30CrMo vs 35CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

30CrMo and 35CrMo are chromium–molybdenum alloy steels commonly specified for forged or machined components that require higher strength and toughness than plain carbon steels. Engineers and procurement professionals typically choose between them when balancing fatigue and wear resistance against manufacturability and cost—examples include shafts, gears, connecting rods, and high-strength fasteners.

The principal selection dilemma is mechanical performance: 35CrMo is specified for higher as‑quenched strength and wear resistance, while 30CrMo provides a slightly more favorable balance of ductility, toughness, and weldability for many applications. These two grades are frequently compared because they share the same alloying system (Cr–Mo) and processing routes, but differ primarily in carbon content and, therefore, in achievable strength and hardenability.

1. Standards and Designations

• Typical standards and designations where these grades appear:
• GB/T (China): 30CrMo, 35CrMo (commonly used national alloy-steel grades).
• EN: similar function to EN 34CrMo4/42CrMo4 series (comparative equivalents, not exact 1:1).
• JIS: there are chromium-molybdenum steels in the JIS family, but direct name equivalents differ.
• ASTM/ASME: no direct ASTM single-number equivalents; comparable grades can be found in AISI/SAE 4130/4140 family for engineering reference.
• Classification: Both 30CrMo and 35CrMo are low-alloy, quenched-and-tempered alloy steels (not stainless, not tool steel, not HSLA in the exact sense). They are designed for high strength and good hardenability.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (weight %). Values shown are representative ranges commonly used in specifications for these GB-style grades.

Notes: - The majority of the mechanical difference comes from carbon (C) content: 35CrMo contains more carbon than 30CrMo, which increases as‑quenched strength and hardness potential. - Chromium (Cr) and molybdenum (Mo) increase hardenability, high-temperature strength, and tempering resistance; they also improve resistance to temper softening relative to plain carbon steels. - Silicon (Si) and manganese (Mn) contribute to strength and deoxidation; excessive Mn increases hardenability and influences weldability. - Elements like V, Nb, or Ti may appear at low levels in specific variants to refine grain size and improve toughness or creep resistance.

How alloying affects performance: - Carbon raises strength and hardness but reduces ductility and weldability if increased excessively. - Cr and Mo enhance hardenability (allowing through‑hardening in larger sections) and temper resistance; Mo is particularly important for maintaining strength after tempering. - Control of tramp elements (P, S) and balancing Mn/Si is crucial for impact toughness and forging quality.

3. Microstructure and Heat Treatment Response

Typical microstructures: - In the annealed or normalized condition, both grades show a mixture of ferrite and pearlite with grain size influenced by forging and normalization parameters. - After quenching from the austenitizing temperature, both form martensite (or bainite depending on section size and cooling rate), with 35CrMo producing a higher volume fraction of untempered martensite for a given quench due to its higher carbon content. - Proper tempering transforms martensite to tempered martensite (cementite dispersion within a ferritic matrix), which sets the final balance of strength and toughness.

Heat treatment routes and effects: - Normalizing: refines grain size, improves mechanical uniformity; used as a pre‑treatment for forging and to ensure consistent microstructure prior to quenching. - Quenching and tempering (Q&T): primary route for both alloys to reach high strength. Higher tempering temperatures lower hardness and increase ductility and toughness. - Thermo‑mechanical processing: controlled forging and cooling cycles can improve grain refinement and resulting toughness; both steels benefit from controlled rolling followed by appropriate heat treatment to maximize fatigue and impact properties. - Practical implication: because 35CrMo has higher carbon, careful control of austenitizing, quench severity, and tempering is required to avoid excessive brittleness. 30CrMo is slightly more forgiving during heat treatment if toughness is critical.

4. Mechanical Properties

Table: comparative mechanical properties (qualitative ranges and direction). Absolute values depend strongly on product form and heat treatment; the table indicates relative behavior typical after comparable quench-and-temper processes.

Explanation: - 35CrMo's higher carbon content increases hardenability and the achievable tensile and yield strengths after quench and temper. This makes it preferable where higher static or fatigue loads are expected. - 30CrMo will typically show better ductility and impact toughness at the same nominal strength level because of its lower carbon and slightly lower martensite brittleness after quench. - Designers must choose heat-treatment parameters to meet the required combination of strength and toughness; for example, tempering at higher temperatures recovers ductility but reduces maximum strength.

5. Weldability

Weldability is strongly influenced by carbon equivalent and hardenability. For alloy steels, standard empirical formulas help assess preheating and post‑weld heat treatment needs:

• A common carbon equivalent used for welded assemblies: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• A more comprehensive parameter for complex alloy mixes: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Because 35CrMo contains higher carbon, its carbon equivalent and $P_{cm}$ will generally be higher than 30CrMo, indicating greater susceptibility to cold cracking and harder heat-affected zones (HAZ). - For both grades, Cr and Mo contribute to hardenability and therefore increase the risk of HAZ hardening; Mo has a notable effect on retained hardness after welding. - Practical welding guidance: preheating, controlled interpass temperatures, and post‑weld heat treatment (PWHT) are frequently required for both grades when joining components thicker than a few millimeters; requirements are more stringent for 35CrMo. - When weldability is a primary requirement, 30CrMo or lower-carbon alternatives (or filler metals matched for toughness) are often preferred to minimize PWHT duty.

6. Corrosion and Surface Protection

• Neither 30CrMo nor 35CrMo are stainless steels; they do not provide significant corrosion resistance by composition alone.
• Typical protections: painting, powder coating, solvent-based primers, and hot-dip galvanizing (for moderate environments). For aggressive environments, additional barriers (linings, sacrificial anodes) or corrosion allowances should be used.
• If stainless analogues are under consideration, corrosion indices such as PREN are used: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• PREN is not applicable to 30CrMo/35CrMo because they are not stainless; their Cr and Mo levels are too low and there is no intentional nitrogen for pitting resistance.
• Practical note: surface hardening (induction, nitriding) can be applied carefully, but nitriding and case hardening require selection of base material and process so as not to introduce excessive distortion or compromise core properties.

7. Fabrication, Machinability, and Formability

• Machinability: 35CrMo tends to be slightly harder to machine in the heat-treated condition due to higher hardness; in annealed condition both grades machine reasonably well, though chip control and tool life depend on carbon and Mn levels. Carbide tools and appropriate cutting speeds are recommended for hardened conditions.
• Formability/bendability: in annealed or normalized condition 30CrMo is easier to bend/form because of lower strength and higher ductility. Cold forming of these alloy steels is limited; hot-forming and post-form heat treatment are common for complex shapes.
• Finishing: grinding, shot peening, and surface treatments are similar for both grades; attention to residual stress and distortion during finishing is necessary, especially for 35CrMo after hardening.

8. Typical Applications

Selection rationale: - Choose 30CrMo when the design requires a good compromise of toughness, ductility, and machinability with elevated strength. - Choose 35CrMo when higher tensile strength and wear resistance are needed and when production can accommodate more stringent heat treatment and weld procedures.

9. Cost and Availability

• Cost: 35CrMo typically commands a small premium over 30CrMo due to higher carbon content and sometimes tighter processing controls to achieve specified toughness. However, raw-material cost differences are modest; heat-treatment and additional welding controls contribute more to total part cost for 35CrMo.
• Availability: Both grades are commonly available in forgings, bars, and billets in regions where GB-style alloys are stocked. Availability by product form (bars, forgings, seamless tubes) varies by supplier and region—procuring special variants with microalloying or certified traceability may increase lead time.

10. Summary and Recommendation

Table summarizing key trade-offs qualitatively:

Conclusions: - Choose 30CrMo if durability with a favorable toughness–ductility balance, easier welding, and less demanding heat-treatment control are priorities. Typical applications include medium-duty shafts, components that require some welding, and parts where impact resistance is important. - Choose 35CrMo if higher ultimate and yield strength and increased hardenability are essential, and the manufacturing plan can accommodate more careful quenching/tempering and welding procedures. Typical uses include heavily loaded gears, high-strength fasteners, and components subject to higher fatigue or wear demands.

Final practical tip: For any critical component, specify the required heat-treatment condition, hardness limits, Charpy impact requirements (if applicable), and weld procedure qualification in the purchase document. Performance differences between 30CrMo and 35CrMo are most reliably managed through controlled heat treatment, validated welding procedures, and inspection (hardness mapping, metallography, or mechanical testing) rather than relying on nominal grade names alone.