GCr18 vs GCr18Mo – Composition, Heat Treatment, Properties, and Applications

Introduction

GCr18 and GCr18Mo are high‑carbon chromium bearing/alloy steels widely used for rolling elements, shafts, and wear‑resistant parts. Engineers, procurement managers, and manufacturing planners commonly weigh tradeoffs between fatigue life, wear resistance, hardenability, weldability, and cost when choosing between them. Typical decision contexts include selecting a grade for deep‑section bearings, designing components that require through‑hardening versus case hardening, or choosing a material that can be produced reliably in large cross sections.

The principal metallurgical difference is the deliberate addition of molybdenum to GCr18Mo to increase hardenability and improve wear and tempering resistance compared with plain GCr18. Because of this, both grades are compared in design and manufacturing where section size, heat‑treatment response, and service wear are critical.

1. Standards and Designations

• Common national and international equivalents and standards:
• GB (China): GCr18, GCr18Mo (Chinese national nomenclature for bearing/alloy steels).
• EN (Europe): Closely related to 100Cr6 / 1.3505 (for GCr18); a Mo‑bearing variant may be specified under modified 1.3505 grades or specific EN designations.
• AISI/SAE: 52100 / SAE 52100 is similar to GCr15/100Cr6; GCr18 is often compared to these bearing steels in chemistry and application, but exact equivalence depends on limits.
• JIS (Japan): Similar bearing steels appear in JIS but direct one‑to‑one designation mapping requires checking composition tables.
• Classification:
• Both GCr18 and GCr18Mo are high‑carbon, chromium‑bearing alloy steels typically used as bearing/tool/wear steels rather than stainless steels or HSLA structural steels.

2. Chemical Composition and Alloying Strategy

The table below shows typical element ranges for representative GCr18 and GCr18Mo formulations. Exact limits vary by standard, producer, and heat treatment specification; users should consult the mill certificate or applicable standard for procurement.

Notes: - Carbon is high to enable high hardness and wear resistance after quenching and tempering. - Chromium provides hardenability, wear resistance, and carbide formation. - Molybdenum in GCr18Mo is added in small amounts to increase hardenability and tempering resistance; it also refines the tempering response and reduces the risk of brittle quench microstructures in larger sections. - Minor elements (V, Nb, Ti) are typically present only in trace amounts or as deliberate microalloying depending on supplier practice.

How alloying affects properties: - C and Cr control achievable hardness and carbide structure—higher C increases hardness but reduces weldability. - Cr at the 1.3–1.7% level contributes to secondary hardening and wear resistance but does not make the steel stainless. - Mo increases hardenability, elevates tempering resistance (retains hardness at higher tempering temperatures), and can improve rolling contact fatigue life. - Mn and Si are deoxidizers and contribute modestly to hardenability and strength.

3. Microstructure and Heat Treatment Response

Typical microstructures and the effect of processing:

• As‑rolled/normalized:

• Both grades present a pearlitic or spheroidized carbide matrix depending on annealing practice. Normalizing/refining cycles produce a fine pearlite and retained carbides suitable for subsequent quench‑and‑temper treatment.

• Quenching and tempering:

• Typical hardening: austenitize (e.g., 780–840°C, depending on section and spec) then oil/quench or air/quench for specific geometries. Subsequent tempering produces tempered martensite with dispersed chromium carbides.
• GCr18: achieves high hardness and fine carbide distribution in smaller sections; in larger sections it is more prone to incomplete transformation (soft core) and higher risk of quench cracks if not carefully controlled.

• GCr18Mo: Mo raises hardenability, promoting a more through‑hardening martensitic structure in larger cross sections. Mo also shifts tempering resistance, so tempered hardness is better retained at higher tempering temperatures.

• Thermo‑mechanical processing:

• Thermo‑mechanical controlled processing and hot rolling affect prior austenite grain size. Finer austenite grains improve toughness and reduce required austenitizing temperatures; Mo helps preserve toughness in coarser sections by improving hardenability.

In summary, microstructural control for both grades centers on achieving a martensitic matrix with dispersed chromium carbides; GCr18Mo is more forgiving for larger sections and higher tempering temperatures.

4. Mechanical Properties

Mechanical properties depend strongly on the exact heat treatment. The table below gives typical post‑quench‑and‑temper ranges used for bearing/wear applications; treat these as representative rather than guaranteed values.

Interpretation: - Peak hardness and tensile strength are primarily controlled by carbon and heat treatment. Both grades attain similar peak hardness in small components. - GCr18Mo offers improved hardenability, so in larger parts it can achieve higher core hardness and strength compared with plain GCr18 processed identically. - Toughness differences are application and treatment dependent; Mo can improve toughness in heavier sections by enabling a more uniform martensitic response and by tempering microstructure stability.

5. Weldability

Weldability for high‑carbon, high‑hardness steels requires careful control because of high carbon equivalent and tendency to form hard, brittle martensite in the heat‑affected zone (HAZ).

Common weldability indices used qualitatively: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - 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: - Both GCr18 and GCr18Mo have high carbon; their $CE_{IIW}$ and $P_{cm}$ values will be high enough to warrant preheating, controlled interpass temperature, and post‑weld heat treatment for critical applications. - The presence of Mo increases $CE_{IIW}$ and $P_{cm}$ slightly; in other words, GCr18Mo is marginally more prone to HAZ hardening and cracking risk than GCr18 when welded without precautions. - For small repairs or non‑critical joints, use matching filler metals, preheat to reduce cooling rate, and perform PWHT (post‑weld heat treatment) to temper martensite in the HAZ. - Where welding must be minimized or eliminated, mechanical joining or machining to assembly fits is preferred.

6. Corrosion and Surface Protection

• Neither GCr18 nor GCr18Mo are stainless steels; their chromium content is insufficient to provide passivation in aqueous environments.
• Typical surface protection strategies:
• Protective coatings: painting, plating (zinc, nickel), or phosphate coatings.
• Galvanizing is an option for certain formed or structural components, though heat treatment after galvanizing is not typical.
• Lubrication and design for drainage are common for rolling contacts to reduce corrosion‑assisted wear.
• PREN (pitting resistance equivalent number) is not applicable to these non‑stainless bearing steels because PREN is used to assess stainless corrosion resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use corrosion‑resistant alloys (stainless bearing steels) when service requires both corrosion resistance and wear/fatigue performance.

7. Fabrication, Machinability, and Formability

• Machinability:
• High carbon and high hardness capability mean that both grades are more difficult to machine in hardened state. Annealed or spheroidized conditions are used for machining to improve tool life.
• GCr18Mo in the tempered/hardened condition may be marginally more difficult due to Mo‑strengthening effects, but differences are minor for pre‑machining conditions.
• Cold forming and bending:
• Not favorable in hardened condition; forming is done in softer annealed state.
• High carbon content limits deep drawing or extensive forming without cracking.
• Grinding and finishing:
• Both respond well to precision grinding after hardening. Carbide distribution and retained austenite can influence grinding behavior and final dimensional stability.
• Heat treatment considerations:
• Spheroidize anneal for machining: reduces cutting forces and prevents edge chipping.
• Controlled quench media, tempering schedules, and stress relief are critical to minimize distortion.

8. Typical Applications

Selection rationale: - Choose GCr18 for small to moderate‑section bearings and wear parts where conventional heat treatment achieves the required hardness and fatigue life economically. - Choose GCr18Mo when section size, expected operating loads, or higher tempering temperatures mean improved through‑hardening, higher retained hardness after tempering, or slightly better rolling contact fatigue life is required.

9. Cost and Availability

• Cost:
• GCr18 is typically less expensive than GCr18Mo because of the absence of molybdenum, a higher‑cost alloying element.
• The incremental cost of GCr18Mo is justified where hardenability, reduced scrap, or performance gains deliver lifecycle savings.
• Availability:
• Both grades are commonly available in bearing steel product forms (bars, rings, sheets, and forged blanks) through specialty steel suppliers.
• Lead times may be slightly longer for tight‑tolerance Mo‑bearing variants or for specific heat treatments; specify mill certificates and heat treatments when ordering.

10. Summary and Recommendation

Recommendation: - Choose GCr18 if you require a cost‑effective bearing/wear steel for small to moderate cross sections where standard quench and temper treatments achieve the required hardness and fatigue life. - Choose GCr18Mo if your components are larger in section, require more uniform hardening through the cross section, need improved tempering resistance, or will benefit from modest improvements in wear and rolling contact fatigue life that molybdenum provides.

Final practical notes: - Always specify exact chemical limits, heat‑treatment schedules, and acceptance testing (hardness, microstructure, non‑destructive testing) on purchase orders. - For welded assemblies or critical fatigue components, involve metallurgical process engineers early to define preheat, interpass temperatures, and required PWHT to avoid HAZ brittleness and service failures.