Soft Annealed Copper for Windings, Cables and Formed Parts

Drawing and rolling crowd the copper's crystal structure with defects called dislocations. They are what make cold-worked copper hard, and they also scatter electrons, which is why hard copper conducts a little worse. Heat the metal enough and new, defect-free grains grow in place of the distorted ones. That is recrystallisation, and it brings copper back to its natural state: low strength, high ductility and full conductivity.

For ETP copper the annealing range runs from roughly 250 °C to 650 °C. Heavily worked wire recrystallises at the low end; lightly worked or thick sections need more heat. Wire is usually annealed continuously on the drawing line by passing current through it, or in batches in a furnace with a protective atmosphere so the surface stays bright.

Yes, by a few percent. Soft ETP must meet 100 % IACS, the reference value written into ASTM B3 as a maximum resistivity of 0.15328 Ω·g/m² at 20 °C. Hard drawn wire is allowed to sit around 96–97 %. That small gap is one reason power cable and winding conductors are always annealed: the conductor is sized for losses and temperature rise, so every percent counts over a long run.

Soft copper wire is not bought on tensile strength. ASTM B3 does not specify any tensile requirement at all. What it checks is elongation, measured over a 254 mm gauge length, because elongation tells you whether the wire is truly soft. The minimum rises with diameter, from 15 % for very fine wire to 35 % for wire of 8.25 mm and above. The full table is below.

For rod and bar, ASTM B187 sets soft O60 copper at 28–37 ksi tensile with 25 % minimum elongation, and asks for a 180° bend without cracking when a bend test is ordered.

• Transformer and motor windings, where strip and wire must form tightly round coils without cracking insulation
• Cable conductors: IS 8130, the Indian standard for conductors in insulated cables, calls for plain or metal-coated annealed conductors
• Buried earth grid conductor and electrode connections, laid loose in a trench and bent to follow the layout
• Flexible jumpers, braids and laminated links that absorb vibration and movement
• Gaskets and sealing washers, where soft copper flows into the sealing faces
• Spun and deep-drawn parts made from copper circles and sheet
• ACR coils for split AC installation, which are annealed so they can be bent and flared by hand

Annealed tempers are not all identical. ASTM also orders soft copper by average grain size, such as OS025 or OS050 (0.025 mm and 0.050 mm). Finer grains give a smoother surface after deep drawing; coarser grains are softer. If your parts show an 'orange peel' surface after forming, ask for a finer-grain anneal.

When a job needs some bendability but must still hold its shape, half hard (H02) sits between O60 and H04. Busbars that are bent once at the panel and then left alone are often specified half hard. See hard drawn copper for the other end of the scale.

Copper work hardens every time it is bent, straightened or hammered. A coil that has been rewound twice, or a flat straightened over a knee, is no longer truly soft. Handle coils on their sides, avoid kinks, and form parts in as few operations as possible. If a part has to be reworked, it can be re-annealed with a torch to a dull red and then cooled. Unlike steel, copper does not harden when quenched in water.

When you order, say how the copper will be used. 'Soft strip for LT coil winding' or 'soft wire for earth grid, laid in trench' tells us far more than a temper code on its own, and it lets us flag early if a hard or half hard condition would actually suit the job better. Sizes are confirmed at enquiry, and the test certificate can report elongation and conductivity together.