annealing

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:

1. Heat to above Ac3 (hypoeutectoid) or between Ac1–Ac3 (hypereutectoid): typically 750–900°C
2. Hold (soak): 1–2 hr per 25 mm section thickness; ensure full austenitization
3. Furnace cool: cool slowly at 10–30°C/hr through transformation range (typically 600°C → 400°C)
4. 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).