Copper busbar temper: soft, half-hard and hard

What temper means on a busbar drawing

Temper describes the amount of cold work and heat treatment in the bar and, with it, the strength and ductility. Soft, or annealed, temper has been heated to recrystallise the grain and remove the work hardening. Hard temper has been cold rolled after the final anneal. Half-hard sits between the two.

The names differ between standards. ASTM uses designations such as O for annealed and H02 or H04 for half-hard and hard. EN 13601 uses a similar system based on the letter H followed by a number, with O for annealed. The drawing should name the designation and the standard rather than relying on the word soft.

Temper is not the same as grade. A C11000 bar can be supplied soft, half-hard or hard, and so can an oxygen-free bar. Grade controls chemistry; temper controls mechanical condition. Both have to be on the drawing.

How the temper is produced

Copper bar is cast, hot rolled or continuously cast, then cold rolled to size. Cold rolling reduces the section and introduces dislocations into the crystal structure, which raises strength and hardness and reduces ductility. An anneal between passes softens the metal again so it can be rolled further.

The final temper is set by the last cold reduction and, where needed, a controlled anneal. A soft temper gets a final anneal that recrystallises the grain. A hard temper stops after a substantial cold reduction. A half-hard temper uses an intermediate reduction or a partial anneal.

Strength, stiffness and springback

Soft copper is weak but very ductile. It deforms plastically at a low stress, which makes it easy to bend and easy to straighten, and also easy to damage in handling. A soft bar that is unsupported over a long span will sag under its own weight.

Hard temper raises the yield strength substantially, often by a factor of several over the annealed condition. The bar resists bending, holds its shape across a span and keeps a flatter joint face under bolt load. That is valuable in a part that is also a structural member or that must stay flat when clamped.

Springback is the practical consequence. When a hard bar is bent, the elastic recovery is larger, so the formed angle opens up more when the tool releases. The tool has to overbend to compensate, and the compensation changes with thickness and with the bend radius. A shop that normally forms soft bar will need a different setup for hard bar.

Bend radius and the risk of cracking

The minimum bend radius grows as the temper gets harder. Soft copper can be bent to a tight radius relative to its thickness. Half-hard needs more. Hard temper needs substantially more, and an edgewise bend at a tight radius is where cracks appear first.

The outside of the bend carries tensile strain. In a hard bar, the ductility left after cold rolling may not be enough to accommodate that strain, so the surface opens up. The crack may be small at first and grow under thermal cycling or vibration, which makes it a reliability problem rather than a cosmetic one.

Three routes avoid the problem. Increase the inside radius. Bend across the rolling direction rather than along it, where the ductility allows. Or form in a softer temper and, if strength is needed, age or anneal afterwards according to the alloy. Each route has a cost, and the drawing should say which is intended.

Conductivity and current capacity

Cold work slightly reduces conductivity. A hard-drawn copper bar may sit a little below the annealed value, while still meeting the minimum for the grade in most cases. The change is small compared with the effect of section, joint quality and temperature.

Where a formed part is annealed after bending, the conductivity returns to the annealed value and the bar softens. That is useful when the design needs a tight bend and the best conductivity, but it also removes the strength that a hard temper would have provided.

Joint faces, bolt load and relaxation

A bolted joint depends on contact pressure. A soft bar conforms to the mating surface, which can help contact at first, but it also creeps and relaxes under sustained bolt load, especially at higher temperature. The joint can lose torque over time and the resistance rises.

A harder temper resists relaxation and holds a flatter face. That is one reason hard or half-hard bar is used for joint plates and for bars that are clamped repeatedly. The trade is that a hard bar conforms less to an imperfect mating surface, so the flatness of both faces matters more.

The washer and the bolt pattern can compensate. A hardened washer spreads the load, and a larger contact area reduces the stress in the copper. For a critical joint, specify the flatness, the surface finish, the plating and the torque, and check the millivolt drop after assembly.

Thermal cycling is the real test. A joint that is sound at room temperature can loosen after repeated heating and cooling. If the bar runs hot, the temper and the joint design should be reviewed together, not separately.

Specifying and verifying temper

Put the temper designation and the material standard on the drawing, together with the properties that matter for the part. If strength is the reason for the temper, state a minimum tensile or yield strength. If formability is the reason, state the bend radius and, where useful, a bend test requirement.

Hardness is a quick check but it is not a substitute for a tensile test when strength is critical. A hardness reading can confirm that a bar is in the expected range, but the conversion to strength depends on the alloy and the product form. Use the mill certificate for the formal result and hardness for routine verification.

A bend test is the most direct check of formability. Bend a sample around a mandrel of the specified radius and inspect the outside of the bend for cracks. Do it on a sample from the lot, not on a hand-picked piece from the bench.

Check the certificate against the order at goods in. It should name the grade, the temper, the lot and the mechanical properties. A certificate that names only copper is not enough to show that the bar will bend or hold a joint.

Matching temper to the part

For a formed busbar with several bends, soft temper is usually the practical starting point. It forms without cracking, and the finished part does not need high strength if it is supported in the assembly. If the design needs strength after forming, the route becomes form then anneal or form then age, and the drawing has to say so.

For a flat bar with holes and a joint face, half-hard is often the best balance. It holds its shape, keeps a flatter face and still bends at a workable radius if a small offset is needed. Hard temper suits a flat part that must resist bending or hold a clamped joint, where the forming is minimal.

The choice should follow the function. Write down what the bar has to do: carry current, locate a component, stay flat, survive a bend. The temper that satisfies the mechanical function, at the conductivity the current allows, is the right one.