20CrMo vs 30CrMo – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners commonly face a trade-off between strength, toughness, cost, and manufacturability when selecting alloy steels for critical mechanical components. 20CrMo and 30CrMo are two chromium–molybdenum alloy steels that are often compared for gears, shafts, and structural parts where fatigue resistance and through-hardening or surface-hardening capability are important.

The principal distinction between these grades lies in their nominal carbon content and the resulting design emphasis: one grade is formulated with lower carbon for improved ductility and toughness and better weldability; the other has higher carbon for greater as-quenched strength and higher achievable hardness after heat treatment. Because chromium and molybdenum contents are similar, designers typically choose between them based on the strength/toughness balance required and downstream processing constraints.

1. Standards and Designations

• Common international and regional standards and designations where these names appear:
• GB/T (China): 20CrMo, 30CrMo (often used in domestic specifications)
• EN (Europe): equivalents are usually expressed as EN 10083-series or 1.xxxx numbers; direct one-to-one names may differ
• JIS (Japan): similar alloy steels exist but under different codes
• ASTM/ASME: alloy steels covered under AISI/SAE series (e.g., AISI 4135/4140 family) present similar chemistries but different naming
• Classification: Both 20CrMo and 30CrMo are alloy steels (low-alloy, Cr–Mo steels). They are not stainless steels, tool steels, or HSLA in the strictest sense; they are often used as engineering alloy steels for quenched-and-tempered or case-hardened parts.

2. Chemical Composition and Alloying Strategy

The following table gives typical compositional tendencies for both grades. Actual ranges vary by standard and producer; always consult mill certificates for procurement and design calculations.

How alloying affects performance: - Carbon: primary lever for strength and achievable hardness; higher carbon enables higher quenched hardness but reduces weldability and ductility. - Chromium and molybdenum: both increase hardenability (depth/extent of martensitic transformation on quenching), temper resistance, and strength at elevated temperature. - Manganese and silicon: assist hardenability and strength; silicon also assists carburizing surface treatments. - Trace microalloying elements refine prior austenite grain size and can improve toughness without a large increase in strength.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses to common thermal processes:

• As-rolled/normalized:
• 20CrMo: ferrite–pearlite matrix with tempered bainite possible depending on cooling; finer grains after normalization improve toughness.
• 30CrMo: higher pearlite fraction and finer carbide distribution; normalization grain size control is critical to achieve good toughness.
• Quenching and tempering:
• Both grades form martensite on quenching from austenitizing temperatures; tempering converts martensite to tempered martensite/tempered bainite, determining final strength–toughness balance.
• 30CrMo reaches higher hardness and tensile strength at comparable tempering temperatures due to higher carbon; however, it can be more prone to temper brittleness if tempering is not optimized.
• Case-carburizing (when surface hardness is required):
• Both can be used as core steels under a carburized case. 20CrMo, with lower core carbon, yields a tougher, more ductile core compared with 30CrMo if used similarly.
• Thermo-mechanical processing:
• Controlled rolling and accelerated cooling can produce bainitic or refined martensitic microstructures with improved toughness; microalloying additions, if present, aid grain refinement.

4. Mechanical Properties

Mechanical properties depend strongly on heat treatment. The table below gives generalized typical ranges for quenched & tempered conditions used in engineering design; verify with tested mill reports.

Explanation: - 30CrMo is stronger in most comparable quenched-and-tempered conditions because its higher carbon content increases martensite fraction and hardness. - 20CrMo is typically tougher and more ductile for comparable strength levels and is easier to achieve good toughness with conservative heat treatment.

5. Weldability

Weldability is governed largely by carbon equivalent and hardenability due to alloying content.

Common carbon-equivalent formulas used for qualitative assessment: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ and a more conservative parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Because 30CrMo has higher carbon, its $CE_{IIW}$ and $P_{cm}$ values will generally be higher than 20CrMo, indicating greater risk of hard, brittle heat-affected zones and cold cracking. Preheat and controlled interpass temperatures are more often required for 30CrMo. - Chromium and molybdenum increase hardenability equally for both grades, so weld procedures must address alloying that promotes martensite formation. - 20CrMo, with lower carbon, is typically easier to weld but still benefits from preheat/post-weld heat treatment (PWHT) when used in critical, high-strength applications.

6. Corrosion and Surface Protection

• Neither 20CrMo nor 30CrMo are stainless grades; corrosion resistance is limited and comparable to other low-alloy steels.
• Typical protection strategies:
• Surface coatings: hot-dip galvanizing, painted systems, powder coating, or specialized corrosion-resistant coatings.
• Plating: for components where wear and mild corrosion protection are required.
• Design considerations: drainage, crevice avoidance, and sacrificial anodes in marine or aggressive environments.
• PREN (pitting resistance equivalent number) is only meaningful for stainless grades and is not applicable to these Cr–Mo alloy steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index should not be used for non-stainless steels like 20CrMo/30CrMo.

7. Fabrication, Machinability, and Formability

• Machinability:
• 20CrMo (lower C) usually machines more readily and with longer tool life than 30CrMo; however, both are tougher to machine than plain-carbon steels of comparable RU due to alloying.
• Higher hardness levels (as-quenched or not fully tempered) reduce machinability and increase tool wear.
• Formability:
• Cold forming is easier with 20CrMo because of lower carbon and higher ductility; 30CrMo is less forgiving and may require higher forming forces or warm forming.
• Surface finishing:
• Both respond well to grinding, polishing, and surface treatments after appropriate tempering; carburizing followed by low-temperature tempering is common for gear surfaces.
• Key fabrication note: for welded or heat-treated assemblies, control of interpass temperature, preheat, and PWHT are essential to minimize cracking and achieve desired toughness.

8. Typical Applications

Selection rationale: - Choose 20CrMo when toughness, ductility, weldability, and post-weld properties are priorities, or when a ductile core under a carburized case is required. - Choose 30CrMo when a higher strength or higher as-quenched hardness is required and when design and fabrication can accommodate stricter welding and heat-treatment controls.

9. Cost and Availability

• Raw material cost: Both grades are similar in alloying elements (Cr, Mo), so raw steel cost differences are modest; 30CrMo may be slightly more expensive on a per-ton basis because of higher carbon content's impact on downstream heat-treatment demands and potential tighter processing controls.
• Heat treatment and processing costs: 30CrMo often incurs higher process costs due to more stringent preheat/PWHT and greater susceptibility to quench cracking if not managed, and sometimes longer tempering cycles.
• Availability by product form: Both are widely available in bar, forgings, plate, and ring forms from major suppliers; lead times depend on required heat-treatment and certification.

10. Summary and Recommendation

Recommendation: - Choose 20CrMo if you need a balanced alloy with better weldability, higher core toughness, easier fabrication, and safer margin against quench-related cracking—typical for components requiring ductile cores, simpler welding procedures, or better fatigue resistance at moderate strength levels. - Choose 30CrMo if the application requires higher quenched-and-tempered strength or higher final hardness (for wear or load reasons), and you can specify controlled welding procedures, adequate preheat/PWHT, and rigorous heat-treatment control to mitigate cracking and toughness loss.

Final note: Always specify the exact standard, required heat-treatment condition, and acceptance test criteria. Confirm mill certificates and, for critical components, request mechanical tests (tensile, CVN), hardness maps, and fracture-toughness data for the exact heat-treatment batch to validate design assumptions.