Heat treatment determines whether your castings meet mechanical property requirements or fail in service. The challenge: most guides explain concepts without providing actionable parameters.
This guide delivers specific temperatures, holding times, and cooling rates for steel, cast iron (including ADI), aluminum, and stainless steel castings. Match the process to your material, target properties, and application requirements using the decision framework and parameter tables below.
Process selection starts with your target property, not your material. Identify what you need to achieve, then match the appropriate treatment.
Match your target property to the correct process:
For most carbon steel castings, normalize-and-temper is more reliable than quench-and-temper unless maximum hardness is required. Quench-and-tempered steels are more susceptible to permanent strength reduction at elevated service temperatures.
| Material Type | Common Processes | When to Use |
|---|---|---|
| Carbon steel (WCB, WCC) | Normalize + temper | Most applications |
| Alloy steel | Quench + temper | High hardness required |
| Gray iron | Ferritizing anneal | Improve machinability |
| Ductile iron | Normalize or anneal | Per ASTM A536 grade |
| ADI | Austempering | High strength-to-weight |
| Aluminum | T6 (solution + age) | Maximum strength |
| Stainless (CF8M) | Solution anneal | Restore corrosion resistance |
Steel castings require precise temperature control and adequate soaking time to achieve uniform properties throughout the section thickness.
ASTM A216 WCB/WCC castings typically use normalizing followed by tempering. This combination meets types of heat treatment in metal casting requirements for most valve and pressure-containing applications.
Process parameters:
Target properties (ASTM A216 WCB):
After air cooling from normalizing temperature, tempering relieves transformation stresses and adjusts final properties.
Lower tempering temperatures retain higher hardness. Higher tempering temperatures maximize ductility but reduce strength. Select based on your application requirements.
Use quench-and-temper when hardness exceeds 400 BHN or high wear resistance is required.
CAUTION: As-quenched hardness must exceed 500 BHN to achieve >400 BHN after tempering. If your quench only achieves 300 BHN when targeting 500+ BHN, the quench rate is too slow. Increase agitation or switch quenchants. Even water can be “too slow” without proper agitation.
Tempering temperature effects:
| Range | Hardness | Elongation | Application |
|---|---|---|---|
| 150-250 C (Low) | ~55 HRC | Low | Wear resistance |
| 350-450 C (Medium) | Moderate | ~8% | Balanced properties |
| 550-650 C (High) | Low | ~15% | Impact loads |
Stress relief removes residual stresses without significantly altering mechanical properties. Apply after:
Parameters:
Cast iron heat treatment differs fundamentally from steel due to the graphite phase. The goal is typically matrix modification rather than full transformation.
Ferritizing Anneal (for machinability):
This treatment converts pearlite to ferrite, significantly improving machinability for custom grey iron casting applications requiring extensive machining.
Graphitizing Anneal (for carbide breakdown):
Stress Relief:
Stress relief is critical for castings that will be machined to tight tolerances. Without it, residual stresses release during machining, causing distortion.
Different iron vs ductile iron grades require different heat treatments to meet specification requirements.
| Grade | Requirement | Heat Treatment |
|---|---|---|
| 60-40-18 | High ductility | Full ferritizing anneal |
| 65-45-12 | Balanced | As-cast or optional anneal |
| 80-55-06 | Higher strength | As-cast or normalize |
| 100-70-03 | High strength | Normalize + temper or Q&T |
| 120-90-02 | Maximum strength | Quench + temper |
Ductile iron normalizing parameters:
Low-temperature anneal:
ADI is twice as strong as standard ductile iron with comparable toughness, and three times stronger than aluminum. Consider ADI before specifying steel for high-strength applications where weight matters.
Austempering process:
Step 1 – Austenitizing:
Step 2 – Salt Bath Quench:
Step 3 – Air Cool:
Lower salt bath temperatures produce higher hardness and strength. Higher temperatures produce greater ductility and toughness.
T6 treatment is the most common heat treatment for aluminum castings, increasing strength by 30-40% compared to the as-cast condition.
Step 1 – Solution Treatment:
Step 2 – Quench:
CAUTION: Transfer time must not exceed 10 seconds. Delayed quench allows precipitate formation during transfer, reducing final strength.
Step 3 – Artificial Aging:
For what is cast aluminum components requiring T6 treatment:
Over-aging reduces strength. If you exceed the aging time window, properties will decrease rather than improve.
Solution annealing restores corrosion resistance in austenitic stainless steel castings by dissolving carbide precipitates.
Purpose: Restore corrosion resistance by dissolving chromium carbides that formed during slow cooling.
Parameters:
CAUTION: Avoid holding at 900 C (1650 F). Exposure exceeding 4 hours causes sensitization – intergranular carbide precipitation that destroys corrosion resistance. This is why can stainless steel rust becomes a concern after improper heat treatment.
If your CF8M casting will operate in corrosive environments, solution annealing is not optional. The cost of treatment is far less than field failures.
Heat treatment defects can render castings unusable. Prevention requires understanding the root causes and controlling process parameters.
Quench Cracking:
Distortion/Warpage:
How do you prevent cracks in metal casting applies equally to heat treatment. The principles are similar: minimize thermal gradients and ensure adequate material ductility during transformation.
Soft Spots:
Decarburization:
| Quenchant | Cooling Rate | Distortion Risk | Cracking Risk | Best For |
|---|---|---|---|---|
| Water | Fastest | High | High | Simple shapes, low alloy |
| Brine | Very fast | Very high | Very high | Maximum hardness (use carefully) |
| Oil | Moderate | Low | Low | Alloy steels, complex shapes |
| Polymer | Adjustable | Controllable | Low | Versatile, modern choice |
| Air | Slowest | Minimal | Minimal | Air-hardening steels |
| Salt bath | Controlled | Low | Low | ADI, isothermal processes |
For most applications, I recommend polymer quenchants as the default choice. They offer the flexibility to adjust cooling rates without maintaining separate oil and water systems.
Match your process to your material and target properties using the parameters in this guide. Verify specific temperature ranges for your alloy grade – the values here cover common grades but your material test report (MTR) may specify tighter requirements.
Ready to discuss heat treatment specifications for your casting project? Contact our engineering team to review your requirements and ensure your custom steel castings or custom iron castings meet your mechanical property targets.
