34CrMoS4 vs 34CrMo4 —
How to Choose the Right Grade
for Your Forging Project

Why the Grade Choice Matters More Than It Appears

Every year, procurement teams and design engineers specify 34CrMo4 when they actually need 34CrMoS4 — and vice versa. The downstream costs are rarely visible on a purchase order. They show up as shortened insert life on a gear-hobbing machine, increased cycle time on a CNC lathe, or a weld repair needed after the forging is already delivered. They accumulate as change orders, delayed builds, and quality hold costs that nobody traces back to the original grade specification on the drawing.

The confusion is understandable. In most reference tables, 34CrMoS4 (1.7226) and 34CrMo4 (1.7220) differ by a single character. Their tensile strength, yield strength, elongation, and Charpy impact values are identical per EN 10083-3. Their chromium and molybdenum contents are identical. Their heat treatment cycles are virtually identical. Most materials reference charts offer almost no practical guidance on which to choose.

This guide addresses the confusion directly: what the one real difference is, why it matters in specific production environments, and a step-by-step decision framework so you specify the right alloy the first time, every time.

The One Real Difference: Controlled Sulfur

The designation "34CrMoS4" encodes the alloy's identity precisely. The trailing "S" is not a minor sub-specification suffix — it is a compositional instruction. Under EN 10083-3, 34CrMoS4 must contain between 0.020% and 0.040% sulfur by mass. This is a controlled band with both a minimum and a maximum — not simply a maximum limit.

34CrMo4, by contrast, specifies only a maximum sulfur of ≤ 0.035% with no lower bound. In practice, steelmakers producing 34CrMo4 suppress sulfur as low as possible, because sulfur is ordinarily treated as a contamination that degrades mechanical isotropy and transverse toughness. A typical 34CrMo4 heat may contain anywhere from near-zero to 0.030% sulfur.

The deliberate reason for adding sulfur is metallurgical: in the presence of manganese (0.60–0.90% in both grades), sulfur precipitates as fine manganese sulfide (MnS) inclusions distributed throughout the steel matrix. These inclusions are softer than the surrounding quenched and tempered martensite and act as natural chip-breakers during cutting operations — reducing local shear strength, causing chips to break short rather than forming long continuous stringers that impair tool life and surface finish.

If you are sourcing custom 34CrMoS4 open die forgings and seamless rolled rings, our product page covers available product forms, weight range, heat treatment options, and certification details.

Chemical Composition Comparison per EN 10083-3

The table below reproduces the full composition requirements from EN 10083-3 for both grades. Every single element except sulfur is specified identically — this is by design in the European standard, making sulfur the sole differentiator between these two closely related alloy steels.

Mechanical Properties After Quench & Temper: Nearly Identical

EN 10083-3 specifies identical minimum mechanical property requirements for both 34CrMoS4 and 34CrMo4 in the quenched and tempered (+QT) condition. Engineers who select between these grades on the basis of strength or notch toughness targets are making a decision the standard itself does not support — both grades deliver the same guaranteed performance envelope.

Machinability Deep Dive: Where 34CrMoS4 Earns Its Premium

The machinability advantage of 34CrMoS4 over 34CrMo4 is not marketing language — it has a specific metallurgical mechanism and is measurable in a production environment. Understanding the mechanism helps engineers evaluate whether it applies to their specific component.

How MnS Inclusions Improve Cutting Performance

During CNC turning, the cutting insert engages the workpiece at shear-zone temperatures of 600–900°C and cutting forces measured in kilonewtons. In a standard CrMo alloy with minimal sulfur content (such as 34CrMo4 as typically produced), chips tend to be long, continuous, and ductile — wrapping around the insert, generating heat, and accelerating cutting-edge wear through adhesive and abrasive mechanisms. In 34CrMoS4, the fine MnS inclusions interrupt the shear plane at frequent intervals. Because MnS has significantly lower shear strength than the surrounding martensite matrix, the chip breaks at each inclusion. The result is short, segmented chips that evacuate the cutting zone cleanly, reduce heat build-up at the cutting edge, and preserve insert geometry across a substantially longer tool life.

Operations That Benefit Most

Weldability & Hot-Crack Sensitivity

Both grades are weldable with proper precautions — neither is a freely weldable structural steel by modern standards. The carbon equivalent of both grades (IIW formula) is approximately 0.56–0.65%, placing them firmly in the range requiring pre-heat to 200–300°C and low-hydrogen (H5 or H4 class) welding consumables.

The distinction between the two grades in welding applications is real but subtle: the controlled sulfur in 34CrMoS4 (0.020–0.040%) slightly increases hot-crack sensitivity (solidification cracking) in the weld metal and heat-affected zone in heavily restrained multi-pass joints. Sulfur reduces the solidification temperature range of grain boundary films — a detrimental effect in fully constrained structural welds with high joint restraint.

Hardenability in Large Section Open Die Forgings

Both grades share identical chromium (0.90–1.20%) and molybdenum (0.15–0.30%) content — the two alloying elements that drive hardenability in the CrMo steel family. As a result, their Jominy hardenability bands are essentially identical: both achieve full martensite transformation to approximately 35–40 mm from the quenched face under standard test conditions.

For large open die forgings in section sizes of 200 mm, 400 mm, or 600 mm+ diameter, the critical factor is the interaction between the steel's hardenability and the quench severity achievable at the forging's actual scale. Neither 34CrMoS4 nor 34CrMo4 holds a meaningful advantage here — the same oil-quench or polymer-quench strategy applies to both grades, and the same heat treatment qualification process is required to verify that specified mechanical properties are achieved at the critical cross-section.

Decision Framework: When to Specify Which Grade

The decision grid below distils our forging and machining experience with both grades since 1997. If your component's characteristics fall clearly into one column, that is your grade. Where characteristics span both columns, evaluate which production stage dominates total cost — and choose accordingly.

Application-by-Application Grade Guide

For full technical details on product forms, weight range (30 kg–30,000 kg), heat treatment options, and EN 10204 certification scope, see our 34CrMoS4 (1.7226) forging product page.

Sourcing & Certification Checklist for Both Grades

Whether you specify 34CrMoS4 or 34CrMo4, verify these five requirements before placing an order with any forging supplier. They separate a traceable, compliant supply chain from a substitution and documentation risk.

At 34CrMoS4 (1.7226) forgings are produced at Jiangsu Liangyi Co., Limited from our own EAF + LF + VOD steelmaking facility in Jiangyin, Jiangsu Province, China. Every heat is analysed by in-house OES spectrometry before tapping, and the numerical sulfur content is recorded and certified on every EN 10204 MTC we issue. Our engineering team is available to review your specification and recommend the most cost-effective grade within 24 hours.

Global Grade Equivalents for 34CrMoS4 (1.7226) and 34CrMo4 (1.7220)

Both grades are European standards (EN 10083-3). The table below maps them to the closest equivalents in major global standards systems. Grade equivalents are always approximate — always verify actual chemistry and mechanical property requirements before treating equivalents as interchangeable in a critical specification.