Bright annealing brass in controlled atmosphere

Abstract

Producers of semifinished brass products have long been interested in bright annealing these products. First, the affinity between zinc and oxygen was investigated, and the possibility of achieving the required reduction potential in a nitrogen-hydrogen atmosphere was checked. The investigation was concentrated on the influence of the trace gases 0₂, CO₂, and CO, and on that of H₂O, i.e. of dewpoint. As the equilibrium equation Zn+H₂O ⇌ H₂+ZnO strongly inclines to the zinc oxide side, the equilibrium factor K = H₂/H₂O is large. The problem of reductive potential having been solved, that of zinc evaporation was considered. At recrystallization temperatures traditionally used for brass, around 540°–560°C, the zinc evaporation pressure is unmanageably high. It was discovered that it was possible to achieve reasonable recrystallization results even at lower recrystallization temperatures, if the recrystallization time was lengthened. It was found, for example, that Cu-37Zn can be fully recrystallized, reaching grain sizes of 35 μm, with a soaking time of 4 h in the region of 400°C. In the case of Cu-30Zn, it is possible to anneal to a grain size of 6–7 μm even at 400°C, with 4 h soaking. At these temperatures, the zinc evaporation pressure can be reduced to a tolerable level. The scope of the investigations of zinc evaporation pressure was extended to other copper-zinc alloys, and it was discovered that these findings are applicable here too. The need for lubricants which would evaporate satisfactorily at these low recrystallization temperatures without leaving residues is also discussed.