If you make a bolt, a fitting or a hydraulic connector by the thousand, the way the material machines becomes one of the biggest items in the cost per part. Free-cutting steels were developed for exactly this: steels that can be machined on automatic lathes at high speed, with long tool life and a good surface finish.
What do sulphur and lead do?
In free-cutting steels the sulphur content is deliberately kept high (about 0.15–0.40%). Sulphur combines with manganese to form manganese sulphide (MnS) inclusions in the steel. These inclusions make the chip break into short pieces instead of long, curling ribbons, so there is less chip wrapping and fewer stoppages at the machine.
Some grades also contain about 0.15–0.35% lead. During cutting, lead acts like a lubricant between tool and chip; it reduces cutting forces and tool wear and allows higher cutting speeds.
How are the names read?
- 11SMnPb30 — about 0.11% carbon, with sulphur (S) and manganese (Mn), leaded (Pb); 30 refers to about 0.30% sulphur. Old DIN name 9SMnPb28, SAE equivalent 12L13.
- 11SMn37 — the lead-free version with about 0.37% sulphur; old name 9SMn36, SAE equivalent 1215.
- SAE 12L14 — in the American system the letter “L” indicates added lead.
Where are they used?
Free-cutting steels are used for small and medium-sized parts where dimensional accuracy and production efficiency matter more than strength:
- Bolts, nuts, pins and special fasteners
- Fittings, nipples and hydraulic-pneumatic connectors
- Bushings, shaft ends and precision turned parts
These steels are usually used as bright (cold-drawn) bar, hexagon and coil that can go straight onto the automatic lathe. Some low-carbon free-cutting steels are suitable for case hardening (e.g. 10S20) and higher-carbon ones for hardening and tempering (e.g. 35S20).
Limitations
- Poor weldability: High sulphur increases the risk of hot cracking when welding.
- Toughness depends on direction: MnS inclusions stretched in the rolling direction lower toughness and fatigue strength, especially across the grain. They are not suitable for safety parts under high dynamic loads and impact.
- Restrictions on lead: Some industries and markets regulate lead content. Check the customer specification before choosing a leaded grade, and choose a lead-free grade if necessary.
Chemical composition (DIN 1651)
Composition ranges of common free-cutting steels, in percent by mass:
| Mat. No. | DIN | C | Mn | S | Pb |
|---|---|---|---|---|---|
| 1.0711 | 9 S 20 | 0.13 | 0.60-1.20 | 0.180-0.250 | - |
| 1.0715 | 9 SMn 28 | 0.14 | 0.90-1.30 | 0.240-0.320 | - |
| 1.0718 | 9 SMnPb 28 | 0.14 | 0.90-1.30 | 0.240-0.320 | 0.15-0.30 |
| 1.0736 | 9 SMN 36 | 0.15 | 1.00-1.50 | 0.320-0.400 | – |
| 1.0737 | 9 SMnPb 36 | 0.15 | 1.00-1.50 | 0.320-0.400 | 0.15-0.30 |
| 1.0721 | 10 S 20 | 0.07-0.13 | 0.50-0.90 | 0.150-0.250 | - |
| 1.0722 | 10SPb20 | 0.07-0.13 | 0.50-0.90 | 0.150-0.250 | 0.15-0.30 |
| 1.0726 | 35 S 20 | 0.32-0.39 | 0.50-0.90 | 0.150-0.250 | - |
| 1.0727 | 40 S 20 | 0.42-0.50 | 0.50-0.90 | 0.150-0.250 | - |