20Cr vs 30Cr – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
20Cr and 30Cr are two commonly specified low-alloy steels used for carburized or quenched-and-tempered components in power transmission, gears, shafts, and structural parts where a balance of surface wear resistance and core toughness is required. Engineers and procurement professionals often choose between them when facing trade-offs among strength, toughness, hardenability, cost, and machinability. Typical decision contexts include specifying a material for a gear set where surface hardness and core ductility matter, or for a shaft that must resist both torsion and occasional shock.
The principal difference between the two grades is that 30Cr is alloyed to achieve higher bulk strength and hardenability than 20Cr—this is accomplished chiefly through modest increases in carbon and chromium (and sometimes other strengthening microalloying elements). Because of this, 30Cr generally provides higher strength and hardenability at the expense of slightly reduced weldability and machinability compared with 20Cr. These contrasts make the pair a useful comparison when selecting steels for moderate-to-high-load mechanical components.
1. Standards and Designations
- Common standards and their nomenclature:
- GB/T (China): 20Cr, 30Cr (often specified for carburizing and quenched components)
- JIS (Japan): similar grades exist under different codes (e.g., SCM, SN) but may not be direct one-to-one equivalents
- EN / ISO: equivalent families would be in the 16MnCr, 20MnCr, or 20CrMn series (check exact part numbers)
- ASTM/ASME: no direct ASTM number exactly named “20Cr” or “30Cr”; equivalents are chosen by chemical and mechanical property match
- Classification: both 20Cr and 30Cr are low-alloy steels (used as carburizing or medium-alloy structural steels), not stainless, tool steel, or HSLA in the narrow sense. They are typically specified for components requiring surface hardening (carburizing) or bulk quench-and-temper treatment.
2. Chemical Composition and Alloying Strategy
The following table shows typical nominal composition ranges (wt%) used in common industrial practice. Actual compositions depend on the chosen standard or mill specification—use the mill certificate for procurement.
| Element | 20Cr (typical range, wt%) | 30Cr (typical range, wt%) |
|---|---|---|
| C | 0.16 – 0.24 | 0.24 – 0.32 |
| Mn | 0.40 – 0.80 | 0.50 – 0.90 |
| Si | 0.10 – 0.35 | 0.10 – 0.35 |
| P | ≤ 0.035 (max) | ≤ 0.035 (max) |
| S | ≤ 0.035 (max) | ≤ 0.035 (max) |
| Cr | 0.50 – 1.10 | 0.80 – 1.30 |
| Ni | ≤ 0.30 (trace) | ≤ 0.30 (trace) |
| Mo | ≤ 0.10 – 0.20 (if specified) | ≤ 0.10 – 0.30 (if specified) |
| V | trace or ≤ 0.05 (if microalloyed) | trace or ≤ 0.05 (if microalloyed) |
| Nb, Ti, B | trace (occasionally used in microalloyed variants) | trace (occasionally used) |
| N | trace | trace |
Notes: - The table gives typical banded ranges; procurement should reference the exact standard or material certificate. - 30Cr commonly has higher carbon and slightly higher chromium and manganese compared with 20Cr. Additional microalloying (V, Nb, Ti) may be present in some variants to improve strength and grain refinement. - Increased alloying (Cr, Mn, and occasional microalloying) raises hardenability and tempering resistance and, together with carbon, controls achievable strength levels.
How alloying affects properties: - Carbon increases strength and hardenability but reduces ductility and weldability. - Chromium increases hardenability, strength, and tempering resistance and can improve wear resistance after surface hardening. - Manganese contributes to hardenability and tensile strength. - Microalloying elements (V, Nb, Ti) refine grain size, increase yield strength via precipitation, and improve fatigue strength.
3. Microstructure and Heat Treatment Response
Typical microstructures and heat-treatment responses differ because of carbon and alloying level:
- 20Cr:
- As-rolled/normalized: ferrite–pearlite microstructure with relatively fine grains if controlled-rolled or normalized.
- After carburizing and quench: a hard martensitic/carburized case with a tougher, lower-carbon core (tempered martensite or tempered bainite depending on the quench/temper cycle).
-
Quench & temper (bulk): can produce tempered martensite with moderate strength and good toughness when adequately tempered.
-
30Cr:
- As-rolled/normalized: higher proportion of pearlite and finer transformed microstructures than 20Cr for comparable cooling due to higher hardenability.
- After identical carburizing/quench: deeper hardenable case and higher case/core strength because of higher carbon and Cr; core may transform to martensite more readily than 20Cr unless cooling is slow.
- Quench & temper (bulk): achieves higher strength levels at similar tempering temperatures but requires careful tempering to retain acceptable toughness.
Heat treatment considerations: - Carburizing is widely used with both grades; 20Cr is often specified where a relatively shallow, hard case with tough core is required. 30Cr is selected where deeper case or higher core strength is needed without increasing section size. - Normalizing before final heat treatment improves uniformity. Quenching media and part section size affect final hardness, especially for 20Cr which has lower hardenability. - Tempering reduces hardness and improves toughness; 30Cr requires tempering regimes tuned to avoid excessive brittleness from higher carbon content.
4. Mechanical Properties
Typical mechanical-property ranges depend strongly on the heat treatment and section size. The table below gives representative ranges for quenched-and-tempered bars or carburized-and-tempered parts; use certified test data for design.
| Property | 20Cr (typical range) | 30Cr (typical range) |
|---|---|---|
| Tensile strength (MPa) | 600 – 950 | 700 – 1100 |
| Yield strength (0.2% proof, MPa) | 350 – 700 | 450 – 850 |
| Elongation (%) | 12 – 20 | 8 – 16 |
| Impact toughness (Charpy V-notch, J) | moderate to good (varies with temper) | generally lower than 20Cr at same hardness |
| Hardness (HRC or HB) | core tempered: HRC ~20–40; carburized case: HRC 55–62 | core tempered: HRC ~22–44; carburized case: HRC 58–64 |
Interpretation: - 30Cr is typically higher in strength and hardenability due to increased carbon and Cr; it can achieve higher tensile and yield strengths, but ductility and impact toughness at a given hardness may be somewhat lower than 20Cr. - 20Cr often offers a better balance of toughness and weldability for applications where absolutely maximum strength is not required. - Always reference supplier material certificates and perform component-level testing for critical applications (fatigue, impact at low temperature).
5. Weldability
Key weldability drivers are carbon content, effective alloying, thickness, and pre/post-heat treatments. Two commonly used empirical indices are the IIW carbon equivalent and the Pcm formula:
-
Use of the IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
Pcm (more conservative for weld cracking susceptibility): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 20Cr, with lower carbon and slightly lower alloy content, will have a lower $CE_{IIW}$ and $P_{cm}$, and therefore better as-welded behavior. Preheat and post-weld heat treatment (PWHT) requirements are lower than for 30Cr. - 30Cr, with higher carbon and chromium, increases hardenability and the risk of martensite formation in the heat-affected zone (HAZ); it often requires higher preheat, controlled interpass temperatures, and in many cases PWHT to avoid cracking and restore toughness. - For both grades, carburized surfaces should not be welded without special procedures; welding can alter local carbon and produce brittle zones. If welding is necessary, follow qualified procedures and perform HAZ toughness checks.
6. Corrosion and Surface Protection
- Neither 20Cr nor 30Cr is stainless steel; both are considered carbon/alloy steels and have limited intrinsic corrosion resistance.
- Typical protection strategies:
- Surface coatings: galvanizing, electroplating, or conversion coatings can be used depending on environment.
- Paints and industrial coatings: epoxy, polyurethane systems for atmospheric protection.
- Case-hardening surfaces: after carburizing, additional finishing coatings are sometimes applied for corrosion protection, recognizing coatings must tolerate the surface hardness.
- PREN (pitting resistance equivalent number) is applicable to stainless grades and is not relevant for 20Cr/30Cr in their standard forms: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Use of PREN is not meaningful for these non-stainless steels.
7. Fabrication, Machinability, and Formability
- Machinability:
- 20Cr generally machines more easily due to lower carbon and slightly lower hardenability.
- 30Cr can be more abrasive on tooling after heat treatment and requires slower cutting speeds or harder tooling when machining hardened sections.
- Formability and cold-working:
- Both grades are workable in annealed or normalized conditions. Higher carbon in 30Cr reduces formability; forming should be done in softer heat-treated states.
- Grinding and finishing:
- Carburized and hardened cases require diamond or CBN tooling for efficient grinding. Surface finishing to achieve required contact fatigue life is more challenging on deeper, harder cases (common with 30Cr).
- Heat-treatment distortion:
- Higher hardenability and retained stresses in 30Cr can increase sensitivity to distortion during quench and temper compared with 20Cr; process control and fixturing are important.
8. Typical Applications
| 20Cr – Typical Uses | 30Cr – Typical Uses |
|---|---|
| Moderate-duty gears, shafts, spline shafts, pinions where good core toughness is required and cost control is important | Heavier-duty gears, larger-diameter shafts, high-load pins and axles where higher core strength and deeper case hardening are required |
| Automotive transmission components with medium loads | Gearbox main shafts and heavy machinery drive components requiring higher fatigue strength |
| General-purpose carburized parts where machinability and weldability are considerations | Applications where increased hardenability allows design simplification (thicker sections, less alloying elsewhere) |
Selection rationale: - Choose 20Cr for components where toughness, easier machining/welding, and lower material cost are priorities, and where section size is small-to-moderate. - Choose 30Cr when the design demands higher bulk strength, deeper hardening, or higher fatigue resistance in larger cross sections.
9. Cost and Availability
- Cost:
- 30Cr is typically priced slightly higher than 20Cr due to higher alloy content and potentially tighter process controls needed to achieve higher strength.
- The price delta depends on market alloying element costs and order volume.
- Availability:
- Both grades are widely produced in many steel markets (bars, forgings, plates) but availability of specific sizes, surface conditions (annealed, normalized, pre-hardened), or microalloyed variants may vary by region and mill.
- Procurement tip: specify required heat-treatment condition and certified mill test reports; for critical components, request mechanical test certificates and chemistry analysis.
10. Summary and Recommendation
| Aspect | 20Cr | 30Cr |
|---|---|---|
| Weldability | Better (lower CE, easier preheat control) | Lower (higher CE, needs more preheat/PWHT) |
| Strength–Toughness balance | Good toughness at moderate strength | Higher strength, somewhat reduced ductility/toughness at equal hardness |
| Cost | Lower (generally) | Higher (generally) |
Conclusion and practical guidance: - Choose 20Cr if: - You need a balanced, cost-effective carburizing or quenched-and-tempered steel with good core toughness, easier machining, and more forgiving weldability; ideal for small-to-medium sections and applications where extreme strength is not required. - Choose 30Cr if: - Your design requires higher bulk strength or deeper hardenability (for larger or heavily loaded parts), and you can accept the need for more careful welding practice, tighter heat-treatment control, and slightly higher material cost.
Final note: the terms 20Cr and 30Cr are convenient shorthand. Always verify the chosen material against the specific standard or mill certificate for exact chemical composition and guaranteed mechanical properties, and qualify heat-treatment and welding procedures for critical components.