100Cr6 vs 100CrMo7 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
100Cr6 and 100CrMo7 are two high-carbon chromium-bearing steels commonly used for rolling-element bearings, shafts, and other wear-critical components. Engineers, procurement managers, and manufacturing planners regularly weigh trade-offs between cost, hardenability, toughness, and processing complexity when choosing between them. Typical decision contexts include whether higher through‑hardening and elevated-temperature strength justify slightly higher alloy cost and heat-treatment control, or whether the simpler chemistry of the baseline grade is preferable for established bearing practice.
The principal metallurgical distinction between these two grades is the deliberate addition of molybdenum in 100CrMo7 to improve hardenability and temper resistance. This single alloying change alters heat-treatment response, retained mechanical properties at elevated temperatures, and, to a lesser extent, weldability and cost — reasons why these grades are frequently compared in component design.
1. Standards and Designations
- 100Cr6
- Common international equivalents: EN designation 100Cr6 (material number 1.3505), ISO bearing steel; often matched to AISI 52100 in U.S. nomenclature.
- Category: High-carbon chromium bearing steel (tool/bearing steel family).
- 100CrMo7
- EN designation: 100CrMo7 (used in European specifications for alloyed bearing steels).
- Category: High-carbon chromium-molybdenum bearing/alloy steel (alloyed bearing/tool steel).
Relevant standards where these appear: EN (European), ISO (bearing steel standards), and various manufacturer specifications. They are not stainless steels; they are high-carbon, alloyed steels intended for hardening and tempering.
2. Chemical Composition and Alloying Strategy
The table below gives typical composition ranges (wt%) cited in commonly used specifications for these grades. Exact limits differ by standard and supplier; refer to the specific standard sheet for certified limits.
| Element | 100Cr6 (typical wt%) | 100CrMo7 (typical wt%) |
|---|---|---|
| C | 0.95 – 1.05 | 0.95 – 1.05 |
| Mn | 0.25 – 0.45 | 0.25 – 0.45 |
| Si | 0.10 – 0.40 | 0.10 – 0.40 |
| P | ≤ 0.025 | ≤ 0.025 |
| S | ≤ 0.025 | ≤ 0.025 |
| Cr | 1.30 – 1.65 | ~0.8 – 1.4 |
| Ni | ≤ 0.30 (traces) | ≤ 0.30 (traces) |
| Mo | ≤ 0.08 (trace) | 0.10 – 0.30 |
| V | Typically ≤ 0.05 | Typically ≤ 0.05 |
| Nb, Ti, B | Typically ≤ trace levels | Typically ≤ trace levels |
How the alloying strategy affects performance: - Carbon (near 1.0%) provides the matrix for high hardenability and achievable hardness after quench and temper; it is the primary driver of wear resistance. - Chromium (~1–1.6%) increases hardenability, contributes to carbide formation (improving fatigue and wear resistance), and refines grain when controlled. - Molybdenum (present in 100CrMo7 at modest levels) increases hardenability more effectively per weight than chromium, improves temper resistance (higher strength retention after tempering), and reduces the risk of quench cracking by allowing slower quench rates for a given core hardness target. - Manganese and silicon are present as deoxidizers and support strength/hardenability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-rolled/normalized: both grades present a pearlitic or ferrito-pearlitic microstructure depending on cooling rate and prior processing. - After quench and temper: martensitic matrix with a population of chromium-rich carbides. 100Cr6 typically forms fine, evenly distributed chromium carbides; 100CrMo7 shows similar carbide chemistry but with molybdenum partitioning to the matrix and carbides, which stabilizes carbides and refines temper response.
Heat-treatment behavior: - Normalizing improves grain size and homogenizes microstructure for both grades. - Hardening (austenitizing followed by quench) transforms the microstructure to martensite. Because molybdenum raises effective hardenability, 100CrMo7 achieves greater through‑hardening (deeper core hardness) for a given section and quench severity than 100Cr6. - Tempering reduces martensitic strength while improving toughness. Molybdenum in 100CrMo7 increases temper resistance, meaning at the same temper temperature 100CrMo7 will retain somewhat higher strength/hardness than 100Cr6 while suffering less softening at elevated tempering temperatures. - Thermo‑mechanical processing (controlled rolling and accelerated cooling) can further refine carbides and martensite in both grades; the Mo-bearing alloy benefits more in thick sections because of improved core hardenability.
4. Mechanical Properties
Mechanical properties depend strongly on heat treatment (hardness target) and section size. The following table shows representative property ranges for hardened-and-tempered conditions typical for bearing applications.
| Property | 100Cr6 (typical, tempered/hardened) | 100CrMo7 (typical, tempered/hardened) |
|---|---|---|
| Tensile Strength (MPa) | ~1000 – 2200 (depending on hardness) | ~1100 – 2300 |
| Yield Strength (MPa) | Not always specified for bearing steels; approximated lower than tensile | Slightly higher at equivalent hardness due to Mo |
| Elongation (%) | 5 – 15 (decreases with increasing hardness) | 5 – 15 (similar ranges) |
| Impact Toughness (Charpy, J) | Lower at very high hardness; moderate with tempered conditions | Typically modestly higher toughness at similar core hardness in larger sections owing to better through-hardening |
| Hardness (HRC) | Typically 58 – 66 HRC for bearing races/balls | Typically 58 – 66 HRC; easier to achieve core hardness in larger sections |
Interpretation: - Strength and hardness achievable are comparable when both are fully hardened; however, 100CrMo7 often attains equivalent or slightly higher core hardness in larger parts because of increased hardenability. - Toughness at a given surface hardness can be better for 100CrMo7 in thicker sections because the core is less likely to be soft and ductile than in 100Cr6 when quench is less severe. - Ductility is limited in both grades due to high carbon; designers should avoid over‑engineering thin sections expecting ductile failure modes.
5. Weldability
Weldability is limited for both grades because of near‑1% carbon content and significant hardenability; preheat and post‑weld heat treatment (PWHT) are commonly required.
Useful carbon-equivalent formulas: - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$ - Dearden and 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: - Both grades produce high $CE_{IIW}$ and $P_{cm}$ values due to high carbon and alloying; this indicates high cracking risk without careful control. - 100CrMo7 typically scores slightly higher on hardenability terms because of molybdenum; this can translate to greater susceptibility to cold cracking in weld heat‑affected zones if the same welding procedures are used. Accordingly, 100CrMo7 generally requires more conservative preheat and slower cooling or mandatory PWHT compared with 100Cr6. - For repairs or welded fabrications, consider alternative designs (mechanical fastening, brazing) or specialist welding procedures performed by qualified welders with post‑weld tempering.
6. Corrosion and Surface Protection
Neither 100Cr6 nor 100CrMo7 are stainless steels; their chromium content (≈1–1.6%) is insufficient to confer stainless behavior. Corrosion protection strategies used in industry include: - Surface coatings: electroplating (zinc, nickel), physical vapor deposition for tooling, conversion coatings. - Galvanizing (for parts where geometry permits and Zn coating is acceptable). - Painting and oil/grease lubrication for bearings and shafts. - For contact with aggressive environments, nitriding or case-hardening plus sacrificial coatings can extend service life.
PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels. If corrosion resistance is a primary design driver, switch to stainless bearing grades (then evaluate using, for example, $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ ) rather than attempting to rely on 100Cr6 or 100CrMo7.
7. Fabrication, Machinability, and Formability
- Machinability: In the annealed condition (soft), both grades machine similarly; the high-carbon content and carbide population reduce machinability relative to low‑carbon steels. Tooling (carbide inserts, rigid setups, proper cutting speeds) and frequent tool changes are common for hardened parts.
- Formability: Cold-forming is limited when the steel is in a fully annealed but still carbon‑rich state; hot forming or forging is typical before final heat treatment. Bending and stamping are not recommended in hardened condition.
- Finishing: Grinding and superfinishing are standard for bearing surfaces. 100CrMo7’s carbide stability may slightly increase tool wear during grinding versus 100Cr6, but benefits are realized in service life.
8. Typical Applications
| 100Cr6 (typical applications) | 100CrMo7 (typical applications) |
|---|---|
| Rolling-element bearings (balls, rollers, races) for general industrial applications | Bearings and bearing components for larger sections or where deeper core hardness is required |
| Shafts, spindles, and precision components where surface hardness and wear resistance are primary | Heavier-duty shafts, large rollers, and components requiring improved through‑hardening and temper resistance |
| Gears and tooling inserts in small-to-medium sizes (with appropriate heat treatment) | Parts exposed to higher operating temperatures or cyclic loads where temper softening resistance is beneficial |
| High-wear pins and bushings in controlled environments | Components where thicker sections must achieve uniform hardness without extreme quench severity |
Selection rationale: - Choose 100Cr6 where small-to-medium parts with well-controlled quench conditions will achieve the required surface and core properties economically. - Choose 100CrMo7 where part geometry or service demands require greater through‑hardening, improved temper resistance, or slightly better toughness in larger sections.
9. Cost and Availability
- Cost: 100CrMo7 is typically more expensive per kilogram than 100Cr6 due to molybdenum content. The delta is modest for small batch purchases but may be significant for high-volume production.
- Availability: 100Cr6 (AISI 52100) is one of the most widely available bearing steels worldwide, supplied in bars, rings, and finished balls. 100CrMo7 is widely available but may be less ubiquitous in some markets and product forms; certain bar sizes and specialty forgings may have lead times.
- Product forms: Both are available as bars, rings, and forgings; specialized suppliers provide vacuum-degassed, high‑cleanliness variants for fatigue‑critical bearings.
10. Summary and Recommendation
Summary table (qualitative):
| Attribute | 100Cr6 | 100CrMo7 |
|---|---|---|
| Weldability | Moderate-to-poor; high preheat/PWHT needed | Slightly worse; higher hardenability increases cracking risk |
| Strength–Toughness at section | High surface hardness; core hardness dependent on quench | Comparable surface hardness; better through‑hardening and temper resistance |
| Cost | Lower | Higher (due to Mo) |
Concluding recommendations: - Choose 100Cr6 if you need a well-established, cost-effective bearing steel for small to medium components where standard quenching practices reliably produce the required surface and core hardness. It is the industry workhorse for many rolling-element applications. - Choose 100CrMo7 if your components are thicker or larger, require more uniform core hardness, or will operate at temperatures and tempering conditions where improved temper resistance and slightly higher retained strength are advantageous — and when the modest increase in material cost and stricter heat-treatment/welding control are acceptable.
Final note: The exact selection should be validated against part geometry, expected service loads, required fatigue life, and available heat‑treatment and finishing capabilities. For critical parts, request certified chemical and mechanical test reports from suppliers and consider fatigue testing on representative samples.