Why this comparison matters
Material selection for large steel forgings is not a catalog exercise. The wrong grade costs money in one of two directions: over-specifying drives up raw material and heat-treatment cost; under-specifying produces a forging that cannot deliver the required properties through its full cross-section — a failure mode that often surfaces during third-party inspection or, worse, in service.
42CrMo4 (1.7225) is one of the world's most widely specified Cr-Mo engineering steels, used in everything from automotive crankshafts to general engineering shafts. Its strength-to-cost ratio is excellent and its manufacturing ecosystem is mature. But it has a fundamental limit: as section size increases beyond roughly 150 mm, its hardenability cannot produce a fully martensitic microstructure through the cross-section. The core becomes a mixed bainite-ferrite structure with meaningfully lower toughness and yield strength than the surface.
23CrNiMo7-4-7 (1.6749) was engineered specifically to address this limitation. The addition of approximately 3.5–4.5% nickel dramatically deepens hardenability, enabling full through-hardening in sections exceeding 400 mm and predictable, uniform properties even in forgings weighing 20 tonnes or more. This is the grade of choice for large steam and gas turbine rotors, heavy-wall pressure vessels, and any rotating component where a low-energy core fracture would be catastrophic. For technical specifications, supply dimensions, and a custom quotation, see our 23CrNiMo7-4-7 open die forging product page.
Both grades are Cr-Mo steels achieving their strength through quench-and-temper heat treatment. The fundamental difference is what happens at the center of a thick section during quenching — and the nickel content of 1.6749 is the mechanism that keeps that center fully martensitic.
Chemical composition side-by-side
The table below reflects EN 10083-3 heat analysis requirements. Both grades carry chromium and molybdenum; the structural difference is the presence of 3.5–4.5% Ni and approximately 0.12% V in 23CrNiMo7-4-7.
| Element | 23CrNiMo7-4-7 (1.6749) % | 42CrMo4 (1.7225) % | Metallurgical role |
|---|---|---|---|
| C — Carbon | 0.19 – 0.26 | 0.38 – 0.45 | Strength foundation; lower C in 1.6749 compensated by Ni/Mo |
| Si — Silicon | ≤ 0.40 | ≤ 0.40 | Deoxidiser; minor strength contribution |
| Mn — Manganese | 0.40 – 0.70 | 0.60 – 0.90 | Hardenability, sulphur fixing |
| Cr — Chromium | 0.35 – 0.65 | 0.90 – 1.20 | Hardenability, oxidation resistance, carbide stability |
| Ni — Nickel | 3.50 – 4.50 | ≤ 0.30 | Deep hardenability; toughness at low temperature; primary cost driver |
| Mo — Molybdenum | 0.55 – 0.70 | 0.15 – 0.30 | Elevated-temp strength; temper embrittlement resistance |
| V — Vanadium | 0.08 – 0.15 | — | Grain refinement via VC precipitation; creep resistance |
| P — Phosphorus | ≤ 0.015 | ≤ 0.025 | Controlled low; embrittlement risk at elevated levels |
| S — Sulphur | ≤ 0.010 | ≤ 0.035 | Lower in 1.6749 due to fracture toughness requirements |
The lower carbon content of 23CrNiMo7-4-7 (0.19–0.26% vs 0.38–0.45%) is deliberate. Higher carbon raises strength but reduces toughness and weldability, and makes steel more susceptible to hydrogen cracking during quenching of large forgings. The Ni-Mo combination achieves equivalent or higher strength without the carbon penalty, and delivers superior impact energy at both room temperature and sub-zero conditions.
Nickel at 3.5–4.5% is the primary cost driver. At current nickel prices, 23CrNiMo7-4-7 forgings carry a raw material premium of approximately 25–50% over 42CrMo4 at equivalent section weights. This premium is fully justified when section thickness demands it — and completely wasteful when it does not.
Hardenability: the real reason to choose 1.6749
Hardenability is the capacity of a steel to form martensite — the hard, strong microstructure responsible for high mechanical properties — throughout its entire cross-section during quenching. It is not the same as surface hardness. For large forgings, the critical question is: what is the maximum ruling section in which this steel can be reliably quenched to a fully martensitic core?
Beyond the limit, 42CrMo4 develops a bainite-ferrite mixed microstructure at the core. This is significantly weaker: yield strength can fall 15–25% below the surface value and Charpy impact energy may drop 40–60%. For non-critical structural components this may be acceptable. For rotating turbine components, high-pressure valve bodies, and pressure-retaining equipment, it is not.
Ruling section below 120 mm → 42CrMo4 adequate and more cost-effective. Above 200 mm → specify 23CrNiMo7-4-7 without substitution. The 120–200 mm range requires engineering judgement based on specific property requirements, safety category, and operating conditions.
Mechanical properties comparison
All values below are for the quenched-and-tempered (QT) condition, measured at the ¼-radius position per EN 10250-1 at room temperature unless noted. Note that 23CrNiMo7-4-7 values are relatively insensitive to section size by design; 42CrMo4 values degrade significantly with increasing section.
| Property | 23CrNiMo7-4-7 QT | 42CrMo4 QT | Notes |
|---|---|---|---|
| Tensile strength Rm | 850 – 1000 MPa | 900 – 1100 MPa (small sec.) | 42CrMo4 higher in thin sections; reverses in thick sections |
| Yield strength Rp0.2 | ≥ 750 MPa | ≥ 650 MPa → falls at thick sec. | 1.6749 more consistent across section sizes |
| Elongation A | ≥ 14% | ≥ 12% | Both adequate for most structural applications |
| Reduction of area Z | ≥ 50% | ≥ 45% | Higher Z in 1.6749 indicates superior ductility in thick sections |
| Charpy KV (RT, transverse) | ≥ 34 J | ≥ 27 J (if within spec section) | Impact advantage of 1.6749 grows dramatically in large sections |
| Charpy KV (−40°C) | ≥ 27 J | Not typically guaranteed | 1.6749 suitable for cold-climate power generation |
| Hardness surface (HBW) | 248 – 302 | 248 – 330 | Surface similar; core diverges in thick sections |
| Fatigue limit (approx.) | ~400 MPa | ~380 MPa (small sec.) → degrades | 1.6749 fatigue maintained in large sections |
Visual comparison at 500 mm ruling section
Heat treatment requirements
Both grades are supplied in the quenched-and-tempered (QT) condition for most engineering applications. Heat treatment parameters differ meaningfully, reflecting the different carbon levels and alloying systems.
- Austenitising: 820–870°C, hold minimum 1 hour per 100 mm ruling section
- Quench medium: Water quench (or fast polymer) required for large sections; oil quench for sections below ~200 mm
- Temper temperature: 550–650°C — higher range preferred for toughness-critical applications
- Temper hold: Minimum 2 hours per 100 mm of ruling section; multi-stage tempering for very large forgings
- Cooling after temper: Water quench or air cool — avoid slow cooling through 250–400°C (temper embrittlement zone)
- Post-QT stress relief: Available on request for weld-adjacent regions
- Austenitising: 830–870°C, similar hold time requirements
- Quench medium: Oil quench standard; polymer acceptable; water quench risks distortion/cracking in complex geometries
- Temper temperature: 550–680°C — upper range for improved toughness
- Temper hold: Minimum 1 hour per 25 mm of section thickness
- Cooling after temper: Air cool acceptable for most sections; avoid slow cooling if temper embrittlement sensitivity is a concern
- Post-QT stress relief: Sometimes specified after welding or heavy machining
The higher nickel content of 23CrNiMo7-4-7 allows water quenching — a more severe quench — without the cracking risk that accompanies water-quenching the higher-carbon 42CrMo4. For large forgings, this means 1.6749 can receive the aggressive water quench needed to drive martensite formation to the core, while 42CrMo4 is limited to oil quench, which is insufficient for sections above ~130 mm.
Where each grade belongs
| Application | Typical ruling section | Recommended grade | Reason |
|---|---|---|---|
| Steam / gas turbine rotor shafts | 500 – 1500 mm dia. | 23CrNiMo7-4-7 | Core toughness and fatigue life under cyclic bending at temperature |
| Large pressure vessel shells / heads | 200 – 600 mm wall | 23CrNiMo7-4-7 | EN 10222-5 compliance; through-wall uniformity for PED certification |
| Heavy valve bodies (Class 600–2500) | 150 – 400 mm | 23CrNiMo7-4-7 | Wall thickness demands full hardenability; pressure containment integrity |
| Generator forged shafts | 400 – 900 mm | 23CrNiMo7-4-7 | Fatigue life under torsional loading; failure containment |
| Mining / crushing equipment shafts | 120 – 280 mm | 23CrNiMo7-4-7 | High-impact cyclic loading; often exceeds 42CrMo4 hardenability limit |
| Machine tool spindles | 60 – 150 mm | 42CrMo4 | Within hardenability range; excellent machinability and cost efficiency |
| Automotive / CV crankshafts | 60 – 120 mm | 42CrMo4 | Industry-standard grade; widely certified in automotive supply chains |
| General engineering shafts and axles | 40 – 180 mm | 42CrMo4 | Cost-appropriate where through-section uniformity is not safety-critical |
| Forged flanges (PN40 and below) | < 100 mm | 42CrMo4 | Thin section; 42CrMo4 fully hardens; cost-effective |
Jiangsu Liangyi manufactures all of the 23CrNiMo7-4-7 component types listed above. For available product forms, maximum dimensions, and lead times, visit the 23CrNiMo7-4-7 forging product page.
Decision guide: which grade to specify
Use the checklist below. If any condition on the left is true for your application, the recommended grade is shown on the right.
A common and serious procurement error is specifying 23CrNiMo7-4-7 and accepting a substitution of 42CrMo4 when 1.6749 is temporarily unavailable or priced higher. These grades are not interchangeable in large sections. The substitution may pass chemical analysis but will fail mechanical testing at depth. Always reject this substitution for rotating components and pressure-retaining forgings.
Procurement and supply notes
Certifications to require
For 23CrNiMo7-4-7 forgings in critical applications, the minimum documentation package should include: EN 10204 3.1 Mill Test Report covering chemical analysis and full mechanical testing; UT report per EN 10228-3 or ASTM A388; heat treatment furnace chart; dimensional inspection report. For PED-classified equipment, EN 10204 3.2 with a nominated third-party inspection body is required. Jiangsu Liangyi supplies all of this documentation as standard — see the full certification and supply details on our product page.
Key questions for your forging supplier
| Question to ask | Why it matters | Red flag answer |
|---|---|---|
| Maximum press capacity (tonnes)? | Insufficient press force means inadequate reduction ratio — microstructure will not be fully worked | "We can forge it" without specifying press tonnage |
| Minimum forging reduction ratio for 1.6749? | Ratio below 3:1 leaves dendritic as-cast structure incompletely broken; properties will be anisotropic | Cannot answer, or states below 3:1 |
| Quench tank capacity and circulation rate? | Undersized or uncirculated bath cannot cool a large forging fast enough to achieve martensite at the core | Single static bath, no circulation data available |
| Do you perform hardness surveys at multiple depths? | Surface hardness does not confirm core hardness; through-section survey confirms hardenability | "We test surface hardness only" |
| Can you supply EN 10204 3.2 with TÜV/DNV witness? | Confirms third-party inspection experience and open-door QC policy | Hesitation, large extra charge, or refusal |
International grade equivalents
| Standard | 23CrNiMo7-4-7 equivalent | 42CrMo4 equivalent |
|---|---|---|
| EN (European) | 1.6749 / 23CrNiMo7-4-7 | 1.7225 / 42CrMo4 |
| DIN (German) | 23CrNiMo7-4 | 42CrMo4 |
| ASTM / AISI (USA) | No direct equivalent; closest AISI 4340 | AISI 4140 / ASTM A29 4140 |
| BS (British) | 817M40 (partial equivalent) | 708M40 |
| JIS (Japanese) | SNCM630 (closest) | SCM440 |
| GB (Chinese) | 40CrNiMoA (approximate) | 42CrMo |
AISI 4340 is sometimes offered as an equivalent, but it has higher carbon (0.38–0.43%) and lower molybdenum, changing temper embrittlement behaviour and weldability. For EN 10222-5 qualified applications, only the EN grade should be accepted unless a formal equivalency review is conducted by a qualified metallurgical engineer.
Frequently asked questions
These are the questions engineers and procurement teams most commonly ask when selecting between these two grades.
23CrNiMo7-4-7 (1.6749) contains 3.5–4.5% nickel, giving it deep hardenability for sections exceeding 400 mm, while 42CrMo4 (1.7225) is a cost-efficient Cr-Mo steel suitable for sections up to ~150 mm. The key difference is through-thickness property uniformity in large forgings — 1.6749 maintains consistent yield strength and Charpy impact at the core even in sections over 500 mm diameter.
Specify 23CrNiMo7-4-7 when: ruling section exceeds 200 mm; the component is a rotating turbine or generator element; EN 10222-5 governs; sub-zero Charpy impact is required (−20°C or below); or the component is pressure-retaining in PED Category III or IV.
42CrMo4 achieves full martensitic hardening only to approximately 100–150 mm ruling section. Beyond this, core properties degrade — yield strength can fall 15–25% and Charpy impact energy 40–60% compared to surface values due to the formation of a mixed bainite-ferrite microstructure at the core.
No. AISI 4340 has higher carbon (0.38–0.43%) and lower molybdenum than 23CrNiMo7-4-7, changing its temper embrittlement behaviour and weldability profile. For EN 10222-5 qualified applications, only the EN grade should be accepted unless formal equivalency review is conducted by a qualified metallurgical engineer.
Minimum: EN 10204 3.1 Mill Test Report (chemical + mechanical), UT report per EN 10228-3 or ASTM A388, heat treatment furnace chart, dimensional inspection report. For PED applications: EN 10204 3.2 with nominated third-party witness (TÜV, BV, DNV, ABS, or Lloyd's Register).
No. These grades are not interchangeable in large sections. 42CrMo4 substituted for 23CrNiMo7-4-7 in rotating components or pressure-retaining forgings with sections above 150 mm will fail to meet required core mechanical properties and may fail in service. Always reject this substitution for critical forgings.
Conclusion
The choice between 23CrNiMo7-4-7 and 42CrMo4 is not primarily a question of strength — it is a question of what happens to strength and toughness when you increase section size. 42CrMo4 is a well-proven, cost-effective grade for sections up to approximately 120–150 mm, and specifying 23CrNiMo7-4-7 where it is not needed wastes meaningful alloy cost.
But for turbine shafts, heavy pressure vessels, large valve bodies, and any rotating component where a core fracture could be catastrophic, 23CrNiMo7-4-7 is not a premium upgrade — it is the minimum correct specification. The nickel content that drives its cost is also the mechanism that ensures the centre of a 600 mm rotor shaft has the same toughness as its surface.
If uncertain which grade applies to your component, define the ruling section first, then consult your forging manufacturer's metallurgy team. A supplier with genuine 1.6749 experience will provide Jominy curves, hardness survey data from comparable sections, and EN 10204 3.1 test reports from previous heats — removing ambiguity before you commit to an order.
Now you know which grade to choose — get the full product details
This guide has covered the engineering case for selecting 23CrNiMo7-4-7 over 42CrMo4. If 1.6749 is the right grade for your application, the next step is reviewing supply dimensions, available product forms, certification scope, and requesting a quotation.