42CrMo vs 42CrMo4 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
42CrMo and 42CrMo4 are medium‑carbon chromium‑molybdenum alloy steels widely used for high‑strength, heat‑treatable components such as shafts, gears, and fasteners. Engineers and procurement professionals often face a selection dilemma between these two labels because they yield very similar metallurgy and mechanical behavior but are referenced in different regional standards and supply chains. Typical decision contexts include balancing required certification (regional or project standard), specifying weldability and post‑weld treatments, and optimizing cost and lead time for specific product forms.
The primary practical distinction is not a dramatic metallurgical mismatch but rather the standards and specification regimes that govern production, inspection, and certification. That difference drives procurement, traceability, and sometimes minor compositional tolerances or permitted impurity levels.
1. Standards and Designations
- EN (Europe): 42CrMo4 — commonly referenced in EN 10083–3 and EN steel numbering systems (often also given as 1.7225 in material databases).
- GB (China): 42CrMo — widely used in Chinese national standards (GB/T) and commonly listed for alloy structural steels.
- ASTM/ASME / AISI (USA): AISI 4140 / UNS G41400 is the closest widely accepted American equivalent and is often interchanged with 42CrMo/42CrMo4 in cross‑reference charts.
- JIS (Japan): SCM440 is a commonly cited Japanese equivalent.
- Classification: both 42CrMo and 42CrMo4 are medium‑carbon, low‑alloy, heat‑treatable steels (alloy steel suitable for quenching and tempering; not stainless; not HSLA in the modern sense).
2. Chemical Composition and Alloying Strategy
| Element | Typical 42CrMo (GB/T) | Typical 42CrMo4 (EN) |
|---|---|---|
| C | 0.38 – 0.45 wt% | 0.38 – 0.45 wt% |
| Mn | 0.60 – 0.90 wt% | 0.60 – 0.90 wt% |
| Si | 0.10 – 0.40 wt% | 0.10 – 0.40 wt% |
| P | ≤ 0.035 wt% | ≤ 0.035 wt% |
| S | ≤ 0.035 wt% | ≤ 0.035 wt% |
| Cr | 0.90 – 1.20 wt% | 0.90 – 1.20 wt% |
| Ni | ≤ 0.40 wt% (not specified) | ≤ 0.40 wt% (not specified) |
| Mo | 0.15 – 0.30 wt% | 0.15 – 0.30 wt% |
| V, Nb, Ti, B, N | trace / not typically specified | trace / not typically specified |
Notes: - These are typical composition ranges; actual tolerances depend on the standard and mill certification. - The alloying strategy centers on Cr and Mo for increased hardenability and tempering resistance while maintaining moderate carbon for strength via heat treatment. Mn and Si are present to aid strength and deoxidation. P and S are controlled to low levels to preserve toughness and fatigue life.
How alloying affects properties: - Carbon: primary strength contributor via martensite formation after quench; higher C increases hardenability but reduces weldability and ductility. - Chromium and molybdenum: increase hardenability, wear resistance, and high‑temperature tempering resistance; they promote deeper hardening in thicker sections. - Manganese: increases hardenability and tensile strength. - Silicon: strengthens ferrite and improves tempering behavior.
3. Microstructure and Heat Treatment Response
Typical microstructures: - In normalized condition: a mixture of ferrite and pearlite, with fine grains if controlled cooling is applied. - After quenching and tempering (the most common route for these steels): tempered martensite with carbides (Cr/Mo rich) dispersed within, delivering high strength with improved toughness.
Processing routes and responses: - Normalizing (air cooling from austenitizing temperature): refines grain size and produces a predictable uniform microstructure for moderate strength and improved machinability. - Quench & temper: austenitize (around 820–880°C depending on section size), quench (oil/water/controlled polymer), then temper at 400–650°C to balance strength and toughness. Result: high tensile and yield strengths with retained toughness; tempering temperature controls final hardness. - Thermo‑mechanical processing: forging plus controlled cooling can produce very fine prior‑austenite grains and improved fatigue resistance but is process‑sensitive.
Both grades respond similarly to these heat treatments; small differences in guaranteed chemistry or impurity limits may slightly affect hardenability and toughness margins in large sections.
4. Mechanical Properties
| Property (typ.) | 42CrMo / 42CrMo4 (normalized) | 42CrMo / 42CrMo4 (quenched & tempered) |
|---|---|---|
| Tensile strength (Rm) | 650 – 850 MPa | 850 – 1100+ MPa (process dependent) |
| Yield strength (Rp0.2) | 360 – 600 MPa | 600 – 950 MPa |
| Elongation (A%) | 12 – 18% | 8 – 15% |
| Impact toughness (Charpy V, room T) | 30 – 80 J | 20 – 60 J (dependent on tempering & section) |
| Hardness | 180 – 260 HB | 220 – 360 HB (or HRC 18–36) |
Interpretation: - Both grades can achieve very similar mechanical properties when processed equivalently. Quench & tempering produces significantly higher tensile and yield strength at the expense of ductility. - Toughness and ductility depend strongly on heat treatment parameters and on cleanliness (inclusion content) — differences between the two labels are typically negligible relative to processing effects.
5. Weldability
Weldability is moderate and governed primarily by carbon content and hardenability from Cr/Mo. Use carbon equivalent formulas to estimate preheat and PWHT needs.
Common indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Typical $CE_{IIW}$ for these steels is moderate (often around 0.4–0.6 depending on exact chemistry), indicating a tendency to form hard martensite in the heat‑affected zone (HAZ) unless appropriate preheat and/or interpass temperatures are used. - Preheat and controlled interpass temperatures reduce cooling rate and HAZ hardness; PWHT (tempering) is recommended for critical, thick, or highly stressed weldments. - Both 42CrMo and 42CrMo4 have similar weldability; selection is driven by acceptance of PWHT and the fabrication environment. Use qualified welding procedures and consider hydrogen control and preheat for crack prevention.
6. Corrosion and Surface Protection
- These grades are not stainless steels; corrosion resistance is low in bare form.
- Surface protection options: painting, oiling, phosphating, electroplating, and hot‑dip galvanizing depending on application and post‑heat‑treatment constraints (galvanizing after tempering can be acceptable; galvanizing before critical heat treatment is not).
- PREN (pitting resistance equivalent number) is not applicable to non‑stainless steels, but for reference the formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ and yields meaningless low values for these low‑Cr alloy steels; therefore, corrosion performance should be managed by coatings or stainless overlays where necessary.
7. Fabrication, Machinability, and Formability
- Machinability: In normalized condition machinability is moderate; in quenched & tempered condition machining is more difficult and may require carbide tooling and reduced feeds. Free‑cutting variants (with added sulfur) are not standard for these grades.
- Formability: Hot forming and forging are straightforward; cold forming is limited by carbon content — severe cold bending may cause cracking unless annealed or used in normalized condition.
- Grinding and finishing: both respond well to precision grinding after heat treatment; surface treatments for fatigue life (shot peening, nitriding) are commonly applied.
8. Typical Applications
| 42CrMo (common uses) | 42CrMo4 (common uses) |
|---|---|
| Shafts, axles, and spindles | Shafts, axles, and spindles |
| Gears and pinions | Gears and pinions |
| High‑strength fasteners and bolts | High‑strength fasteners and bolts |
| Connecting rods, crankshafts in smaller or medium sections | Machinery components under high cyclic load |
| Pressurized parts (after appropriate heat treatment) | Automotive and heavy equipment parts requiring EN traceability |
Selection rationale: - Both grades are chosen for high strength and good fatigue resistance after quench & temper. - Choose on the basis of required certification, specified standard, supply chain (which mill and region), and acceptance of required post‑weld heat treatment.
9. Cost and Availability
- Both materials are widely available worldwide in bars, forgings, seamless tubes, and plates. Availability varies by region and stockholder preferences.
- 42CrMo4 is very common in European markets and often more convenient when EN certification is required.
- 42CrMo (GB/T) and equivalents (AISI 4140 / SCM440) are typically easier to source in Asia and North America.
- Relative cost differences are normally small and dominated by product form, section size, and heat‑treatment or mill certification requirements rather than by the nominal grade label.
10. Summary and Recommendation
| Criterion | 42CrMo | 42CrMo4 |
|---|---|---|
| Weldability | Moderate; preheat/PWHT often required | Moderate; preheat/PWHT often required |
| Strength–Toughness (heat treated) | High (dependent on process) | High (dependent on process) |
| Cost / Availability | Widely available in Asia/Americas; competitive | Widely available in Europe; competitive |
Recommendations: - Choose 42CrMo if you need to source material within a GB/AISI supplier ecosystem, or if project procurement calls for GB or American specification equivalence and you require typical 4140‑class mechanical performance with local traceability. - Choose 42CrMo4 if the project specification calls for EN material certification, European supply chain alignment, or if the client/project contract explicitly states EN standards (42CrMo4 / EN 1.7225).
Final note: metallurgical and mechanical performance of these two labels overlap significantly; the deciding factors are often non‑metallurgical (standard compliance, mill certification, traceability, and local availability). For critical components always specify required heat treatment, mechanical property acceptance criteria, and NDT/post‑weld heat treatment procedures rather than relying on grade name alone.