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

Introduction

30CrMo and 35CrMo are two widely used medium‑carbon, low‑alloy steels specified in regional and national standards for components that require a balance of strength, toughness, and toughness retention after heat treatment. Engineers, procurement managers, and manufacturing planners frequently face a selection dilemma between slightly lower‑carbon, more ductile grades and slightly higher‑carbon, higher‑strength grades — balancing machinability and weldability against achievable strength and fatigue life.

The principal technical distinction between these two grades is a modest difference in carbon and alloy content that shifts hardenability and final strength: the 35CrMo family is typically specified with a higher carbon level and similar chromium/molybdenum content, giving it greater as‑quenched strength and hardenability but generally lower ductility and somewhat more demanding welding requirements. Because their chemistries are close, they are often compared when choosing materials for shafts, gears, axles, and forgings where heat treatment and fatigue performance matter.

1. Standards and Designations

• Common or relevant standards and designation systems where equivalents or similar grades appear:
• GB/T (Chinese national standards): 30CrMo, 35CrMo.
• EN / DIN: 35CrMo4 (often written 1.7225) and related grades; 30CrMo equivalents exist in regional specs.
• AISI/SAE: No exact one‑to‑one AISI names, but mechanical property equivalents often compared to the 41xx family (e.g., 4140) for many engineering applications.
• JIS: Similar high‑strength alloy steels appear under SNCM/SNCM4xx designations.
• Classification: both 30CrMo and 35CrMo are medium‑carbon, low‑alloy steels (not stainless, not tool steels) intended for quenching & tempering or normalizing and tempering. They fall into the general category of heat‑treatable structural/alloy steels used for critical machine parts.

2. Chemical Composition and Alloying Strategy

Typical composition ranges are given as weight percent; exact limits depend on the specific standard or supplier certificate.

Element	Typical 30CrMo (wt%)	Typical 35CrMo (wt%)
C	0.27 – 0.34	0.32 – 0.39
Mn	0.50 – 0.80	0.50 – 0.90
Si	0.15 – 0.35	0.15 – 0.35
P	≤ 0.025 – 0.035	≤ 0.025 – 0.035
S	≤ 0.035	≤ 0.035
Cr	0.80 – 1.20	0.90 – 1.20
Ni	often ≤ 0.30 (trace)	often ≤ 0.30 (trace)
Mo	0.15 – 0.30	0.15 – 0.30
V, Nb, Ti, B	trace to ≤ 0.05 (if microalloyed)	trace to ≤ 0.05 (if microalloyed)
N	trace	trace

Alloying strategy: - Carbon is the principal determinant of achievable strength and hardness after quench & temper. A higher carbon content (as in 35CrMo) increases strength and wear resistance but reduces ductility and weldability. - Chromium and molybdenum increase hardenability, tempering resistance, and high‑temperature strength. They also contribute to fatigue resistance when properly heat treated. - Manganese and silicon are deoxidizers and strengthen the ferrite/pearlite matrix; they modestly affect hardenability. - Microalloying (V, Nb, Ti) is sometimes used to refine grain size and improve toughness, but these are not primary alloying elements in standard 30CrMo/35CrMo grades.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses: - As‑rolled/normalized: both grades produce a ferrite–pearlite or fine tempered martensite/ferrite microstructure depending on cooling and normalization. Normalizing refines grain size and produces a uniform structure for subsequent machining or tempering. - Quenching and tempering (Q&T): both steels are commonly hardened by austenitizing (typical austenitizing temperatures depend on section size and standard), oil or water quenching, then tempering to achieve the desired strength‑toughness balance. Because 35CrMo usually has a slightly higher carbon content, its as‑quenched microstructure forms a higher fraction of martensite at similar quench rates, making it harder and stronger after tempering. - Thermo‑mechanical processing: controlled rolling and accelerated cooling can further refine grain size and improve toughness; both grades benefit, but 30CrMo can be optimized for better ductility while 35CrMo is optimized for higher strength and fatigue life. - Grain size control and tempering practice are critical for toughness. Over‑tempering reduces strength; under‑tempering risks embrittlement.

4. Mechanical Properties

Mechanical properties depend heavily on heat treatment, section size, and supplier practice. The table shows typical property ranges after a standard quench & temper regimen used for high‑strength components.

Property (typical Q&T ranges)	30CrMo	35CrMo
Tensile strength (MPa)	800 – 1,050	850 – 1,200
Yield strength (MPa)	600 – 900	650 – 1,000
Elongation (%), A5	10 – 16	8 – 14
Charpy V‑Notch impact (J, room temp)	30 – 70 (depending on temper)	25 – 60
Hardness (HRC)	22 – 36	25 – 40

Interpretation: - 35CrMo generally achieves higher tensile and yield strengths and higher hardness for a given heat‑treatment severity because of its higher carbon fraction and similar Cr/Mo levels, which increase hardenability. - 30CrMo tends to be slightly tougher and more ductile at equivalent strength levels, making it preferable where energy absorption and forming are important. - Impact toughness can be engineered by tempering; lower tempering temperatures raise strength but reduce impact toughness.

5. Weldability

Weldability is governed primarily by carbon equivalent and the presence of hardenability‑increasing elements. Common indices:

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

and

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

Qualitative interpretation: - Because 35CrMo typically has higher carbon, its $CE_{IIW}$ and $P_{cm}$ will be higher than 30CrMo, indicating greater susceptibility to cold cracking and a greater need for preheat, controlled interpass temperature, and post‑weld heat treatment (PWHT). - 30CrMo exhibits better weldability than 35CrMo but still often requires preheat and controlled procedures for thick sections or high restraint. - Use of matching or overmatching filler metals, stress‑relief tempering, and hydrogen‑control procedures are common for both grades when welded in critical applications.

6. Corrosion and Surface Protection

• Neither 30CrMo nor 35CrMo is stainless; corrosion resistance is similar to plain carbon/low‑alloy steels and driven by surface finish and service environment.
• Typical protection strategies: painting, powder coating, solvent‑borne coats, hot‑dip galvanizing, or local plating depending on exposure. For long term exposure in moist or corrosive atmospheres, galvanizing or application of barrier coatings is recommended.
• PREN (pitting resistance equivalent number) is not applicable to these non‑stainless grades. For stainless grades one would use:

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

but this has no meaning for 30CrMo/35CrMo.

7. Fabrication, Machinability, and Formability

• Machinability: 30CrMo, with slightly lower carbon, is generally easier to machine in normalized or annealed conditions compared to 35CrMo. Both become harder to machine after quenching & tempering.
• Formability/bending: easier in normalized or annealed conditions; avoid forming in fully hardened condition. 30CrMo accepts cold bending and forming slightly better due to lower as‑treated strength.
• Grinding and finishing: both respond well to standard machining and grinding practices for alloy steels; surface treatments or residual stress control may be needed to meet fatigue performance.
• Heat treatment prior to forming (e.g., normalize or anneal) is common practice to improve formability.

8. Typical Applications

30CrMo — Typical uses	35CrMo — Typical uses
Shafts, spindles, medium‑duty gears, connecting rods, fasteners where toughness and machinability are priorities	Heavy‑duty shafts, gear wheels, axles, crankshafts, high‑fatigue machine components requiring higher strength and hardenability
Forged components where good ductility is helpful for forming	Components for heavy machinery and off‑highway equipment where higher section size hardenability is required
Parts requiring welding with moderate preheat controls	High‑strength quenched & tempered parts where strength and fatigue life are primary design drivers

Selection rationale: - Choose 30CrMo where a balance of strength and toughness plus easier machining/welding is needed and component section sizes are moderate. - Choose 35CrMo where higher strength, deeper hardening for larger cross‑sections, and improved fatigue resistance are required, and where welding and fabrication controls can be accommodated.

9. Cost and Availability

• Cost: 35CrMo is typically slightly more expensive than 30CrMo due to the marginally higher carbon and alloy content and because purchasing specifications for higher‑strength variants often have tighter controls. The price difference is usually modest relative to total component cost.
• Availability: Both grades are widely available in bar, rod, and forgings from major mills and distributors. 35CrMo (35CrMo4 / 1.7225) is a very common European grade; 30CrMo is common in markets using GB/T designations. Lead times are generally short for standard product forms; special chemistries or premium bar/forging sizes may require longer delivery.

10. Summary and Recommendation

Criteria	30CrMo	35CrMo
Weldability	Better (lower CE)	More demanding (higher CE, requires preheat/PWHT)
Strength–Toughness balance	Balanced—good toughness at moderate strength	Higher strength and hardenability, modestly lower ductility/toughness for same temper
Cost	Lower / cost‑effective	Slightly higher

Choose 30CrMo if: - You need a balanced combination of strength and toughness with better machinability and easier welding procedures. - Component sizes are moderate, and you prefer more forgiving tempering and forming behavior. - Cost and fabrication simplicity are important.

Choose 35CrMo if: - The design demand prioritizes higher tensile/yield strength, deeper hardening for larger sections, or enhanced fatigue life. - You can accommodate stricter welding controls (preheat, interpass limits, PWHT) and tighter process control during heat treatment. - Use cases include heavy‑duty shafts, axles, or high‑stress gears where higher as‑quenched strength is decisive.

Final note: exact performance depends strongly on the chosen standard, the specific supplier chemical limits, and the heat‑treatment schedule. For critical components specify the required mechanical properties, inspection criteria (hardness, impact, microstructure), and weld procedure qualification in the purchase order and collaborate with heat‑treat and welding specialists early in design to select the correct grade and process envelope.