35CrMo vs 42CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

35CrMo and 42CrMo are two closely related chromium–molybdenum alloy steels used for structural and mechanically loaded components. Engineers, procurement managers, and manufacturing planners routinely weigh trade-offs between strength, toughness, machinability, weldability, and cost when choosing between them. Typical decision contexts include whether to favour slightly higher strength and wear resistance (for example for heavy shafts or gears) versus easier fabrication and improved toughness for dynamic parts.

The key metallurgical difference is the intentionally different carbon content and consequent hardenability strategy: 42CrMo has higher carbon to increase strength and hardenability, while 35CrMo has a lower carbon level and similar Cr–Mo additions to balance toughness and fabrication. Because both rely on Cr and Mo as core alloying elements, they are commonly compared in designs that require a balance of strength, toughness, and heat-treatment response.

1. Standards and Designations

• Common international designations:
• EN/ISO: 35CrMo4 (approx. 1.7220), 42CrMo4 (approx. 1.7225)
• AISI/ASTM equivalents: 35CrMo ≈ some grades similar to 4130 family; 42CrMo ≈ AISI 4140 (note: exact equivalence depends on local standard specifications)
• GB (China): 35CrMo, 42CrMo (standard chemical ranges)
• JIS: comparable Cr–Mo steels exist but naming differs (confirm against the JIS catalogue)
• Classification:
• Both are alloy steels (Cr–Mo steels). They are not stainless or HSLA in the strict sense; they are used as quenched-and-tempered (Q&T) structural/engineering steels.

2. Chemical Composition and Alloying Strategy

The following table shows typical composition ranges by weight percent for commonly specified commercial grades. Values are representative ranges found in standard grade specifications; final composition must be confirmed against the exact standard or mill certificate.

Element	35CrMo (typical range, wt%)	42CrMo (typical range, wt%)
C	0.32 – 0.40	0.38 – 0.45
Mn	0.50 – 0.80	0.60 – 0.90
Si	0.17 – 0.37	0.17 – 0.37
P	≤ 0.035	≤ 0.035
S	≤ 0.035	≤ 0.035
Cr	0.90 – 1.20	0.90 – 1.20
Ni	≤ trace	≤ trace
Mo	0.15 – 0.30	0.15 – 0.30
V	≤ trace	≤ trace
Nb	≤ trace	≤ trace
Ti	≤ trace	≤ trace
B	≤ trace	≤ trace
N	≤ trace	≤ trace

Notes: - “Trace” means usually not intentionally added; only residual amounts may appear. - The main deliberate alloying differences are carbon and manganese levels; Cr and Mo are similar because they provide hardenability, strength, and tempering resistance. - The lower carbon in 35CrMo is part of an alloying strategy to optimize a balance of ductility/toughness and weldability, while Cr–Mo additions maintain hardenability and high-temperature strength.

How alloying affects properties: - Carbon increases strength and hardness but reduces ductility and weldability. - Chromium increases hardenability, strength, and tempering resistance; it also improves wear resistance. - Molybdenum substantially increases hardenability and creep resistance and helps maintain toughness after tempering. - Silicon and manganese act as deoxidizers and contribute to strength/hardening behavior.

3. Microstructure and Heat Treatment Response

Typical microstructures and response to common thermal processes:

• As-rolled / normalized:
• Both grades develop a ferrite–pearlite structure after normalization, with 42CrMo typically presenting a finer pearlite and higher dislocation density due to greater carbon, yielding higher strength.
• Quenched and tempered (Q&T):
• Quenching from the austenitizing temperature produces martensite (and possibly bainite depending on cooling rate); tempering reduces brittleness and provides a tailored strength–toughness combination.
• 42CrMo, because of its higher carbon and slightly higher hardenability (with Cr/Mo), can achieve higher ultimate and yield strengths at equivalent Q&T treatments, but requires carefully controlled tempering to avoid excessive brittleness.
• 35CrMo can reach high strength with slightly higher retained toughness for a given tempering regime because of lower carbon.
• Thermo-mechanical processing:
• Controlled rolling followed by appropriate heat treatment refines prior austenite grain size and can improve toughness and fatigue resistance in both grades. Both steels respond well to TMCP for improved mechanical property combinations.

Practical implication: heat-treatment parameters (austenitizing temperature, quench medium and severity, tempering temperature/time) must be selected with the steel grade and section thickness in mind to avoid hard, brittle HAZ microstructures and to meet mechanical property targets.

4. Mechanical Properties

Mechanical properties vary strongly with heat treatment and section size. The table below gives typical property ranges for quenched-and-tempered conditions commonly used in engineering practice. These are representative ranges; specify exact heat-treatment and test condition for procurement.

Property (Q&T typical range)	35CrMo	42CrMo
Tensile Strength (MPa)	Moderate–High	High (higher than 35CrMo)
Yield Strength (MPa)	Moderate	Higher
Elongation (%, A)	Better ductility	Lower ductility at same hardness
Impact Toughness (Charpy)	Generally higher at equal strength	Generally lower unless tempered to lower strength
Hardness (HRC / HB)	Achievable wide range depending on temper (lower peak than 42CrMo)	Can achieve higher peak hardnesses for wear-resistant parts

Interpretation: - 42CrMo is typically the stronger and more hardenable of the two because of its higher carbon combined with Cr–Mo. For equal Q&T cycles, 42CrMo will usually produce higher tensile and yield strengths and higher hardness. - 35CrMo will usually offer better toughness and ductility at matched or slightly lower strength levels due to its lower carbon content. - Designers must specify required toughness (e.g., impact energy at temperature) and acceptable hardness; otherwise, higher-carbon 42CrMo can unintentionally produce brittle components or make welding more difficult.

5. Weldability

Weldability depends primarily on carbon, carbon equivalent (hardenability from alloying), and thickness.

Useful empirical indices: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (weldability index): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 42CrMo’s higher carbon increases $CE_{IIW}$ and $P_{cm}$ relative to 35CrMo, indicating a higher propensity for forming hard martensitic microstructures in the heat-affected zone (HAZ) and a greater risk of cold cracking. Preheat, controlled interpass temperatures, and post-weld heat treatment (PWHT) are often required for thicker sections. - 35CrMo, with lower carbon, generally welds more readily and may need less preheat and milder PWHT, making it preferable where weld fabrication is routine and economic. - For both grades, filler metal selection and PWHT must be planned based on thickness and service conditions to restore toughness and relieve residual stresses.

6. Corrosion and Surface Protection

• Neither 35CrMo nor 42CrMo is stainless; corrosion resistance is typical of low-alloy steels and must be achieved by coating or surface engineering for corrosive environments.
• Typical protection strategies: galvanizing, painting, powder coating, plating (zinc/nickel), cladding, or applying corrosion-resistant barriers combined with cathodic protection if required.
• Stainless indices such as PREN are not applicable to these Cr–Mo steels, but for reference the PREN formula for stainless alloys is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use PREN only for stainless alloys; for Cr–Mo low-alloy steels, corrosion protection strategy should be based on expected environment (atmospheric, salt spray, chemical) and cost.

7. Fabrication, Machinability, and Formability

• Machinability: Higher carbon and higher hardness capability of 42CrMo reduce machinability relative to 35CrMo at comparable hardness levels. For machining, both steels are usually specified in normalized or annealed conditions; 35CrMo may machine faster or produce longer tool life under the same conditions.
• Formability/bending: Lower carbon 35CrMo typically has better cold formability. 42CrMo can be formed when soft-annealed but springback and cracking risk increase if hardness is high.
• Grinding and finishing: Both can be ground and finished effectively when supplied in the correct condition. Surface treatments (nitriding, carburizing) are common for wear-critical components.
• Welding and distortion control: 35CrMo offers easier welding and lower HAZ hardness; 42CrMo requires more thermal control, filler selection, and PWHT to avoid cracking and restore optimum properties.

8. Typical Applications

35CrMo – Typical Uses	42CrMo – Typical Uses
Medium-duty shafts, bolted structural parts, medium-load gears, forged components requiring good toughness	Heavy-duty shafts and axles, high-strength gears, crankshafts, Connecting rods, high-load pins and studs
Components requiring frequent welding or complex fabrication	Components where higher strength, wear resistance or high hardenability are primary drivers
Hydraulic cylinder rods, couplings where ductility/toughness prioritized	Off‑road machinery, heavy machinery, high-torque drivetrain parts

Selection rationale: - Choose 35CrMo where a balance of strength and toughness is needed along with easier fabrication and welding. - Choose 42CrMo where higher tensile strength, wear resistance, and hardenability are required and where the manufacturing process can accommodate stricter welding and heat-treatment controls.

9. Cost and Availability

• Availability: Both grades are widely available worldwide in bars, forgings, billets, and plate. 42CrMo (AISI 4140 family) is one of the most commonly stocked alloy steels in many markets.
• Cost: Material cost difference is typically small; 42CrMo can be marginally more expensive due to higher carbon content and sometimes tighter processing requirements. Total part cost, however, must include heat treatment, welding/PWHT, and machining—areas where 42CrMo may incur higher processing costs.
• Procurement tip: Specify the exact grade, required heat treatment state, mechanical properties, and mill test certificates to avoid mismatches between supplier and design intent.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	35CrMo	42CrMo
Weldability	Good	Fair–Moderate (requires more control)
Strength–Toughness balance	Moderate strength with higher toughness	Higher strength, lower ductility at same hardness
Cost (material + processing)	Lower–Moderate	Moderate–Higher

Recommendations: - Choose 35CrMo if: - The part requires better toughness or ductility at given strength. - Frequent welding or complex fabrication is expected without extensive PWHT. - Slightly better machinability and formability are important. - You target lower overall processing costs and easier QA for HAZ toughness. - Choose 42CrMo if: - Higher tensile strength, wear resistance, or hardenability for thick sections is the primary objective. - Part is subject to high static or cyclic loads where strength outweighs welding convenience. - Manufacturing can support required preheat, interpass control, and PWHT.

Final note: Both grades are robust engineering steels; the correct choice depends on the specific load case, required toughness, welding and heat-treatment capabilities, and cost constraints. Always specify the required heat-treatment condition, mechanical property targets, and testing/inspection requirements in procurement documents to ensure the delivered material meets design intent.