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

Introduction

Engineers and procurement professionals routinely decide between similar alloy steels when balancing strength, toughness, weldability, and cost. 30CrMo and 35CrMo are two commonly specified low-alloy, medium‑carbon steels used for mechanical components where strength and fatigue resistance matter. Typical decision contexts include choosing between slightly higher as‑quenched strength versus better weldability and ductility, or when specifying heat treatment windows for parts such as shafts, gears, and high‑stress fasteners.

The principal practical difference between these two grades is their relative carbon/alloy content: 35CrMo is specified with a somewhat higher carbon (and often marginally higher alloy additions) than 30CrMo. That difference shifts the balance toward higher attainable strength and hardness in 35CrMo after quenching and tempering, while 30CrMo generally offers easier fabrication, improved weldability, and greater ductility for equivalent heat treatments.

1. Standards and Designations

• Common standards where these names appear:
• GB (China): 30CrMo, 35CrMo (typical Chinese designation system)
• EN / ISO: comparable materials exist (e.g., Cr–Mo steels such as 34CrMo4, 42CrMo4), but direct equivalence requires checking composition limits and mechanical property tables in the applicable standard.
• ASTM / ASME: AISI/SAE series (e.g., 4130 family) are often referenced as functional analogues for engineering selection; exact interchange requires verification.
• JIS: similar Cr–Mo grades exist; confirm matching chemical/microstructure requirements.
• Classification: Both 30CrMo and 35CrMo are medium‑carbon, low‑alloy steels used as alloy structural steels (not stainless, not tool steels, and not HSLA in the modern sense). They are designed for strength and hardenability via heat treatment (normalizing, quench & temper).

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (weight %, indicative). Actual values depend on the supplier and the governing standard — treat these as representative ranges for engineering comparison, not as procurement specifications.

How alloying affects performance - Carbon: primary control of strength and hardenability. Slightly higher carbon in 35CrMo raises the attainable hardness and tensile strength after quench & temper, but reduces ductility and weldability if carbon equivalent rises. - Chromium and molybdenum: improve hardenability and tempering resistance; both grades rely on Cr and Mo to achieve through‑thickness mechanical properties in larger sections. - Manganese and silicon: strengthen as deoxidizers and contribute to hardenability. - Microalloying elements (V, Nb, Ti) may appear in microalloy variants to refine grain size and improve toughness, but are not mandatory in the basic 30/35CrMo designations.

3. Microstructure and Heat Treatment Response

Typical microstructures - In the normalized condition both grades show a mixture of ferrite and pearlite with grain sizes determined by hot‑work and cooling. Normalizing improves machinability and toughness. - After quenching and tempering, both develop tempered martensite (or bainitic/tempered bainite depending on cooling rate and section size). Higher carbon in 35CrMo promotes a higher fraction of hard martensite for a given quench, raising strength and hardness. - Thermo‑mechanical processing (controlled rolling) followed by accelerated cooling can produce finer bainitic/tempered martensitic structures that yield excellent strength–toughness combinations.

Heat treatment responses - Normalizing: refines as‑rolled structure, improves machinability and prepares for quenching. - Quenching & tempering (Q&T): primary route for achieving design strength. Both grades respond predictably — 35CrMo typically attains higher as‑tempered strength at the cost of somewhat lower elongation and potentially reduced impact toughness if over‑tempered incorrectly. - Tempering: necessary to reduce brittleness of as‑quenched martensite. 35CrMo often requires slightly different temper schedules to preserve toughness while achieving target strength.

4. Mechanical Properties

Table: indicative mechanical properties after representative quench & temper processing (engineer to verify actual spec and heat treatment).

Interpretation - Strength: 35CrMo is generally capable of higher tensile and yield strengths under comparable quench & temper cycles because of its higher carbon and similar Cr/Mo hardenability. - Toughness and ductility: 30CrMo usually demonstrates greater elongation and can be tougher in transient conditions, particularly if careful tempering and grain control are applied. - The actual strength–toughness balance is highly dependent on section size, cooling rate, and tempering; specification must define these parameters.

5. Weldability

Weldability of alloy steels depends on carbon and alloying — summarized qualitatively using accepted carbon equivalent expressions.

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

Qualitative interpretation - 35CrMo, with its higher carbon content, will produce a higher $CE_{IIW}$ / $P_{cm}$ than 30CrMo under similar Cr/Mo levels and therefore is more demanding to weld. Higher CE suggests increased risk of hard, brittle heat-affected zones (HAZ) and cold cracking unless mitigated. - Practical welding controls: preheat, controlled interpass temperature, use of matching or over‑matching filler metals, and post‑weld heat treatment (PWHT) are more often required for 35CrMo, especially in thicker sections. 30CrMo often permits less stringent preheat and may be welded more readily with standard Cr–Mo filler rods, though PWHT is still recommended for load‑bearing components. - For both grades, follow relevant welding procedure specifications (WPS) and confirm via PWHT and hardness checks in the HAZ.

6. Corrosion and Surface Protection

• Neither 30CrMo nor 35CrMo are stainless alloys; corrosion resistance is similar to carbon steel and depends on surface finish and environment.
• Typical protection methods:
• Hot‑dip galvanizing for general atmospheric protection (check how galvanizing affects dimensional tolerances and fatigue-critical surfaces).
• Organic coatings: primers, paints, and powder coatings for industrial environments.
• Specialized plating (e.g., cadmium, zinc‑nickel) for particular functional requirements or thin components.
• Stainless indices such as PREN are not applicable to these Cr–Mo low‑alloy steels, because they are not corrosion‑resistant stainless grades. For context, PREN is defined as: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but it is meaningful only for stainless steels containing significant Cr and N. For Cr–Mo alloy steels, corrosion mitigation relies on coatings and cathodic protection, not intrinsic passivity.

7. Fabrication, Machinability, and Formability

• Machinability:
• 30CrMo typically machines more easily than 35CrMo in comparable conditions because of its lower carbon content and lower hardness in the normalized state.
• When parts are specified in the quenched & tempered condition, both grades are harder to machine; recommended practice is to do heavy machining in the normalized or annealed condition and finish‑machine after final heat treatment where feasible.
• Formability:
• Cold forming and bending are easier with 30CrMo. Higher carbon in 35CrMo reduces ductility and increases risk of cracking during severe forming.
• When forming is required, perform operations prior to final heat treatment or use higher‑temperature forming strategies.
• Surface finishing:
• Both respond well to standard grinding and shot‑peening for fatigue life improvements; higher hardness in 35CrMo can require more robust tooling and abrasives.

8. Typical Applications

Table: typical uses and selection rationale.

Selection rationale - Choose 30CrMo when the design requires easier welding, greater formability, or when parts will be repaired in the field. It is also favorable when cost control is important and the ultimate strength requirements are moderate. - Choose 35CrMo when higher as‑tempered strength, wear resistance, and fatigue endurance at elevated static stresses are the primary drivers, and when controlled welding/PWHT is feasible.

9. Cost and Availability

• Relative cost: 35CrMo is typically somewhat more expensive than 30CrMo due to slightly higher alloy (and carbon) content and tighter heat‑treatment requirements for high‑performance applications. The incremental cost is usually modest but can be significant for large volumes.
• Availability by product form: both grades are commonly available as bars, forgings, and pressed or rolled sections via industrial steel suppliers. Inventory depth depends on regional supplier networks; 30CrMo may be more widely stocked in general engineering sizes due to its broader use in repairable and welded structures.
• Procurement tip: specify chemical and mechanical acceptance criteria, heat‑treatment requirements, and any PWHT needs to avoid surprises and to obtain competitive quotes.

10. Summary and Recommendation

Table: short comparison snapshot.

Conclusions - Choose 35CrMo if you need higher quenched and tempered strength or hardness for fatigue‑critical, high‑load, or wear‑prone components and you can accommodate stricter welding controls (preheat, PWHT) and slightly higher material cost. - Choose 30CrMo if your priorities are better weldability, easier forming/machining, greater ductility, simpler field repairs, and lower cost while still achieving good strength after suitable heat treatment.

Final practical note: Always confirm grade selection against the exact chemical and mechanical requirements in the applicable standard or drawing. For weldment design, calculate the carbon equivalent for the proposed composition and consult your welding engineer to define preheat, interpass temperature, filler metal, and PWHT to ensure component integrity.