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

Introduction

35CrMo and 42CrMo are two closely related chromium–molybdenum alloy steels commonly used for structural and mechanical components where a balance of strength, toughness, and hardenability is required. Engineers and procurement teams frequently face the choice between them when specifying shafts, gears, fasteners, or intermediate-pressure components — a decision that trades strength and wear resistance against ductility, impact performance, and manufacturability.

The principal differentiator between these grades is their nominal carbon level and the resulting effect on strength and temper resistance at elevated operating temperatures. Because chromium and molybdenum are present in both grades to promote hardenability and tempering resistance, their behavior during quenching and tempering and their suitability for applications at moderate elevated temperatures are common reasons these two alloys are compared in design and manufacturing.

1. Standards and Designations

• Common standards and equivalents:
• GB/T (China): 35CrMo, 42CrMo
• EN: often compared with EN 41xx series steels (e.g., 35CrMo ≈ 1.7035/34CrMo; 42CrMo ≈ 1.7225/42CrMo4, though precise equivalents depend on spec)
• AISI/SAE: approximate equivalents are 35CrMo ≈ 4135, 42CrMo ≈ 4140 (note: direct equivalence depends on product form and spec)
• JIS: similar grades exist in JIS G4105/G4106 families
• Classification:
• Both are low-alloy structural steels (alloyed carbon steels) — not stainless, not HSLA in the modern sense; used as heat-treatable alloy steels for forgings, bars, and machine parts.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (wt %). These are representative ranges found in common commercial specifications; always consult the specific mill certificate or standard for procurement.

Element	35CrMo (typical range)	42CrMo (typical range)
C	0.30 – 0.38	0.38 – 0.45
Mn	0.50 – 0.80	0.60 – 1.00
Si	0.15 – 0.35	0.15 – 0.40
P	≤ 0.035	≤ 0.035
S	≤ 0.035	≤ 0.035
Cr	0.80 – 1.10	0.90 – 1.20
Ni	≤ 0.30 (trace)	≤ 0.30 (trace)
Mo	0.15 – 0.30	0.15 – 0.30
V	≤ 0.05 (trace)	≤ 0.05 (trace)
Nb, Ti, B	— (trace microalloying possible)	— (trace microalloying possible)
N	≤ 0.012	≤ 0.012

Notes: - The key compositional distinction is higher carbon in 42CrMo, which increases hardenability, strength, and wear resistance but tends to reduce ductility and weldability if not properly preheated and post-weld heat treated. - Cr and Mo are the principal alloying elements here: chromium increases hardenability, strength, and corrosion resistance slightly; molybdenum improves hardenability and tempering resistance (i.e., maintains strength at elevated tempering temperatures). - Trace microalloying elements (V, Nb, Ti) may be present in some commercial variants to refine grain size and improve strength without large increases in carbon.

3. Microstructure and Heat Treatment Response

Microstructure: - In the annealed or normalized condition, both steels are typically composed of ferrite + pearlite, with pearlite fraction increasing with carbon. - After quenching from the austenitizing temperature, a martensitic (or bainitic + martensitic) structure develops, with retained austenite content depending on cooling rate and composition.

Heat treatment effects: - Normalizing: refines grain size, produces a fine ferrite–pearlite microstructure. 35CrMo typically yields slightly finer, more ductile microstructures at the same cooling rate because of lower carbon. - Quench & temper: both respond well. 42CrMo, with higher carbon, reaches higher hardness and tensile strength after hardening; it also requires cautious tempering to avoid excessive brittleness. Molybdenum content helps both grades resist softening at higher tempering temperatures (improved temper resistance). - Thermo-mechanical processing: controlled rolling and accelerated cooling produce bainitic or fine martensitic microstructures with improved toughness; microalloying and finishing temperature are important to control grain growth. - Practical implication: for a given quench-and-temper regime, 42CrMo attains higher achievable strength but will need different tempering schedules to balance toughness, especially where elevated tempering or operating temperatures are encountered.

4. Mechanical Properties

Table: typical mechanical property ranges. These ranges depend strongly on product form and heat treatment; values shown are representative for normalized and quenched & tempered (Q&T) conditions used in engineering practice.

Property	35CrMo (normalized)	35CrMo (Q&T)	42CrMo (normalized)	42CrMo (Q&T)
Tensile strength (MPa)	550 – 750	760 – 1000	600 – 800	900 – 1100
Yield strength (0.2% Rp0.2, MPa)	350 – 550	600 – 900	400 – 600	700 – 950
Elongation (%)	16 – 22	10 – 16	14 – 20	8 – 14
Impact toughness (Charpy V-notch, J)	30 – 80 (norm)	20 – 60 (Q&T, depends on temper)	25 – 70 (norm)	15 – 50 (Q&T, depends on temper)
Hardness (HRC / HB)	20 – 26 HRC (Q&T ranges)	26 – 40 HRC	22 – 28 HRC	28 – 45 HRC

Interpretation: - 42CrMo generally achieves higher tensile and yield strengths after quench and temper due to higher carbon content; it also attains higher hardness for wear resistance. - 35CrMo tends to offer higher ductility and slightly better impact performance when tempered to comparable strength levels, making it preferable where toughness and fatigue resistance are priorities. - Actual mechanical properties are a function of the heat-treatment parameters (austenitizing temperature, quench medium, and tempering temperature/time) and product geometry.

5. Weldability

Weldability is influenced primarily by carbon equivalent and alloy content. Two commonly used predictors are the IIW carbon equivalent and the more conservative $P_{cm}$:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 42CrMo, with its higher carbon content, has a higher carbon equivalent than 35CrMo for identical Cr–Mo levels; this translates into a higher risk of hard, brittle heat-affected zones (HAZ) and cold cracking if welded without preheat and controlled interpass temperatures. - Both grades contain Cr and Mo which increase hardenability; welding procedures typically require preheating, low-hydrogen consumables, and post-weld heat treatment (PWHT) when strength or critical applications are involved. - 35CrMo welds more readily and often requires less aggressive PWHT than 42CrMo for equivalent component performance, but proper welding practice remains critical for both.

6. Corrosion and Surface Protection

• Neither 35CrMo nor 42CrMo are stainless steels; their chromium content is not sufficient to form a continuous passive film for general corrosion resistance.
• Typical protective strategies:
• Barrier coatings (paint systems, powder coatings)
• Galvanizing (hot-dip) where suitable — note that galvanizing can affect heat treatment and property targets on small sections and requires post-galvanizing treatments if hardness/precision is critical
• Cladding or use of corrosion-resistant overlays where localized corrosion is a concern
• PREN formula for stainless alloy ranking is not applicable to these carbon-alloy steels, since their chromium and molybdenum levels are too low to rely on passivity:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• Use corrosion-resistant grades or protective measures when service environments are corrosive; neither 35CrMo nor 42CrMo should be selected solely for corrosion resistance.

7. Fabrication, Machinability, and Formability

• Machinability: Lower-carbon 35CrMo is generally easier to machine than 42CrMo when in comparable normalized conditions because of lower hardness and lower cutting forces. After quench & temper, both grades become more difficult; 42CrMo at higher hardness levels increases tool wear.
• Formability: 35CrMo shows better cold formability and bending performance than 42CrMo in annealed or normalized states. Deep drawing is limited by carbon content in both; forming should typically be done in soft-annealed condition.
• Grinding, surface finishing, and hard turning are common for both when hardened; 42CrMo requires more robust tooling for hard machining.
• Heat treatment distortions and residual stresses: both require attention to section thickness, quench media, and fixture design to control distortion.

8. Typical Applications

Table: representative uses

35CrMo	42CrMo
Shafts (where toughness and fatigue resistance are important)	High-load shafts and axles requiring higher strength
Gears in moderate-load applications	Gears for higher-stress applications and power-transmission parts
Bolts and fasteners requiring good toughness	High-strength fasteners and studs
Connecting rods, crankshafts for medium-duty service	Heavy-duty machine components, hydraulic cylinders, mandrels
Forged parts requiring good ductility	Wear-prone parts needing higher hardness after Q&T

Selection rationale: - Choose 42CrMo where higher static strength, hardness, and wear resistance are required and where controlled heat treatment and welding procedures are available. - Choose 35CrMo where better ductility, impact resistance, or fatigue performance at comparable tempering levels is needed, or where ease of fabrication is prioritized.

9. Cost and Availability

• Cost: Prices vary with market conditions, product form (bar, forging, plate), and finished condition. Generally, the raw-material cost difference between 35CrMo and 42CrMo is modest because the primary alloying additions (Cr, Mo) are similar; 42CrMo may be slightly more expensive due to higher carbon-grade processing and tighter controls needed for welding-critical applications.
• Availability: Both grades are widely produced and available in bars, forgings, and round steel. 42CrMo (4140 equivalents) has particularly broad availability in global supply chains, as it is a very common engineering alloy.

10. Summary and Recommendation

Table: quick comparison

Attribute	35CrMo	42CrMo
Weldability	Better (lower CE)	More demanding (higher CE)
Strength–Toughness balance	Better toughness at comparable strength	Higher achievable strength and hardness
Cost (relative)	Slightly lower or similar	Slightly higher in processing/weld control

Recommendation: - Choose 35CrMo if you need a balanced combination of toughness, ductility, and reasonable strength with easier fabrication and less stringent welding/PWHT requirements. It is well suited to components where impact resistance, fatigue life, or ductile behavior is prioritized. - Choose 42CrMo if your design requires higher static strength, greater hardenability, and superior wear resistance after quench and temper. It is appropriate for heavily loaded shafts, gears, and components subjected to higher mechanical stresses or where higher temper resistance at moderate elevated temperatures is required — provided welding and heat-treatment controls are in place.

Final note: Neither 35CrMo nor 42CrMo are intended for sustained high-temperature service (creep) without detailed materials selection. For elevated-temperature or creep-critical applications, consider purpose-designed Cr–Mo–V or stainless creep-resistant alloys and consult creep/tempering data specific to the intended service temperature and time. Always verify mill certificates and perform heat-treatment qualification tests (tensile, impact, hardness) for critical components.