Case hardening vs through hardening: which one for screws and bolts?
The two main processes on our continuous mesh-belt furnace — choose by what the part has to do, not by maximum hardness alone.
Case hardening vs Through hardening
Short answer: tapping screws are case hardened, class 8.8/10.9 bolts are through hardened
Case hardening (carburizing) diffuses carbon into the surface of low-carbon steel before quenching, giving a hard skin of about 550–700 HV over a tough, break-resistant core — right for self-tapping and self-drilling screws that cut threads into steel. Through hardening (quench & temper) hardens medium-carbon or alloy steel uniformly through the section, which is how bolts reach property classes 8.8, 10.9 and 12.9.
Case hardening versus through hardening comparison
| Decision factor | Case hardening (carburizing) | Through hardening (Q&T) |
|---|---|---|
| Principle | Carbon diffused into the surface (~850–940°C), then quenched | Whole part heated, quenched and tempered |
| Material | Low-carbon steel: SWCH10A–22A (C ≤ ~0.25%) | Medium-carbon/alloy steel: S45C, SCM435, SCM440 |
| Surface hardness | About 550–700 HV (~52–60 HRC) | Per class, e.g. 22–39 HRC (8.8–10.9) |
| Core hardness | Much softer than the skin — tough core | Close to surface — uniform through section |
| Key spec variables | Case depth + surface + core hardness | Hardness window / property class + test point |
| Common standard | ISO 2702 (heat-treated tapping screws) | ISO 898-1 (bolt property classes) |
| Example parts | Tapping, self-drilling and drywall screws, washers | Class 8.8–12.9 bolts and nuts, pins |
| Strength | Wear-resistant skin + impact-tough core | Full-section tensile strength per class |
How to choose
Case hardening
Choose case hardening when the part is formed from low-carbon steel and the job is a hard surface — cutting threads into sheet steel, resisting surface wear — while the core must stay tough enough not to snap.
Through hardening
Choose through hardening when the part carries tensile or shear load through its whole section to a property class — bolts of class 8.8 and up, or parts specified with uniform hardness.
Limits before putting it on the drawing
- Low-carbon steel cannot be through hardened, and high-carbon steel should not be carburized — material and process must match.
- Case depth on small screws is tens to hundreds of microns; post-hardening grinding can remove the entire case.
- HV/HRC ranges are typical mesh-belt small-part guidance, not guarantees — verify by test.
- Hardened parts that are zinc plated afterwards need hydrogen-embrittlement assessment and baking by grade.
What to send the heat treater
- 1Part type, material and mill certificate
- 2Reference standard, e.g. ISO 2702 or ISO 898-1
- 3Surface hardness and case depth (carburizing) or hardness window/class (Q&T)
- 4Test location and method (HV/HRC)
- 5Lot quantity/weight and downstream finish
Frequently asked questions
Which process is used for self-tapping screws?
Case hardening (carburizing). Low-carbon screws get a hard case that cuts threads into steel while the core stays tough. Through hardening is for medium-carbon or alloy bolts that need full-section strength.
Can any steel be through hardened?
No. Low-carbon steel cannot be through hardened and high-carbon steel should not be carburized — the material and process must match. Send the grade and we will confirm the right route.