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Heat Treatments: Nitriding

Nitriding is a thermo-chemical surface hardening treatment applied to steels—and other alloys as well—aimed at combating abrasive, adhesive, and erosive wear, rolling/sliding contact, and mechanical or thermomechanical fatigue.
This process chemically alters the steel surface by diffusing atomic nitrogen into the iron matrix, leading to the formation of nitrides.

Figure 1 – Typical microstructure of the surface of a nitrided component – Courtesy of TTN SpA
Figure 1 – Typical microstructure of the surface of a nitrided component – Courtesy of TTN SpA

Steels and the Hardening Mechanism

Many steels can undergo nitriding, but the process is typically applied to quenched and tempered steels or nitriding steels containing nitride-forming alloying elements such as Al, Cr, Mo, V, and Nb.
These elements are responsible for the post-treatment hardness of the material.

Graph – Effect of nitride-forming alloying elements on achievable hardness after nitriding
Graph – Effect of nitride-forming alloying elements on achievable hardness after nitriding

The resulting case depths are typically in the range of tenths of a millimetre. For hot work steels, hardness levels between 1000 and 1100 HV can be achieved.

Figure 3 – Micrograph at 1500X of Ti-6Al-4V titanium alloy with 2μm TiN composite layer – Courtesy of TAG Srl
Figure 2 – Microstructure and hardness indentations in cross-section of a tool steel component after nitriding process

Types of Nitriding

Nitriding can be performed using different methods, each with specific characteristics:

  • Salt Bath Nitriding
  • Gas Nitriding (in dissociated ammonia)
  • Plasma Nitriding (also called Ionic Nitriding)
Figure 3 – Micrograph at 1500X of Ti-6Al-4V titanium alloy with 2μm TiN composite layer – Courtesy of TAG Srl
Figure 3 – Micrograph at 1500X of Ti-6Al-4V titanium alloy with 2μm TiN composite layer – Courtesy of TAG Srl

Advantages and Critical Aspects

One of the main advantages of nitriding is the low process temperature (480–550 °C), which avoids changes to the microstructure.
This allows near-net shape components to be treated without distortion or significant dimensional changes.

However, the process must be precisely controlled to prevent the excessive formation of nitrides, which may accumulate along grain boundaries in a network structure and increase brittleness.

Figure 4 – Examples of over-nitriding
Figure 4 – Examples of over-nitriding

Conclusion

Nitriding is an effective heat treatment for increasing the surface strength of steels and other alloys, without introducing significant distortion.
Selecting the correct cycle and tightly controlling process parameters is essential to achieving the desired benefits without compromising mechanical properties.

Bonomi Acciai, Italian representative of the Swiss Steel Group, supports engineers and heat treaters in selecting the most suitable material and treatment for each application.

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Daniele Magistroni

Daniele Magistroni

Powder & Cold Work Tool Steels Application Manager

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    Daniele Magistroni

    Powder & Cold Work Tool Steels Application Manager

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