It is one of the most common dilemmas in machine design: should the part be made from case-hardening steel or from quenched and tempered steel? Both are used for high-performance parts, but they deliver strength by different routes and in different places.
Case-hardening steels: hard case, tough core
Case-hardening steels are low in carbon (about 0.10–0.25% C). As they are, they do not harden enough when quenched, so carbon is first diffused into their surface.
- Carburising: The part is held at about 880–950 °C in a carbon-rich atmosphere. The carbon content at the surface rises to 0.7–0.9%.
- Quenching: The carbon-enriched surface layer becomes very hard; the low-carbon core stays tough.
- Low-temperature tempering: Usually at 150–200 °C, to reduce internal stresses.
The resulting surface hardness is typically around 58–62 HRC; the case depth is usually chosen between 0.5 and 2 mm depending on the application. The surface resists wear and contact pressure while the core absorbs impact.
Typical grades: C15, 16MnCr5, 20MnCr5, 20NiCrMo2 (SAE 8620), and 17CrNiMo6 and 18CrNi8 for heavily loaded gears.
Quenched and tempered steels: strength throughout the section
Quenched and tempered steels have medium carbon (about 0.25–0.60% C) and are usually alloyed with chromium, molybdenum and nickel. The heat treatment has two steps:
- Hardening: The steel is heated to its austenitising temperature (about 820–880 °C depending on the grade) and cooled quickly in oil or water.
- High-temperature tempering: Held at about 540–680 °C, part of the hardness is traded for toughness.
After this treatment the material has high yield strength and toughness not just at the surface but throughout the section. 42CrMo4, for example, reaches a tensile strength of around 900–1,300 MPa in the quenched and tempered condition, depending on section, up to a diameter of about 100 mm; at larger diameters the value falls. As sections grow, alloying matters more so that the centre can harden too — which is why nickel-bearing grades such as 34CrNiMo6 are preferred for large-diameter shafts.
Typical grades: C45, 41Cr4, 34CrMo4, 42CrMo4 (SAE 4140), 50CrMo4, 34CrNiMo6 (SAE 4340).
Side-by-side comparison
| Case-hardening steel | Quenched and tempered steel | |
|---|---|---|
| Carbon content | Low (≈ 0.10–0.25%) | Medium (≈ 0.25–0.60%) |
| Heat treatment | Carburising + quenching + low-temperature tempering | Hardening + high-temperature tempering |
| Surface | Very hard (≈ 58–62 HRC) | Medium hardness; induction hardened if needed |
| Core | Tough, relatively low strength | High strength and toughness |
| Best against | Surface contact, wear, impact | Bending, torsion, fatigue |
| Typical part | Gears, pinions, pins, camshafts | Axles, crankshafts, shafts, high-strength bolts |
Which one for which part?
- Gears and pinions: The tooth flank sees contact pressure and wear, the tooth root impact and bending. Since a hard case over a tough core is needed, case-hardening steel is the standard choice.
- Axles, crankshafts, drive shafts: These carry torsion and alternating bending through the whole section. Quenched and tempered steel is the better fit.
- Shafts with bearing seats: A common solution is to make them from quenched and tempered steel and induction-harden only the bearing areas, giving both core strength and surface hardness.
- Pins and bushings: Where wear dominates, case-hardening steel; where load-bearing dominates, quenched and tempered steel.
Points to check before ordering
- Case hardening can cause distortion and dimensional change, so a grinding allowance is left on precision surfaces.
- For quenched and tempered steels, stating the delivery condition at the quoting stage (e.g. quenched and tempered +QT or soft annealed +A) makes it easier to plan the following operations.
- For large sections, choose the grade with the diameter in mind so that the required values can be reached.