Quick Answer
Heat treatment for castings — heat treatment changes the microstructure and mechanical properties of cast metals to achieve the strength, hardness, toughness, or wear resistance the application requires. For OEM buyers, the key is understanding which heat treatment your part needs, specifying it correctly on the drawing, and verifying it with documentation from the foundry.
Why Castings Need Heat Treatment
As-cast metal has a coarse, uneven microstructure from the solidification process. Heat treatment refines the microstructure to achieve:
- Higher strength: Yield and tensile strength can be doubled
- Better toughness: Resistance to impact and shock loading
- Improved machinability: Some heat treatments make machining easier
- Wear resistance: Surface hardening for wear-critical applications
- Dimensional stability: Reduces internal stresses that cause distortion
Common Heat Treatments for Castings
1. Annealing
What it does: Heats metal to a specific temperature, holds, then cools slowly. Produces the softest, most ductile condition.
Why use it: Relieves internal stresses from casting, improves machinability, refines grain structure.
Common for: Carbon steel, alloy steel, stainless steel castings that will be heavily machined or need maximum ductility.
2. Normalizing
What it does: Heats above the upper critical temperature, then cools in air. Produces a uniform, fine-grained microstructure.
Why use it: Improves strength and toughness while maintaining good machinability.
Common for: Carbon and low-alloy steel castings for structural applications.
3. Quenching and Tempering
What it does: Heats to austenitizing temperature, quenches rapidly in water/oil/polymer, then tempers at lower temperature.
Why use it: Achieves the highest strength and hardness in alloy steels.
Common for: High-strength steel castings for gears, shafts, structural members.
Important: Not all steel castings can be quenched and tempered — only those with sufficient hardenability (alloy content). Carbon steel castings (WCB, WCC) have limited hardenability.
4. Solution Treatment and Age Hardening
What it does: Solution treats at high temperature (dissolves alloying elements), then ages at lower temperature (precipitates hardening phases).
Why use it: Achieves high strength in precipitation-hardening alloys (aluminum, stainless, precipitation-hardening steels).
Common for: Aluminum castings (A356, A357), stainless steel (CF8M, 17-4 PH).
5. Stress Relief
What it does: Heats to a moderate temperature (typically 500–650°C), holds, then cools slowly.
Why use it: Reduces internal stresses from casting, machining, or welding. Prevents distortion in subsequent machining.
Common for: Large steel castings, machined castings, castings that will be welded.
6. Surface Hardening
What it does: Hardens only the surface while keeping the core tough.
Methods: Carburizing, nitriding, induction hardening, flame hardening.
Common for: Wear surfaces, gear teeth, bearing surfaces.
Heat Treatment for Specific Materials
| Material | Common Heat Treatments | Properties Achieved | Notes |
|---|---|---|---|
| Carbon Steel (WCB, WCC) | Normalize, Normalize + Temper, Anneal | Improved strength and toughness | Limited hardenability; Q&T not effective for large sections |
| Low Alloy Steel | Q+T (Quench + Temper) | High strength (up to 900 MPa) | Requires sufficient alloy content for through-hardening |
| Ductile Iron | Anneal, Normalize, Q+T, Austemper (ADI) | Various strength levels, ADI for highest | Austempering (ADI) gives exceptional strength+toughness |
| Aluminum (A356, A357) | Solution + Age (T6) | High strength (up to 350 MPa) | Solution treat + artificial age for peak properties |
| Stainless CF8/CG8M | Solution treat | Corrosion resistance, ductility | Quenches rapidly after solution treat |
| Stainless 17-4 PH | Solution + Age (H900, H1150) | High strength + corrosion | Precipitation hardening; multiple temp options |
Specifying Heat Treatment on Drawings
Clear heat treatment specification prevents disputes:
Drawing Note Format
Example:
“Heat treatment: Quench and Temper to achieve minimum 620 MPa yield strength, 750 MPa tensile strength, 15% elongation. Per AMS 2770 (aluminum) or AMS 2759 (steel).”
Information to Include
- Heat treatment type (e.g., “Quench and Temper”)
- Target hardness or mechanical properties (or both)
- Acceptance criteria (minimum values)
- Applicable specification (ASTM A802, AMS 2770, AMS 2759, etc.)
- Mark locations on drawing if partial heat treatment
Heat Treatment Documentation
Always require heat treatment documentation with each shipment:
- Furnace records: Time-temperature chart showing the heat treatment cycle
- Hardness test report: Hardness readings (Brinell, Rockwell) from the part or test coupon
- Mechanical test report: Tensile, yield, elongation from test bars cast with production parts
- Batch identification: Heat treatment batch number linked to part batch
Heat Treatment Cost Factors
| Heat Treatment | Typical Cost/kg | Cost Factors |
|---|---|---|
| Stress relief | $0.30–$0.80/kg | Furnace size, time at temperature |
| Annealing | $0.40–$1.00/kg | Temperature, cooling rate |
| Normalizing | $0.40–$1.00/kg | Temperature, air cool capacity |
| Q+T (Steel) | $0.80–$2.00/kg | Quench medium, tempering, material hardenability |
| Solution + Age (Al) | $0.60–$1.50/kg | Solution temp, quench method, age time |
| Carburizing | $1.50–$4.00/kg | Case depth, method |
Common Buyer Mistakes
- Specifying Q+T on carbon steel: Carbon steel has low hardenability; Q+T won’t achieve the expected properties in large sections. Use normalizing or specify low-alloy steel.
- Forgetting to specify the standard: “Heat treat to T6” is not enough. Specify the standard and acceptance criteria.
- Accepting parts without documentation: No furnace record = no proof of heat treatment. Reject or hold parts pending documentation.
- Partial heat treatment: If only part of the casting needs heat treatment, mark it clearly on the drawing. Partial HT requires careful masking and control.
Before finalizing the sourcing decision, many OEM buyers also compare Heat Treatment for Cast Steel Parts, Material Selection, Quality Assurance, and Sand Casting to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
Buyer Checklist
- Does the drawing specify heat treatment type and acceptance criteria?
- Is the material capable of the specified heat treatment (hardenability, alloy content)?
- Is the applicable standard specified (ASTM, AMS, etc.)?
- Is the heat treatment batch number recorded on the material cert?
- Is furnace time-temperature record available for the heat treatment batch?
- Are hardness or mechanical test results provided with each shipment?
- Have you verified that the heat treatment was done in-house or by an approved sub-supplier?
- Is NADCAP accreditation required for aerospace applications?
FAQ
Can you heat treat sand castings?
Yes — most sand castings are heat treated after casting to improve properties. Sand casting does not prevent heat treatment; the material chemistry determines what heat treatments are possible.
What is the difference between T5 and T6 for aluminum castings?
T5: Artificial aging only (cooler, faster, lower strength). T6: Solution treatment + artificial aging (higher strength, peak properties). T6 is the most common for A356/A357 aluminum castings.
What is a test coupon?
A test coupon is a small casting poured from the same heat and mold as the production part. It receives the same heat treatment and is tested to verify mechanical properties. Test coupon results represent the production part properties.