What Is Tempering? Metal Tempering Process and Applications

Published Sep 21, 2026, updated Sep 21, 2026

10 min

What Is Tempering?
Why Is Steel Tempered?
How Does the Tempering Process Work?
How Does Tempering Temperature Affect Steel Properties?
How Does Tempering Affect CNC Machined Parts?
Tempering vs Other Heat Treatment Processes
Where Is Tempered Steel Used?
Tempering FAQs
Conclusion About Tempering

Key Takeaways

In conventional quench-and-temper treatment, tempering reheats previously hardened steel below its lower critical temperature.
Temperature and holding time affect the final mechanical properties.
Tempered steel is used for tools, gears, shafts, and loaded components.
CNC planning must account for heat-treatment distortion and final tolerances.
Tempering differs from quenching, annealing, and normalizing in purpose and sequence.

Tempering steel parts in an industrial furnace

Hardening gives steel high hardness. But the quenched condition can leave it brittle and internally stressed. Tempering addresses this condition. It reheats hardened steel below the lower critical temperature, holds it for a selected period, and cools it under controlled conditions. In practice, tempering adjusts the properties of hardened steel by reducing as-quenched hardness and residual stress while increasing toughness and ductility. The target condition depends on the steel grade, tempering temperature, holding time, and service requirements.

What Is Tempering?

In conventional quench-and-temper treatment, tempering is a subcritical heat treatment applied to previously hardened steel. The steel is reheated below its lower critical temperature to adjust the condition left by quench hardening. Tempering reduces as-quenched brittleness and relieves part of the residual stress while bringing hardness, strength, toughness, and ductility into the required service range. Some reduction in hardness and strength is expected. The result is tempered steel with a property balance suited to its intended service.

Why Is Steel Tempered?

Reduce Brittleness After Hardening

Quenching produces very hard martensite. But the as-quenched structure can have poor fracture toughness and limited resistance to impact. Metal tempering lowers this brittleness. It changes the hardened microstructure in a controlled way while retaining enough hardness for wear resistance and demanding service conditions.

Improve Toughness and Ductility

A hardened component might need to resist cracking as well as surface wear. Tempering trades part of the as-quenched hardness for better ductility and toughness. It produces a more useful hardness-toughness balance for parts exposed to shock, bending, or repeated loading.

Relieve Internal Stress

Rapid quenching can leave residual stresses from uneven thermal contraction and phase changes within the steel. Metal tempering allows part of these stresses to relax. Tempering reduces residual stress that can contribute to quench cracking and later dimensional change. It can improve dimensional stability, but heat-treatment distortion still needs to be accounted for during machining and inspection.

How Does the Tempering Process Work?

Steel tempering process from quenching to cooling

Heating and Holding at the Tempering Temperature

The tempering process begins by reheating hardened steel to a selected temperature below the lower critical temperature, Ac1. Once the part reaches the target temperature throughout its section, it is held there for a specified period to allow the intended metallurgical changes to develop. The holding period allows the selected tempering reactions to proceed through the part thickness.

Cooling After Tempering

After soaking, the component is cooled using the method specified for the steel grade and heat-treatment cycle. Many conventional steel tempering cycles are air-cooled, but cooling practice can be important for alloy steels that are susceptible to temper embrittlement. This stage completes the thermal cycle without heating the material into the austenitic region again.

What Happens to the Steel Microstructure

During tempering, supersaturated martensite undergoes carbon redistribution and carbide precipitation. The reactions and resulting carbides depend on tempering temperature, holding time, and alloy composition. Depending on the steel and thermal cycle, retained austenite may decompose during tempering or during subsequent cooling. Higher-temperature tempering can promote recovery and further carbide development, while some alloy steels may require multiple tempering cycles to achieve the specified microstructure and properties.

Starting With Quenched or Hardened Steel

All of these stages assume a hardened starting condition, which is dominated by martensite with possible retained austenite and undissolved carbides. The tempering process acts on this nonequilibrium structure through controlled reheating and holding. This produces a tempered martensite whose final microstructure depends on thermal cycle and alloy chemistry.

How Does Tempering Temperature Affect Steel Properties?

Note

Tempering temperature is grade-specific, so these ranges should be treated as general examples rather than universal settings.

Low-Temperature Tempering

At lower tempering temperatures (which might be between 150-250°C for many steels), hardness and tensile strength are comparatively high. Brittleness declines to a limited degree. This range is often selected when relatively high hardness must be retained, such as in some tool and wear-resistant applications. The required temperature must be selected for the specific steel grade and target mechanical properties.

Medium-Temperature Tempering

Going to an intermediate range produces more pronounced softening. Hardness and strength decrease as toughness and ductility increase. The relationship is not identical for every grade. Alloy steels which contain carbide-forming elements can show secondary hardening at certain temperatures. This makes material chemistry an important part of temperature selection.

High-Temperature Tempering

At higher temperatures, which might be around 500-650°C for conventional quenched steels, recovery and carbide development progress further. Higher-temperature tempering generally produces a tougher, more ductile condition with lower hardness and strength, although the exact response depends strongly on alloy composition. Engineers therefore choose the final temperature from the steel grade, loading conditions, wear demands, and required mechanical properties. One setting should not be applied to every component.

How Does Tempering Affect CNC Machined Parts?

Tempering workflow for CNC machined steel parts

Tempering can change the hardness, residual stress, and dimensional stability of hardened steel, so the heat-treatment cycle should be considered when planning the machining process. The exact sequence depends on the steel grade, part geometry, required hardness, and final tolerance.

Machining Sequence and Final Finishing

For some heat-treated steel parts, rough machining is performed before hardening and tempering, with enough material left on critical surfaces for final machining or grinding after heat treatment. This approach allows heat-treatment movement to be corrected during the finishing stage. Pre-hardened materials and surface-hardening processes may require a different sequence.

Final machining method also depends on the hardness after tempering. Parts that remain relatively hard may require tooling and cutting parameters suited to hardened steel, while grinding may be preferred for highly critical surfaces.

Distortion, Tolerance, and Final Inspection

Heat treatment can cause dimensional movement or geometric distortion even when residual stress is reduced by tempering. For tight CNC tolerances, the machining plan should account for expected movement rather than treating the post-machining dimensions as fixed before heat treatment.

Critical features such as bores, mounting faces, and datum-related surfaces should be verified after the final heat-treatment and finishing operations. Where necessary, machining allowance can be left on functional surfaces so that the remaining stock can be removed after heat treatment to achieve the required geometry and tolerance.

Tempering vs Other Heat Treatment Processes

Heat Treatment	Main Purpose	Position in Heat Treatment Sequence	Typical Heating and Cooling Approach	Resulting Effect on Steel Properties
Tempering	Adjust the properties of hardened steel by improving toughness and ductility as well as relieving quenching stresses.	Follows quench hardening.	Reheats steel below the lower critical temperature, holds it for a selected time, and cools it at a suitable rate.	Reduces as-quenched hardness to a controlled level and improves toughness, ductility, stress condition, and dimensional stability.
Quenching	Rapidly cool austenitized steel to produce a hardened microstructure.	Follows heating into the austenitizing range and precedes tempering.	Steel is heated to the required austenitizing temperature and rapidly cooled using a quenching medium.	Produces high hardness and strength, though the resulting condition can contain residual stress and greater brittleness.
Annealing	Reduce hardness, improve machinability or formability, and obtain a more stable microstructure.	Used before machining, forming, or further heat treatment, depending on the manufacturing route.	Full annealing heats steel into the austenitizing range, followed by very slow furnace cooling.	Produces a softer condition with improved machinability; full annealing typically produces a relatively coarse ferrite-pearlite structure in suitable carbon and alloy steels.
Normalizing	Refine and equalize the microstructure and grain condition of steel.	Used as a preparatory or property-adjustment treatment before machining or further heat treatment.	Steel is heated above its transformation range and cooled in air.	Produces a uniform and refined structure and properties that differ from the softer condition obtained through full annealing.

Where Is Tempered Steel Used?

Cutting Tools and Tooling

Cutting tools provide a practical example of why steel is tempered. High-speed and cold-work tool steels are quenched and tempered for drills, taps, reamers, milling cutters, punches, dies, and shear blades. These tools need enough hardness to resist wear while retaining sufficient toughness to reduce edge chipping and fracture in service.

Gears, Shafts, and Machine Components

Shafts, axles, and many machine components are commonly made from quenched-and-tempered alloy steels when a through-hardened combination of strength and toughness is required. Gears may use a different heat-treatment route, such as carburizing followed by quenching and tempering, when the design calls for a hard wear-resistant surface and a tougher core.

Automotive and Industrial Components

Q&T steels are also used for components subjected to repeated mechanical loading, including some connecting rods, shafts, machine-tool components, and other structural parts. The required heat-treatment condition depends on the component geometry, load case, wear requirement, and whether a hardened surface or through-hardened structure is needed. These components may encounter vibration, cyclic loading, impact, and abrasive contact. This makes tempered steel useful when a component must withstand cyclic or impact loading without giving up all of the hardness gained during hardening.

Tempering FAQs

Does Tempering Reduce Steel Hardness?

Usually, yes. For many quenched steels, increasing tempering temperature reduces hardness and strength while increasing toughness and ductility. However, some alloy tool steels can show secondary hardening at specific temperatures, so the response is not always a simple monotonic decrease in hardness.

Can Steel Be Tempered More Than Once?

Yes. Multiple tempering cycles are used for some tool and alloy steels to achieve the required hardness and toughness and to further stabilize the microstructure after the initial temper. ASM heat-treatment guidance specifically recognizes multiple tempering as an established practice.

Does Tempering Always Follow Quenching?

In conventional quench-and-temper treatment, tempering follows quenching. ASM also uses the term for certain subcritical treatments applied to normalized steel and for stress relief after welding, forming, or machining. These applications should not be treated as identical to conventional martensitic tempering.

Is Aluminum Tempered Like Steel?

No. Aluminum uses temper designations such as T4 and T6 to describe specific processing conditions, including solution heat treatment and aging. These temper designations are not the same process as tempering hardened martensitic steel.

How Long Does Steel Tempering Take?

Tempering time depends on the steel grade, section thickness, tempering temperature, and required mechanical properties. The holding period is selected to allow the part to reach the required temperature throughout its section and for the intended tempering reactions to develop.

Conclusion About Tempering

Tempering is used to bring hardened steel into a condition that is suitable for its intended service, rather than simply retaining the highest possible hardness. For CNC-machined parts, the heat-treatment condition also affects how much stock should be left for finishing and when critical dimensions should be verified. A suitable tempering treatment therefore needs to be considered as part of the overall manufacturing process, not as an isolated heat-treatment step.