20Cr vs 20CrMnTi – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
20Cr and 20CrMnTi are two widely used carburizing steels in power transmission and machinery components. Engineers, procurement managers, and manufacturing planners commonly face the choice between the simpler 20Cr and the microalloyed 20CrMnTi when specifying gears, shafts, splines, and other case-hardened parts. Typical decision contexts include balancing cost against core strength and fatigue resistance, selecting a grade for higher hardenability or better grain control, and choosing a steel that meets heat-treatment and welding constraints.
The principal technical distinction is that 20CrMnTi includes additional manganese and titanium (and other microalloying adjustments) to enhance hardenability, grain refinement, and the stability of the carburized case. These differences make 20CrMnTi favorable where deeper case hardening, improved core properties, and better resistance to temper embrittlement are required, while 20Cr remains an economical choice for moderate-duty carburized parts.
1. Standards and Designations
- Common national and international references where these grades appear or have equivalents:
- GB (China): 20Cr, 20CrMnTi (widely used designations in Chinese standards)
- JIS (Japan): carburizing steels with similar chemistry (e.g., SC series equivalents)
- EN (Europe): comparable to some 15–20Cr series carburizing steels (but check exact EN numbers)
- ASTM/ASME: carburizing steels covered under general specifications for alloy steels for quenching and tempering; direct AISI/ASTM one-to-one designations may not exist so cross-referencing is required
- Classification: both are alloy carburizing steels (not stainless, not tool steel, not HSLA). They are designed for case hardening to provide a hard wear-resistant surface and a tougher ductile core.
2. Chemical Composition and Alloying Strategy
The table below shows typical composition ranges (wt%) used in industry descriptions. Exact ranges vary by standard and producer—use mill certificates for procurement.
| Element | 20Cr (typical wt%) | 20CrMnTi (typical wt%) |
|---|---|---|
| C | 0.17–0.24 | 0.17–0.24 |
| Mn | 0.25–0.60 | 0.50–0.80 |
| Si | 0.15–0.35 | 0.15–0.35 |
| P | ≤0.035 | ≤0.035 |
| S | ≤0.035 | ≤0.035 |
| Cr | 0.90–1.30 | 0.90–1.30 |
| Ni | ≤0.30 | ≤0.30 |
| Mo | ≤0.10 | ≤0.10 |
| V | —/trace | —/trace |
| Nb | —/trace | —/trace |
| Ti | —/trace (typically none) | 0.02–0.08 |
| B | — | — |
| N | trace | trace |
How the alloying strategy affects properties: - Carbon: set low-to-medium to allow carburizing (low core carbon for toughness) while permitting a high-carbon surface after carburization for hardness. - Chromium: improves hardenability and tempering resistance of the carburized case and helps wear resistance. - Manganese: increases hardenability and tensile strength; higher Mn in 20CrMnTi increases depth of hardening and core strength. - Titanium: microalloying with Ti refines grain size, ties up nitrogen, stabilizes carbides/nitrides, and can improve fatigue resistance and toughness after heat treatment. - Silicon: assists deoxidation and can slightly strengthen ferrite. - Low P, S: for improved toughness and fatigue resistance.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-carburized (before final quench): a low-carbon ferritic/pearlitic core with an enriched austenitic case (higher carbon) in both grades. - After quench & temper (typical carburizing route): a hardened martensitic or bainitic case with retained austenite pockets near the extreme surface; a tempered, lower-carbon martensitic or ferritic–pearlitic core.
How heat-treatment routes affect each grade: - Normalizing: both grades respond to normalizing with refined, uniform ferritic–pearlitic microstructures; 20CrMnTi’s Ti content improves grain refinement on normalizing. - Carburizing + quench + temper (standard route): both are designed for this route. 20Cr will produce an adequate case and ductile core for standard-duty gears. 20CrMnTi, due to higher Mn and Ti, achieves deeper hardenability and generally a tougher core after tempering; it also maintains a finer prior-austenite grain size, which improves fatigue resistance. - Quenching & tempering without carburizing: not typical for these low-carbon steels because their base carbon is low; benefit of microalloying is less leveraged. - Thermo-mechanical processing: microalloyed 20CrMnTi benefits from controlled rolling to further refine grain and improve toughness; 20Cr gains less from microalloying effects.
4. Mechanical Properties
Mechanical properties are strongly dependent on the specific heat-treatment schedule (case depth, quench severity, temper temperature). The table below compares the grades qualitatively under typical carburized and quenched/tempered conditions.
| Property | 20Cr | 20CrMnTi |
|---|---|---|
| Tensile strength (post HT) | Moderate (case-dominated surface strength) | Higher (improved core and through-hardening) |
| Yield strength (core) | Moderate | Higher (better core hardenability) |
| Elongation (ductility, core) | Better ductility in typical moderate heat treatment | Slightly reduced ductility if highly hardened, but better retained toughness due to grain refinement |
| Impact toughness (core) | Good to moderate | Improved toughness for comparable strength, due to Ti grain refinement |
| Hardness (surface case) | High achievable (after carburizing) | Similar surface hardness achievable; tends to maintain case integrity better under load |
Explanation: - 20CrMnTi commonly achieves higher core tensile and yield strengths after the same carburizing and quench because higher Mn increases hardenability and Ti refines grain size. The result is better load-bearing capacity and fatigue performance for high-demand parts. - Surface hardness achievable by case hardening is comparable for both when identical carburizing cycles are used, because surface carbon determines martensite hardness. Differences emerge in case integrity, retained austenite stability, and resistance to tempering.
5. Weldability
Weldability depends principally on carbon equivalent and hardenability. Two common indices:
Display formula for IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
Display formula for 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: - 20Cr: moderate CE; reasonable preheat and controlled cooling can avoid cold cracking. Typical practice is to avoid welding in highly carburized condition or to perform post-weld heat treatment (PWHT) when required. - 20CrMnTi: slightly higher weldability risk due to higher Mn (increases $CE_{IIW}$) and microalloying elements like Ti affecting local microstructure and potential for hardening in the heat-affected zone. $P_{cm}$ flags Ti as a contributor to weldability risk. In practice, welding both grades requires preheat, low interpass temperatures, and appropriate filler materials; welding of carburized surfaces is generally avoided when function depends on case properties. - Practical note: Repairs on carburized surfaces should follow strict procedures (grind out hardened case, local preheat, use compatible weld metal, PWHT), and designers often specify bolted or forged joints where welding would compromise performance.
6. Corrosion and Surface Protection
- Both 20Cr and 20CrMnTi are non-stainless alloy steels; they do not provide inherent corrosion resistance for aggressive environments.
- Common protection strategies: painting, oiling, phosphating, black oxide, and galvanizing (hot-dip or electro) where appropriate. For heavily loaded carburized parts, galvanizing is often avoided on contact surfaces because of potential coating fracture and hydrogen embrittlement risks if not properly controlled.
- PREN (pitting resistance equivalent number) is not applicable to these non-stainless steels; for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ But this applies only to stainless alloys—do not use for 20Cr/20CrMnTi selection.
- Corrosion allowance: where corrosion is a concern, select surface treatments compatible with case hardness and fatigue requirements (e.g., duplex systems, thin ceramic coatings applied before final grinding only with validated processes).
7. Fabrication, Machinability, and Formability
- Machinability: both grades are relatively machinable in the normalized or annealed condition. 20CrMnTi may be slightly more difficult to machine due to higher Mn and the presence of Ti carbides/nitrides; differences are modest in practice.
- Hard machining: after carburizing and quenching, both grades require grinding or hard machining techniques for finishing the case. Case-hardened surfaces should be ground to final dimensions; hard turning with appropriate tooling is possible in production.
- Formability and bending: in low-carbon base condition (pre-carburizing), both form similarly. Post-carburizing forming is not recommended.
- Heat-treatment distortion: 20CrMnTi’s higher hardenability can increase risk of quench-induced distortion if not controlled; design of geometry and fixture, selection of quench medium, and tempering practice are important.
8. Typical Applications
| 20Cr (common uses) | 20CrMnTi (common uses) |
|---|---|
| Moderate-duty gears, pinions, splines | Heavy-duty gears, large pinions, and high-load transmission shafts |
| Shafts and axles for general machinery | Components requiring deeper case and higher core strength (wind gearbox shafts, heavy vehicle gears) |
| Sprockets, camshafts in lighter service | Parts exposed to dynamic loading and fatigue where grain refinement is beneficial |
| General carburized fasteners and sleeves | High-fatigue carbureted components and safety-critical drivetrain parts |
Selection rationale: - Choose 20Cr when cost sensitivity is significant and duty cycles are moderate—adequate case hardness with a ductile core at a lower material cost. - Choose 20CrMnTi when deeper hardenability, better core strength, improved fatigue life, and refined microstructure are required despite higher material cost and potentially more stringent heat-treatment control.
9. Cost and Availability
- Cost: 20Cr is typically the more economical option because it contains fewer alloying additions and simpler melt/processing control. 20CrMnTi commands a premium for additional Mn and Ti microalloying and tighter processing control.
- Availability: Both grades are commonly produced by steel mills supplying forgings, bars, and blanks in regions with heavy machinery manufacturing. 20Cr tends to be more widely available in standard bar and forging stock; 20CrMnTi may require ordering from mills or distributors that supply microalloyed carburizing steels.
- Product forms: both are available as bars, forgings, and machined blanks. Specify mill certificates and heat-treatment conditions to ensure traceability.
10. Summary and Recommendation
Summary table (qualitative):
| Attribute | 20Cr | 20CrMnTi |
|---|---|---|
| Weldability | Moderate (better than highly alloyed steels) | Slightly lower (higher CE and microalloy content) |
| Strength–Toughness balance | Adequate for standard duty | Improved core strength and toughness for similar case hardness |
| Cost | Lower | Higher |
Recommendation: - Choose 20Cr if: - The application requires a standard carburizing steel for moderate loads and fatigue demands. - Cost and broad availability are priorities. - Case depth requirements are shallow to moderate and conventional heat treatment suffices. - Choose 20CrMnTi if: - The part requires deeper hardenability, higher core strength, or superior fatigue resistance. - Grain refinement and improved temper resistance are important (e.g., high-cycle fatigue parts). - You accept a slightly higher material cost for better performance and can control heat treatment (quench, temper) precisely.
Final note: Always specify the required carburized case depth, surface hardness, core hardness/toughness, and any post-weld heat-treatment or surface protection in procurement and drawings. Confirm mill certificates and heat-treatment records for each lot: the practical performance depends primarily on the exact chemistry delivered and the rigor of the processing route rather than the nominal grade name alone.