Quenched and tempered steel explained

Quenched and tempered (Q&T) steel obtains its final properties through a deliberate two-stage heat-treatment route: first hardening by quenching, then tempering to restore a useful balance of strength, toughness and ductility.

The important procurement point is that Q&T is a delivery condition and manufacturing route, not merely a strength label. A normalized, TMCP and quenched-and-tempered steel can have similar nominal yield strength while still being different materials for fabrication and substitution purposes.

What happens during quenching?

The steel is first heated into the austenitic range. It is then cooled rapidly enough to create a hardened microstructure.

The precise austenitizing temperature, cooling medium and cooling rate depend on the grade, product form, thickness and mill process. Heavy plate producers typically use controlled water-quench equipment; other product forms can use different qualified routes.

Quenching can provide high strength, but the as-quenched condition is generally too hard and stressed to be the desired final condition.

What does tempering do?

After quenching, the material is reheated to a lower temperature and held under controlled conditions.

Tempering is used to modify the hardened microstructure and obtain the specified combination of:

• yield and tensile strength;
• toughness;
• ductility;
• hardness;
• dimensional and metallurgical stability.

The result is not simply "softened quenched steel". The complete quench-and-temper cycle is designed around the grade and required properties.

For high-strength structural plate, EN 10025-6 covers flat products supplied in the quenched-and-tempered condition.

How Q differs from N and M

The three delivery routes should not be treated as interchangeable.

A Q&T grade normally uses heat treatment after rolling to establish its final properties. TMCP, by contrast, develops properties through controlled deformation and cooling during the rolling process. Normalizing uses a different thermal route again.

See Normalizing steel: what the heat treatment does and TMCP / thermomechanical rolling explained.

Where Q&T steels are used

Q&T routes are common where relatively high strength must be combined with useful toughness. Examples include:

• high-strength structural plate;
• offshore structural steels;
• line pipe grades with a Q delivery suffix where permitted;
• pressure and alloy steels;
• wear-resistant and engineering steels.

The application standard matters. The letter Q in one standard should not be assumed to create an automatic equivalent in another standard.

Q&T does not automatically mean "better"

Higher strength can allow thinner sections or lower structural weight, but that does not make a Q&T grade a universal upgrade from a normalized or TMCP grade.

A substitution check can still require comparison of:

• product standard and product form;
• yield and tensile properties across the actual thickness;
• Charpy requirements;
• chemistry, CEV and/or Pcm;
• through-thickness or NDT requirements;
• forming limits;
• welding procedure qualification;
• permitted heat treatment after fabrication;
• project or design approval.

The designer may prefer a different delivery route because fabrication behaviour, toughness, fatigue design or weldability governs the application rather than nominal yield strength alone.

Welding Q&T steel

Welding deserves particular attention because the base material properties were produced by a controlled heat-treatment cycle.

The weld heat-affected zone can experience local re-heating that changes hardness and strength. Depending on the steel and heat input, parts of the HAZ may be re-hardened or over-tempered.

This is why fabrication documents can impose controls on:

• preheat;
• interpass temperature;
• heat input;
• consumable strength and hydrogen level;
• cooling;
• hardness;
• impact testing;
• repair welding;
• PWHT.

The correct limits come from the material producer, application code and qualified WPS—not from a generic rule for all Q&T steels.

Q&T steel and PWHT

A common mistake is to assume that PWHT is harmless because Q&T steel has already been heat treated.

It is not that simple.

A later thermal cycle can alter the microstructure that the original quench-and-temper route created. Some Q&T grades have explicit maximum temperatures or qualified PWHT windows; other grades or applications may prohibit the proposed treatment.

Before specifying PWHT, check:

1. the material standard and grade;
2. the mill's fabrication recommendations;
3. the proposed PWHT temperature and duration;
4. the number of thermal cycles;
5. whether the procedure qualification reproduces the intended PWHT;
6. whether mechanical testing after simulated PWHT is required.

See Post-weld heat treatment (PWHT): when it appears in steel specifications.

What to check on the MTC and order

For Q&T material, verify at least:

• complete grade designation;
• governing product standard and edition;
• stated delivery condition;
• dimensions and thickness range;
• tensile results;
• impact test temperature and results;
• chemistry and relevant carbon-equivalent values;
• required NDT/supplementary tests;
• heat number and traceability;
• any project-specific fabrication restrictions.

Do not infer Q&T merely from high strength. The required delivery condition should be stated and supported by the product specification and certificate.

Procurement takeaway

Q&T is a controlled metallurgical route used to create a specific property combination.

It should not be substituted freely for normalized or TMCP material, and it should not be exposed to unqualified fabrication heat cycles simply because its nominal strength remains similar.

When Q&T is specified, treat the delivery condition, welding procedure and any later heat treatment as part of the material requirement.

References and verification sources

• DIN EN 10025-6:2023-06 — high-yield structural steels in the quenched and tempered condition
• voestalpine — Quenching and tempering
• SSAB — Heat affected zone after thermal cutting of Q&T steels