bronze (1.75% lead). Free-cutting brass provides optimum machinability.
• The tin brasses are copper-zinc alloys with small amounts of tin. The tin improves corrosion resistance. Naval brass and manganese bronze are widely used for products requiring good corrosion resistance and high strength, particularly in marine equipment.