Heat treatment is a critical metalworking process used to modify the physical—and sometimes chemical—properties of fasteners, ensuring they meet demanding performance requirements. For screws and bolts, especially in military and aerospace applications, heat treatment enables control over hardness, strength, toughness, fatigue resistance, and environmental durability.
1. Core Heat Treatment Processes
Quench and Temper
Standard for high-carbon or alloy steel fasteners (e.g., SAE Grade 8, ISO 10.9/12.9), this process involves heating to austenitizing temperatures (~850–900 °C), rapid quenching (oil, water, or air), then tempering to balance hardness and toughness. Benefits include significantly increased tensile strength and fatigue resistance.
Annealing & Normalizing
Slows cooling to refine grain structure, relieve stress, and improve ductility or machinability. Used for fasteners requiring formability or softer cores while retaining some strength.
Case-Hardening (Carburizing/Nitriding)
Imparts a hard surface layer with a ductile core—ideal for wear resistance in self-drilling screws and high-cycle fasteners. Typical case depths are 0.5 mm.
Precipitation Aging
Applied to non-ferrous alloys (e.g., 17-4PH stainless steel, Ti‑6Al‑4V), this involves solution heat treatment followed by aging, delivering high strength with retained toughness. Common in aerospace-grade materials.
2. Benefits for Performance
- Enhanced Tensile Strength & Hardness
Heat-treated fasteners show superior mechanical strength; for instance, tempered SAE Grade 8 bolts can have nearly double the tensile strength of untreated Grade 2 bolts. - Improved Fatigue and Wear Resistance
Surface treatments like carburizing or shot peening, combined with core hardening, delay crack initiation under cyclic loads. - Temperature & Corrosion Stability
Inconel, titanium, or age-hardened stainless alloys retain structural properties under thermal stress and corrosive environments—ideal for military use. - Optimized Toughness & Ductility
Tempering avoids excessive brittleness, ensuring aluminium and steel fasteners balance strength with impact resistance.
Heat Treatment Methods & Applications
| Process | Benefit | Typical Use |
|---|---|---|
| Quench & Temper | High strength, fatigue & wear resistance | Grade 10.9/12.9 bolts, SAE Grade 8 Fasteners |
| Annealing/Normalizing | Stress relief, improved machinability | Formed parts, rivets, threaded rods |
| Case-Hardening | Hard surface, ductile core | Self-drilling screws, wear-prone shafts |
| Precipitation Aging | High-performance non-ferrous alloys | 17‑4PH SS bolts, Ti‑6Al‑4V fasteners |
3. Aerospace & Military Applications
- Aerospace: Titanium and superalloy fasteners undergo multi-stage aging to endure flight loads and temperature.
- Military: DFARS-compliant steel bolts are quench-and-tempered for required tensile strength and fatigue performance.
- Automotive/Heavy Equipment: SAE Grade 8 bolts, quench-and-tempered, provide rugged joint strength suitable for military vehicles.
4. Quality Control & Testing
- Strength Verification: Tensile and shear testing ensures compliance with ISO 898 and SAE standards.
- Hardness Checks: Rockwell scales verify heat-treatment success and batch accuracy.
- Microstructure Inspections: Ensure desired grain structure is achieved (martensite vs. bainite vs. pearlite).
- Batch Markings: Fastener heads are stamped post-treatment to indicate grade and traceability.
Why It Matters
Heat treatment transforms ordinary fasteners into mission- and safety-critical components. Without it, fasteners would suffer from deformation, fatigue failures, environmental degradation, or catastrophic joint collapse—unacceptable in military, aerospace, or industrial favorites.
Partner with Metric Bolt for Certifiable Heat-Treated Fasteners
At Metric Bolt, our engineered fasteners feature industry-standard heat treatments—quench-and-temper, case-hardening, precipitation aging—with documentation for DFARS, MIL-SPEC, and aerospace compliance. We provide detailed heat-treatment certs, material traceability, and metallurgical support.
Need heat-treated fasteners built to exacting specs?
Contact Metric Bolt for traceable sourcing, strength data, or project-specific guidance.
