GCr15 vs GCr18 – Composition, Heat Treatment, Properties, and Applications

Introduction

GCr15 and GCr18 are closely related high‑carbon chromium steels widely used for bearings, wear parts, and precision components. Engineers and procurement managers commonly weigh tradeoffs among achievable hardness, through‑hardening ability, wear resistance, and cost when selecting between them. Typical decision contexts include: specifying a bearing race where fatigue life and surface hardness are paramount, choosing a shaft or roller that requires deeper hardening, or optimizing purchase cost versus service life.

The principal metallurgical distinction between these grades is an increased chromium level in GCr18 relative to GCr15. That higher chromium concentration shifts the alloying balance toward greater hardenability and carbide formation, which in turn affects heat‑treatment response, wear behavior, and fabrication considerations. Because both are high‑carbon, chromium‑bearing steels intended for similar applications, they are often compared directly in design and manufacturing choices.

1. Standards and Designations

• Common international references and equivalents:
• GB (China): GCr15, GCr18 (Chinese national grades used in bearing and wear components).
• EN / ISO: 100Cr6 (EN) is commonly equated to GCr15/AISI 52100 in practice.
• JIS: SUJ2 is commonly compared with GCr15.

• ASTM/ASME: no one‑to‑one universal ASTM designation for these specific GB grades, but AISI 52100 is the common U.S. analogue to GCr15.

• Classification:

• Both GCr15 and GCr18 are high‑carbon chromium non‑stainless bearing steels (high‑carbon alloy steels focused on wear and fatigue resistance). They are not stainless grades, nor are they structural low‑alloy HSLA steels.

2. Chemical Composition and Alloying Strategy

Table: Typical composition (wt%, approximate; consult the specific standard or mill certificate for exact limits)

Element	GCr15 (typical)	GCr18 (typical)
C	0.95–1.05	0.95–1.05
Mn	0.25–0.45	0.25–0.45
Si	0.17–0.37	0.17–0.37
P	≤0.025 (max)	≤0.025 (max)
S	≤0.025 (max)	≤0.025 (max)
Cr	1.40–1.65 (approx.)	1.70–2.00 (approx.; higher than GCr15)
Ni	≤0.30 (trace)	≤0.30 (trace)
Mo	≤0.10 (usually absent)	≤0.10 (usually absent)
V, Nb, Ti, B, N	Trace or controlled impurities	Trace or controlled impurities

Notes: - Values above are indicative typical ranges used in industry practice; always verify against mill test certificates or the relevant GB/T specification. - The key compositional change is the deliberate elevation of chromium in GCr18 relative to GCr15; other elements remain comparable and generally low.

How alloying affects properties: - Carbon provides the basis for hardenability and achievable martensitic hardness; both grades are high‑carbon to support high hardness and wear resistance. - Chromium increases hardenability, carbides (chromium carbides), and temper resistance. Higher Cr improves through‑hardening and wear resistance, and raises tempering stability. - Manganese and silicon act as deoxidizers and modest hardenability contributors; trace alloying elements or microalloying (V, Nb) will influence fine carbide dispersion if present.

3. Microstructure and Heat Treatment Response

Typical microstructures and heat‑treatment responses: - Annealed / spheroidized condition: both grades are commonly delivered or processed into a spheroidized pearlitic or spheroidized ferrite + carbide structure to improve machinability and formability prior to final heat treatment. - Quenched and tempered condition: heat treatment produces martensite tempered to the required hardness with a dispersion of chromium‑rich carbides. The carbide morphology and volume fraction are influenced by Cr level; GCr18 tends to form a slightly higher fraction of stable carbides and may show finer or more numerous Cr‑carbides at comparable heat treatments. - Normalizing: restores a fine pearlitic/tempered microstructure before finish machining or hardening; effect similar for both grades. - Influence of higher Cr in GCr18: - Increased hardenability: GCr18 achieves deeper martensitic structures for the same quench severity or allows lower severity quench to reach a target hardness, improving uniformity in larger sections. - Carbide stability/volume: more Cr tends to stabilize carbides and can reduce temper softening for a given tempering temperature, which improves wear resistance but can reduce toughness if carbide size/continuity increases.

Thermo‑mechanical processing that refines prior austenite grain size and disperses carbides can benefit both grades; GCr18's higher Cr gives more margin for through‑hardening in thicker sections.

4. Mechanical Properties

Table: Comparative mechanical behavior (qualitative, dependent on heat treatment and section size)

Property	GCr15	GCr18
Tensile strength (hardened)	High; typical bearing steel range	Similar to slightly higher (due to higher hardenability)
Yield strength	High (Heat‑treatment dependent)	Similar to marginally higher in deeper‑quenched sections
Elongation (ductility)	Low to moderate after hardening	Similar or marginally reduced if carbide volume increases
Impact toughness	Generally better in comparable conditions (slightly more forgiving)	Slightly lower at equivalent hardness if carbide fraction increases
Hardness (hardened/tempered)	Can reach typical bearing hardnesses (very high)	Comparable peak hardness; easier to achieve through thickness

Interpretation: - Both grades are designed for high hardness and fatigue resistance. GCr18’s higher chromium content improves through‑hardening and temper stability, enabling similar or slightly higher tensile strength for thicker components or under milder quench regimes. However, increased carbide content can reduce notch toughness and ductility slightly, so designers must balance hardness and toughness based on application.

5. Weldability

Weldability considerations: - High carbon content in both grades limits weldability; preheat, controlled interpass temperature, and post‑weld heat treatment (PWHT) are commonly required to avoid cold cracking and brittle martensite in heat‑affected zones. - Increased hardenability (from higher Cr) in GCr18 increases the risk of hard, brittle HAZ microstructures following welding, requiring more conservative welding procedures than GCr15 in some cases.

Useful carbon equivalent formulas (interpret qualitatively — do not substitute for procedure qualification): - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Pcm for weldability assessment: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - Both grades yield high $CE_{IIW}$ and $P_{cm}$ relative to low‑carbon structural steels; the higher Cr in GCr18 increases these indices modestly, indicating a slightly higher propensity for HAZ hardening and cracking if welding is attempted without controls. - Practical recommendation: minimize welding on critical bearing surfaces; if welding is unavoidable, use qualified preheat, consumables with matching composition, narrow groove geometry, and PWHT to temper the HAZ.

6. Corrosion and Surface Protection

• Neither GCr15 nor GCr18 is stainless; corrosion resistance is limited and largely a function of surface finish, lubricants, and environmental controls.
• Standard protective approaches: oil or grease lubrication for bearings, phosphating for corrosion resistance before painting, hot‑dip galvanizing or painting for structural/wear components where appropriate.
• PREN (pitting resistance equivalent number) is a stainless‑steel index: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to GCr15/GCr18 because they are not stainless alloys (insufficient Cr and essentially no Mo/N to provide passive film formation).

Practical note: GCr18’s slightly higher Cr gives marginally better corrosion resistance in purely chemical terms, but the difference is small and irrelevant for environments requiring true corrosion resistance — such applications need stainless steels or surface coatings.

7. Fabrication, Machinability, and Formability

• Machinability:
• Both grades are challenging to machine in hardened condition; machining is normally performed in annealed or spheroidized condition to protect tooling life and dimensional accuracy.
• GCr18 may present slightly higher abrasive wear on cutting tools because of increased carbide content; tool material and cutting conditions should be selected accordingly (carbide inserts, coolant, appropriate feed/speed).
• Formability:
• High carbon content reduces ductility in the hardened condition; cold forming is limited and typically requires annealing first.
• For bending and forming operations, full anneal or spheroidized anneal is standard to avoid cracking.
• Surface finishing:
• Grinding and finishing operations for bearing surfaces are standard; higher Cr may increase wheel wear but also supports better wear resistance for the finished part.

8. Typical Applications

GCr15 (typical uses)	GCr18 (typical uses)
Precision ball and roller bearings (races, balls)	Bearings and rollers where deeper hardening or slightly improved wear resistance is needed
Shafting and spindles for machine tools	Heavier‑section rollers, shafts and wear rings requiring through‑hardening
Wear rings, bushings, cams (where high surface hardness required)	Components operating under higher contact stresses or larger cross sections
Precision hardened components requiring high fatigue life	Applications that benefit from improved temper resistance or marginally higher carbide content

Selection rationale: - Choose GCr15 when standard bearing steel performance, broad availability, and established processing routes are primary considerations. - Choose GCr18 when section thickness or geometry makes through‑hardening difficult for GCr15 or when a modest improvement in temper resistance/wear performance is desired and a small tradeoff in toughness is acceptable.

9. Cost and Availability

• Cost: GCr18 typically commands a modest premium over GCr15 due to the higher chromium content and more specialized demand. The price delta varies with alloying element market prices and supplier practices.
• Availability: GCr15 is extremely common and widely stocked in bar, ring, and finished bearing product forms. GCr18 is available but less ubiquitous — it may be stocked by specialty suppliers or produced to order for heavier or higher‑performance components.
• Product forms: both grades are available as hot‑rolled and cold‑drawn bars, rings, and forged blanks; finished bearing parts are a mature supply chain for GCr15.

10. Summary and Recommendation

Table: Quick summary

Attribute	GCr15	GCr18
Weldability	Challenging (high C); better than GCr18 for same conditions	Slightly worse due to higher hardenability
Strength–Toughness balance	Good balance for many bearing applications	Slightly higher strength/through‑hardening at cost of marginally lower toughness
Cost	Lower / widely available	Higher / less common

Recommendations: - Choose GCr15 if: - You require a proven bearing steel with mature processing routes and broad supplier availability. - The component is relatively thin or can be aggressively quenched so that through‑hardening is not limiting. - Cost and standardized supply are primary constraints.

• Choose GCr18 if:
• Section size, design, or quench limitations make deeper hardening desirable to ensure consistent properties through thickness.
• Application benefits from improved temper resistance or a modest increase in wear resistance and the design tolerates a small reduction in notch toughness.
• You accept a modest cost premium and potentially longer lead times for specialized supply.

Final note: Both grades require careful specification of heat treatment, surface finish, and lubrication regimes to realize fatigue and wear performance. For critical bearings or high‑reliability rotating components, work with material suppliers and heat‑treatment specialists to produce and qualify the exact condition (hardness profile, microstructure, residual stresses) required by the application.