If quenching gives bolts their hardness, tempering gives them their usefulness. As‑quenched martensite is hard but brittle – useless for any real application. Tempering transforms that brittle martensite into tempered martensite, the microstructure that delivers the right balance of strength, toughness, and ductility for property classes 8.8, 10.9, and 12.9. In this article, we answer five critical questions about tempering for fasteners, based on our shop floor experience, to help you understand how to achieve consistent, reliable mechanical properties.
Tempering is a heat treatment process in which hardened (as‑quenched) steel is reheated to a temperature below the lower critical point (typically 400–650°C for most fastener steels), held for a specified time, and then cooled – usually in still air. The purpose is to:
Reduce brittleness – As‑quenched martensite is very hard but extremely brittle; a quenched bolt can snap like glass.
Relieve internal stresses – Rapid cooling during quenching creates high residual stresses that can cause distortion or delayed cracking.
Adjust mechanical properties – By choosing the tempering temperature, we can achieve the exact combination of strength, hardness, and toughness required for a given property class.
The complete quench‑and‑temper (Q&T) sequence for high‑strength fasteners:
Cold‑headed or forged blank → austenitizing (830–880°C) → quench (rapid cool) → as‑quenched martensite (50–55 HRC, brittle) → temper (400–650°C) → tempered martensite (28–44 HRC, tough) → final product.
Real‑world case – the danger of skipping tempering:
A small bolt maker once sent us a sample of “grade 10.9” bolts for testing. When we torqued them to specification, they snapped with a clean, flat fracture. Microstructure revealed untempered martensite – they had skipped tempering to save time. Every bolt was rejected. Tempering is not optional; it transforms a dangerously brittle fastener into a reliable one.
During tempering, as quenched martensite – a supersaturated solid solution of carbon in iron – decomposes into a mixture of ferrite and fine carbide particles. The higher the temperature, the more the carbides grow and coalesce, reducing strength but increasing ductility and toughness.
Effect of tempering temperature on mechanical properties (typical for 40Cr or SCM435):
| Tempering Temperature (°C) | Resulting Microstructure | Hardness (HRC) | Tensile Strength (MPa) | Ductility / Toughness | Typical Fastener Grade |
|---|---|---|---|---|---|
| 150–200 (low temp) | Tempered martensite (very fine carbides) | 50–55 | >1800 | Very low (brittle) | Not used – too brittle |
| 400–480 (medium temp) | Tempered martensite (fine carbides) | 39–44 | 1200–1400 | Moderate | Grade 12.9 |
| 500–550 (medium‑high temp) | Tempered martensite (coarser carbides) | 32–38 | 1000–1200 | Good | Grade 10.9 |
| 550–600 (high temp) | Tempered martensite / tempered sorbité | 28–34 | 800–1000 | High | Grade 8.8 |
| 650–700 (very high temp) | Tempered sorbité / spheroidized | <25 | <700 | Very high | Over‑tempered – too soft for grade |
Key takeaway:
Lower tempering temperature → higher strength, lower toughness.
Higher tempering temperature → lower strength, higher toughness.
The art of heat treatment is to hit the exact temperature that gives the required property class with adequate safety margin.
Real‑world case – hitting the target:
A customer required SCM435 M14 bolts with a tight 10.9 spec: 32–36 HRC. Using 860°C austenitizing and oil quench, we tried tempering at 480°C – hardness 38–40 HRC (too high). At 520°C – 34–36 HRC (within spec). At 550°C – 30–32 HRC (too low). We established 510–530°C as the stable process window, achieving 34±1 HRC.
Even with the right temperature, tempering can go wrong. Here are the most common defects, causes, and prevention methods:
| Defect | Appearance / Detection | Root Cause | Prevention |
|---|---|---|---|
| Temper embrittlement | Low impact strength (Charpy test); intergranular fracture | Holding too long in 375–575°C range (especially alloy steels with impurities) | Avoid slow cooling through 375–575°C; quench after tempering; use cleaner steel (low P, Sn, Sb) |
| Over‑tempering (too soft) | Hardness below specification | Temperature too high or hold time too long | Calibrate furnace; use shorter soak; validate with test coupons |
| Under‑tempering (too hard/brittle) | Hardness above specification, low elongation | Temperature too low or hold time too short | Raise temperature or extend time; verify with hardness and tensile tests |
| Non‑uniform tempering | Hardness variation across batch (e.g., 5+ HRC spread) | Poor furnace temperature uniformity; parts stacked unevenly | Ensure proper fan circulation; separate parts; calibrate furnace zones |
| Tempering cracks | Fine cracks, often at stress risers | Quenching stresses not fully relieved; original quench cracks propagate | Normalize before Q&T; avoid sharp corners; temper immediately after quenching |
| Surface oxidation / decarburization | Blue/brown scale; soft surface layer | No protective atmosphere in tempering furnace | Use controlled atmosphere; short cycle time minimizes scale |
Real‑world case (temper embrittlement):
A batch of 42CrMo M20 bolts (grade 10.9) passed hardness and tensile tests but failed impact testing (Charpy V‑notch) at -20°C. Fracture surface showed intergranular cracking – classic temper embrittlement. The customer had slow‑cooled the bolts from 520°C instead of air cooling. Switching to air cooling (forced fan) eliminated the embrittlement.
Inspection methods after tempering:
Hardness test: Rockwell C (HRC) or Brinell (HB). Must be uniform across batch.
Tensile test: Confirms proof load, yield strength, and elongation (especially for 10.9 and 12.9).
Impact test (Charpy): Required for cold‑climate or dynamic load applications.
Microstructure: Tempered martensite – no untempered martensite, no ferrite/pearlite.
Different materials respond differently to tempering. The table below provides typical tempering parameters for common fastener steels after full quenching.
| Material | Typical Grade | Target Property Class | Tempering Temperature (°C) | Soaking Time (minutes per 25mm section) | Resulting Hardness (HRC) |
|---|---|---|---|---|---|
| Medium‑carbon steel | 35K, 45# | 8.8 | 550–600 | 60–90 | 28–34 |
| Low‑alloy steel | 40Cr | 10.9 | 500–550 | 60–90 | 32–38 |
| Low‑alloy steel | SCM435 | 10.9 | 500–540 | 60–90 | 32–38 |
| Low‑alloy steel | SCM435 | 12.9 | 400–450 | 60–90 | 39–44 |
| Medium‑alloy steel | 42CrMo | 10.9 | 520–560 | 60–90 | 33–38 |
| Medium‑alloy steel | 42CrMo | 12.9 | 430–480 | 60–90 | 40–44 |
| Martensitic stainless steel | 410, 420, 431 | (Custom) | 550–700 (tempering after quench) | 60–120 | 30–45 (varies) |
| Austenitic stainless steel | 304, 316 | (Not Q&T) | N/A – hardened by cold work | – | – |
Special notes for stainless steel:
Martensitic stainless fasteners (410, 420) are quenched and tempered like alloy steels, but tempering temperatures are often higher (550–700°C) to achieve good toughness.
Austenitic stainless bolts (304, 316) are not quenched and tempered; they are work‑hardened. Do not apply Q&T to them.
Real‑world case (different materials, same furnace):
A customer ran both 40Cr (10.9) and SCM435 (10.9) bolts in the same tempering batch at 520°C. The 40Cr bolts came out at 35 HRC (good), but the SCM435 bolts were 38 HRC (borderline too high). Reason: SCM435 contains molybdenum, which slows softening during tempering. The solution: separate batches or adjust temperature by 20–30°C.
If quenching gives bolts their hardness, tempering gives them their usefulness. As‑quenched martensite is hard but brittle – useless for any real application. Tempering transforms that brittle martensite into tempered martensite, the microstructure that delivers the right balance of strength, toughness, and ductility for property classes 8.8, 10.9, and 12.9. In this article, we answer five critical questions about tempering for fasteners, based on our shop floor experience, to help you understand how to achieve consistent, reliable mechanical properties.
Tempering is a heat treatment process in which hardened (as‑quenched) steel is reheated to a temperature below the lower critical point (typically 400–650°C for most fastener steels), held for a specified time, and then cooled – usually in still air. The purpose is to:
Reduce brittleness – As‑quenched martensite is very hard but extremely brittle; a quenched bolt can snap like glass.
Relieve internal stresses – Rapid cooling during quenching creates high residual stresses that can cause distortion or delayed cracking.
Adjust mechanical properties – By choosing the tempering temperature, we can achieve the exact combination of strength, hardness, and toughness required for a given property class.
The complete quench‑and‑temper (Q&T) sequence for high‑strength fasteners:
Cold‑headed or forged blank → austenitizing (830–880°C) → quench (rapid cool) → as‑quenched martensite (50–55 HRC, brittle) → temper (400–650°C) → tempered martensite (28–44 HRC, tough) → final product.
Real‑world case – the danger of skipping tempering:
A small bolt maker once sent us a sample of “grade 10.9” bolts for testing. When we torqued them to specification, they snapped with a clean, flat fracture. Microstructure revealed untempered martensite – they had skipped tempering to save time. Every bolt was rejected. Tempering is not optional; it transforms a dangerously brittle fastener into a reliable one.
During tempering, as quenched martensite – a supersaturated solid solution of carbon in iron – decomposes into a mixture of ferrite and fine carbide particles. The higher the temperature, the more the carbides grow and coalesce, reducing strength but increasing ductility and toughness.
Effect of tempering temperature on mechanical properties (typical for 40Cr or SCM435):
| Tempering Temperature (°C) | Resulting Microstructure | Hardness (HRC) | Tensile Strength (MPa) | Ductility / Toughness | Typical Fastener Grade |
|---|---|---|---|---|---|
| 150–200 (low temp) | Tempered martensite (very fine carbides) | 50–55 | >1800 | Very low (brittle) | Not used – too brittle |
| 400–480 (medium temp) | Tempered martensite (fine carbides) | 39–44 | 1200–1400 | Moderate | Grade 12.9 |
| 500–550 (medium‑high temp) | Tempered martensite (coarser carbides) | 32–38 | 1000–1200 | Good | Grade 10.9 |
| 550–600 (high temp) | Tempered martensite / tempered sorbité | 28–34 | 800–1000 | High | Grade 8.8 |
| 650–700 (very high temp) | Tempered sorbité / spheroidized | <25 | <700 | Very high | Over‑tempered – too soft for grade |
Key takeaway:
Lower tempering temperature → higher strength, lower toughness.
Higher tempering temperature → lower strength, higher toughness.
The art of heat treatment is to hit the exact temperature that gives the required property class with adequate safety margin.
Real‑world case – hitting the target:
A customer required SCM435 M14 bolts with a tight 10.9 spec: 32–36 HRC. Using 860°C austenitizing and oil quench, we tried tempering at 480°C – hardness 38–40 HRC (too high). At 520°C – 34–36 HRC (within spec). At 550°C – 30–32 HRC (too low). We established 510–530°C as the stable process window, achieving 34±1 HRC.
Even with the right temperature, tempering can go wrong. Here are the most common defects, causes, and prevention methods:
| Defect | Appearance / Detection | Root Cause | Prevention |
|---|---|---|---|
| Temper embrittlement | Low impact strength (Charpy test); intergranular fracture | Holding too long in 375–575°C range (especially alloy steels with impurities) | Avoid slow cooling through 375–575°C; quench after tempering; use cleaner steel (low P, Sn, Sb) |
| Over‑tempering (too soft) | Hardness below specification | Temperature too high or hold time too long | Calibrate furnace; use shorter soak; validate with test coupons |
| Under‑tempering (too hard/brittle) | Hardness above specification, low elongation | Temperature too low or hold time too short | Raise temperature or extend time; verify with hardness and tensile tests |
| Non‑uniform tempering | Hardness variation across batch (e.g., 5+ HRC spread) | Poor furnace temperature uniformity; parts stacked unevenly | Ensure proper fan circulation; separate parts; calibrate furnace zones |
| Tempering cracks | Fine cracks, often at stress risers | Quenching stresses not fully relieved; original quench cracks propagate | Normalize before Q&T; avoid sharp corners; temper immediately after quenching |
| Surface oxidation / decarburization | Blue/brown scale; soft surface layer | No protective atmosphere in tempering furnace | Use controlled atmosphere; short cycle time minimizes scale |
Real‑world case (temper embrittlement):
A batch of 42CrMo M20 bolts (grade 10.9) passed hardness and tensile tests but failed impact testing (Charpy V‑notch) at -20°C. Fracture surface showed intergranular cracking – classic temper embrittlement. The customer had slow‑cooled the bolts from 520°C instead of air cooling. Switching to air cooling (forced fan) eliminated the embrittlement.
Inspection methods after tempering:
Hardness test: Rockwell C (HRC) or Brinell (HB). Must be uniform across batch.
Tensile test: Confirms proof load, yield strength, and elongation (especially for 10.9 and 12.9).
Impact test (Charpy): Required for cold‑climate or dynamic load applications.
Microstructure: Tempered martensite – no untempered martensite, no ferrite/pearlite.
Different materials respond differently to tempering. The table below provides typical tempering parameters for common fastener steels after full quenching.
| Material | Typical Grade | Target Property Class | Tempering Temperature (°C) | Soaking Time (minutes per 25mm section) | Resulting Hardness (HRC) |
|---|---|---|---|---|---|
| Medium‑carbon steel | 35K, 45# | 8.8 | 550–600 | 60–90 | 28–34 |
| Low‑alloy steel | 40Cr | 10.9 | 500–550 | 60–90 | 32–38 |
| Low‑alloy steel | SCM435 | 10.9 | 500–540 | 60–90 | 32–38 |
| Low‑alloy steel | SCM435 | 12.9 | 400–450 | 60–90 | 39–44 |
| Medium‑alloy steel | 42CrMo | 10.9 | 520–560 | 60–90 | 33–38 |
| Medium‑alloy steel | 42CrMo | 12.9 | 430–480 | 60–90 | 40–44 |
| Martensitic stainless steel | 410, 420, 431 | (Custom) | 550–700 (tempering after quench) | 60–120 | 30–45 (varies) |
| Austenitic stainless steel | 304, 316 | (Not Q&T) | N/A – hardened by cold work | – | – |
Special notes for stainless steel:
Martensitic stainless fasteners (410, 420) are quenched and tempered like alloy steels, but tempering temperatures are often higher (550–700°C) to achieve good toughness.
Austenitic stainless bolts (304, 316) are not quenched and tempered; they are work‑hardened. Do not apply Q&T to them.
Real‑world case (different materials, same furnace):
A customer ran both 40Cr (10.9) and SCM435 (10.9) bolts in the same tempering batch at 520°C. The 40Cr bolts came out at 35 HRC (good), but the SCM435 bolts were 38 HRC (borderline too high). Reason: SCM435 contains molybdenum, which slows softening during tempering. The solution: separate batches or adjust temperature by 20–30°C.
