What Is Tempering? | Proces, Temperatura & Effects on Steel

Tempering is one of the most foundational yet widely misunderstood heat treatment processes in metal manufacturing.

Almost every quenched steel component undergoes tempering as a mandatory post-quench secondary operation, yet many engineers and production teams treat it as a trivial, standardized step rather than a precision-controlled process that defines final strength, žilavost, internal stress state, i radni vijek.

U praksi, quenching locks in a hard, krhka, stress-laden martensitic structure that is functionally unusable for most engineering applications.

Tempering — carefully controlled heating below the critical transformation temperature — tailors that as-quenched microstructure to deliver the exact balance of hardness, žilavost, dimenziona stabilnost, and fatigue resistance required for service.

Tempering is therefore commonly used in the manufacture of tool steels, matrice čelika, opružni čelici, komponente ležaja, zupčanici, osovine, Automobilski dijelovi, kalupi, and structural machine components.

1. What Is Tempering?

In metallurgy, kaljenje is a heat-treatment process in which a previously hardened ferrous alloy is reheated to a temperature ispod donje kritične temperature (A₁), held for a specified period, and then cooled under controlled conditions.

For plain-ugljenični čelici, the eutectoid temperature associated with the A₁ line is approximately 727° C (1341° F), although the actual critical temperatures vary with chemical composition.

Tempering is primarily applied after quenching to modify the as-quenched martensitic structure.

The main objectives are to reduce brittleness and residual stress while retaining an appropriate level of hardness and strength.

By selecting the tempering temperature and holding time, manufacturers can adjust the balance between hardness, žilavost, duktilnost, otpornost na habanje, i dimenzionalna stabilnost.

The principal effects of tempering include:

The tempering temperature is selected according to the steel grade and the desired final properties.

A low-temperature temper may be used when high hardness must be retained, whereas a higher tempering temperature generally produces greater toughness and ductility with a corresponding reduction in hardness.

Tempering should not be confused with annealing. Annealing is normally intended to soften steel and promote a relatively stable, ductile microstructure, often involving heating above a critical transformation temperature followed by slow cooling.

Kaljenje, u kontrastu, is applied to hardened steel and is performed below the critical transformation range to modify rather than completely eliminate the hardened condition.

In practical steel manufacturing, tempering is therefore best understood as a controlled adjustment of the microstructure created by quenching.

The objective is not simply to make hardened steel softer, but to convert an excessively brittle as-quenched structure into one that provides a more useful engineering combination of properties.

2. Why Tempering Is Necessary: The Science Behind Quenched Steel

To understand tempering, one must first understand what quenching does.

When steel is heated into the austenite phase field—typically above 800–900°C depending on composition—its crystal structure becomes face-centered cubic (FCC).

Austenite can dissolve significant amounts of carbon. If the steel is then cooled rapidly, usually in water, ulja, or polymer, the carbon atoms do not have time to diffuse out.

The crystal structure attempts to transform into body-centered cubic (BCC) ferita, but the trapped carbon distorts the lattice into a body-centered tetragonal (Bct) structure known as martenzit.

Martensite is characterized by:

• Veoma visoka tvrdoća.
• High yield strength.
• Low ductility and toughness.
• High residual stresses.
• A tendency toward delayed cracking.
• The presence of retained austenite in many alloys.

In this as-quenched condition, steel is often too brittle for practical use. A cutting tool might chip. A shaft might crack during assembly. The gear tooth might fracture under shock loading.

Tempering reduces these risks by allowing controlled diffusion and microstructural rearrangement.

The necessity of tempering is therefore both scientific and practical.

Naučno, it allows the steel to approach equilibrium by relieving lattice strain and precipitating carbides. Praktično, it enables the steel to survive real-world service conditions.

3. What Are the Main Tempering Temperature Ranges?

Tempering temperature is one of the most important variables in steel heat treatment because it directly controls the extent of microstructural change after quenching.

As the tempering temperature increases, carbon diffusion, taloženje karbida, oporavak, and the transformation of retained austenite generally become more pronounced.

There is no single temperature range that applies to every steel.

Kaljenje na niskim temperaturama

Low-temperature tempering is commonly used when the component must retain a high level of hardness and wear resistance while reducing the brittleness associated with fresh martensite.

Typical temperatures are approximately 150–250°C (300–480°F), although the suitable range depends on the steel.

U ovoj fazi, carbon begins to redistribute from supersaturated martensite, fine carbide phases may develop, and residual stresses from quenching are reduced.

The reduction in hardness is usually limited compared with higher-temperature tempering.

This treatment is common for applications such as:

• Alati za rezanje
• Umire
• Komponente otporne na habanje
• Certain bearing components
• High-hardness tool-steel parts

The trade-off is that toughness remains relatively limited compared with medium- or high-temperature tempered conditions.

Kaljenje na srednjim temperaturama

Medium-temperature tempering, otprilike 250–450°C (480–840°F), produces a greater modification of the quenched microstructure.

Carbide precipitation becomes more developed, martensite becomes less supersaturated, and internal stresses continue to decrease.

This region can be useful when a component requires a more balanced combination of snaga, tvrdoća, žilavost, i otpornost na habanje.

The exact response varies considerably by steel. Some steels also exhibit temper embrittlement phenomena in particular temperature ranges, which means that both the tempering temperature and subsequent cooling practice may require attention.

Visokotemperaturno kaljenje

High-temperature tempering is generally performed at approximately 450–700°C (840–1290°F), provided the temperature remains appropriate for the specific steel and below the relevant critical transformation range.

Na ovim temperaturama, the microstructure undergoes much more extensive recovery and carbide evolution.

Residual stresses are substantially reduced, and the material generally develops significantly higher toughness and ductility than in the as-quenched or low-tempered condition.

High-temperature tempering is widely associated with quenched-and-tempered structural steels, Legura čeli, osovine, zupčanici, pressure-containing components, and heavily loaded machine parts.

A particularly important distinction is that high-temperature tempering does not necessarily mean poor strength.

Properly alloyed and heat-treated steels can retain substantial yield and tensile strength while gaining much better fracture resistance.

Secondary Hardening During Tempering

Certain highly alloyed tool steels behave differently. Instead of continuously decreasing in hardness as the tempering temperature increases, they may show a secondary-hardening peak, often within a higher tempering range.

This occurs when alloying elements such as chromium, molibdenum, volfram, or vanadium precipitate as fine, stable alloy carbides.

These particles impede dislocation movement and can increase or preserve hardness.

Secondary hardening is especially important for high-speed steels and selected tool steels intended for elevated-temperature service.

4. What Is the Tempering Process?

The tempering process is a controlled thermal cycle performed after hardening or another treatment that has produced a sufficiently hardened microstructure.

Although the procedure appears straightforward, achieving consistent results requires control over temperature, vrijeme, furnace uniformity, loading, hlađenje, and the condition of the quenched steel.

A typical industrial tempering sequence can be represented as:

Quenching → Transfer to Tempering → Heating → Soaking → Controlled Cooling → Inspection

Korak 1: Quench the Steel

Tempering normally begins with a properly quenched component.

The steel is first austenitized and then rapidly cooled at a rate sufficient to produce the intended hardened structure.

The quenching medium may include:

• Ulja
• Voda
• Polymer solution
• Pressurized gas
• Molten salt

The correct medium depends on the alloy and component geometry.

The objective is to achieve the required hardenability and microstructure while minimizing cracking and distortion.

Korak 2: Transfer the Component to the Tempering Furnace

Nakon gašenja, the component should be tempered according to the specified production procedure.

For many grades, prompt tempering is desirable because the freshly quenched condition contains high residual stresses.

The transfer procedure must also avoid unnecessary thermal shock or uncontrolled cooling that could affect the final heat-treatment condition.

Za kritične komponente, furnace records should document the actual temperature cycle rather than relying only on the furnace set point.

Korak 3: Heat to the Specified Tempering Temperature

The component is heated to the selected tempering temperature, which may range from roughly 150°C to 700°C depending on the steel.

Heating should be sufficiently controlled to prevent excessive temperature gradients, especially in large or complex components.

The furnace should provide suitable:

• Temperature uniformity
• Atmosphere control where required
• Load circulation
• Instrument calibration
• Temperature recording

For precision heat treatment, The actual workpiece temperature is more important than the programmed furnace temperature.

Korak 4: Soak at Temperature

Once the component has reached the required temperature, it is held for a specified period.

During this soaking stage, the microstructure undergoes the tempering reactions appropriate to the steel:

• Carbon redistribution
• Precipitacija karbida
• Martensite decomposition
• Opuštanje od stresa
• Matrix recovery
• Retained-austenite transformation
• Alloy-carbide precipitation in secondary-hardening steels

The required soaking time depends on component size, geometrija, legura, furnace type, and applicable specification.

Korak 5: Controlled Cooling

After the required holding time, the component is cooled according to the process specification.

Many steels can be cooled in air after tempering, while certain alloy systems or critical components may require a specific cooling practice.

Cooling conditions can influence dimensional stability and, in some steels, susceptibility to temper embrittlement. Stoga, “cooling” should not be treated as an insignificant final step.

Korak 6: Repeat Tempering When Required

Some steels require double or triple tempering rather than a single cycle.

Repeated tempering may be used to:

• Further stabilize retained austenite
• Temper newly formed martensite after retained-austenite transformation
• Improve dimensional stability
• Develop a more stable carbide structure
• Achieve consistent properties throughout the component

This practice is especially common for high-alloy tool steels.

5. Why Do Some Steels Require Double or Triple Tempering?

A single tempering cycle is sufficient for many quenched steels, but high-alloy alatni čelici and certain high-carbon steels often require two or even three tempering cycles.

The main reason is that the first tempering treatment can change the stability of retained austenite and produce fresh martensite during subsequent cooling.

A second or third temper then tempers this newly formed martensite and improves the stability of the final microstructure.

Retained Austenite Is the Main Reason

Nakon gašenja, some steels retain a portion of the original austenite because the martensitic transformation does not proceed to completion.

This is particularly relevant in steels containing relatively high levels of carbon and alloying elements.

During the first tempering cycle, the stability of retained austenite can change. On cooling after tempering, some of this retained austenite may transform into fresh, untempered martensite.

This creates a new problem: although the original martensite has been tempered, the newly formed martensite has not.

A second tempering cycle is therefore used to temper this fresh martensite.

Why Is Fresh Martensite a Concern?

Fresh martensite has the same basic disadvantages as the original as-quenched structure:

• Visoka tvrdoća
• High residual stress
• Niska duktilnost
• Relatively poor toughness
• Increased susceptibility to cracking

Leaving this structure untreated can produce local variations in hardness and toughness. In precision tool steels, it can also contribute to dimensional instability.

Double tempering therefore helps ensure that both the original martensite and martensite formed after the first tempering cycle are properly tempered.

When Is Triple Tempering Used?

Triple tempering is used when an even higher degree of microstructural stabilization is required.

It is particularly common in some high-speed steels, hot-work tool steels, and highly alloyed die steels, depending on the manufacturer’s heat-treatment specification.

A third cycle may be selected to:

• Further stabilize retained austenite
• Temper any newly formed martensite
• Refine the balance between hardness and toughness
• Improve dimensional stability
• Establish the required secondary-hardening condition

Triple tempering should not be applied automatically. It is justified only when the steel grade and required properties call for it.

Typical Steels Requiring Repeated Tempering

Repeated tempering is particularly relevant to steels such as:

• H13 hot-work tool steel
• Brzorezni čelici
• High-alloy cold-work tool steels
• Selected hot-work and die steels
• Certain precipitation or secondary-hardening tool steels

The actual number of tempering cycles should always follow the specific steel grade and qualified heat-treatment procedure. More cycles do not automatically mean better performance.

6. What Is Secondary Hardening During Tempering?

Secondary hardening is a phenomenon in which certain highly alloyed steels develop increased hardness during tempering at relatively high temperatures, even though ordinary tempering of carbon steels generally causes hardness to decrease.

This behavior is characteristic of steels containing significant amounts of carbide-forming alloying elements, posebno molibdenum, volfram, vanadij, i hrom.

Why Does Secondary Hardening Occur?

During conventional tempering, the hardness of quenched martensite normally decreases as carbon leaves the supersaturated matrix and carbides become more stable.

In highly alloyed tool steels, međutim, a different process can occur at higher tempering temperatures. Alloying elements diffuse and form very fine alloy carbides.

These precipitates can be extremely effective at resisting dislocation movement.

The resulting strengthening can compensate for, and sometimes exceed, the hardness lost during earlier stages of tempering.

The general sequence is:

Quenching → Initial Tempering/Softening → Alloy-Carbide Precipitation → Secondary Hardening

Alloying Elements Involved

The most important elements include:

The exact carbide type depends on steel composition and heat-treatment history.

Why Is Secondary Hardening Important?

Secondary hardening allows certain tool steels to achieve high hardness after tempering at temperatures substantially higher than those used for conventional low-temperature tempering.

This is valuable because high-temperature tempering can also improve:

• Temper resistance
• Dimenzijska stabilnost
• Structural stability
• Hot hardness
• Resistance to softening during service

For tools operating at elevated temperature, retaining hardness during service may be more important than achieving the maximum possible room-temperature hardness.

Secondary Hardening and H13 Tool Steel

H13 is a representative hot-work tool steel in which alloy-carbide precipitation contributes to its response during high-temperature tempering.

Its heat treatment is designed to establish an appropriate balance of hardness, žilavost, thermal-fatigue resistance, and resistance to softening.

The exact secondary-hardening response depends on the steel’s chemistry, austenitizing temperature, quenching method, temperatura kaljenja, and holding time.

It is therefore incorrect to treat secondary hardening as simply “hardening by reheating.”

The phenomenon is the result of controlled precipitation of alloy-rich carbides within a previously hardened microstructure.

7. How Does Tempering Affect Mechanical Properties?

Tempering changes the mechanical properties of steel by modifying the highly supersaturated and stressed martensitic structure produced by quenching.

The magnitude and direction of these changes depend on the steel grade and tempering condition, so the relationship should be understood as a general trend rather than a fixed rule.

Overall Effect of Tempering

*General trends for conventional quenched steels; highly alloyed steels may show different behavior, particularly where secondary hardening occurs.

The practical objective of tempering is therefore to select the right property balance rather than maximize any single mechanical property.

A cutting tool may require maximum practical hardness and wear resistance, while a gear or shaft may require considerably greater toughness and fatigue resistance.

By controlling the tempering temperature and time for the specific steel, manufacturers can tailor the final microstructure to the actual service requirements.

8. What Is Tempering vs Annealing vs Normalizing vs Stress Relieving?

Kaljenje, žarljivost, normalizacija, and stress relieving are all thermal treatment processes, but they are ne zamjenjiva.

Their temperature ranges, starting microstructures, cooling methods, metallurgical mechanisms, and intended results are different.

9. Tempering of Different Steel Types

Different steel types respond differently to tempering. Understanding these differences is essential for selecting the right tempering schedule.

Plain Carbon Steels:

Simple to temper. Niski karbonski čelici (<0.3% C) are rarely quenched and tempered because they do not harden significantly.

Medium carbon steels (0.3–0.6% C) are commonly quenched and tempered for shafts, zupčanici, i osovine. High carbon steels (>0.6% C) are used for tools, Springs, i noževi.

legirani čelici:

Contain elements such as chromium, nikl, molibdenum, i vanadij.

They have better hardenability and respond well to tempering. Secondary hardening is common in alloys with strong carbide formers.

Alat Steels:

Include water-hardening, oil-hardening, air-hardening, velike brzine, and hot-work steels.

Tempering schedules vary widely. High-speed steels are triple tempered at 540–560°C. Hot-work steels like H13 are double tempered at 500–550°C.

Nerđajući čelici:

Martenšitski nehrđajući čelici (E.g., 410, 420, 440C) can be hardened and tempered. Austenitni nehrđajući čelici (E.g., 304, 316) cannot be hardened by quenching and are not tempered.

Ferritic stainless steels are generally not hardened. Precipitation-hardening stainless steels are aged, not tempered.

Maraging Steels:

These are low-carbon, high-nickel alloys that are strengthened by aging, not by martensite tempering.

Međutim, they may undergo a solution treatment and aging process that is conceptually similar to tempering.

10. Tempering H13 Tool Steel: A Practical Example

H13 alatni čelik is a chromium-molybdenum-vanadium hot-work tool steel widely used for die casting dies, kovane matrice, extrusion tooling, and other tools exposed to repeated heating and cooling.

Njegova kombinacija vruće snage, žilavost, temper resistance, and thermal-fatigue resistance makes it a representative example of why tempering must be carefully controlled in alloy tool steels.

After austenitizing and quenching, H13 develops a hard martensitic structure with residual stresses and some retained austenite.

The subsequent tempering treatment is therefore designed not only to reduce brittleness, but also to develop the required secondary hardening response and microstructural stability.

Typical H13 Tempering Schedule

H13 is commonly double tempered, with a third temper sometimes specified for large or particularly critical components.

Exact parameters should follow the applicable material specification and qualified heat-treatment procedure.

The actual austenitizing and quenching conditions are also important.

H13 is commonly austenitized at approximately 1,020–1,050°C, followed by controlled cooling such as air or gas quenching, zavisno od veličine sekcije, oprema, and the required properties.

Typical Resulting Hardness

A properly heat-treated H13 component commonly reaches a final hardness in the region of approximately 48–52 HRC, although the actual hardness depends on the austenitizing condition, quenching rate, temperatura kaljenja, vrijeme zadržavanja, and material chemistry.

Within the stated tempering range, the general trend is:

• Around 500°C: veća tvrdoća, with less tempering-related softening.
• Around 550°C: somewhat lower hardness, generally with greater toughness and stress relief.

The relationship is not strictly linear, and H13 can exhibit secondary hardening because of precipitation of fine alloy carbides during high-temperature tempering.

Why Is H13 Double Tempered?

The principal reason for double tempering is the behavior of zadržani austenit after quenching.

Following the first tempering cycle, some retained austenite may transform during cooling into fresh martensite.

This newly formed martensite has not yet undergone tempering and therefore may contain relatively high residual stress and brittleness.

The second tempering cycle treats this fresh martensite and helps produce a more uniform final microstructure:

Quenching → First Temper → Retained Austenite Transformation → Fresh Martensite → Second Temper

Double tempering also helps complete the desired precipitation reactions and improve the consistency of hardness and dimensional stability.

For large or highly critical H13 components, a third temper may be used when required by the heat-treatment specification, particularly where additional stabilization is beneficial.

Inženjerski značaj

The objective of H13 tempering is not simply to maximize hardness.

The final condition must balance hardness, vruća snaga, žilavost, thermal-fatigue resistance, i dimenzionalna stabilnost.

Iz tog razloga, an H13 die tempered at approximately 500–550°C is engineered to retain sufficient hardness for wear resistance while developing the toughness and temper resistance required for repeated thermal cycling and mechanical loading in service.

11. Industrial Applications of Tempered Steel

Tempered steel is widely used in industrial manufacturing because it provides a controllable balance between strength, tvrdoća, žilavost, duktilnost, Otpornost na umora, i dimenzionalna stabilnost.

Gears and Transmission Components

Gears commonly undergo quenching followed by tempering to obtain high strength and adequate toughness.

The tempering treatment reduces the brittleness and residual stresses associated with the hardened condition while maintaining sufficient hardness for tooth contact and wear resistance.

Tipične komponente uključuju:

• Transmission gears
• Pinions
• Drive gears
• Lančanici
• Shafts and gear shafts

For heavily loaded gears, the final heat-treatment condition is usually designed around fatigue strength, kontaktni stres, and resistance to tooth fracture rather than hardness alone.

Osovine, Osovine, and Pins

Quenched-and-tempered alloy steels are widely used for shafts, osovine, pinovi, and similar machine elements subjected to combined bending, torzija, uticaj, and cyclic loading.

Compared with an as-quenched structure, a properly tempered martensitic structure provides substantially better toughness and resistance to crack propagation, making it more suitable for dynamically loaded components.

Springs

Spring steels require high elastic strength and fatigue resistance.

Tempering after hardening is used to adjust the hardness and relieve internal stresses while maintaining the strength required for repeated elastic loading.

Aplikacije uključuju:

• Coil springs
• Leaf springs
• Torsion springs
• Komponente ovjesa
• Valve springs

The tempering condition must be carefully controlled because both excessive softness and excessive brittleness can reduce spring life.

Dies and Tooling

Tool steels are commonly tempered to achieve a controlled combination of hardness, otpornost na habanje, žilavost, and resistance to softening.

Hot-work tool steels such as H13 are tempered at relatively high temperatures to obtain thermal stability and resistance to repeated heating and cooling.

Cold-work and high-speed tool steels use different tempering schedules depending on their alloy systems and required cutting or forming performance.

Automotive Components

Tempered steel is extensively used in automotive manufacturing for components that must withstand repeated mechanical loading and impact.

Primjeri uključuju:

• Axle shafts
• Steering components
• Drive shafts
• Connecting components
• Delovi vešanja
• Komponente prijenosa

Quench-and-temper treatment allows these components to achieve high strength without sacrificing the toughness needed for real-world service conditions.

Construction and Heavy Equipment

Heavy machinery components often experience high loads, šok, abrazija, and cyclic stress. Tempered alloy steels are therefore widely used for:

• Pins and bushings
• Hidraulične komponente
• Excavator and loader parts
• Komponente krana
• Agricultural machinery components
• Mining equipment parts

U ovim aplikacijama, the required tempering condition is typically selected according to the dominant combination of impact resistance, snaga umora, i otpornost na habanje.

Pressure and Energy Equipment

Certain pressure-bearing and energy-system components use quenched-and-tempered steels because the treatment can provide high strength together with improved toughness.

Prijave mogu uključivati:

• High-strength pressure components
• Turbine-related mechanical parts
• Industrial fasteners
• Power-transmission components
• Heavy-duty structural components

Za sigurnosno kritične komponente, the heat-treatment process is normally supported by hardness testing, mehaničko ispitivanje, dimenzionalni pregled, and metallurgical verification.

12. Tempering Beyond Steel: Staklo, Chocolate, and Other Materials

The word “tempering” is also used in other fields, but the mechanisms are different.

Glass Tempering

Glass tempering involves heating glass to around 600–700°C and then rapidly cooling the surfaces with air jets.

The surface contracts faster than the interior, creating compressive stresses on the surface and tensile stresses in the core.

This makes tempered glass much stronger and causes it to break into small, blunt fragments rather than sharp shards. It is used in car windows, shower doors, and architectural glazing.

Chocolate Tempering

Chocolate tempering is a controlled heating and cooling process that encourages cocoa butter to crystallize in a stable form, known as Form V (beta crystals).

Properly tempered chocolate has a glossy finish, a crisp snap, and a smooth melt. Untempered chocolate may bloom, feel grainy, or soften too easily.

Ostali materijali

Some non-ferrous alloys, such as certain aluminum and copper alloys, undergo precipitation hardening or aging treatments that are sometimes loosely called tempering.

Međutim, the term is most precise when applied to ferrous metallurgy.

13. Zaključak

Tempering is a fundamental heat-treatment process for transforming quenched, visoko pod stresom, and relatively brittle steel into a more stable engineering material.

Its importance lies not simply in reducing hardness, but in controlling the microstructural evolution of martensite and establishing the required balance between hardness, snaga, žilavost, duktilnost, i dimenzionalna stabilnost.

The tempering temperature is the most influential process variable, but it must be considered together with holding time, steel composition, prior austenitizing and quenching conditions, component size, and the required final properties.

The correct tempering treatment is therefore steel-grade specific rather than universal.

Karbonski čelici, alloy structural steels, visokougljeničnih čelika, alatni čelici, brzorezni čelici, hot-work steels, and martensitic stainless steels can all exhibit different tempering responses.

U industrijskoj proizvodnji, successful tempering should be judged by the final performance of the component—not by temperature alone.

A properly controlled tempering process converts the extreme hardness of the quenched condition into a reliable combination of mechanical properties suitable for gears, osovine, Springs, umire, Automobilski dijelovi, teške opreme, and other demanding applications.

 

FAQs

What is tempering in steel?

Tempering is a heat-treatment process in which hardened steel is reheated to a temperature below its critical transformation range, held for a specified time, and then cooled.

It is primarily used to reduce brittleness and residual stress while obtaining the required balance of hardness, snaga, i žilavost.

Does tempering reduce hardness?

For most conventional quenched steels, increasing the tempering temperature generally reduces hardness and strength while improving toughness and ductility.

Highly alloyed tool steels can behave differently because of secondary hardening.

What is the difference between tempering and annealing?

Tempering is normally performed on hardened steel below the critical transformation range to modify martensite.

Annealing is generally intended to soften steel and establish a more ductile, Stabilna mikrostruktura, often using slow controlled cooling.

How is tempered steel inspected?

Typical inspection may include hardness testing, dimenzionalni pregled, metallographic examination, and mechanical testing, with additional surface or non-destructive testing where required by the application or specification.