Heat Treatment Selection

Anneal. Quench.
Temper. Age.
Right process, right material.

Heat treatment transforms material properties — soft to hard, brittle to tough, low to high strength. Different processes for different materials. This guide covers selection for common engineering needs.

Free DFM Review Heat treatment overview

01 · Process types

Heat treatment processes.

Annealing

Soften + stress relief

Heat to specific temperature, slow cool. Softens material (reverses cold work), relieves internal stress, improves machinability. Used after forging, cold working, machining.

Normalizing

Refine grain

Similar to annealing but faster cooling (air cool). Refines grain structure, improves mechanical uniformity. Standard for steel forgings and weldments before machining.

Through hardening

Quench + temper

Heat to austenitizing temperature, quench (oil, water, or air), then temper at lower temperature. Produces hard material throughout thickness. Standard for tool steels, structural parts.

Case hardening

Surface hard core tough

Hardened surface + soft tough core. Processes: carburizing (add carbon to surface), nitriding (add nitrogen), induction hardening (selective surface hardening). For gears, shafts, wear surfaces.

Precipitation aging

Al, Ti, PH steel

Age at specific temperature to precipitate strengthening particles. Used for: 7075-T6 aluminum, 17-4 PH stainless, Inconel 718, beryllium copper. Produces high strength.

Solution treatment

Pre-aging

Heat to high temperature to dissolve precipitates, quench. Produces soft, workable state. Typically followed by aging to final properties. Critical first step for precipitation hardening.

02 · Case hardening

Surface hard processes.

Process	Surface hardness	Case depth	Time	Distortion
Carburizing	60-62 HRC	0.5-2 mm	8-12 hours	Moderate
Carbonitriding	58-60 HRC	0.1-0.5 mm	4-6 hours	Low
Nitriding	1000-1300 HV	0.2-0.5 mm	24-60 hours	Minimal
Gas nitrocarburizing	700-900 HV	0.05-0.5 mm	3-6 hours	Low
Induction hardening	55-60 HRC	1-6 mm	30 sec - 5 min	Moderate
Flame hardening	55-60 HRC	2-10 mm	1-10 min	Moderate
Salt bath nitriding	700-1000 HV	0.1-0.3 mm	1-4 hours	Low
Plasma nitriding	800-1100 HV	0.1-0.6 mm	8-24 hours	Very low

03 · By material

Material-specific workflows.

Carbon & alloy steels

• 4140 Q&T: quench from 850°C (oil), temper 425-650°C
• 4340 Q&T: quench from 830°C (oil), temper 205-650°C
• 8620 (case hardening): carburize 900°C, quench, temper 150°C
• 1045 (medium carbon): normalize from 870°C, then induction harden
• O1 tool steel: harden 800°C (oil), temper 175-315°C to 58-62 HRC
• D2 tool steel: harden 1020°C (air), temper 175-540°C to 58-62 HRC

Stainless steels

• 304/316L: solution anneal 1040°C, water quench. No hardening possible.
• 17-4 PH: solution 1040°C, age 480°C (H900) for max strength 1310 MPa
• 17-4 PH H1025: 1025°F/552°C → 1000 MPa yield, better ductility
• 15-5 PH: similar to 17-4, stainless with higher Cr for better corrosion
• Martensitic 420: quench from 980°C (oil), temper to 45-55 HRC
• Duplex 2205/2507: solution anneal only, no age hardening

Aluminum alloys

• 6061-T6: solution 530°C (water quench), age 180°C × 8h
• 7075-T6: solution 485°C (water quench), age 120°C × 24h
• 2024-T3: solution 495°C (cold water), natural age at room temperature
• Cast aluminum A356-T6: solution 540°C (water), age 155°C × 3-5h
• Soft aluminum 6061-O: anneal at 415°C, furnace cool
• Artificial aging increases strength but reduces corrosion resistance

Titanium & superalloys

• Ti-6Al-4V: solution 955°C (air cool), age 540°C × 4h for STA condition
• Ti-6Al-4V annealed: 705°C × 1-4h, air cool — standard mill condition
• Inconel 718: solution 980°C + age 720°C + age 620°C = full strength
• Inconel 625: solid solution strengthened — no aging heat treatment
• Hastelloy C-276: solution anneal 1120°C, water quench, no aging
• Haynes 230: solution anneal 1175°C, air cool

04 · Selection guidance

Which process when.

For structural parts needing through-strength: Quench and temper (4140 Q&T standard workhorse). Achieves uniform hardness and strength throughout thickness. Specify hardness range (e.g., 28-32 HRC, 38-42 HRC) based on strength requirements.

For wear-critical surfaces with tough core: Case hardening. Gears: carburizing (standard). Shafts in moderate wear: nitriding (minimal distortion, lower temperature). Selective wear surfaces on large parts: induction hardening (only hardens where needed).

For maximum strength at room temperature: Precipitation hardening. 17-4 PH H900 condition — 1310 MPa yield, corrosion resistant, non-magnetic in annealed state. Inconel 718 — 1035 MPa yield at 650°C service.

For improved machinability: Full annealing (slow cool from austenitizing temperature). Reduces hardness to typical 180-200 HB. Standard for: tool steels before machining (harden after machining), forged alloy steels, cold-worked stainless.

For stress relief after machining: Stress relief annealing. Heat to 500-650°C for steel, hold 1-4 hours, slow cool. Relieves residual stress from machining without changing hardness. Important for: thin-wall parts, welded assemblies, precision parts, parts for dimensional stability.

NADCAP qualified heat treatment: For aerospace and medical, we coordinate with NADCAP-qualified heat treatment partners. Full process documentation, calibrated furnaces, qualified procedures. Lead time adds 5-10 days but traceability and quality essential for regulated applications.

FAQ

Carburizing vs nitriding?

Carburizing: adds carbon to surface, requires quench. Case depths 0.5-2mm. 60+ HRC surface. Higher temperature process (900°C) causes distortion. Used for gears (deep case needed), high-load wear parts. Nitriding: adds nitrogen to surface, no quench (already hard). Case depths 0.2-0.5mm. 1000+ HV (65+ HRC equivalent). Lower temperature (500-550°C) means minimal distortion. Used for precision parts, thin parts, finished parts needing hardening. For gears that need post-hardening grinding: nitriding avoids distortion. For gears where distortion isn't critical: carburizing produces deeper case.

What's distortion risk during heat treatment?

Through hardening with water quench: severe distortion potential, especially for thin or complex parts. Oil quench: less severe but still significant. Air hardening (A2, D2, H13 tool steels): minimal distortion. Nitriding: virtually no distortion (low temperature). For precision parts, design with oversize allowance for post-hardening grinding, or use low-distortion process (nitriding, vacuum heat treat). Budget 0.05-0.2 mm distortion typical.

Aging for 17-4 PH conditions?

17-4 PH conditions: H900 (aged at 480°C / 900°F) — highest strength 1310 MPa, some brittleness. H1025 (552°C / 1025°F) — 1000 MPa, better ductility, most common. H1075 (580°C) — 965 MPa, best toughness. H1150 (620°C) — 790 MPa, maximum ductility. Specify condition based on strength/ductility needs. H1025 is default for most applications.

Aerospace heat treatment qualification?

NADCAP-certified heat treatment required for aerospace parts. NADCAP audit covers: process specifications (AMS or OEM), equipment calibration, operator qualification, process controls, quality records. Heat treat lot documentation required with aerospace parts. We partner with NADCAP-certified heat treatment suppliers — adds 5-10 days to lead time but essential for aerospace compliance.

Stress relief — when needed?

Stress relief required for: (1) Precision parts with tight dimensional stability. (2) Welded assemblies (especially thick section welds). (3) Heavily-machined parts with deep material removal. (4) Parts cycled through thermal or pressure service. (5) Castings before machining. Process: heat to 500-650°C (steel), hold 1-4 hours, slow cool. Reduces residual stress by 70-90% without changing hardness significantly.

Vacuum heat treatment — when?

Vacuum heat treatment: heating in vacuum or inert gas atmosphere. Benefits: no surface oxidation, bright finish retained, clean surfaces for subsequent operations. Used for: aerospace parts (oxidation control), precision tooling (dimensional stability), stainless steels (maintain passivation), nickel alloys (Inconel). Premium over atmosphere heat treatment: 30-50% cost increase but improved quality for critical parts.