42CrMoS4 alloy steel: properties and machinability

Key takeaways

• 42CrMoS4 is the free-machining variant of 42CrMo4, with added sulfur (0.020–0.040%) to improve machinability.

• The sulfur content produces shorter chips, reduces tool wear, and enables higher cutting speeds in CNC machining.

• After quenching and tempering, 42CrMoS4 offers mechanical properties comparable to 42CrMo4, making it suitable for high-strength engineering components.

• It is commonly used for shafts, gears, hydraulic components, and other high-volume precision-machined parts.

• 42CrMoS4 can be heat treated, induction hardened, and nitrided to improve hardness and wear resistance.

• Welding is generally not recommended because the sulfur content increases the risk of weld cracking.

• If a part requires extensive grinding, polishing, photo-etching, or welding, 42CrMo4 is generally the better choice.

42CrMoS4 (material number 1.7227, EN 10083-3) is a chromium-molybdenum alloy steel designed for high-volume CNC production. It is chemically almost identical to 42CrMo4 (1.7225), with one key difference: a sulfur content of 0.020–0.040%, which improves machinability.

This single element is why production engineers specify it over the standard grade. The "S" in the designation determines how the steel cuts, how long inserts last, and how profitable a production run is.

Both grades conform to EN 10083-3. For sourcing purposes, 42CrMoS4 maps closely to AISI 4140. Confirm the sulfur specification on the datasheet before substituting.

What is 42CrMoS4 alloy steel

Chromium (0.90–1.20%) and molybdenum (0.15–0.30%) provide the alloy's core mechanical properties. Chromium increases hardenability, enabling the steel to form martensite throughout sections up to approximately 60 mm (2.4 in.) in diameter during quenching. Molybdenum refines the grain structure, improves tempering resistance, and enhances toughness under repeated dynamic loading.

Sulfur reacts with the manganese already present in the alloy to form manganese sulfide (MnS) inclusions. In the cutting zone, these inclusions fracture ahead of the cutting edge, producing shorter, more manageable chips. This reduces heat transfer to the cutting insert and can extend tool life while maintaining high machining speeds, depending on the application.

Mechanical properties in the quenched and tempered (+QT) state

Matched-heat comparisons between 42CrMo4 and 42CrMoS4 show tensile and yield strength within 2-3% of each other at equivalent heat treatment. The trade-off is a modest reduction in transverse fatigue strength and through-thickness impact toughness. This is acceptable for components loaded axially or torsionally.

Machinability comparison: 42CrMoS4 vs. 42CrMo4

In automated CNC turning and milling of shafts, gear blanks, and hydraulic components, the controlled-sulfur grade delivers three concrete advantages:

• Higher cutting speeds without accelerating tool wear. MnS inclusions reduce adhesive bonding between the chip and tool face, which causes 15-30% higher top speeds. This is the primary driver of flank wear in carbide tooling.

• Better surface finish at equivalent feed rates. Chip breaking is more predictable, which reduces built-up edge formation on the insert.

• Shorter cycle times. Where 42CrMo4 may need chip-breaking pauses or reduced depth of cut, 42CrMoS4 runs continuously at the same parameters.

There is one important limitation: if the part will undergo grinding, polishing, or photo-etching after machining, 42CrMo4 is generally the better choice.

Heat treatment processes

42CrMoS4 follows the same heat treatment sequence as 42CrMo4. The sulfur content does not affect the transformation temperatures.

Heat treatment processes run in three stages. Soft annealing at 710–750°C with a slow furnace cool brings the hardness below 241 HB, which is the working range for heavy rough machining before final hardening. Once the part geometry is established, hardening takes place at 820–860°C. Hold the metal until it is through-heated. Quench it in oil or polymer.

The quench severity must match section thickness; insufficient agitation in heavy bars risks a soft core. The final stage is tempering at 150 to 680°C for a minimum of 2 hours, which sets the hardness-toughness balance for the application.

Surface hardening: induction hardening and nitriding

When only the working surface needs wear resistance, selective surface hardening is more efficient than through-hardening.

Induction hardening austenitizes the surface locally using an induction coil, then quenches immediately. Surface hardness of 54 to 58 HRC is achievable on gear teeth and journal surfaces without affecting its core properties.

Gas nitriding at 500–530°C over 20 to 80 hours produces a compound layer of 5–20 µm and a diffusion zone to 0.2 to 0.6 millimeter depth. Surface hardness reaches 650–750 HV with minimal distortion. This material is well suited for crankshafts, camshafts, and precision gear flanks where dimensional stability after treatment matters.

Welding and fabrication: precautions for alloy steels

Welding 42CrMoS4 is generally not recommended because its sulfur content increases the risk of hot cracking in the weld metal and heat-affected zone. Its relatively high carbon equivalent also raises the likelihood of hydrogen-induced cold cracking.

If welding is unavoidable, preheat the material to 150–250°C (302–482°F), depending on the section thickness. Use a low-hydrogen filler material with strength matched to the base metal, then perform stress-relief heat treatment at 580–620°C (1,076–1,148°F), followed by slow cooling.

For weldments where sulfur inclusions are a structural concern, specify 42CrMo4. It delivers better weld integrity with no compromise on mechanical properties.

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