V.S. Heat Treatment
Heat treatment & metal finishing experts
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V.S. Heat Treatment
Heat treatment & metal finishing experts
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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
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.
| 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 |
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.
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.