Carburizing Steel: Practical Guidance for Design Engineers

Carburizing is one of the most widely used heat treatment processes in mechanical systems, particularly when a component needs very high surface hardness combined with a tough, ductile core. It is especially common in gears, shafts, and wear-critical components where through hardening would either create excessive brittleness or unacceptable distortion.

For newer engineers, carburizing is often confusing because it's described as “surface hardening,” yet it involves both carbon diffusion and quench hardening. Understanding how those two steps interact—and why certain steels are chosen specifically for carburizing—makes the process much easier to apply correctly.

What Carburizing Actually Is

Carburizing is a thermochemical heat treatment that increases the carbon content at the surface of a steel part.

In simplified terms:

1. The part is heated into the austenitic range
2. Carbon is introduced at the surface (gas, vacuum, or solid media)
3. Carbon diffuses into the surface over time
4. The part is quenched, transforming the high-carbon surface into hard martensite
5. The part is tempered to reduce brittleness

The result is:

• A very hard outer case
• A lower-carbon, tougher core
• A gradual hardness transition between the two

Why Carbon Content Matters So Much

Carburizing works because it changes the surface carbon content of the steel.

This is why:

• Plain low-carbon steels (≤0.25% C) are commonly carburized
• Special alloy steels with ~0.20% carbon are widely used for carburizing applications

The low-carbon core remains tough and impact-resistant, while the carburized surface can reach very high hardness levels after quenching.

Common Steels Used for Carburizing

Plain Low-Carbon Steels

Examples:

• 1018
• 1020

These steels carburize readily and are inexpensive. They are commonly used for moderate-duty parts where toughness is more important than maximum strength.

Limitations:

• Lower core strength
• Less control over hardenability in thicker sections

Carburizing Alloy Steels (~0.20% Carbon)

These are purpose-designed for carburizing and are extremely common in machinery.

8620

• ~0.20% carbon
• Nickel, chromium, and molybdenum alloyed
• Excellent toughness and fatigue resistance
• Very predictable carburizing behavior

8620 is often the default choice for gears, splines, and shafts that require a hardened case and a durable core.

4120

• Similar carbon level to 8620
• Lower nickel content
• Good hardenability and wear resistance
• Often used where cost is a concern

Other carburizing steels follow the same pattern: low carbon, alloyed for hardenability, and optimized for controlled case depth and predictable quenching response.

Case Depth and Hardness

Carburizing produces a deep hardened case compared to nitriding.

Typical values:

• Case depth: ~0.020-0.080 in (0.5-2.0 mm)
• Surface hardness: 58-62 HRC (sometimes higher)

Case depth is controlled by:

• Temperature
• Time
• Carbon potential of the atmosphere

Unlike nitriding, carburizing can produce deep wear layers suitable for long service life under load.

Dimensional Change and Distortion

Carburizing always involves quenching, which introduces risk.

Key considerations:

• Distortion is common, especially on long or asymmetric parts
• Gears and shafts often require post-heat-treat grinding
• Allowances should be designed in from the start

This is one of the main trade-offs compared to nitriding, which avoids quenching altogether.

What Carburizing Does Well

Very high surface hardness

Excellent for wear, rolling contact, and pitting resistance.

Tough core

Low-carbon core absorbs shock and impact.

Deep wear layer

Much deeper than nitriding, making it suitable for long-life components.

Excellent fatigue performance

Especially in gears and cyclically loaded components.

Limitations and Trade-Offs

Distortion risk

Quenching almost always requires follow-up machining or grinding.

Process complexity

Carburizing cycles are long and tightly controlled.

Material selection matters

Steels not intended for carburizing may produce unpredictable results.

Not ideal for finished parts

Most carburized components are heat treated before final finishing.

Carburizing vs Through Hardening vs Nitriding (High-Level)

• Through hardening: uniform strength, limited surface hardness
• Carburizing: deep hard case, tough core, distortion risk
• Nitriding: shallow hard case, minimal distortion

Each process solves a different design problem. Carburizing is chosen when deep wear resistance and fatigue life outweigh the challenges of distortion control.

Typical Applications

• Gears and pinions
• Splined shafts
• Camshafts
• Transmission components
• Heavy-duty wear parts
• Automotive and industrial drivetrains

These components benefit from a hard surface that resists wear and contact stress, paired with a core that won't crack under load.

When Carburizing Is the Right Choice

Carburizing is often the best option when you need:

• Very high surface hardness
• Deep wear resistance
• High fatigue strength
• A tough, impact-resistant core

When tight tolerances or minimal distortion dominate the design, nitriding or through hardening may be better alternatives.