Why Brass Is Annealed Rather Than Hardened
Brass is a copper-zinc alloy, and in the common grades it is a binary copper-zinc system with no useful precipitation hardening response. According to structure it is divided into single-phase alpha brass, with copper content roughly between 62.4% and 100% by mass, and two-phase alpha plus beta brass, with copper content roughly between 56.6% and 62.4%. The solubility of zinc in copper increases as temperature falls, which is why quench-and-age treatments used on aluminium alloys and steels have no equivalent here.
The exception is brass containing aluminium, which can respond to heat treatment, but for ordinary grades the purpose of heating is threefold: to soften cold worked metal so that further drawing, stamping or machining is possible, to control grain size so that the mechanical properties of the semi-finished product stay predictable, and to remove residual stress that would otherwise lead to stress corrosion cracking in service.
How Structure and Cold Work Govern Properties
The mechanical properties and cold forming behaviour of annealed brass semi-finished products depend mainly on grain size, and the general rules are simple. Fine grain gives higher hardness and strength; coarse grain gives better ductility and a lower yield point. Cold deformation raises hardness through work hardening, and a low annealing temperature leaves more of that hardness in place; at the same temperature a longer soak softens more, and at the same soak time a higher temperature softens more.
Brass with low zinc content work hardens only mildly, so it needs a finer grain to reach a given hardness. Brass containing more than about 20% zinc retains residual stress after cold deformation and cracks readily in a humid atmosphere containing ammonia or ammonium salts, or in contact with mercury and mercury salt solutions; that combination is the classic season cracking failure, and stress relief annealing is the standard protection.
Intermediate Annealing Temperatures by Grade and Size
During cold working of tube, rod and wire, the intermediate anneal is set by grade and reduced as the effective section size falls, because a thin section heats through faster and needs less thermal input to reach the same softening.
| Grade | Over 5 mm | 1 to 5 mm | 0.5 to 1 mm | Under 0.5 mm |
|---|---|---|---|---|
| H96 | 560 to 600 C | 540 to 580 C | 500 to 540 C | 450 to 500 C |
| H90 | 650 to 700 C | 620 to 680 C | 560 to 620 C | 450 to 560 C |
| H80 | 650 to 700 C | 580 to 650 C | 540 to 600 C | 500 to 560 C |
| H68 | 580 to 650 C | 540 to 600 C | 500 to 560 C | 450 to 520 C |
Two practical notes apply to every line of that table. First, high-zinc two-phase grades tolerate a wider temperature window than low-zinc single-phase grades, which overheat and coarsen quickly. Second, most brasses do not require rapid quenching after annealing; controlled furnace cooling is normal, and only the grades that respond to thermal treatment need a specific cooling path.
Final Annealing and Stress Relief Practice
Soft or fully annealed temper for deep drawing and severe bending is produced in the upper part of the recrystallisation range, typically around 550 to 650 C for the common H68 and H62 grades, with soak time chosen to reach a uniform grain size.
Half-hard and quarter-hard tempers are produced by a controlled final cold reduction after the last full anneal, or by a low-temperature recovery anneal that removes part of the work hardening while leaving the grain size fine.
Stress relief annealing is a separate operation carried out at a much lower temperature, typically in the range of 200 to 300 C for one to several hours, which is enough to relax internal stress without softening the part or changing its grain size.
Atmosphere control matters: bright annealing of tube and wire for the electrical and sanitary markets is done in a reducing or inert atmosphere or under a protective cover, because oxide scale on thin wall tube is removed only by pickling that also removes metal.
Heating and cooling rates should be controlled on thick rod and on welded tube so that thermal gradients do not re-introduce stress or distort dimensions.
Post-Annealing Property Checks and Standards
The wrought designations and composition of the grades being annealed are set by GB/T 5231, and the product standards that fix temper, dimension and mechanical requirements include GB/T 2059 for strip, GB/T 4423 for drawn rod, GB/T 1527 for tube, EN 12163 and EN 12167 for rod and profile, and ASTM B36 for brass plate, sheet and strip. Testing after annealing usually covers tensile strength and elongation in the specified temper, hardness by the Vickers or Brinell method under ISO 6507-1 or ISO 6506-1, and grain size determination with a comparison chart or intercept method under ASTM E112.
Stress corrosion resistance of a finished part can be verified with an ammonia test to ISO 6957, which is used mostly on cold-formed fittings and on components intended for humid or ammoniacal service. For export orders the release package typically combines the heat analysis, tensile and hardness results for the delivered temper, grain size where the drawing requires it, dimensional records and an inspection certificate to EN 10204 type 3.1.
Two mistakes recur in production. The first is using an intermediate anneal temperature intended for a thin section on a thick one and then wondering why the core is still hard; the second is stress relieving at a temperature high enough to soften the finished part and lose the temper the customer ordered. Both are avoided by tying the schedule to grade and to measured section size rather than to a single house temperature.
Frequently Asked Questions
Q: Can brass be hardened by heat treatment?
A: In ordinary copper-zinc grades, no. Brass is strengthened by cold work and softened by annealing. Only brass containing aluminium, or a few special grades, responds to a thermal hardening treatment.
Q: What is the purpose of stress relief annealing?
A: To remove residual stress left by cold forming, which otherwise causes stress corrosion cracking in a humid ammonia-bearing atmosphere. It is carried out at a much lower temperature than softening anneal so that temper and grain size are not changed.
Q: Why do annealing temperatures fall as section size falls?
A: Because a thin section reaches temperature faster and needs less heat input for the same degree of softening. Using the temperature intended for a thick section on thin wire or strip causes grain coarsening and loss of ductility.
Q: Is water quenching needed after annealing brass?
A: Normally not. Controlled cooling in the furnace is standard practice, since most brasses do not respond to a quench and fast cooling can introduce new thermal stress, especially in thick rod and welded tube.
Q: How are annealed properties verified?
A: By tensile testing in the specified temper, hardness testing under ISO 6507-1 or ISO 6506-1, and grain size measurement by ASTM E112, with an ammonia stress corrosion test to ISO 6957 where the end use is exposed to humidity or ammonia vapour.
Q: What happens if the annealing temperature is too high?
A: Grain coarsening. The material softens beyond the ordered temper, surface finish deteriorates and, in thin strip and tube, mechanical properties fall below specification and subsequent drawing or bending behaviour becomes unpredictable.
