Recrystallizing Annealing Process For Treating Semifinished Brass Stock

Abstract

The semifinished stock is annealed at a temperature of 350.degree.-600.degree. C. in a hydrogen-nitrogen atmosphere which consists of 3-99.5% hydrogen, less than 0.08% carbon dioxide, less than 0.01% water vapour, less than 0.001% oxygen, up to 5% carbon monoxide, balance nitrogen.

Description

Because brass can be cold-formed only to a limited extent, semifinished brass stock must be process-annealed when it has been cold-formed about 50-70%. This annealing has been performed so far in a normal air atmosphere because no inert gases are available which prevent an oxidation, which is inevitable during a heat treatment in air. Before the cold-forming is continued, however, the semifinished brass stock must have an unblemished surface. For this reason, a wet chemical pickling treatment is required to remove the copper-zinc oxide which has formed on the surface of the brass during the annealing in air. Sulphuric acid is usually employed for pickling, although its use involves considerable difficulties as regards the purification of water. Besides, a pickling plant involves a high capital requirement particularly when the stock to be pickled consists of bundles of long brass tubes and an attempt is made to accomplish a uniform pickling of the inside surface of the tubes throughout the length in a vacuum in a so-called autoclave. Detrimental substances cannot be avoided even if the waste water is optimally neutralized. For this reason, the process previously employed to pickle semifinished brass stock after the process-annealing is not satisfactory.

It is an object of the invention to eliminate these disadvantages and to provide a process by which semifinished brass stock can be process-annealed without a formation on the surface of the brass of a copper-zinc oxide which would require a subsequent pickling and in which a bright annealing is accomplished.

This object is accomplished according to the invention in that the semifinished stock, which may have been degreased, if desired, is annealed at a temperature of 350.degree.-600.degree. C., preferably 430.degree.-530.degree. C., in a hydrogen-nitrogen atmosphere which contains 3-99.5% hydrogen, less than 0.08% carbon dioxide, less than 0.01% water vapour, less than 0.001% oxygen, up to 5% carbon monoxide, balance nitrogen. Compared to its hydrogen content, this hydrogen-nitrogen inert gas contains only very small amounts of oxygen, water vapour and carbon dioxide and due to this high purity has a very high reduction potential which in fact results in a bright annealing of the brass. This has not been possible before and is due to the fact that the zinc vapour pressure at the selected recrystallizing temperature is such that it does not adversely affect the bright recrystallization.

When the surface of the semifinished products has been degreased, the subsequent annealing will not result in a discolouration of the surface, which would otherwise be caused by the cracking of adhering lubricants or the like. Because the annealing is thus a real bright annealing, there is no longer a need for any subsequent surface treatment by pickling or the like, which would involve the disadvantages and difficulties which have been described.

It will be particularly desirable if the semifinished stock are degreased by being heated to 250.degree.-400.degree. C. under a vacuum so that the lubricant or the like adhering to the surface evaporates. The degreasing may be accomplished in the same furnace and there is no need to change the charge or to cool the stock before the actual annealing.

The process according to the invention will be explained more fully with reference to the following example:

Semifinished brass stock (brass wire, cold-rolled brass strip, cold-drawn brass tubes) of Ms 63 grade brass, free of lead, were heated to 350.degree. C. within 3 hours under a vacuum in a vacuum pot annealing furnace provided with means for circulating inert gas. During that time, the lubricant which adhered to the surface as a result of the cold working of the stock evaporated without leaving a residue. The furnace was then scavenged with inert gas; its temperature was raised to 470.degree. C. and held for 3 hours on that level. The inert gas was vigorously circulated during that time. The inert gas had the following composition: 15% H.sub.2, less than 0.001% CO.sub.2, less than 0.0005% H.sub.2 O, less than 0.0001% O.sub.2, less than 2% CO, balance N.sub.2. The charge was then cooled to room temperature during 12 hours, while the inert gas was kept in circulation. The resulting semifinished stock was prefectly bright and had been completely recrystallized.

Claims

What is claimed is:

1. A recrystallizing annealing process for treating semifinished brass stock, which comprises annealing the semifinished stock at a temperature of 350.degree.-600.degree. C. in a hydrogen-nitrogen atmosphere which consists of 3-99.5% hydrogen, less than 0.08% carbon dioxide, less than 0.01% water vapour, less than 0.001% oxygen, up to 5% carbon monoxide, balance nitrogen.

2. A process as set forth in claim 1, in which said semifinished brass stock is degreased before it is thus annealed.

3. A process as set forth in claim 2, in which said semifinished stock is degreased by being heated to 250.degree.-400.degree. C. under a vacuum.

4. A process as set forth in claim 1, in which said semifinished stock is thus annealed at a temperature of 430.degree.-530.degree. C.