20CrMo vs 42CrMo – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
20CrMo and 42CrMo are two widely used low-alloy steels encountered in power transmission components, gears, shafts, and heavy machinery. Engineers and procurement managers frequently must choose between the two when balancing core strength, surface hardness, hardenability, weldability, and cost. Typical decision contexts include whether a component needs a hardened case with a ductile core (carburizing designs) versus a through-hardened, higher-strength shaft where uniform mechanical properties are required.
The primary operational distinction is that one grade is tailored for carburizing and surface hardening strategies producing a relatively lower bulk carbon but improved case properties, while the other contains higher bulk carbon and alloying to produce higher through-thickness strength and toughness after quench and tempering. Because both grades are low-alloy steels with chromium and molybdenum additions, they are commonly compared for similar rotating or loaded parts where heat treatment route drives final performance.
1. Standards and Designations
- 20CrMo
- Commonly referenced standards: GB (China) designations (e.g., 20CrMo), EN equivalents (carburizing steels such as 5120/20Cr), and JIS variations. Often classified as low-alloy carburizing steel.
- Category: Low-alloy steel designed for carburizing (case hardening).
- 42CrMo
- Commonly referenced standards: GB 42CrMo (42CrMo4), EN 1.7225 / 42CrMo4, AISI/SAE 4140 (close equivalent), JIS. Classified as chromium–molybdenum alloy steel for through-hardening.
- Category: Low-alloy, quenched-and-tempered steel (structural/alloy steel).
Both are not stainless steels; they are alloy steels (not HSLA in the strictest sense, but alloyed to improve hardenability and strength).
2. Chemical Composition and Alloying Strategy
Below are typical element ranges used as guidance (ranges reflect common specifications; exact limits depend on the selected standard and heat-treatment condition).
| Element | Typical 20CrMo (wt%) | Typical 42CrMo (wt%) |
|---|---|---|
| C | 0.17–0.25 | 0.38–0.45 |
| Mn | 0.35–0.65 | 0.50–0.90 |
| Si | 0.15–0.35 | 0.15–0.35 |
| P | ≤ 0.025 | ≤ 0.025 |
| S | ≤ 0.035 | ≤ 0.035 |
| Cr | 0.40–0.70 | 0.90–1.20 |
| Ni | ≤ 0.30 (minor) | ≤ 0.30 (minor) |
| Mo | 0.08–0.20 | 0.15–0.30 |
| V | ≤ 0.05 (trace) | ≤ 0.05 (trace) |
| Nb | typically trace | typically trace |
| Ti | typically trace | typically trace |
| B | typically trace | typically trace |
| N | typically trace | typically trace |
How alloying affects properties: - Carbon: primary control on strength and hardenability. Lower bulk carbon in carburizing steels (20CrMo) facilitates a ductile core and good case/ core gradient after carburizing. Higher carbon in 42CrMo yields higher as-quenched strength and hardness throughout section. - Chromium and molybdenum: increase hardenability, tempering resistance, and strength; both grades use Cr and Mo but 42CrMo generally has higher Cr and Mo to enable through-hardening to higher strength levels. - Manganese and silicon: contribute to strength and deoxidation. - Microalloying elements (V, Nb, Ti) may be present in trace amounts to control grain size and improve toughness; not primary alloying elements in these grades.
3. Microstructure and Heat Treatment Response
Typical microstructures and responses: - 20CrMo - As-rolled/normalized: predominantly ferrite–pearlite or fine-grained microstructure depending on normalization. - After carburizing + quench & temper: a hard martensitic/carburized case with controlled carbon gradient; core is tempered martensite or tempered ferrite–pearlite with relatively lower strength and higher ductility. Carburizing makes 20CrMo ideal where surface wear resistance is needed without sacrificing core toughness. - 42CrMo - As-rolled/normalized: ferrite-pearlite; good grain size control due to Cr and Mo. - After quench & temper: transforms to martensite on quenching and, after tempering, achieves high strength and toughness throughout the cross-section. Tempering temperature controls the strength–toughness balance; higher temper lowers strength and raises toughness. - Thermo-mechanical processing: Both grades respond to controlled rolling and accelerated cooling to refine grain size and improve mechanical properties; however, 42CrMo’s higher carbon and alloy content make it more responsive to strengthening via quench & temper.
4. Mechanical Properties
Mechanical properties depend strongly on heat treatment and section size. The table shows representative ranges for commonly used heat-treatment conditions (normalized, carburized/quenched-tempered, or quenched & tempered). These are indicative; specify supplier datasheets or test certificates for design-critical values.
| Property | 20CrMo (typical post-carburize/core temper) | 42CrMo (typical quenched & tempered) |
|---|---|---|
| Tensile Strength (MPa) | Core: ~500–800 | ~800–1200 |
| Yield Strength (MPa) | Core: ~300–600 | ~600–1000 |
| Elongation (%) | Core: moderate to good (10–18%) | Varies with temper (8–16%) |
| Impact Toughness (J, room temp) | Good core toughness after temper | Good to very good with appropriate tempering; depends on temper level |
| Hardness (HRC or HB) | Case hardness high (HRC 55–62), core low (HB 170–250) | Through-hardness achievable (e.g., HRC ~25–55 depending on temper) |
Explanation: - Which is stronger? In through-hardened condition, 42CrMo delivers higher bulk tensile and yield strength because of higher carbon and alloy content. - Which is tougher/ductile? 20CrMo’s lower core carbon after carburizing yields superior core ductility and toughness while still providing a wear-resistant case. 42CrMo can be engineered for a toughness–strength balance via tempering but will typically have higher strength and lower ductility than the core of a carburized 20CrMo part when both are optimized for their respective use cases.
5. Weldability
Weldability considerations: - Carbon content and hardenability are key. Higher bulk carbon and alloying increase risk of hard, brittle martensite in the heat-affected zone (HAZ) and thus increase preheat/interpass and post-weld heat treatment (PWHT) requirements. - 20CrMo: lower bulk carbon improves weldability for non-carburized sections, but if the part is carburized, welding must account for the carburized layer (avoid welding through the case without proper procedures). Carburized components generally require pre- and post-weld attention. - 42CrMo: higher carbon and alloying result in a higher propensity for HAZ hardening; controlled preheat and PWHT are commonly required for structural welds to avoid cracking. - Use of carbon equivalent formulas helps assess weldability. For example: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - Interpretation: Higher $CE_{IIW}$ or $P_{cm}$ values indicate greater hardenability and more stringent welding controls. In practice, 42CrMo will typically yield higher carbon-equivalent measures than 20CrMo, implying more restrictive welding preheat and PWHT.
6. Corrosion and Surface Protection
- Neither 20CrMo nor 42CrMo is stainless; both require protective measures where corrosion resistance is needed.
- Common protections: painting, powder coating, oiling, phosphating, or hot-dip galvanizing depending on environment. For parts with tight dimensional/heat-treated surfaces, mechanical coatings or lubricative finishes may be preferable.
- Stainless indices like PREN are not applicable to these carbon/alloy steels. For reference on stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This is not relevant for 20CrMo or 42CrMo because their Cr and Mo levels and matrix chemistry are not designed for pitting-corrosion resistance.
7. Fabrication, Machinability, and Formability
- Machinability:
- 20CrMo: moderate machinability in annealed or normalized condition; the low bulk carbon improves ease of machining at core regions. Carburized surfaces are difficult to machine after hardening.
- 42CrMo: poorer machinability than low-carbon steels when hardened; in normalized or annealed condition machining is manageable but chattering and tool wear are considerations due to higher carbon and alloy content.
- Formability:
- 20CrMo (annealed/normalized): better cold-forming and bending capability because of lower core carbon. Post-carburizing forming is not typical.
- 42CrMo: limited cold formability in higher-strength conditions; design for forming in the annealed/normalized state before final heat treatment.
- Surface finishing: Both respond well to grinding, shot-peening, and surface finishing. Grinding hardened components requires proper tool selection and coolant control.
8. Typical Applications
| 20CrMo (carburizing grade) | 42CrMo (through-hardened grade) |
|---|---|
| Gears (transmission gears with carburized case) | Shafts and axles requiring high torsional strength |
| Spline shafts and pinions with hard wearing surface | High-strength fasteners, heavy-duty shafts |
| Bushings or components requiring wear-resistant surface and tough core | Crankshafts, heavy machinery components needing uniform strength |
| Components designed for carburizing process to combine wear resistance and toughness | Structural parts where through-hardening and predictable bulk properties are required |
Selection rationale: - Choose 20CrMo if the design benefits from a hard, wear-resistant surface (case) with a ductile, tough core — typical of gears and highly-loaded mating surfaces where contact wear is critical. - Choose 42CrMo if the application requires higher uniform bulk strength and fatigue resistance across the cross-section and where through-hardening is acceptable or needed.
9. Cost and Availability
- Relative cost: 42CrMo typically costs more per tonne than plain-carbon steels because of higher alloy content and tighter processing requirements; 20CrMo may be priced similarly or slightly lower depending on grade and market but may incur extra process costs (carburizing).
- Availability by product form: Both grades are widely available globally in bars, forgings, and plate forms from specialist steel mills and distributors. 42CrMo (or equivalents like AISI 4140 / 42CrMo4) is a standard alloy often stocked; carburizing grades like 20CrMo are also common but may be supplied as normalized or pre-carburized blanks.
- Total cost of ownership: account for heat treatment (carburizing cycle cost for 20CrMo), post-heat-treatment machining/grinding, and any additional nondestructive testing or surface protection. A seemingly cheaper base-grade can become costlier after carburizing and finishing steps.
10. Summary and Recommendation
| Criterion | 20CrMo | 42CrMo |
|---|---|---|
| Weldability | Better for core welding; avoid welding through carburized case without controls | More restrictive — higher preheat/PWHT often required |
| Strength–Toughness balance | Excellent case/core combination for wear + toughness | Higher through-thickness strength; adjustable toughness by tempering |
| Cost | Competitive base cost; added process cost for carburizing | Higher alloy cost; simpler heat-treat route for through-hardening |
Recommendation: - Choose 20CrMo if you need a hardened surface for wear resistance while preserving a ductile, tough core — typical for gears, pinions, and carburized shafts. - Choose 42CrMo if you require higher through-thickness strength and a predictable quenched-and-tempered condition for shafts, axles, or heavily loaded structural components where uniform properties are critical.
Final note: Always correlate material selection with the specific design life, fatigue loading, dimensional constraints, and post-fabrication treatments (carburizing, nitriding, quench & temper, PWHT). Confirm exact chemical and mechanical limits from the mill certificate or applicable standard before finalizing procurement or design specifications.