HEAT TREATMENT
Heat treatment is a controlled thermal process used to change the metallurgical microstructure of steel and obtain the required mechanical properties. Depending on the process, heat treatment can increase a fastener’s strength, hardness, toughness, wear resistance and resistance to elevated temperatures.
HEAT TREATMENT PROCESSES AT A GLANCE
The appropriate heat-treatment process depends on the steel composition, fastener dimensions, required property class and intended application.
| Process | Approximate Temperature or Method | Purpose | Typical Fastener Applications |
|---|---|---|---|
| Annealing | Just below 721°C | Softens and spheroidises the steel for easier forming | Wire rod and blanks before cold heading |
| Normalising | 800–920°C, followed by air cooling | Refines coarse grain after hot working | Hot-rolled or forged fastener blanks |
| Stress Relieving | 550–650°C | Reduces residual stresses created during cold forming | Cold-headed lower-property-class fasteners where specified |
| Hardening | Usually above 800°C, followed by quenching | Forms a hard martensitic microstructure | First stage of quenching and tempering |
| Tempering | Approximately 200–650°C | Reduces brittleness and adjusts hardness and toughness | Applied after hardening |
| Quenching and Tempering | Hardening and quenching, followed by tempering at approximately 340–650°C | Balances high strength with sufficient toughness and ductility | Property classes 8.8, 10.9 and 12.9 |
| Case Hardening | Carbon-rich atmosphere, followed by hardening | Creates a hard, wear-resistant surface with a tougher core | Tapping, thread-forming, thread-cutting, chipboard and some self-drilling screws |
| Induction Hardening | Local high-frequency induction heating, followed by quenching | Provides localised hardness and wear resistance | Threaded bars and special components |
Note: Temperatures are indicative. The exact cycle depends on the steel composition, fastener dimensions, equipment, required properties and applicable product standard.
ANNEALING
The steel is maintained at a temperature just below 721°C for several hours and then allowed to cool slowly. During this process, the structure changes from hard lamellar pearlite into softer globular or spheroidised pearlite. This produces a condition that is particularly suitable for cold heading and other forming operations.
NORMALISING (RECRYSTALLISATION)
The steel is heated to between approximately 800°C and 920°C for a controlled period and then cooled in air. This refines the coarse grain structure that can result from processes such as hot rolling or hot forging. Reducing the grain size can improve yield strength and impact resistance without significantly reducing tensile strength.
STRESS RELIEVING
Cold forming introduces residual stresses and work hardening into the material. Heating steel fasteners to approximately 550°C to 650°C for a controlled period can relieve much of this residual stress. The fasteners are then cooled slowly and uniformly to avoid creating new thermal stresses. Depending on the material and manufacturing route, this process may be used for some cold-headed fasteners in lower property classes.
HARDENING
When steel containing sufficient carbon is heated above its critical transformation temperature—often above 800°C for fastener steels—and then rapidly quenched in oil, water or another suitable medium, a hard but brittle martensitic microstructure can form.
The resulting hardness depends on the steel’s carbon content, alloy composition, section size and cooling rate. Thin fasteners made from suitable carbon steel may harden through to the core. In larger diameters, heat cannot be extracted from the core as quickly, so alloying elements such as boron, manganese, chromium, nickel and molybdenum may be used to increase hardenability.
Oil is commonly used to quench fasteners because it provides a more controlled cooling rate than water. Water cools more aggressively but can increase the risk of distortion and quench cracking.
TEMPERING
As hardness increases, hardening stresses and brittleness also increase. A second heat treatment, known as tempering, should therefore follow hardening as soon as practicable. At temperatures up to approximately 200°C, brittleness is reduced only slightly and hardness is largely retained. At higher tempering temperatures, internal stresses and hardness decrease while toughness and ductility improve.
QUENCHING AND TEMPERING
Quenching and tempering is a combined process in which hardening and quenching are followed by high-temperature tempering, commonly within a range of approximately 340°C to 650°C. It is one of the most important heat treatments used for high-strength fasteners. The process provides a balance between high tensile and yield strength and the toughness needed to withstand external and dynamic loads. Property classes 8.8, 10.9 and 12.9 are produced using controlled quenching and tempering in accordance with the applicable requirements.
FASTENER PROPERTY CLASSES AND HEAT TREATMENT
Property classes define minimum mechanical performance rather than one mandatory commercial steel grade. The material and manufacturing route are selected to achieve the required mechanical and physical properties.
| Property Class | Typical Processing or Heat Treatment | Minimum Tensile Strength Rm (MPa) | Typical Material Family |
|---|---|---|---|
| 4.6 | Normally not quenched and tempered; process selected to meet specified properties | 400 | Low-carbon steel |
| 4.8 | Normally not quenched and tempered; cold working or stress relieving may be used | 420 | Low-carbon steel |
| 5.6 | Normally not quenched and tempered; process selected to meet specified properties | 500 | Carbon steel |
| 5.8 | Normally not quenched and tempered; cold working or stress relieving may be used | 520 | Low-carbon or carbon steel |
| 6.8 | Normally not quenched and tempered; process selected to meet specified properties | 600 | Carbon steel |
| 8.8, d ≤ 16 mm | Quenched and tempered | 800 | Carbon or boron-alloyed steel |
| 8.8, d > 16 mm | Quenched and tempered | 830 | Boron-alloyed or other alloyed steel |
| 10.9 | Quenched and tempered | 1,040 | Medium-carbon, boron-alloyed or alloy steel |
| 12.9 | Quenched and tempered | 1,220 | Alloy steel |
Note: Minimum tensile-strength values are based on ISO 898-1 for applicable carbon-steel and alloy-steel bolts, screws and studs tested at ambient temperature. Material grades, chemical composition and production routes may vary by manufacturer, fastener dimensions and product specification.
CASE HARDENING
Case hardening is a carburising process carried out in a carbon-rich atmosphere. Carbon diffuses into the outer surface of the heated component, creating a thin layer that can be hardened to provide high wear resistance while the core remains tougher and more ductile. Case hardening is commonly applied to tapping screws, thread-forming screws, thread-cutting screws, chipboard screws and some self-drilling screws. Related treatments include carbonitriding, salt-bath nitriding and gas nitriding.
INDUCTION HARDENING
For special applications, a wear-resistant layer can be produced by rapidly heating a selected area with a high-frequency induction coil without direct contact with the workpiece. The heated area is then quenched in oil, water or another controlled medium.
Induction hardening is used when only part of a component requires increased hardness and wear resistance. It can also be suitable for long components such as threaded bars. The chart below relates steel carbon content to heat-treatment temperatures, steel categories and resulting mechanical properties.
FREQUENTLY ASKED QUESTIONS ABOUT HEAT TREATMENT
Why do high-strength bolts in property classes 8.8, 10.9 and 12.9 need quenching and tempering?
Quenching forms the hard martensitic structure required for high strength, but the fastener is too brittle in the as-quenched condition. Tempering is therefore applied immediately afterwards to reduce internal stresses and restore toughness while retaining most of the strength. Together, the processes provide the balance of tensile strength, yield strength and ductility required for high-strength bolted joints.
What is the difference between case hardening and through hardening?
Through hardening changes the microstructure across the fastener’s full cross-section, normally through quenching and tempering. Case hardening produces a thin, hard and wear-resistant outer layer while retaining a tougher, more ductile core. It is commonly used for tapping, thread-forming, thread-cutting, chipboard and some self-drilling screws.
Can heat treatment be reversed?
Heat treatment cannot simply be reversed in a way that guarantees the original properties. Annealing can soften hardened steel, after which a new controlled heat-treatment cycle may be applied. However, grain growth, decarburisation, distortion and previous processing can affect the final result. Welding or uncontrolled heating can reduce the strength of a heat-treated fastener and should not be carried out unless an approved procedure specifically permits it.
How do I know whether my fasteners have been properly heat treated?
Head markings identify the declared property class and manufacturer, but they do not by themselves prove that a particular production batch was correctly heat treated. Verification can include hardness testing, tensile testing, chemical analysis and metallographic examination. Fabory’s accredited laboratory can assess mechanical properties and microstructure.
Does heat treatment affect the surface coating of fasteners?
Heat treatment is normally completed before coating. Electroplating can introduce hydrogen into high-strength or case-hardened steel, so the coating process must follow ISO 4042 requirements and appropriate risk-reduction measures. Post-plating baking can reduce, but does not guarantee elimination of, hydrogen-embrittlement risk. Hot dip galvanising is governed by ISO 10684 for fasteners, and compatibility depends on the property class, material, pickling process, galvanising temperature and original tempering temperature. High-strength fasteners should always be assessed for coating compatibility.
SHOP HEAT-TREATED FASTENERS AT FABORY
- High-strength bolts in property classes 8.8, 10.9 and 12.9
- High-strength nuts
- Tapping and self-drilling screws
- Socket products and set screws
- Threaded rods and stud bolts
RELATED TECHNICAL GUIDES
- Hot Dip Galvanising — Learn how HDG coatings interact with heat-treated fasteners and why ISO 10684 applies to threaded fasteners.
- Electro Zinc and Zinc Alloy Plating — Understand electroplated coating systems and hydrogen-embrittlement considerations.
- Quality and Engineering Laboratory — Explore Fabory’s testing capabilities for tensile strength, hardness, chemical composition and microstructure.
Last updated: July 2026