Nitriding steels

02Definition and Fundamental Characteristics of Nitriding Steels

Nitriding steels are iron-carbon alloys specially formulated with specific alloying elements such as chromium, aluminium, molybdenum and vanadium, designed to respond optimally to the thermochemical processes of surface nitrogen enrichment. The distinctive feature of these materials lies in their ability to develop an extremely hard, wear-resistant surface layer while simultaneously maintaining a tough core that is resistant to mechanical shock.

The mechanical properties of nitriding steels resulting from the process combine high surface hardness (typically 700-1100 HV, up to 1200 HV for high-aluminium steels under optimal conditions), superior wear resistance, excellent dimensional stability and improved corrosion resistance, characteristics that make them irreplaceable for critical applications in the automotive and aerospace industries and in the production of specialised tools.

2.1Principles of the Nitriding Process

The nitriding process consists of the controlled surface enrichment of nitrogen through atomic diffusion at temperatures between 480 °C and 570 °C. At these relatively low temperatures, atomic nitrogen penetrates the crystalline structure of the steel, forming nitrides with the alloying elements present and creating a compound layer and an underlying diffusion zone characterised by finely dispersed nitride precipitates.

The nitriding depth can range from 0.1 mm up to 0.8 mm depending on the treatment time, the temperature and the composition of the base steel. The hardness gradient from the surface to the core ensures an optimal distribution of stresses and prevents delamination or flaking of the nitrided layer.

2.2Optimised Chemical Composition

The chemical composition of nitriding steels is designed to maximise the effectiveness of the thermochemical process. The key elements include:

Chromium (Cr): A crucial element present in concentrations from 1% to 5%, it forms extremely stable CrN nitrides that contribute significantly to surface hardness and corrosion resistance.

Aluminium (Al): Present in contents of 0.8-1.5%, it forms AlN nitrides characterised by high hardness and thermal stability, particularly effective for high-temperature applications.

Molybdenum (Mo): It contributes to the formation of complex nitrides and improves the hardenability of the core, ensuring uniform mechanical properties even on large cross-sections.

Vanadium (V): It forms extremely hard carbonitrides that increase resistance to abrasive wear, particularly important for cutting tools and dies.

2.3Mechanisms of Nitride Formation

Nitride formation occurs through interstitial and substitutional diffusion mechanisms of nitrogen within the ferritic matrix. Nitrogen atoms, owing to their small size, initially position themselves in the interstitial sites of the crystalline structure, subsequently reacting with the alloying elements to form nitrides that are coherent or semi-coherent with the matrix.

The kinetics of nitride formation are governed by Fick's laws of diffusion, with diffusion coefficients that vary exponentially with temperature according to the Arrhenius equation. The presence of nitride-forming elements facilitates the nucleation and growth of the precipitates, optimising the efficiency of the process.

03Classification of Nitriding Steels According to International Standards

3.1European ENStandards for Nitriding Steels

The classification of nitriding steels at the European level is mainly regulated by standard EN ISO 683-5:2021 (formerly EN 10085) “Nitriding steels - Technical delivery conditions”, which establishes chemical compositions, mechanical properties and quality requirements.

The European classification uses the alphanumeric system that identifies the chemical composition according to EN 10027, with specific designations for steels with a high content of nitride-forming elements such as chromium and aluminium.

3.2ASTM and JIS Standards for Nitridable Steels

International standards for nitriding steels include regional technical specifications, with the SAE-AISI system using four-digit numerical designations and the JIS system using alphanumeric designations with specific prefixes. These standards show substantial correspondence with the European standards, while retaining regional specificities in their designation systems and quality control parameters.

3.3International Designation Systems

The international designation systems for nitriding steels vary by region but maintain technical consistency in the fundamental compositional aspects:

• European system: Alphanumeric designation according to EN 10027
• SAE-AISI system: Four-digit numbering with a prefix for the category
• JIS system: Alphanumeric designation with a regional prefix

04Nitriding Processes and Technological Parameters

4.1Gas Nitriding (Ammonia)

Gas nitriding in an ammonia atmosphere is the most widely used process in industry, owing to its versatility and precise parameter control. The process involves heating the components to 520-570 °C in an ammonia (NH₃) atmosphere, which dissociates and releases atomic nitrogen according to the reaction: 2NH₃ → 2[N] + 3H₂.

The degree of ammonia dissociation, controlled by means of the nitriding potential, determines the composition and thickness of the surface compound layer. Typical dissociation values range from 15% to 85%, depending on the desired surface characteristics.

4.2Ion Nitriding (Plasma)

Ion nitriding uses electrical discharges in a rarefied atmosphere of nitrogen and hydrogen to generate a plasma containing highly reactive nitrogen ions and atoms. This process, carried out at temperatures of 450-550 °C, allows precise control of the nitriding depth and significantly reduces treatment times compared with conventional gas nitriding.

The advantages include superior treatment uniformity, the absence of hydrogen embrittlement and the ability to treat complex geometries with uniform results.

4.3Salt Bath Nitriding

Molten salt bath nitriding uses mixtures of alkaline cyanates and carbonates at temperatures of 550-570 °C. The process ensures excellent thermal uniformity but has environmental limitations due to the toxicity of the salts used, making specialised fume treatment systems necessary.

4.4Process Parameters and Quality Control

The critical parameters for process control include:

• Temperature: 480-570 °C with tolerances of ±5 °C
• Time: 10-100 hours depending on the required depth
• Atmosphere: Controlled composition with continuous monitoring
• Nitriding potential (KN): the key parameter governing compound-layer formation and thickness (typically 0.1-10 atm^(-1/2))
• Nitrogen diffusion coefficient at 520 °C: ~10⁻¹¹ m²/s in ferrite
• Layer growth rate: 0.01-0.02 mm/h for the first 20 hours
• Optimal compound layer thickness: 5-20 μm for mechanical applications

05Limitations and Contraindications of Nitriding

The main limitations and contraindications of nitriding are:

• Hydrogen embrittlement: high risk for steels with Re >1200 MPa
• Element incompatibility: S >0.035%, Pb, Se interfere with the process
• Geometric limitations: difficulty with cavities having an L/D ratio >10:1
• Mandatory masking for areas that are not to be treated (threads, mating surfaces)
• Maximum operating temperature: 500 °C in order to maintain the acquired properties
• Re-treatment not possible: the process is not reversible

06Mechanical Properties and Performance Characteristics

6.1Surface Hardness and Nitriding Depth

The surface hardness of nitrided steels can reach typical values of 700-1100 HV, up to 1200 HV for steels with a high aluminium content under optimal conditions. Chromium-aluminium steels develop higher hardness thanks to the formation of particularly stable AlN and CrN nitrides.

The nitriding hardness depth (NHD), defined per ISO 18203:2016 (formerly DIN 50190-3) as the depth at which hardness falls to the core hardness + 50 HV, typically ranges from 0.1 to 0.8 mm. The hardness distribution follows a decreasing trend from the surface to the core, ensuring a gradual transition of the mechanical properties.

6.2Wear Resistance and Fatigue Strength

The wear resistance of nitrided steels is significantly higher than that of untreated steels, with reduced friction coefficients and excellent resistance to adhesive and abrasive wear. The presence of finely dispersed nitrides in the diffusion zone contributes to wear resistance while maintaining adequate toughness.

Fatigue strength is improved by the residual compressive stresses induced by the nitriding process, which counteract the initiation and propagation of surface cracks.

6.3Dimensional Stability and Distortion

Nitriding, carried out at relatively low temperatures, ensures minimal distortion compared with conventional heat treatments. Dimensional variations are typically less than 0.02% for components with a regular geometry, making the process ideal for precision components that require tight tolerances.

6.4Corrosion Resistance

The nitrided layer provides improved corrosion resistance in humid and atmospheric environments. The presence of chromium nitrides contributes particularly to corrosion resistance, with performance comparable to that of protective coatings for applications that are not especially aggressive.

07Most Widely Used Nitriding Steels

7.1CrMoV and CrAl Nitriding Steels (31CrMoV9, 41CrAlMo7-10)

Chromium-aluminium alloy steels for nitriding are the highest-performing category for critical applications. 31CrMoV9 is widely used for aluminium die-casting dies, ensuring excellent resistance to thermal and mechanical wear.

41CrAlMo7-10 (1.8509) has a composition optimised with aluminium for automotive applications where high surface hardness combined with core toughness is required. The presence of aluminium facilitates the formation of AlN nitrides that are stable up to 500 °C.

7.2Tool Steels for Nitriding (X38CrMoV5-1)

X38CrMoV5-1 (W.Nr. 1.2343, equivalent to AISI H11) is the most widely used hot-work tool steel for nitriding. Its composition with approximately 5% chromium, 1.2% molybdenum and 0.4% vanadium ensures the formation of stable nitrides and resistance to the thermal cycling typical of forging and die-casting applications.

7.3Special Steels for Critical Applications

Special steels include grades modified with elements such as tungsten and cobalt for aerospace and nuclear applications, where exceptional performance is required under extreme conditions of temperature and radiation.

7.4Mould and Die Steels for Nitriding

Steels for nitriding moulds and dies include the CrMoV and CrAlMo series specifically designed for plastic forming and die-casting applications. Nitriding these steels makes it possible to significantly extend the service life of the moulds and dies while reducing maintenance costs.

08Industrial Applications of Nitrided Steels

8.1Automotive Industry (Shafts, Gears, Cylinders)

The industrial applications of nitrided steels in the automotive industry include camshafts, timing gears, hydraulic cylinders and injection system components. Nitriding ensures wear resistance and exceptional durability under severe operating conditions.

Nitrided camshafts offer 300-500% higher wear resistance than untreated components, significantly reducing tappet wear and improving engine efficiency.

8.2Tools and Dies for Manufacturing Operations

Cutting tools and dies for plastic deformation are traditional applications of nitrided steels. Nitriding of hot-working tools ensures resistance to thermal shock and erosive wear, extending service life by up to 200-400%.

8.3Components for Engines and Compressors

Components for internal combustion engines and reciprocating compressors benefit significantly from nitriding. Nitrided cylinders, pistons, valves and valve seats offer superior wear and corrosion resistance, improving reliability and efficiency.

8.4Aeronautics and Aerospace Sector

In the aeronautics sector, nitrided steels are used for jet engine components, landing gear systems and actuators, where exceptional performance is required under high temperature and high stress conditions.

09Quality Control and Characterisation

9.1Hardness and Microhardness Testing

Quality control of nitrided steels involves surface hardness measurements according to ISO 6507-1 (Vickers) and microhardness profiles to characterise the hardness distribution from the surface to the core. Measurements are taken with loads of 10-100 gf to minimise the influence of the compound layer on the readings.

9.2Metallographic Inspection and Composition

Metallographic inspections include examination of the microstructure to verify the correct formation of nitrides and the absence of defects such as porosity or cracks. Chemical analysis of the nitrided layer can be carried out using electron probe microanalysis (EPMA) to verify the distribution of the elements.

9.3Wear and Fatigue Testing

Wear testing according to ASTM G99 and rotating bending fatigue testing make it possible to characterise the in-service performance of nitrided steels. Results typically show improvements of 200-500% in wear resistance and 50-100% in fatigue strength compared with untreated steels.

9.4Non-Destructive Testing

Non-destructive testing includes magnetic particle inspection to detect surface cracks and eddy current testing to verify the thickness of the nitrided layer. These inspections are particularly important for critical components where surface integrity is essential for safety.

10Comparison with Other Thermochemical Treatments

10.1Nitriding vs Carburising

Nitriding differs from carburising in its lower process temperatures (520 °C vs 920 °C) and the reduced distortion that results. While carburising produces higher surface hardness (60-64 HRC), nitriding offers better dimensional stability and corrosion resistance.

10.2Nitriding vs Carbonitriding

Carbonitriding combines carbon and nitrogen enrichment, producing hardness values intermediate between carburising and nitriding. Pure nitriding ensures better corrosion resistance and thermal stability up to 500 °C.

11Comparative Advantages and Disadvantages

Advantages of Nitriding:

• Minimal distortion
• Improved corrosion resistance
• Excellent dimensional stability
• Superior wear resistance

Disadvantages:

• Limited treatment depth
• Higher process costs for large batches
• Long treatment times

12Design and Optimisation

12.1Base Steel Selection Criteria

Selecting the base steel for nitriding must take into account:

• Chromium content of approximately 1-3% for the formation of stable nitrides
• Presence of aluminium for high hardness values
• Core hardenability for uniform properties
• Machinability for complex geometries

12.2Geometric Optimisation for Nitriding

Designing components for nitriding must take into account heating uniformity, the accessibility of the nitriding atmosphere and the need to minimise distortion. Generous fillet radii and symmetrical geometries promote uniform results.

12.3Economic and Process Considerations

The cost-benefit analysis of nitriding must take into account:

• Treatment costs vs performance benefits
• Reduction in maintenance costs
• Potential for structural weight reduction
• Impact on productivity

13Frequently Asked Questions about Nitriding Steels

What is the main difference between nitriding and carburising in terms of applications?

Nitriding is preferred for precision components that require minimal distortion and corrosion resistance, whereas carburising is used for components that require maximum surface hardness and must withstand high loads. Nitriding operates at 520 °C vs the 920 °C of carburising.

Which chemical elements are essential in nitriding steels?

Chromium (typically 1-3%) is essential for the formation of stable nitrides, while aluminium (0.8-1.5%) significantly increases surface hardness. Other elements such as molybdenum and vanadium contribute to the formation of specific nitrides for particular applications.

How is the nitriding depth controlled?

The depth is controlled primarily through process time and temperature. Times of 20-80 hours at 520-570 °C make it possible to achieve depths from 0.1 to 0.8 mm. The composition of the steel influences the kinetics of nitrogen diffusion.

Why does nitriding ensure minimal distortion?

The relatively low temperatures (520 °C) minimise phase transformations and thermal gradients, limiting stresses and deformations. Dimensional variations are typically <0.02% for regular geometries.

What are the main defects of nitriding and how can they be prevented?

Defects include surface brittleness, thickness non-uniformity and cracks. Prevention requires rigorous control of process parameters, correct composition of the base steel and optimised geometric design to minimise stress concentrations.

Is nitriding possible after conventional heat treatments?

Yes, nitriding is normally carried out after hardening and tempering of the base steel. Tempering must be performed at a temperature higher than that of nitriding to prevent softening of the core during the thermochemical treatment.

Nitriding steels represent a mature and reliable technology for applications requiring superior surface performance, offering unique advantages in terms of wear resistance, dimensional stability and corrosion resistance for the advanced manufacturing industry.

Nitriding is a fundamental thermochemical treatment for the modern mechanical engineering industry, with a process temperature of 480-570 °C and surface hardness of up to 1200 HV for optimised chromium-aluminium steels.