Room temperature tissue
Ordinary brass is a copper-zinc binary alloy, and its zinc content varies widely, so its microstructure at room temperature is also very different. According to the Cu-Zn binary state diagram (fig. 6), there are three kinds of microstructure of brass at room temperature: brass with zinc content below 35%, and the microstructure at room temperature is composed of single-phase α solid solution, which is called α brass; The microstructure of brass with zinc content in the range of 36% ~ 46% at room temperature is composed of (α+β) two phases, which is called (α+β) brass (two-phase brass); Brass containing more than 46% ~ 50% zinc, whose microstructure at room temperature is only composed of β phase, is called β brass.
Press processability
α-single-phase brass (from H96 to H65) has good plasticity and can withstand cold and hot working. However, α-single-phase brass tends to be brittle at medium temperature during hot working such as forging, and its specific temperature range varies with Zn content, generally between 200℃ and 700℃. Therefore, the temperature during hot working should be higher than 700℃. The main reason for the brittle zone of single-phase α brass at medium temperature is that there are two ordered compounds, Cu3Zn and Cu9Zn, in the α phase zone of Cu-Zn alloy system, which undergo ordered transformation when heated at medium and low temperature, making the alloy brittle. In addition, there are a few harmful impurities of lead and bismuth in the alloy, which form a eutectic film with low melting point and distribute on the grain boundary, resulting in intergranular fracture during hot working. Practice shows that adding trace cerium can effectively eliminate the brittleness at medium temperature.
In two-phase brass (from H63 to H59), besides α phase with good plasticity, β solid solution based on electronic compound CuZn appears in the alloy structure. The β phase has high plasticity at high temperature, while the β' phase (ordered solid solution) at low temperature is hard and brittle. Therefore, (α+β) brass should be forged in hot state. β brass with zinc content greater than 46% ~ 50% can't be pressed because of its hard and brittle properties.
