Quick Answer
Quenching and tempering (Q&T) is a two-step heat treatment that trades a casting’s soft as-cast structure for high strength and controlled toughness. Quenching — rapid cooling from around 850–900°C in water, polymer or oil — creates hard but brittle martensite. Tempering — reheating to 450–650°C — sacrifices a little hardness to restore toughness. The tempering temperature is the tuning knob: higher temperature means tougher and softer, lower means harder and more brittle.
What Each Stage Does
| Stage | Typical temperature | Structure change | Result |
|---|---|---|---|
| Austenitize | 850–900°C (steel dependent) | Carbon dissolves into austenite | Prepares the steel for transformation |
| Quench | To near room temperature, fast | Austenite → martensite | Maximum hardness, high internal stress |
| Temper | 450–650°C for castings, typically | Martensite decomposes to tempered martensite | Toughness recovered, stress relieved |
Why Castings Specifically Benefit
As-cast steel has a coarse, uneven grain structure from slow solidification in the mold. Q&T (often preceded by normalizing) refines this structure and typically lifts tensile strength by 30–70% versus the annealed state, depending on grade. Parts that commonly specify Q&T:
- Mining and construction machinery wear parts and teeth
- High-load brackets, links and lifting components
- Shafts, pins and gears machined from cast blanks
- Ductile iron in the 120-90-02 class (yes — iron can be Q&T too)
The Trade-Offs Buyers Should Understand
- Distortion. Fast cooling moves metal — long thin parts and unbalanced sections move most. Leave machining stock and machine after heat treatment for critical dimensions, or specify a straightening step.
- Cracking risk. Sharp internal corners and abrupt section changes concentrate quench stress. A foundry review at the design stage costs nothing; a cracked batch costs weeks.
- Hardness window, not a number. Specify a range (e.g., 280–320 HBW), never a single value. A single number is unmeasurable in practice and invites disputes.
- Section thickness matters. The core of a 100 mm section cools slower than its skin and will not reach the same hardness as a 20 mm section of the same steel. Discuss hardenability with the foundry for heavy parts.
Q&T vs Normalizing vs Annealing
Not every casting needs Q&T. Normalizing alone (air cooling from austenitizing) suits general-purpose parts; annealing (slow furnace cooling) maximizes machinability and ductility at the lowest strength. The comparison deserves its own article — see our guide to annealing vs normalizing for castings — but the short version: pay for Q&T only when the load case demands the strength.
What to Put in Your Purchase Order
- Material grade and standard (e.g., ASTM A148 90-60, or A536 120-90-02)
- Condition: “quenched and tempered” with a hardness range and test location
- Mechanical test requirements: tensile bars per heat or per lot
- NDT after heat treatment for crack-sensitive geometry (MPI is standard)
- Machining sequence: which surfaces finish before vs after Q&T
At YCUMETAL, heat treatment is scheduled into the production plan with hardness verified in-house through our quality assurance process, and finish machining after treatment runs on the same site through CNC machining — which is how distortion stops being the buyer’s problem.
FAQ
Does quenching always mean water?
No. Water is fastest and harshest; oil and polymer solutions cool slower and reduce cracking risk on alloyed or complex parts. The foundry picks the quenchant to match steel hardenability and geometry.
Can stainless steel castings be quenched and tempered?
Martensitic grades (CA15, CA6NM, 17-4 PH family) — yes, that is how they gain strength. Austenitic grades (CF8, CF8M) — no; they do not harden by heat treatment at all.
Why did my Q&T casting arrive with different hardness readings at different spots?
Section thickness differences and surface decarburization cause spread. This is why hardness is specified as a range at an agreed test location.