| name | annealing |
| description | Annealing heat treatment — full annealing, process annealing, recrystallization, stress relief, spheroidize annealing, atmosphere control, steel/aluminum/copper, grain growth, ASTM A322. |
| metadata | {"priority":7,"promptSignals":{"phrases":["annealing","anneal heat treatment","recrystallization annealing","stress relief anneal","full annealing","spheroidize anneal"],"minScore":3}} |
Annealing Heat Treatment — Complete Skill
Types of Annealing
Full Annealing (Steels)
Purpose: maximum softness; relieve all hardness from prior processing; best machinability
Process:
- Heat to above Ac3 (hypoeutectoid) or between Ac1–Ac3 (hypereutectoid): typically 750–900°C
- Hold (soak): 1–2 hr per 25 mm section thickness; ensure full austenitization
- Furnace cool: cool slowly at 10–30°C/hr through transformation range (typically 600°C → 400°C)
- Air cool below 400°C
Microstructure result: coarse pearlite + ferrite (hypoeutectoid); soft and machinable
Typical hardness: 140–200 HB for low-carbon steels; 170–220 HB for medium carbon
Ac1 and Ac3 temperatures (approximately):
Ac1 ≈ 723°C (eutectoid); Ac3 ≈ 723 + 23(%Si) - 12(%Mn) - 17(%Ni) [°C; increases with Si, decreases with Mn/Ni]
Process Annealing (Partial; Subcritical)
Purpose: restore ductility of work-hardened low-carbon steel without full phase change
Temperature: 550–700°C (below Ac1)
Process: heat + hold 1–4 hr; air cool
Microstructure: recovery/recrystallization without phase transformation; retains some work-hardened strength
Use for: cold-drawn wire, sheet metal after deep draw operations
Recrystallization Annealing
Temperature (steel):
T_recryst ≈ 0.4 × T_melt [K] = 450–600°C for steel [rule of thumb]
For cold-worked steel: 550–700°C (below Ac1)
Grain growth after recrystallization:
d² - d₀² = K × t × exp(-Q / (R T))
d = grain diameter after time t; K = constant; Q = activation energy; R = gas constant; T = temperature [K]
At high T or long time: significant grain coarsening → reduced strength
Minimum strain for recrystallization (critical strain):
ε_crit ≈ 5–10% cold work; below this → incomplete recrystallization; abnormal grain growth risk
Stress Relief Annealing
Purpose: reduce residual stress without significantly changing hardness or microstructure
Temperature: 450–650°C (well below Ac1) for steels
Hold: 1 hr per 25 mm thickness; minimum 1 hr
Cool: slow air cool to prevent new thermal stresses
Residual stress reduction:
At 550°C, 1 hr: σ_residual reduced 60–80% of initial
At 450°C, 2 hr: σ_residual reduced 40–60%
Applications: welded fabrications (PWHT), castings, machined parts, AM builds
Spheroidize Annealing (High-Carbon Steels)
Purpose: convert lamellar pearlite to globular (spheroidal) carbides → maximum softness; excellent machinability
Process:
- Subcritical cycle: 690–720°C (just below Ac1), long hold 8–24 hr
- Or: cycle above/below Ac1 (heat to 740°C; cool to 680°C; repeat 3–5 cycles)
Hardness result: 170–210 HB for tool steels (much softer than annealed pearlite at 250 HB)
Applications: bearing steels (52100), high-carbon wire rod before drawing, cold-heading steels
Aluminum Annealing
Strain hardening alloys (1xxx, 3xxx, 5xxx):
Annealing T: 340–415°C; hold 1–3 hr; air cool
Result: full softness (O temper); UTS drops; elongation maximized
Solution heat treat alloys (2xxx, 6xxx, 7xxx):
Solution anneal: 450–540°C (above solvus); hold 20 min–2 hr; quench
Followed by age hardening (T6) or stabilize (T73)
Not "annealing" in softening sense; used to dissolve precipitates
Recovery (stress relief, Al):
200–250°C, 2–4 hr; partial recovery without full recrystallization; preserves most strength
Copper and Brass Annealing
Copper (C11000): T_anneal = 375–650°C; hold 1 hr; water quench OK
Brass (70/30): T_anneal = 425–750°C; quench prevents dezincification
Bronze: 500–700°C depending on alloy
Season cracking prevention (brass):
Stress relief at 250–300°C, 1 hr after cold drawing/forming → eliminates residual tensile stress
Atmosphere Control
| Atmosphere | Use | Notes |
|---|
| Air (open) | Low-carbon; temporary; scale acceptable | Oxidation → scale; decarburize surface |
| Nitrogen (N₂) | Most metals; clean; low cost | Non-reactive at anneal T |
| Dissociated NH₃ (75H₂/25N₂) | Bright anneal; Cu, SS, steel | Reducing; scale-free |
| Hydrogen (H₂) | Stainless; magnetics | Best reducing; explosion hazard |
| Vacuum | Tool steel; Ti; superalloys | Most expensive; cleanest |
| Endothermic gas | Steel; carburizing base | CO/H₂/N₂ mix; controlled atmosphere |
Decarburization (in air):
C in steel surface oxidizes to CO₂; depth δ ≈ 0.1–0.5 mm after 1 hr in air at 900°C
Prevent by using controlled atmosphere or pack annealing (surround part with cast iron chips)
Furnace Types
Batch (box/car-bottom): flexible; all sizes; temperature uniformity ±5–10°C
Continuous (conveyor/pusher): production annealing; uniform cycle; high throughput
Bell furnace: coils of wire/sheet; controlled atmosphere; uniform temperature
Vacuum furnace: reactive metals; tool steels; no scaling; no quench capability (cool under vacuum)
Grain Size Control
ASTM grain size number G:
d = 2^((G-1)/2) × 0.025 mm [d = mean grain intercept length [mm]]
G = 6: d = 0.044 mm; G = 8: d = 0.022 mm; G = 10: d = 0.011 mm
Fine grain (G 6–8): better fatigue, toughness, yield strength
Coarse grain (G 2–4): better creep resistance at high T; easier machinability
Grain control methods:
- Aluminum grain refiners (TiB₂ in Al): nucleates many crystals → fine grain
- Steel (Al-treated): AlN particles pin grain boundaries
- Niobium/vanadium in steel: carbide particles prevent grain growth during austenization
Standards
| Standard | Scope |
|---|
| ASTM A322 | Bar steel; includes annealing conditions |
| AMS 2759/7 | Hydrogen atmosphere annealing |
| ASTM E112 | Grain size measurement |
| SAE J423 | Steel annealing hardness methods |
Output
Provide: annealing type (full/process/recrystallization/stress-relief/spheroidize), temperature T [°C] and hold time t [hr], cooling rate or method (furnace/air/quench), atmosphere (air/N₂/H₂/vacuum), target hardness after treatment [HB], grain size target [ASTM G number], expected residual stress reduction [%], applicable material and grade, and applicable standard (ASTM A322, AMS 2759).
