1. Annealing the wire (before forming)
| Process | Typical temperature | Purpose |
|---|---|---|
| Spheroidise / soft annealing | about 680–760 °C, long hold, then slow cooling | Softens medium-carbon, boron and alloy steels for cold heading |
| Example: 42CrMo4 soft annealing | 680–720 °C, cooled about 15 °C per hour to 600 °C | Supplier datasheet value |
2. Hardening (austenitising) and quenching
| Steel | Used for | Typical hardening temperature | Quench |
|---|---|---|---|
| Boron steel: 10B21, 20MnB4, 23MnB4 | 8.8, some 10.9 | about 870–900 °C (one published 10B21 study used 870 °C) | Oil |
| Medium carbon: C35, C45, 1035, 1045 | 8.8 larger sizes | about 830–870 °C | Oil or polymer (water for some sizes) |
| Cr-boron: 32CrB4 | 10.9 | about 850–880 °C | Oil |
| Cr-Mo: 34CrMo4, 42CrMo4, SCM435/440, 4140 | 10.9 large, 12.9, B7 studs | 840–880 °C | Oil |
The atmosphere in the furnace is controlled so the surface neither loses nor gains carbon. Parts are held long enough for the whole section to reach temperature, then quenched quickly so the core reaches about 90% martensite.
3. Tempering: the rules and typical values
| Grade | Minimum tempering (standard) | Typical practice | Target hardness |
|---|---|---|---|
| 8.8 (ISO 898-1) | 425 °C | roughly 450–550 °C (one 10B21 study tempered at 480 °C) | HRC 22–32 (≤ M16) |
| 10.9 (ISO 898-1) | 425 °C | roughly 425–500 °C | HRC 32–39 |
| 12.9 (ISO 898-1) | 425 °C (380 °C allowed for certain boron-alloyed steels) | close to the minimum, about 425–460 °C | HRC 39–44 |
| ASTM A325 / A449 / A354 BC | 800 °F (427 °C) | – | per specification |
| ASTM A193 B7, F1554 Gr 105 | 1100 °F (593 °C) | – | B7: 35 HRC max |
The higher the tempering temperature, the softer and tougher the bolt. Makers temper as high as they can while still meeting the minimum hardness and strength. That is why 8.8 is tempered hotter than 12.9.
4. After heat treatment
| Process | Typical temperature | Note |
|---|---|---|
| Hydrogen relief baking after electroplating | about 200 °C for several hours (often 8 h or more) | Start soon after plating; for 10.9, 12.9 and case-hardened screws |
| Hot-dip galvanising bath | about 455–480 °C | Hotter than the 425 °C tempering minimum, so bolts for galvanising should be tempered above the bath temperature. Not recommended for 12.9. |
| Zinc flake curing | about 200–320 °C, depending on product | No hydrogen risk |
5. What happens if the temperature is wrong
- Hardening too cold or quench too slow: soft core, bolt fails the tensile or proof load test.
- Tempering too low: hardness above the maximum, brittle bolt, failed wedge test, high risk of hydrogen cracking.
- Tempering too high: hardness below the minimum, bolt stretches under preload.
- Wrong furnace atmosphere: decarburised (soft) or carburised (brittle) surface on the thread.
This is why hardness is checked on every heat-treatment batch, and why a test certificate for the lot is worth asking for. More in testing and quality checks.
EN grades commonly used in India
| Indian trade name | Close equivalent | Typical hardening | Tempering |
|---|---|---|---|
| EN8 (080M40) | C40 / C45, AISI 1040–1045 | about 830–860 °C, water or oil | set by required hardness; usually 550–660 °C for tough studs |
| EN19 (709M40) | 42CrMo4, AISI 4140, SCM440 | 840–880 °C, oil | 425 °C minimum for 10.9 / 12.9 fasteners; higher (540–680 °C) for tougher, lower-strength parts |
| EN24 (817M40) | AISI 4340 (Ni-Cr-Mo) | about 830–850 °C, oil | set by required strength; used for large, highly loaded studs |
EN grades are not fastener grades. The finished stud or bolt must still meet the property class or ASTM grade on the drawing, which is proved by testing.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.