THE qualities, testing and uses OF cold-rolled brass.

Brass includes the workable alloys of copper and zinc, with a range of copper content from 58% upwards. From the standpoint of the metallurgist, commercial brasses fall into two groups, the one merging into the other at about 63% of copper. Richer mixtures, roughly those above this percentage, are the " Alpha Brasses," and consists of homogeneous aggregates of similar type crystals ; mixtures, poorer in copper, and known as the " alpha + beta " brasses, show a heterogeneous mixture of two different types of crystal. A further characterisation is that brass containing between 58% and 63% of copper becomes plastic at high temperatures and can be readily hot-rolled, extruded or hot-stamped. But with increasing copper content the conditions for satisfactory hot-working become very much restricted, and ready plasticity at high temperatures does not again become a characteristic of the alloys until they contain over 90% of copper. Nevertheless, with due attention to purity of metal and control of temperature, the hot-working of the commonly cold-rolled brasses is possible, although not generally commercially carried out. For certain uses the hot-rolled Brass sheet and strip, containing generally less than 61% of copper may be used, but owing to the restricted ductility of such alloys, and also on account of the relatively rough finish that a hot-rolled metal usually assumes, such brasses do not satisfactorily replace the cold-rolled brasses containing higher percentages of copper.

Generally speaking the copper content in cold-rolled brass is not below 62%. The cold-rolled brass and Copper Association basis quality is up to 64%.

With such a variety of alloys, it is next to impossible to speak of " quality " in general terms. The term " quality " has become so closely connected with the copper content that its fuller meaning is not always appreciated. The copper content is naturally a ready standard against which to fix price, but quality should be considered in wider terms, such as regularity of anneal or temper, freedom from superficial imperfections, and correctness of gauge. Whatever the composition of the alloy may be, its subsequent quality is largely decided in the casting process.

In the case of soft annealed metal, the uniformity and correct softness are only obtained by exercising every care, and strict control of temperature and time in the annealing muffles. Finally, the finished annealed metal is pickled and cleaned so as to leave it in a condition suitable for subsequent working. In the case of metal to be finished to some degree of temper, the final rolling requires accurate control, since very slight variations in the amount of cold work lead to considerable differences in final result.

The effect of the presence of small quantities of metals other than the constituents copper and zinc on the quality of brass is one which can only be commented upon in a general way. Broadly speaking, where high ductility is demanded, the brass should be an alloy of as pure copper and zinc as is obtainable. Here the Trade has, within recent years, taken a big step forward in that basic prices to-day are founded upon the use of electrolytic copper and good-class spelters, as compared with the old days, when they were based upon " Best Select " and "G.O.B." It is generally considered that the higher the percentage of copper in the brass, the greater will be the influence of any impurities on its physical properties. Lead and iron are, probably, the two most frequent impurities which are to be found in appreciable quantities in brass. Lead facilitates the cutting properties, but it reduces ductility. Iron gives increased tensile strength and hardness, but again at the expense of ductility. Owing to the purity of the raw materials generally used, other impurities, such as arsenic, antimony, bismuth, sulphur, etc., are seldom found save in such small quantities that their presence does not affect the mechanical properties of the brass. Antimony and bismuth are the most deleterious of these infrequent impurities, and should not be present in more than traces. Arsenic, though by itself not so injurious, becomes markedly more so in the presence of other impurities, such as antimony and, therefore, if present should only be in very small quantities.

Sometimes larger quantities of lead, iron, tin, nickel, manganese or aluminium are definitely added for the purpose of giving to the brass some special property, such as free machinability, resistance to corrosion, increased strength, etc., such are known as " special brasses," and it is not intended to consider such here.

In the annealing of a hard cold-rolled brass, three definite stages are recognisable. Rolled brass, after being annealed at about 260° C. shows a slight hardening. Above 300° C. rapid softening sets in. Above about 450° C. the rate of softening slows down considerably. The sudden fall in hardness between 300° C. and 450° C. is recognised as a period of re-crystallization, when the broken-down material commences to recrystal-lize. Above the temperature of recrystallization the small reformed crystals grow with rise of temperature. The most satisfactory temperature, therefore, at which to carry out the annealing process is one above that at which recrystallization has set in, but below that at which the crystals have grown to large dimensions. The physical properties of a metal depend so largely on the grain size of the crystals that these may be taken as a very good criterion of its suitability for any specific purpose. A grain size of 0.04 m.m. to 0.06 m.m. (which means that these are the average diameters of the crystal grains when viewed under the microscope after polishing and etching the surface of the metal), is suitable for subsequent cold-working, either by pressing or drawing. A smaller grain size will indicate less ductility, whilst a larger grain size means little increase in ductility, but reduced strength, and a roughening of the surface of the metal during cold work which is undesirable.