AFNOR 20NCD2 Chrome Moly Alloy Steel Bars

Product Description

AFNOR 20NCD2 is a classic French standard (Association Française de Normalisation) low-alloy structural steel. While it contains chromium and molybdenum, it is technically a Nickel-Chromium-Molybdenum triple-alloy steel formulated specifically for case-hardening (carburizing).

If you are sourcing or machining 20NCD2 bar stock, it is the exact historical French equivalent to the globally ubiquitous AISI 8620 or European 20NiCrMo2-2. It is chosen for high-wear, high-fatigue components that require a glass-hard outer shell but an incredibly tough, shatter-proof core.

1. Decoding the Alphanumeric Name

The French AFNOR naming convention tells you exactly what is inside the bar:

• 20: Represents $\sim 0.20\%$ average Carbon content ($0.17\% - 0.23\%$). This low initial carbon is deliberate, ensuring the core remains ductile and doesn't get brittle during quenching.

• N (Nickel): Promotes high core toughness and shock resistance.

• C (Chromium): Enhances depth of hardenability and wear resistance.

• D (Molybdenum): Increases core strength and prevents temper brittleness.

• 2: Indicates the major alloying elements are present at low, balanced levels (typically under $1\%$ each).

2. Chemical Composition Breakdown

3. Core Mechanics & Heat Treatment Strategy

Because 20NCD2 bars have relatively low baseline carbon, you cannot get them fully hard straight out of the box by standard quenching. It is designed to undergo a multi-step thermal cycle:

The Carburizing Process

Components are machined in their soft state, then exposed to a carbon-rich atmosphere at $880^\circ\text{C} - 950^\circ\text{C}$. Carbon diffuses into the outer layer (case), raising the surface carbon content to $\sim 0.80\%$.

Quenching and Tempering (+QT)

Following carburization, the bars are oil or water quenched (typically from $810^\circ\text{C} - 830^\circ\text{C}$) and low-temperature tempered at $150^\circ\text{C} - 200^\circ\text{C}$.

• The Result: A surface hardness of $58 - 63 \text{ HRC}$ (excellent wear and abrasion resistance) wrapped around a core with a tensile strength of $700 - 1100 \text{ MPa}$ (retains elasticity to prevent fracturing under load).

4. Common Technical Applications

AFNOR 20NCD2 bars are heavily utilized for components that see extreme contact friction paired with sudden torque spikes:

• Automotive & Drivetrain: Heavy-duty gearboxes, driving bevel gears, crown wheels, splined shafts, and universal joints.

• Heavy Equipment: Link pins, high-speed bearings, steering knuckles, and heavy-duty transmission components for agricultural and mining vehicles.

• Fasteners: High-stress safety bolts and anchor pins subject to shear forces.

5. Direct Global Equivalents

If you need to source this material locally or cross-reference mill certificates, 20NCD2 matches seamlessly with:

• United States: AISI 8620 / SAE 8620 / UNS G86200

• Europe / Germany: 20NiCrMo2-2 / W.Nr. 1.6523

• United Kingdom: BS 805M20

• Japan: JIS SNCM220 / SNCM220H

• Russia: GOST 20KhGNM (20ХГНМ)