11SMnPb30, 11SMn37, 9SMn28 – why free-cutting steel remains the first choice despite the steel crisis.

Free-cutting steels determine a large part of turned parts economics. Excellent machinability, a low material price, good mechanical properties for non-corrosive applications – they are the standard when no corrosive or special requirements exist.

The most important grade is 11SMnPb30 (material no. 1.0718), formerly designated 9SMnPb28. Sulfur as a chip breaker, lead as an additional chip breaker. Very good machinability – 30–40 % faster to machine on the same machine than quenched and tempered steel 42CrMo4. Low material price. Mechanical strength sufficient for non-load-bearing components (Rm 380–530 MPa).

With the RoHS and REACH regulations, lead has become problematic in many applications. The lead-free alternative is 11SMn37 (1.0736). Machinability is roughly 10–15 % poorer than the leaded variant; mechanical properties are comparable. It is increasingly becoming the standard, especially in corporate purchasing.

For components where sulfur is tolerated but lead is not, there is 9SMnPb28 without lead: 9SMn28 (1.0715). Practically identical to 11SMnPb30 without lead – but the sulfur remains. Rarely suitable for medical and food-contact components.

The quenched and tempered counterpart: 42CrMo4 (material no. 1.7225) is used for mechanically highly loaded components. Poorer machinability (60–70 % more machine time), but tensile strength above 1,000 MPa after quenching and tempering. The standard for pins, shafts and axles in mechanically stressed applications.

A practical tip from 77 years of turned parts manufacturing: a blanket specification of 'quenched and tempered steel' is a common cost trap. In roughly 30–40 % of cases, a free-cutting steel is functionally sufficient – with a clear unit price advantage. Discussing material selection with the manufacturer often saves more than negotiating afterwards.

Corrosion is the Achilles heel of all free-cutting steels. Without surface protection (black oxide, zinc plating, chromium plating) they rust quickly in humid or aggressive environments. For corrosive requirements, stainless steel is the right choice – not a cheap free-cutting steel with makeshift 'on-site maintenance'.

If a turned part needs a hard, wear-resistant surface layer with a tough core, case-hardening steels such as 16MnCr5 (1.7131) or 20MnCr5 (1.7147) are used. These materials machine well in the soft, annealed condition; only after mechanical production does case hardening follow, in which the surface zone is carburized and hardened to a depth of roughly 0.3 to 1.5 mm. This produces surface hardnesses of around 58 to 62 HRC. Typical applications are pins, small gears, shafts and bushings that must withstand locally high contact pressure or friction.

For corrosion protection, several processes are available. Black oxide creates a thin, decorative oxide layer of less than 1 µm and offers only limited protection in combination with oil. Electrolytic zinc plating with clear or yellow chromating offers considerably more protection depending on coating thickness (typically 5 to 15 µm); zinc-nickel withstands salt spray tests for over 720 hours. Chromium plating serves as wear protection on functional surfaces. For high-strength, hardened parts, hydrogen embrittlement must be considered; here, hydrogen relief baking after the electroplating process is mandatory.

Whether free-cutting steel is sufficient depends on the load. For non-load-bearing or lightly stressed parts, 11SMnPb30 or 11SMn37 with a tensile strength of around 380 to 570 MPa is enough. As soon as components carry dynamic loads, bending or torsional moments, a quenched and tempered steel such as 42CrMo4 makes sense, achieving tensile strengths of 900 to 1100 MPa in the tempered condition. Where high surface hardness with a tough core and fatigue strength are required, for example in gearing, the path leads to case-hardening steel with a surface-hardened zone around 60 HRC. Please share your load assumptions with us.

From a regulatory perspective, sulfurized and especially leaded free-cutting steels are coming under increasing pressure. Lead is listed in REACH as a substance of very high concern, and the scope under the RoHS directive is also narrowing. That is why many corporations prefer the lead-free 11SMn37 (1.0736) for new parts. Without lead, chip breaking and tool life change noticeably; chips become longer, so we adapt cutting parameters and chip breaker geometry. Alternatives include bismuth-alloyed free-cutting steels or optimized machining strategies, which we coordinate with you in advance.

An important design note concerns weldability. The very additives that make free-cutting steels so machinable also significantly worsen their welding behavior: sulfur and especially lead lead to cracks and pores in the weld seam. If a turned part is to be welded later, a free-cutting steel is the wrong choice. In that case, weldable structural or quenched and tempered steels with low sulfur content are preferable. Tell us about any planned welding operations on the part, so we choose the right material from the start and avoid joining problems later.

We check feasibility, suggest optimizations and get back to you within 48 hours with a first assessment.