Q+T Quench and temper (QT)
Quench and temper (QT) · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “42CrMo4 +QT · 900 MPa min” states a quench and temper (qt) requirement.
What controls
A full quench-and-temper cycle applied through the entire cross-section of the part, not just the surface, to achieve a balanced set of mechanical properties: tensile strength, yield strength, toughness and hardness. It is the usual delivery condition for many alloy structural steels. What is verified is not only hardness: when the drawing calls for mechanical properties, they are demonstrated by tests on test specimens reported in the material or heat treatment certificate.
How to verify
Hardness tester for hardness at the location specified on the drawing, and tensile and impact tests on test specimens for the mechanical properties, reported in the corresponding certificate. On heavy-section parts, when the drawing requires it, verification is done at a defined depth or on a specimen taken from a specific location, not at the surface. Without a certificate for the heat treatment batch, surface hardness is only an indication.
Workshop guideline values
- General
- Hardness in the order of 200–300 HB on heat-treated structural steels.
- Accuracy
- Hardness in the order of 280–350 HB with mechanical properties guaranteed by certificate.
- High precision
- Narrow ranges and location-specific tests within the part when the section is large and the requirement is structural.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Confusing it with surface hardening: here the whole section is treated and there is no distinct case and core.
- Accepting surface hardness as proof of the required mechanical properties.
- Ignoring the effect of section thickness: the same specification is not achieved the same way in a thin part as in a thick one.
How to get it in the workshop / if it comes out of tolerance
If the material arrives already heat treated, you are machining hard, tough material: reduce speeds, use robust tool geometries and expect more tool wear. If heat treatment is done after machining, leave stock, because the part will move. In heavy sections the core does not reach the same properties as the surface, and that is inherent to the process, not a defect.
Properties below requirement at the core of a heavy part: this is a hardenability limitation of the steel for that section size, and the fix is to change the material or the design, not to repeat the cycle. Surface hardness OK but test results poor: check where the specimen was taken from. Distortion after heat treatment: machine in two stages with an intermediate stress relief.
Related symbols
Other symbols in Heat treatment
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