Heat Treating Copper: Annealing C101 and C110, and Why Pure Copper Doesn't Harden

Copper is not age-hardenable and has no allotropic phase transformation, so unlike steel there is no quench-and-temper route to harden it. Pure copper grades C101 (oxygen-free, OFHC) and C110 (electrolytic tough pitch, ETP) gain strength only through cold working, drawing, rolling, or peening, which deforms and elongates the grains and raises dislocation density. The harder a copper part is, the more cold work it carries. Annealing reverses this. Heating cold-worked copper into the recrystallization range (roughly 700 to 1200F depending on prior cold work and grade) lets new strain-free grains nucleate and grow, dropping hardness and restoring ductility, full-hard C110 at around 90 HRB can be annealed back to dead-soft at 40 HRB or below. This is the only thing copper heat treatment does for strength, and it does it by removing strength. The buyer takeaway: temper in copper (soft, half-hard, full-hard) is a cold-work condition, not a heat-treatable state. You order copper to a temper, and annealing is used to reset a part to soft so it can be formed again, not to add hardness.

C110 ETP contains a small amount of dissolved oxygen as cuprous oxide, and that oxygen is a liability at high temperature in a reducing atmosphere. If C110 is heated above about 700F in an atmosphere containing hydrogen (brazing, welding, or some annealing furnaces), the hydrogen diffuses in, reacts with the internal oxide to form steam, and the trapped high-pressure steam blows microscopic voids and cracks along grain boundaries. The part looks fine but is fractured internally, this is classic hydrogen embrittlement. The answer is C101 oxygen-free copper, which has the oxygen removed during manufacture so there is no oxide to react with. Any copper part that will see high-temperature brazing, hydrogen-furnace annealing, or vacuum-electron applications should be C101, not C110. This is exactly why semiconductor, vacuum, and high-reliability electronics use OFHC copper, the oxygen-free grade can be heated in reducing atmospheres without embrittling. For buyers, the grade choice is driven by the thermal process the part will see. If there is any high-temperature reducing-atmosphere step in the part's life, specify C101. C110 is fine for parts that stay cold or are heated only in neutral or oxidizing atmospheres.

Tellurium copper (C145) adds about 0.5 percent tellurium for free machining, the tellurium forms discontinuous particles that break chips, giving copper a machinability rating near 85 percent versus the gummy 20 percent of pure copper. It is still essentially pure copper electrically and thermally, and it is not age hardenable either, so it is annealed and stress relieved the same way, with the same hydrogen-embrittlement caution if any oxygen-bearing variant is used. Stress relief at lower temperatures (around 400 to 600F) removes residual stress from machining or forming without fully recrystallizing the grains, so you keep most of the cold-worked strength while stabilizing dimensions, useful on precision machined copper parts that would otherwise creep or distort. Bright annealing in vacuum or pure inert/hydrogen atmosphere (for C101) produces clean, oxide-free surfaces, important for parts that will be plated, brazed, or used in vacuum service. The practical guidance: use a full anneal to soften for forming, a low-temperature stress relief to stabilize machined parts while keeping their temper, and choose oxygen-free C101 plus an inert atmosphere when surface cleanliness or hydrogen exposure is a concern.