Heat Treatment for Castings: T6, Annealing & Stress Relief

Standards pulled from AFS und NADCA. We also linked to our metal casting guide, casting alloy selection page, and the common casting defects article.

What Casting Heat Treatment Actually Changes

Each heat treatment changes three things in the metal: residual stress, grain structure, and mechanical properties. Pick the cycle based on what’s wrong — your part warps during machining, stretches too little in service, or leaks through porosity.

• Residual stress relief. A typical green-sand iron casting can carry 80–150 MPa of internal stress as-cast. Stress relief at 550–650 °C drops that below 30 MPa and is the difference between a bracket that holds tolerance and one that walks off the datum after a few weeks.
• Microstructure refinement. Solution treatment dissolves alloying elements into a single-phase matrix; subsequent aging precipitates them as fine, evenly distributed particles. In A356 aluminum this is what pushes tensile strength from ~170 MPa (as-cast F temper) to ~260 MPa (T6 temper), with elongation staying around 6–8%.
• Hardness and wear. For tool-steel and high-chrome iron castings, quench-and-temper cycles push hardness into the 45–55 HRC band that as-cast pearlite cannot reach.

Why as-cast properties are rarely enough

A casting pulled from the mold has uneven grains and locked-in thermal stress — it’s not in a stable state. AFS data shows a clear jump: run a simple normalizing cycle on a 0.3% carbon steel casting, and shock resistance goes up 30–60% over the as-cast version. For ductile iron, an austempering heat treatment (called ADI) replaces the as-cast structure with a tougher one called ausferrite, giving 3–4× the strength of standard 65-45-12 ductile iron without changing the part.

The Four Casting Heat Treatment Paths B2B Buyers Actually Order

Most foundries offer the same four cycles under different names. The trick is matching the name to the property you actually need, not to the cheapest line item on the quote.

Stress relief

A sub-critical soak, typically 550–650 °C for 1–2 hours followed by slow furnace cooling. Used for gray iron, ductile iron, and large steel castings that will be machined or assembled. No phase change, no hardness change — just stress reduction.

Annealing (full, partial, normalizing)

Full anneal softens the casting for maximum machinability (good for C355 and A201 aluminum that would otherwise load up a cutter). Normalizing refines grain in steel castings by air-cooling from above the upper critical temperature — cheaper than quench-and-temper and a common ask for carbon-steel valve bodies per ASTM A216.

Solution + age (T6 and T7)

The classic aluminum casting heat treatment: heat to ~530 °C, hold long enough to dissolve Mg and Si into solid solution, quench in water (or polymer for thick sections to limit distortion), then age at 155–175 °C for 6–12 hours. T6 is the peak-strength condition; T7 is over-aged to a slightly lower strength (~85% of T6) but with better dimensional stability and corrosion resistance.

Quench and temper

For low-alloy steel castings (e.g., 8630, 4340) and high-chrome white iron. Austenitize, oil or polymer quench, then temper at 200–600 °C depending on the target hardness. The trade-off is well documented: every 50 °C of tempering temperature costs roughly 1–2 HRC.

Heat Treatment Behavior by Casting Process

Not every casting process responds to every heat treatment. Pick the process first, then the cycle. For a side-by-side process comparison, see our sand vs investment casting und die vs sand casting pages.

Sand castings

Cooler cooling rates give coarse pearlite in iron and coarse dendrites in aluminum — both respond well to normalizing (iron/steel) or T6 (aluminum). Sand castings get the most heat treatment — they need it to hit spec.

Investment castings

Faster cooling than sand means finer starting microstructure, so the response to T6 is more uniform. Investment cast aluminum A356-T6 typically lands within 5–10 MPa of the wrought equivalent. Stainless steel investment castings are normally supplied in the solution-annealed condition per ASTM A351.

Die castings

Conventional die castings contain trapped gas porosity that blisters when heated above ~415 °C. Full T6 is rarely used; when heat treatment is needed, foundries lean on lower-temperature aging (T5) or vacuum die casting — see our aluminum die casting guide for details.

Common Heat Treatment Defects and How to Avoid Them

Bad heat treatment looks like a casting defect at first glance. Many of the issues we cover in common casting defects actually originate in the furnace, not the mold.

Quench cracking

Caused by thermal gradients during rapid cooling. Switch from water to polymer (PAG) quench for sections above 25 mm, and consider interrupted quench (timed in the martensite-start region) for heavy sections.

Distortion during machining

Rough-machine before heat treatment (semi-finish leaving 1–2 mm stock), then finish after. This sequence is standard for the cast + machined components discussed in our CNC vs casting page, and it controls the distortion that wastes machine time.

Incomplete aging or under-solutionizing

Foundries short on furnace time sometimes under-hold. Ask for a time-temperature recording chart attached to the certification packet — it’s the cheapest way to confirm your casting heat treatment actually ran the cycle you paid for.

How to Specify Casting Heat Treatment in Your RFQ

The clearest RFQs name three things: the standard, the property target, and the test method. Vague asks like “heat treated” or “hardened” force the foundry to pick — and they will pick the cheapest cycle that meets the spec.

• Reference the standard. Common ones: ASTM B26 / B969 for aluminum, ASTM A216 / A217 for steel castings, ASTM A536 for ductile iron temper, AMS 2774 for heat treatment of aluminum alloys.
• Name the temper. “T6” is precise; “heat treated to peak strength” is not. Pair with a UTS / yield / elongation number from the foundry’s certified test bar.
• Decide test location. Test bar attached to the casting? Test bar from a separately poured keel? Or test specimen machined from a representative section of the part itself — the latter costs more but is the only way to verify heat treatment penetration on thick sections.
• Require documentation. Time-temperature charts, Brinell or Rockwell readings, and a material certification with the actual values, not just “conforms.”

Cost & Lead Time: What Drives the Line Item

Heat treatment adds roughly 8–25% to the casting price depending on cycle and alloy. Three knobs move the number the most:

• Batch size. Furnaces charge per cycle, not per kilo. A 50-piece batch pays the same furnace time as a 500-piece batch, so the per-piece cost falls sharply with volume.
• Quench medium. Water is cheapest. Polymer quench (typical 10–20% PAG) costs more in chemistry but reduces scrap on heavy sections — usually a wash.
• Outside processing. If the foundry doesn’t own a heat treatment furnace, you’ll pay a third-party processor and a logistics hop. Ask whether heat treatment is in-house; if not, the lead time is roughly +5 to +10 working days.

Alloy choice is upstream of all of this; revisit the casting alloy selection matrix before you finalize a heat treatment cycle you may not need.

When Casting Heat Treatment Is Not Required

FAQ

What is the most common casting heat treatment for aluminum?

T6 — solution treat at roughly 530 °C, water quench, then age at 155–175 °C for 6–12 hours. You’ll see T6 most often on A356, A357, and 356 aluminum sand and investment castings used in aerospace and automotive structural parts.

Can die castings be T6 heat treated?

Conventional high-pressure die castings cannot — internal gas porosity blisters above ~415 °C. Vacuum die castings or low-pressure castings can. For conventional die castings, T5 (age-only at lower temperature) is the available casting heat treatment option.

How long does casting heat treatment take?

A typical T6 cycle on aluminum runs 14–18 hours total (heat-up, hold, quench, age). Stress relief on iron runs 8–10 hours. Quench-and-temper on steel is similar. Lead time at the foundry usually adds 3–7 working days for in-house furnaces, 7–14 days if outsourced.

Does heat treatment change casting tolerances?

Yes — every thermal cycle introduces some distortion, typically 0.1–0.3% of the part dimension. For tighter tolerances, rough-machine before heat treatment and finish afterward, leaving 1–2 mm of stock for the final pass.

*Data referenced from AFS / NADCA industry publications, ASTM B26 / A216 / A536 standards, and Supro MFG shop-floor records.