35CrMo vs 42CrMo – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
35CrMo and 42CrMo are two common chromium–molybdenum low‑alloy steels used for structural, power‑transmission, and engineering components. Engineers and procurement teams often face a selection dilemma between the two when balancing strength, toughness, weldability, cost, and manufacturability. Typical decision contexts include choosing a grade for heavily loaded shafts or gears (where strength and hardenability matter) versus specifying material for welded sub‑assemblies or components requiring higher impact resistance.
At a glance, the primary technical distinction lies in their alloy balance and carbon content: the higher‑numbered grade tends to have a higher nominal carbon and alloy content that yields greater hardenability and strength after quench & temper, while the lower‑carbon variant trades some peak strength for improved toughness and easier fabrication. Because both grades are widely used in similar product classes, designers compare them to optimize heat treatment, welding requirements, and lifecycle cost.
1. Standards and Designations
- GB/T (China): both grades are commonly specified under GB/T quenched‑and‑tempered steel standards (e.g., GB/T 3077/GB/T 1220 family references).
- EN (Europe): 42CrMo is commonly associated with EN 42CrMo4 (EN 1.7225); 35CrMo equivalents exist but are less universally standardized in EN and often mapped against domestic designations—verify the specific standard referenced on purchase orders.
- AISI/SAE: 42CrMo is generally considered equivalent to the 41xx family (notably AISI 4140) in many industrial contexts; 35CrMo is roughly analogous to lower‑carbon 41xx variants but check spec sheets before substitution.
- JIS: Japanese JIS grades for Cr–Mo steels have similar families (e.g., SCM series); cross‑reference required.
- Classification: both are low‑alloy quenched‑and‑tempered steels (not stainless, not tool steels, and not HSLA in the modern sense). They are used where alloying and heat treatment provide higher strength and toughness than plain carbon steels.
2. Chemical Composition and Alloying Strategy
Typical composition ranges vary by standard and supplier; the table below shows commonly referenced approximate ranges. Always use the purchaser’s material certificate or the referenced standard for procurement.
| Element | Typical range — 35CrMo (approx.) | Typical range — 42CrMo (approx.) |
|---|---|---|
| C | 0.32–0.40 wt% | 0.38–0.45 wt% |
| Mn | 0.50–0.80 wt% | 0.50–0.80 wt% |
| Si | 0.17–0.37 wt% | 0.17–0.37 wt% |
| P | ≤0.035 wt% (max) | ≤0.035 wt% (max) |
| S | ≤0.035 wt% (max) | ≤0.035 wt% (max) |
| Cr | ~0.6–1.0 wt% | ~0.9–1.2 wt% |
| Mo | ~0.15–0.25 wt% | ~0.15–0.25 wt% |
| Ni | typically ≤0.30 wt% (often not added) | typically ≤0.30 wt% (often not added) |
| V, Nb, Ti, B, N | trace or not present; depends on microalloying version | trace or not present; depends on microalloying version |
Notes: - The values are approximate ranges used in common industry practice; exact limits come from the applicable standard or mill certificate. - 42CrMo typically has the higher nominal carbon and slightly higher chromium content, increasing hardenability and the potential for higher tempered strengths. Molybdenum content in both grades aims to increase hardenability and tempering resistance; small differences in Mo percentage can influence section‑size hardening and temper resistance.
Alloying effects: - Carbon principally controls hardenability and ultimate strength but reduces weldability and ductility when increased. - Chromium and molybdenum increase hardenability and strength at temperature, improve wear resistance, and help tempering resistance. - Manganese and silicon act as deoxidizers and contribute modestly to hardenability and strength. - Microalloying elements (V, Nb, Ti) may be present in low ppm to refine grain size and improve toughness; these are not primary to the grade differences discussed here.
3. Microstructure and Heat Treatment Response
- Typical microstructures:
- In annealed or normalized conditions both grades present a ferrite–pearlite or fine pearlite structure. After quenching, both form martensite (or martensite + bainite depending on cooling rate and section size). Tempering produces tempered martensite with carbides.
- Heat‑treatment behavior:
- 42CrMo, with higher carbon and slightly higher Cr and often similar Mo, exhibits greater hardenability: it forms martensite more readily through thicker sections compared with 35CrMo under the same quench severity.
- 35CrMo, with a lower carbon baseline, yields slightly finer and tougher tempered microstructures at comparable tempering temperatures; its lower hardenability reduces the risk of forming untempered martensite in large weld heat‑affected zones but may limit achievable strength in very thick sections.
- Processing routes:
- Normalizing improves uniformity of as‑forged billets and produces a uniform starting microstructure for downstream quenching.
- Quenching and tempering is the common route to obtain high strength and good toughness; temper temperature controls the strength–toughness balance.
- Thermo‑mechanical processing can further refine grain size and improve toughness for both grades; the effect is often more pronounced in the lower carbon grade.
4. Mechanical Properties
Mechanical properties depend heavily on heat treatment, section size, and tempering targets. The table below summarizes relative performance and typical behavior rather than absolute certified numbers; for procurement use, rely on the mill test report and the specified heat‑treatment condition.
| Property | 35CrMo (typical behavior) | 42CrMo (typical behavior) |
|---|---|---|
| Tensile strength | Moderate to high after Q&T; generally lower than 42CrMo at equivalent tempering | Higher achievable tensile strength due to higher C/hardenability |
| Yield strength | Moderate; good yield/tensile ratio when properly tempered | Higher yield strength in similar heat‑treatment states |
| Elongation (ductility) | Slightly better ductility and elongation at comparable strength levels | Slightly reduced elongation at the same tensile level |
| Impact toughness | Generally better toughness for a given strength because of lower carbon content and finer tempered microstructure | Still good toughness when properly tempered, but tends to be lower than 35CrMo at the same tensile level |
| Hardness (HRC or HV) | Achievable hardness is lower for the same quench/temper schedule; easier to achieve a ductile tempered hardness | Higher hardness achievable; more sensitive to quench severity and section size |
Interpretation: - For the same target hardness or tensile strength, 42CrMo will typically require more cautious preheat and PWHT for welding and may have higher residual stress and risk of brittle microstructures without appropriate treatment. - 35CrMo provides a more forgiving compromise between strength and toughness in welded or multi‑piece assemblies, especially when deep hardening is not required.
5. Weldability
Weldability depends largely on carbon content, alloying that increases hardenability, and impurity levels. Common indices help predict preheat and interpass controls:
-
Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
International Pcm formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - A higher $CE_{IIW}$ or $P_{cm}$ indicates a greater propensity to form hard, brittle martensite in the weld heat‑affected zone, necessitating preheat, controlled interpass temperatures, or post‑weld heat treatment (PWHT). - Because 42CrMo typically has higher carbon and slightly higher Cr, its calculated carbon equivalent is normally higher than that of 35CrMo, meaning more stringent welding controls are required. - 35CrMo tends to be easier to weld, with lower preheat/PWHT demands for similar section sizes, but welding procedure qualification is still necessary for critical applications.
6. Corrosion and Surface Protection
- These are non‑stainless alloy steels; corrosion resistance is limited relative to stainless grades.
- Common protections:
- Hot‑dip galvanizing for atmospheric corrosion resistance where acceptable.
- Conversion coatings (e.g., phosphating) and paint or powder coatings for environmental protection.
- Oil or wax for temporary protection of machined surfaces.
- PREN (Pitting Resistance Equivalent Number) is a stainless‑steel index and not applicable to Cr–Mo carbon steels. For reference, the PREN formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but it is not relevant here because neither 35CrMo nor 42CrMo are stainless steels.
- In service environments where active corrosion protection is required (marine, chemical), consider protective coatings or a corrosion‑resistant alloy instead.
7. Fabrication, Machinability, and Formability
- Machinability:
- Both grades machine well in the annealed or normalized condition; increased carbon and prior hardening reduce machinability.
- 42CrMo in hardened/tempered condition will be more difficult to machine than annealed 35CrMo.
- Formability and bending:
- Forming is best performed in the annealed condition. Lower carbon 35CrMo is marginally easier to cold‑form without cracking.
- Heat treatment distortion:
- 42CrMo is more prone to quench distortion and cracking risk in complex geometries due to higher hardenability and internal stresses after quenching.
- Surface finishing:
- Both take typical finishing operations (grinding, honing, shot peening) well when heat treated correctly; attention to residual stress management is important for fatigue components.
8. Typical Applications
| 35CrMo — Typical uses | 42CrMo — Typical uses |
|---|---|
| Medium‑duty shafts, pinions, structural forged parts, welded assemblies requiring good toughness and reasonable strength | High‑load shafts, crankshafts, heavily loaded gears, hydraulic cylinders, high‑stress axles and mechanical parts requiring high hardenability |
| Bolts, studs, and fasteners where some weldability is required | Components subjected to high torsional or bending stresses and where deeper section hardening is needed |
| Machinery components with frequent welding or repair operations | Large quenched components where high section properties after Q&T are required |
Selection rationale: - Choose the lower‑carbon option (35CrMo) when weldability, toughness, and fatigue resistance in welded structures are priorities and extreme peak strength is not necessary. - Choose 42CrMo when maximum strength, wear resistance, and the ability to harden through thick sections are key design drivers.
9. Cost and Availability
- Availability: Both grades are widely available in bars, forgings, plate, and seamless tubing from major mills. 42CrMo (AISI 4140 family) is one of the more commonly stocked alloy steels worldwide.
- Relative cost: 42CrMo can be slightly more expensive than 35CrMo due to its higher carbon/alloy content and higher demand for high‑strength applications. Actual price differences depend on market conditions, form, and heat‑treatment state.
- Lead times: special heat treatments, custom chemistries, or certification (e.g., NDT, PMI, specific mill tests) will extend lead times for either grade.
10. Summary and Recommendation
| Criterion | 35CrMo | 42CrMo |
|---|---|---|
| Weldability | Better (more forgiving) | More demanding (higher preheat/PWHT likely) |
| Strength–Toughness balance | Good toughness at moderate strength | Higher peak strength and hardness; toughness compared at same strength is lower |
| Cost | Generally lower or comparable | Slightly higher in many markets |
Recommendations: - Choose 35CrMo if you need a balanced material with good toughness and easier fabrication/welding, for components that will be welded, repaired, or require better ductility and fatigue resistance at moderate strength levels. - Choose 42CrMo if your design prioritizes maximum quenched & tempered strength, wear resistance, and deep section hardenability for heavy‑duty shafts, gears, or large sections where achieving and retaining high tempered strength is critical.
Final note: Always specify the complete purchase condition (chemical standard, heat‑treatment requirement, hardness/tensile targets, and welding/PWHT requirements) and request mill test certificates. Differences between suppliers and the selected heat‑treatment route usually have a larger practical effect on part performance than the small nominal compositional differences between these two common Cr–Mo alloy steels.