GCr15 vs 100Cr6 – Composition, Heat Treatment, Properties, and Applications

Introduction

GCr15 and 100Cr6 are two industrially important high‑carbon chromium bearing steels used globally for rolling‑element bearings, balls, rollers, races, and other wear‑resistant components. Engineers, procurement managers, and manufacturing planners routinely face a choice between them when specifying material for bearing components, high‑wear parts, or legacy assemblies. Typical decision drivers include compatibility with regional standards and specifications, availability in required product forms, heat‑treatment practice, and the balance between wear resistance, toughness, and manufacturability.

Although chemically and metallurgically equivalent in function, one designation is anchored in a national/regional standard system while the other follows a separate international/European standard; this leads to differences in ordering codes, documentation, and sometimes in batch traceability or supply chain logistics. Because both grades are optimized for high hardness and rolling fatigue resistance, they are often compared directly during design, sourcing or qualification.

1. Standards and Designations

• GCr15: Chinese standard designation commonly cited under GB/T standards for bearing steels. Equivalent in application to bearing steels standardized elsewhere.
• 100Cr6: European/EN designation for a chromium bearing steel widely used in EN countries and internationally; often treated as the EN equivalent to AISI 52100.
• Related standards and designation systems commonly encountered:
• EN (European): 100Cr6
• GB (China): GCr15
• AISI/SAE: 52100 (commonly used reference)
• JIS (Japan): SUJ2 (analogous composition/type)
• Classification: both are high‑carbon, chromium bearing steels (not stainless). They are classified as high‑carbon alloy tool/ bearing steels optimized for high hardness and rolling contact fatigue resistance.

2. Chemical Composition and Alloying Strategy

The following table summarizes typical compositional ranges and the alloying intent for each element. Both grades are engineered to the same compositional family; differences are primarily in designation and tolerances specified by the standards.

How alloying affects properties: - Carbon and chromium together enable formation of a tempered martensitic matrix with dispersed carbides (mainly cementite and chromium‑enriched carbides) that provide wear resistance and rolling‑contact fatigue strength. - Chromium increases hardenability and carbide stability; it also contributes minor corrosion resistance but not to the level of stainless steels. - The relatively low levels of other alloying elements keep chemistry simple, maintaining predictable heat‑treatment response and microstructure.

3. Microstructure and Heat Treatment Response

Typical microstructures and responses: - Annealed / spheroidized condition: The steel is often supplied or processed to a spheroidized/soft annealed microstructure for ease of machining. Microstructure consists of ferrite with globular carbides (spheroidized cementite/chromium carbides). - Quenched condition: After austenitizing and quenching (commonly oil quench for these grades), the matrix transforms to martensite with finely distributed carbides. Rapid quenching is used to achieve full martensite because of high carbon content. - Tempered condition: Tempering reduces brittleness and adjusts hardness; tempering temperature and time control the final hardness/toughness balance. Tempering causes secondary hardening phenomena to be limited (unlike high‑alloy steels), producing tempered martensite and tempered carbides optimized for rolling fatigue life.

Effect of heat‑treatment routes: - Normalizing may refine grain size but is not typically used alone for bearing components. - Spheroidizing anneal (soft anneal) is used prior to machining to maximize machinability. - Quench & temper is the standard route for final parts to achieve the required hardness and fatigue life. Fast cooling and appropriate tempering are critical because of the high carbon level—improper quench can produce retained austenite or cracking. - Thermo‑mechanical processing for bar production can influence inclusion morphology and cleanliness, which are important for fatigue life of bearings.

4. Mechanical Properties

Mechanical properties depend strongly on heat treatment; the table below gives comparative descriptors and typical hardness ranges rather than absolute single values.

Which is stronger/tougher/ductile: - In practical use both grades can be heat treated to essentially the same strength and hardness levels. Toughness and ductility are tuned primarily by the selected tempering temperature and the metallurgical quality (inclusions, segregation), rather than by small designation differences.

5. Weldability

Both GCr15 and 100Cr6 are considered challenging to weld due to the combination of high carbon content and chromium which increase hardenability. Hardenability raises the risk of forming hard martensitic microstructures in the heat‑affected zone (HAZ) that are susceptible to cold cracking.

Common weldability indices used to assess risk: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (international weldability index): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Both grades typically give relatively high carbon equivalent values because of ~1.0 wt% C and ~1.4 wt% Cr. High $CE_{IIW}$ or $P_{cm}$ values indicate a need for preheat, controlled interpass temperature, low hydrogen consumables, and in many cases post‑weld heat treatment (PWHT) to temper the HAZ and reduce cold cracking risk. - When welding is unavoidable, best practice is to weld in a soft (spheroidized) condition or to use specialized filler metals and controlled procedures followed by tempering PWHT.

6. Corrosion and Surface Protection

• Neither GCr15 nor 100Cr6 are stainless steels; they do not provide corrosion resistance comparable to stainless grades. The modest chromium content is primarily for hardenability and carbide formation, not continuous passive film formation.
• Typical protection strategies:
• Surface coatings: zinc galvanizing, electroplating, or specialized wear coatings.
• Painting, lacquers, or preservation oil for temporary protection.
• For rolling elements, surface lubrication and appropriate sealing to minimize corrosion and wear are essential.
• PREN is not applicable to these carbon chromium bearing steels, but for reference the PREN formula used for stainless alloys is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Apply that index only to stainless alloys that develop passive films; it is not meaningful for GCr15/100Cr6.

7. Fabrication, Machinability, and Formability

• Machinability: Best when the material is supplied in a soft annealed or spheroidized condition. In the hardened condition, machining is difficult and grinding or abrasive processes are used. Carbide tooling and appropriate speeds/feeds are required for pre‑hardened (annealed) stock operations.
• Formability: Bending and forming should be done in the soft condition. Cold forming in the hardened condition is not recommended except with specific processes.
• Finishing: Precision grinding and lapping are common to achieve geometry and surface finish required for bearing components. Surface integrity (avoidance of grinding burns) is crucial for fatigue performance.
• Surface treatments: Induction hardening or case hardening is not typical for through‑hardened bearing steels, but local induction hardening can be used for specific designs; most bearing components are through‑hardened and ground.

8. Typical Applications

Selection rationale: - Both grades are chosen for rolling‑contact fatigue resistance, high hardness, and wear performance. Selection between them is commonly driven by specification (regional standard preference), supply chain, documentation requirements, and traceability rather than major metallurgical differences.

9. Cost and Availability

• Cost: Material cost for both grades is broadly comparable because their compositions are similar. Prices depend on market conditions, alloying element costs, and processing (bar, ring, finished component).
• Availability: Availability tends to map to regional markets—100Cr6 is ubiquitous in Europe and among suppliers following EN standards, while GCr15 is commonly supplied in China and regions using GB standards. Both are produced worldwide and are available in bars, rings, sheets (limited), and finished components.
• Product form impacts lead time and cost—precision rings, calibrated balls or custom heat‑treated components carry higher lead times and processing premiums.

10. Summary and Recommendation

Recommendation: - Choose GCr15 if your supply chain, inspection and procurement are aligned with Chinese GB standards, or if you require locally certified materials and short lead times in markets where GCr15 is the standard designation. - Choose 100Cr6 if your project or assembly is governed by European/EN specifications, if you require consistency with EN documentation, or if supplier certification and traceability are organized around EN/AISI equivalents.

Final note: Metallurgically GCr15 and 100Cr6 fulfill the same functional role. The deciding factors in practice are specification compatibility, documentation and traceability, and the specific heat‑treatment/processing route your manufacturing or maintenance operation uses. For critical bearing or fatigue‑sensitive components, specify heat‑treatment cycles, hardness targets, inclusion cleanliness, and post‑processing inspection to ensure interchangeability irrespective of the local grade designation.