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

Introduction

Engineers, procurement managers, and manufacturing planners frequently have to choose between closely related chromium–molybdenum steels for forged components, shafts, gears, and structural parts. The selection dilemma typically revolves around balancing achievable strength and fatigue performance against ductility, weldability, and overall production cost. In many specifications the choice comes down to two similar grades: 42CrMo and 35CrMo.

The practical difference between these two grades is primarily driven by their carbon content and the resulting changes in strength and hardenability. Because chromium and molybdenum levels are similar, the higher-carbon grade attains higher strength and hardness after quench & temper, while the lower-carbon grade retains relatively better toughness and weldability for a given heat-treatment target. These trade-offs make the pair a common comparison in design and manufacturing decisions.

1. Standards and Designations

• 42CrMo
• Common equivalents/standards: EN 42CrMo4 (1.7225), AISI/ASTM commonly referenced as 4140 family for similar composition and use. GB/T designation in China: 42CrMo.
• Classification: Medium-alloy quenched & tempered steel (alloy steel).
• 35CrMo
• Common equivalents/standards: Found in some national standards as 35CrMo or 35CrMo4 (EN naming varies); used in GB/T specifications. Less commonly used as a direct AISI analog but comparable to lower-carbon Cr–Mo steels in the 4100 series.
• Classification: Medium-alloy quenched & tempered steel (alloy steel).

Both grades are alloy steels (not stainless, not tool steels). They are typically supplied in bar, forging, and plate forms for subsequent heat treatment.

2. Chemical Composition and Alloying Strategy

The table below lists typical composition ranges used for design and specification comparisons. Actual certified composition should be taken from the mill test certificate for each purchase lot.

Element	42CrMo (typical ranges)	35CrMo (typical ranges)
C (carbon)	0.38 – 0.45 wt%	0.32 – 0.40 wt%
Mn (manganese)	0.50 – 0.90 wt%	0.50 – 0.80 wt%
Si (silicon)	0.17 – 0.37 wt%	0.17 – 0.37 wt%
P (phosphorus)	≤ 0.025 wt% (max)	≤ 0.025 wt% (max)
S (sulfur)	≤ 0.025 wt% (max)	≤ 0.025 wt% (max)
Cr (chromium)	0.90 – 1.20 wt%	0.80 – 1.10 wt%
Mo (molybdenum)	0.15 – 0.30 wt%	0.15 – 0.30 wt%
Ni (nickel)	≤ 0.30 wt% (trace)	≤ 0.30 wt% (trace)
V, Nb, Ti, B, N	typically not specified / trace only	typically not specified / trace only

How the alloying strategy works - Carbon is the principal variable controlling the as-quenched martensite fraction and the temper response; higher carbon raises achievable strength and hardness but reduces ductility and weldability. - Chromium and molybdenum contribute to hardenability (deepen the hardenable section), tempering resistance, and strength at temperature. As both grades have similar Cr and Mo, their hardenability is comparable when carbon is equal, but the higher carbon in 42CrMo increases the final strength. - Manganese and silicon support hardenability and deoxidation; low P and S are controlled to preserve toughness and fatigue performance.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses under common processing routes:

• As-rolled/normalized
• Both grades when normalized show a ferrite–pearlite matrix with fine carbides. The 42CrMo, with higher carbon, will have a slightly higher pearlite fraction and a finer carbide dispersion after suitable thermal cycling.
• Quenching and tempering (Q&T)
• Quench: Both grades will form martensite in sufficiently thick sections given Cr–Mo hardenability. 42CrMo will produce a harder, higher-strength martensitic structure due to higher carbon.
• Tempering: Tempering reduces brittleness and tailors toughness. Because 42CrMo starts with higher hardness, tempering schedules must be adjusted to reach the same strength–toughness balance as 35CrMo.
• Normalizing + tempering / Thermo-mechanical processing
• Thermo-mechanical treatments that refine prior-austenite grain size improve toughness and fatigue resistance in both grades. The relative behavior is similar; the higher carbon content in 42CrMo places greater emphasis on controlled cooling and tempering to avoid temper embrittlement or excessive residual stresses.

Key takeaways - 42CrMo achieves higher ultimate strength and hardness after equivalent Q&T cycles. - 35CrMo offers a slightly more forgiving tempering window for toughness and better performance in thicker cross sections where through-hardening is more challenging.

4. Mechanical Properties

The exact mechanical properties depend strongly on product form and heat treatment. The table below gives qualitative comparisons and typical property trends rather than absolute cert values. Use mill certificates and contractually specified HT conditions for procurement.

Property	42CrMo	35CrMo
Tensile Strength (typical)	Higher maximum tensile strength potential after Q&T (stronger in same HT condition)	Slightly lower ultimate tensile strength potential for a given HT
Yield Strength	Higher yield at equal tempering/hardness targets	Lower yield, higher ductility margin
Elongation / Ductility	Lower elongation at high strength levels (trade-off with strength)	Better elongation and ductility at comparable tempering
Impact Toughness	Can be excellent if properly tempered; more sensitive to heat-treatment and section size	Generally less sensitive; can provide better toughness-to-strength ratio at lower hardness
Hardness	Higher achievable hardness after quench (requires more tempering for toughness)	Lower achievable peak hardness for the same HT schedule

Explanation - Because carbon increases the martensite start and the carbon content of martensite, a higher C in 42CrMo produces higher strength and hardness for a given quench. That strength gain comes at the cost of lower toughness and ductility unless tempering is used to trade off hardness for toughness. - For fatigue-critical components, designers often specify controlled tempering to hit the required balance, and select the grade that minimizes sensitivity to section size and heat input.

5. Weldability

Weldability is a function of carbon equivalent and hardenability; practical assessment usually uses carbon-equivalent formulas. Two commonly cited indices:

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

• International Pcm 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 for 42CrMo vs 35CrMo - 42CrMo (higher carbon) will have a higher $CE_{IIW}$ and $P_{cm}$ than 35CrMo all else equal, meaning greater cold cracking risk and a greater need for preheat, interpass control, low-hydrogen consumables, or post-weld heat treatment (PWHT). - 35CrMo’s lower carbon makes it more weldable in common shop practice and reduces the required preheat/PWHT severity for the same thickness. - Both grades are weldable with standard procedures for Cr–Mo steels when appropriate preheat and PWHT are applied. For critical or thick sections, perform procedure qualification (PQR) and include PWHT to relieve hydrogen and temper the HAZ.

6. Corrosion and Surface Protection

• Neither 42CrMo nor 35CrMo are stainless steels; corrosion resistance is limited to that of low-alloy carbon steels. Selection criteria typically rely on coating and finishing rather than alloying.
• Common protection strategies: painting, cathodic protection, hot-dip galvanizing (when geometry allows), or plating where appropriate.
• PREN is not applicable for these non-stainless low-alloy steels. For context, the PREN formula for stainless assessment is: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ But this index is irrelevant for plain chromium–molybdenum structural steels because their chromium content is too low to confer stainless behavior.

7. Fabrication, Machinability, and Formability

• Machinability
• 35CrMo is typically easier to machine than 42CrMo at the same hardness level due to lower carbon and reduced tendency to work-harden. Tool life and cutting forces will be more favorable for 35CrMo.
• When parts are supplied in softer normalized or annealed conditions, machinability improves for both; hardened conditions will require carbide tooling.
• Formability and bending
• Lower carbon in 35CrMo yields better cold formability. Requiring less springback reduction is typical when bending or forming.
• 42CrMo requires tighter control of bend radii and may require intermediate heat treatments for significant plastic deformation.
• Surface finishing and grinding
• Both steels can be ground and finished to high-quality surface conditions; higher hardness in 42CrMo increases abrasiveness on grinding wheels and tooling.

8. Typical Applications

42CrMo (higher-carbon Cr–Mo)	35CrMo (lower-carbon Cr–Mo)
Shafts, high-strength axles, crankshafts, heavily loaded gears, bearing housings where higher strength and wear resistance are required after Q&T	Axles, structural forgings, bolts & high-strength fasteners, components requiring higher ductility or easier welding
High-stress rotating parts and components with fatigue-critical sections that can be reliably heat treated	Parts that require easier fabrication or more frequent joining operations; intermediate-strength structural components
Machine tool components where hardness and wear resistance are beneficial	Components where toughness, ductility, and cost-effectiveness are prioritized

Selection rationale - Choose 42CrMo where higher post-HT strength and wear resistance are required and welds or complex forming are minimized or can be controlled by robust welding procedures. - Choose 35CrMo where forming, welding, or improved toughness-to-strength ratio is more important and where slightly lower peak strength is acceptable.

9. Cost and Availability

• Relative cost: Both grades are cost-competitive; 42CrMo can be marginally more expensive due to slightly higher carbon processing and demand for higher-strength applications. Price differences are typically small compared to heat-treatment and post-processing costs.
• Availability by product form: Both are widely available in bars, forgings, and seamless tubes in markets that supply Cr–Mo steels. 42CrMo (or AISI 4140 equivalent) tends to be more commonly stocked globally because of its broad use; 35CrMo may be more regional depending on standardization. Always confirm mill lead times, certification, and batch traceability.

10. Summary and Recommendation

Summary table (qualitative)

Criterion	42CrMo	35CrMo
Weldability	Moderate — needs stricter preheat/PWHT at thickness	Better — lower preheat/PWHT requirements
Strength – Toughness balance	Higher maximum strength; requires careful tempering to maintain toughness	Better toughness margin for similar processing; slightly lower peak strength
Cost (material only)	Comparable; slightly higher in some markets	Comparable; often marginally lower
Machinability / Formability	Less favorable at equal hardness	More favorable at equal hardness

Recommendations - Choose 42CrMo if the primary design drivers are higher post-heat-treatment strength, wear resistance, or when a smaller cross section requires maximum allowable strength for fatigue-critical rotating components and when you can control welding and heat treatment procedures. - Choose 35CrMo if the design favors improved ductility, easier welding and fabrication, reduced risk of hydrogen-assisted cracking, or cost-sensitive production where slightly lower peak strength is acceptable.

Final note Always specify the exact heat-treatment condition, hardness limits, and acceptance testing in procurement documents. For welded assemblies or thick sections, require a welding procedure qualification (PQR) and consider PWHT. For critical parts subject to fatigue or high-cycle loading, combine metallurgical selection with validated heat-treatment and non-destructive inspection to achieve reliable field performance.