20MnTi vs 20CrMnTi – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers and procurement professionals often choose between 20MnTi and 20CrMnTi when specifying medium‑carbon steels for components that require a balance of strength, toughness, and wear resistance. Typical decision contexts include whether to prioritize through‑hardness and weldability for structural parts, or to prioritize case strength and contact fatigue resistance for gears and shafts after surface hardening.
The principal distinction between the two grades is the intended alloying and hardenability strategy: 20MnTi is a manganese‑titanium stabilized medium‑carbon steel optimized for good mechanical properties and toughness, while 20CrMnTi is a chromium‑bearing variant formulated for improved hardenability and case hardening performance. Because both are used for similar components (shafts, gears, pins), they are commonly compared during material selection for cost, heat‑treatment route, and service conditions.
1. Standards and Designations
- Common standards where these grades appear (nomenclature and exact chemistry vary by standard body):
- GB (China): 20MnTi, 20CrMnTi (common Chinese designations for medium carbon alloy steels).
- JIS (Japan), EN (Europe), ASTM/ASME (USA): Equivalent grades or nearest alternatives exist under different names; direct one‑to‑one equivalence requires checking the specific composition tolerances.
- Classification:
- 20MnTi: Classified as a medium‑carbon alloy steel (not stainless, not tool steel); micro‑alloyed with Ti for grain refinement/stabilization.
- 20CrMnTi: Classified as a medium‑carbon low‑alloy, case‑hardening (carburizing) steel with chromium and micro‑alloying (Ti); optimized for carburized surface hardness and a tough core.
2. Chemical Composition and Alloying Strategy
Table: typical composition ranges (wt%). These are indicative industry ranges used to guide specification and are not a substitute for the exact limits given in a particular standard or procurement specification.
| Element | 20MnTi (typical range, wt%) | 20CrMnTi (typical range, wt%) |
|---|---|---|
| C | 0.16 – 0.24 | 0.16 – 0.24 |
| Mn | 0.60 – 1.10 | 0.60 – 1.20 |
| Si | 0.15 – 0.35 | 0.15 – 0.35 |
| P | ≤ 0.035 | ≤ 0.035 |
| S | ≤ 0.035 | ≤ 0.035 |
| Cr | ≤ 0.30 (trace) | 0.60 – 1.20 |
| Ni | ≤ 0.30 | ≤ 0.30 |
| Mo | ≤ 0.08 | ≤ 0.08 |
| V | trace ≤ 0.10 | trace ≤ 0.10 |
| Nb | trace ≤ 0.03 | trace ≤ 0.03 |
| Ti | 0.02 – 0.06 | 0.02 – 0.06 |
| B | – | – |
| N | not typically specified | not typically specified |
How the alloying elements influence performance: - Carbon controls hardness potential and strength; both grades are medium carbon to enable through‑hardening and strong tempered cores or effective case hardening. - Manganese increases hardenability and tensile strength; typical levels are similar in both grades. - Chromium in 20CrMnTi increases hardenability and improves carbide formation during carburizing, supporting higher achievable case hardness and better wear resistance. - Titanium functions as a deoxidizer and forms carbonitrides that refine grain size and tie up nitrogen, improving toughness and resistance to intergranular embrittlement. - Silicon, molybdenum and small additions of vanadium or niobium can further affect hardenability, tempering resistance and grain size control depending on mill practice.
3. Microstructure and Heat Treatment Response
Microstructures depend on composition and heat treatment route:
- 20MnTi:
- Typical microstructure after normalization or quench & temper: tempered martensite/tempered bainite with retained ferrite/pearlite constituents depending on cooling rate and section size.
- Microalloying with Ti refines austenite grain size prior to transformation, improving toughness.
-
Responds well to direct quench & temper cycles; achieves a balance of strength and ductility without extensive surface hardening processes.
-
20CrMnTi:
- Designed for carburizing: a low‑to‑medium carbon core chemistry with Cr to promote hardenability of the near‑surface layer after carburizing and quenching.
- After carburizing + quenching + tempering: case microstructure is martensitic (high hardness), core is tempered martensite or ferrite/pearlite depending on processing, engineered to have a ductile core to resist crack propagation.
- Cr promotes formation of alloy carbides and increases hardenability so thicker sections can obtain a hard case with a tough core.
Effects of specific heat treatments: - Normalizing: refines microstructure, modest strength increase; useful as a preparatory step. - Quench & temper: increases strength and toughness; both steels respond, but 20CrMnTi gains more in case hardness when carburized prior to quench. - Carburizing (20CrMnTi): introduces a high carbon surface case enabling very high surface hardness after quench; 20MnTi is less commonly used for deep carburized applications because it lacks the higher Cr hardenability.
4. Mechanical Properties
Table: indicative mechanical properties after typical processing. Values are representative ranges used in industry; final properties depend on precise heat treatment, section size, and exact chemistry.
| Property (typical condition) | 20MnTi (normalized or QT) | 20CrMnTi (carburized case + tempered core or QT) |
|---|---|---|
| Tensile strength (MPa) | ~400 – 650 | Core: ~600 – 900 (post‑carburize and QT depends); surface much higher after carburizing |
| Yield strength (MPa) | ~250 – 420 | Core: ~350 – 700 (varies by treatment) |
| Elongation (%) | ~12 – 20 | Core: ~8 – 18 (carburized parts often trade ductility for case hardness) |
| Impact toughness (J, room temp) | Generally good — higher than carburized counterparts of same hardness | Core toughness engineered to be high; case is hard and less tough |
| Hardness (HRC or HB) | As‑normalized: ~170–240 HB; after QT: can be higher (HRC scale variable) | Case hardness after carburizing: can exceed 58–64 HRC locally; core typically 200–260 HB (varies) |
Which is stronger, tougher, or more ductile: - Strength: In core properties after heavy quench & tempering, 20CrMnTi may achieve comparable or higher strength due to Cr enhanced hardenability, especially after carburizing where surface hardness is much higher. - Toughness: 20MnTi often exhibits better through‑toughness in through‑hardened conditions unless 20CrMnTi is specifically heat treated to optimize core toughness; however, carburized 20CrMnTi provides a tough core with a very hard wear‑resistant case—a desirable combination for contact fatigue applications. - Ductility: 20MnTi tends to display higher ductility in through‑hardened conditions (no carburized hard case).
5. Weldability
Weldability depends mainly on carbon equivalent and microalloying content. Use of carbon‑equivalent assessments helps predict preheat/post‑weld heat‑treatment requirements.
Common carbon equivalent formulas: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Dearden & O'Neill / Pcm formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 20MnTi: moderate carbon and limited alloy content usually yield moderate carbon equivalents and generally acceptable weldability with standard preheat and controlled interpass temperatures. Ti microalloying can complicate weld filler selection marginally but generally the grade is weldable for many fabrications. - 20CrMnTi: Cr increases the carbon equivalent and hardenability, so weldability is lower than 20MnTi in general. Carburized components require special weld procedures, preheat and post‑weld heat treatment to avoid hydrogen cracking and to restore core properties. For repair welding of carburized surfaces, follow appropriate preheat/PWHT and use compatible filler metals.
6. Corrosion and Surface Protection
- Both 20MnTi and 20CrMnTi are non‑stainless, low‑alloy steels; they are susceptible to general corrosion and require protective coatings or environmental control in corrosive service.
- Common protections: painting, solvent‑borne or powder coatings, phosphating, and hot‑dip galvanizing; choice depends on geometry and post‑heat‑treatment requirements (note: galvanizing after carburizing/quenching may be impractical for some applications).
- PREN (pitting resistance equivalent number) is not applicable to these non‑stainless grades, but for reference, stainless assessments use: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- If corrosion resistance is a primary design driver, select stainless or corrosion‑resistant alloys rather than these carbon/alloy steels.
7. Fabrication, Machinability, and Formability
- Machinability:
- 20MnTi: medium machinability typical of medium‑carbon steels; machinability can be improved after appropriate annealing or normalization.
- 20CrMnTi: slightly lower machinability if Cr content is higher or if material is carburized/hardened; machining of hardened case requires grinding rather than conventional cutting.
- Formability and bending:
- Both grades are formable in annealed or normalized condition; 20MnTi is slightly more forgiving for forming due to slightly lower hardenability.
- After heat treatment (QT or carburized), formability and bendability decrease substantially.
- Surface finishing:
- Grinding and polishing are common for carburized 20CrMnTi components to meet surface finish and tolerance requirements.
8. Typical Applications
| 20MnTi — Typical Uses | 20CrMnTi — Typical Uses |
|---|---|
| Shafts, pins, fasteners, structural parts where moderate strength and good toughness are required; forgings and axles that are through‑hardened or quenched & tempered | Gears, gear shafts, sprockets, cams, heavy duty pins, splines and bearings requiring a hard, wear‑resistant case with a tough ductile core (carburized and quenched) |
| General mechanical components requiring good machinability and lower cost heat treatment | High contact stress components where a hard surface and fatigue resistance are critical |
Selection rationale: - Choose 20MnTi when the component requires uniform properties through the section, simpler heat treatment, or when weldability and lower cost are priorities. - Choose 20CrMnTi when surface wear, contact fatigue, and the need for a hard case with a ductile core drive the decision; it is the usual choice for carburized gears and high‑contact components.
9. Cost and Availability
- Relative cost:
- 20MnTi: generally lower material cost because of simpler chemistry and widespread production; machining and heat treatment costs are moderate.
- 20CrMnTi: marginally higher material cost due to Cr addition and the common requirement for carburizing and more complex heat treatment; overall fabricated part cost can be higher because of processing (carburizing furnace time, quench oil, grinding).
- Availability by product form:
- Both grades are commonly available in bar, forging and rolled product forms in regions with established industrial steel production; 20CrMnTi may be more frequently stocked in forms targeted for carburizing (bars for gears, shafts).
10. Summary and Recommendation
Table summarizing qualitative attributes:
| Attribute | 20MnTi | 20CrMnTi |
|---|---|---|
| Weldability | Good (moderate CE) | Fair to poor (higher hardenability; requires more care) |
| Strength–Toughness balance | Good through‑hardened balance | Excellent case/core combination after carburizing; core toughness engineered |
| Cost (material + processing) | Lower | Higher (due to Cr and heat‑treatment processing needs) |
Recommendation: - Choose 20MnTi if you need a cost‑effective, weldable medium‑carbon steel with good through‑section toughness and straightforward heat treatment (quench & temper or normalization), and when no heavy surface hardening is required. - Choose 20CrMnTi if the design requires a wear‑resistant hard case with a tough ductile core (e.g., gears, camshafts, heavily loaded pins) and you can accommodate carburizing/quenching/tempering and associated process controls and costs.
Final note: Always confirm the exact chemical and mechanical limits in the procurement specification or applicable standard for your region and application. Heat‑treatment schedules, section size, and intended service environment will materially affect the final performance of either grade.