Annealing
Also: anneal, full annealing, process annealing, spheroidize annealing, spheroidizing, solution annealing, intermediate annealing, bright annealing, softening
Annealing heats metal and cools it slowly to soften it, restore ductility after cold work and improve machinability: full, process and solution anneals.
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Annealing is the reverse of hardening: heat the metal, hold it, and cool it slowly so it ends up in its softest, most ductile, lowest-stress condition. In steel, a full anneal heats above the upper critical temperature, around 790-900 °C (1450-1650 °F) depending on carbon content, and cools in the furnace, leaving coarse ferrite and pearlite. A process (subcritical) anneal at about 550-700 °C (1020-1290 °F) recrystallizes cold-worked low-carbon steel between drawing or forming operations. A spheroidize anneal holds high-carbon and alloy steels near the lower critical temperature, around 700 °C (1300 °F), until the carbides ball up, which gives the best machinability and cold formability for 52100, 1095 and tool steels.
Solution annealing is a different process under the same name. Austenitic stainless (304, 316) is heated to about 1040-1120 °C (1900-2050 °F) to dissolve chromium carbides, then cooled fast by water quench or rapid air or gas cooling to keep them from re-forming; slow cooling through 425-815 °C (800-1500 °F) sensitizes the steel to intergranular corrosion. It softens work-hardened stainless for further forming and restores corrosion resistance after heavy forming or welding. Copper and brass are annealed at roughly 400-650 °C (750-1200 °F) between drawing steps, and aluminum is annealed to the O temper at about 345-415 °C (650-775 °F).
For buyers, annealing matters in two places. Most alloy and tool steel bar and plate is sold annealed, because that is how it machines and forms best, and parts are hardened after machining. And parts that are drawn, spun, bent or stamped heavily in stages, such as deep-drawn cups, spun stainless and bent copper busbar, may need intermediate anneals to keep from cracking. The cost is strength: an annealed part is at its weakest, so annealing is rarely the final condition of a load-bearing steel part.
At a glance
| Typical tolerances | Annealing is specified as a maximum hardness rather than a band: for example 4140 at 197 HB max or tool steel at 235 HB max, or a Rockwell B maximum on sheet and stainless. Annealed hardness varies little within a heat, so a maximum is easy to meet. There is no case depth. Distortion is moderate: slow cooling adds little stress, but long or thin parts can sag at full-anneal or solution-anneal temperature, and stainless solution annealed with a water quench can move as much as a hardened part. Anneal before finish machining, not after. |
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| Size limits | Box, bell and car-bottom furnaces take annealing loads from small batches to weldments and coils of many tonnes; mills anneal strip and wire in continuous furnaces. Commercial heat treaters commonly handle parts up to about 900 x 1,200 mm (36 x 48") in batch furnaces and much larger in car-bottom furnaces. Stainless solution annealing is limited by quench tank size and by how fast a heavy section can be cooled through the sensitization range. |
| Surface finish | Annealing in air leaves scale on steel and a dark oxide on copper and stainless; a protective atmosphere or vacuum (bright annealing) leaves a clean surface. Full anneals in a poorly controlled atmosphere can decarburize the surface, so leave stock on surfaces that will be hardened later. Stainless annealed in air must be descaled by pickling or blasting and passivated (ASTM A380); stainless bright annealed in hydrogen or vacuum needs neither. Copper and brass are pickled to remove oxide. |
| Lead time | Commercial annealing in 3-7 business days. Spheroidize anneals run long cycles, often 10-30 hours, and stainless solution annealing with certification can take 1-2 weeks. Most machine shops avoid the wait by buying bar and plate already annealed. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon and alloy steels (1045, 4140, 52100, 1095) are full or spheroidize annealed for machining and forming; low-carbon 1018 usually machines better cold drawn or normalized, because fully annealed low-carbon steel is gummy and tears. Tool steels (D2, A2, O1, H13) are supplied spheroidize annealed at about 190-255 HB and must be re-annealed before a hardened part can be re-machined. Austenitic stainless (304, 316) is solution annealed. Copper, brass and aluminum anneal readily at low temperatures to reverse work hardening. Titanium (Ti-6Al-4V) is mill annealed at about 700-785 °C (1300-1450 °F) in vacuum or argon. Precipitation-hardening alloys are not annealed in the ordinary sense; they are solution treated, which is the first step of their hardening sequence.
What drives the cost
- Cycle time: spheroidize and full anneals with slow furnace cooling tie up a furnace for a day or more
- Atmosphere: vacuum or bright annealing costs more than air but saves descaling
- Weight and furnace loading, with a minimum lot charge for small jobs
- Quenching and fixturing for stainless solution anneals
- Descaling and pickling after an air anneal
- Hardness testing and certification
When to use it
- Softening a hardened or work-hardened part so it can be re-machined, re-formed or repaired
- Between deep-drawing, spinning or bending stages once the metal has work hardened
- Improving the machinability of high-carbon and tool steels (spheroidize anneal) before machining and hardening
- Solution annealing stainless after heavy forming or welding to restore corrosion resistance
- Making copper and brass soft and ductile for bending busbar, tubing and terminals
When not to
- When the final part needs strength: annealed steel is at its weakest
- Low-carbon steel for machining: a full anneal makes 1018 gummy, and cold-drawn or normalized bar cuts cleaner
- Heat-treated aluminum parts that must keep their temper: annealing to O throws away the T6 strength
- Only to reduce distortion from machining or welding: stress relieving at a lower temperature does that without softening the part
Design tips
- Specify the anneal by result, a maximum hardness ("anneal to 197 HB max") or a temper designation (6061-O, 304 annealed), and name the type if it matters: full, spheroidize or solution anneal.
- Buy material already annealed where possible; mill-annealed bar is cheaper than annealing parts.
- For stainless weldments that will be solution annealed, allow for movement in the quench and leave finish stock.
- On multi-stage deep drawing or spinning, ask the shop where intermediate anneals fall in the routing; each adds cost and a cleaning step.
- If an air anneal is acceptable, plan for scale removal; if the surface must stay clean, specify bright or vacuum annealing.
- To re-machine a hardened tool steel part it must be fully annealed, then re-hardened, and it moves each time; a new part is often cheaper.
Frequently asked questions
- What is the difference between annealing and normalizing?
- Both heat steel above its critical temperature. Annealing cools slowly in the furnace for the softest, most machinable result. Normalizing cools in still air, which is faster and gives a finer, stronger, more uniform structure; it refines the grain of forgings and weldments rather than making the steel as soft as possible.
- Does annealing make steel easier to machine?
- For medium- and high-carbon steels and tool steels, yes: a spheroidize anneal gives the best machinability, and tool steel is sold that way. For low-carbon steel like 1018 it can make things worse, since fully annealed low-carbon steel is soft and gummy; cold-drawn or normalized bar cuts cleaner.
- What is solution annealing for stainless steel?
- Heating austenitic stainless such as 304 or 316 to about 1040-1120 °C (1900-2050 °F) and cooling it quickly. It dissolves chromium carbides and softens work-hardened material, restoring full corrosion resistance. Cooling too slowly through 425-815 °C (800-1500 °F) lets carbides re-form at the grain boundaries (sensitization), which ASTM A262 tests for.
- Can you anneal hardened steel?
- Yes. A full or spheroidize anneal softens hardened steel back to a machinable state so it can be reworked and hardened again. Expect scale or decarburization unless it is done in vacuum or a protective atmosphere, some size change, and the cost of re-hardening.
- Does annealing soften aluminum?
- Yes. Annealing 6061, 5052 or 7075 at about 345-415 °C (650-775 °F) with a slow cool gives the O temper, the softest condition, used for severe forming. Heat-treatable alloys such as 6061 can then be solution treated and aged back to T6.
