What is Nitriding?

Basis of Nitriding

Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. The added nitrogen increases hardness and wear resistance without significantly affecting the metallurgical properties of the core. During nitriding, the metal is heated to 500-550°C in a nitrogen-rich atmosphere. Nitrogen diffuses into the surface up to 0.5 mm deep and reacts with alloying elements like aluminum, vanadium, and chromium to form hard nitrides. These nitrides increase surface hardness up to 1300 HV, improving wear and corrosion resistance.

Purposes of Nitriding

Nitriding is used to improve the surface properties of metals for several key purposes:

Increase Surface Hardness

The primary purpose of nitriding is to increase the surface hardness of metals. The added nitrogen reacts with alloying elements to form hard nitride compounds. This diffusion layer increases hardness up to 1300 HV, which improves wear resistance and load capacity.

Improve Fatigue Strength

The hardened case depth created by nitriding improves the fatigue strength of components like gears and shafts. The compressive stresses formed in the nitrided case increase resistance to crack initiation and propagation.

Enhance Corrosion Resistance

Nitriding improves corrosion resistance in two ways. First, the nitride compounds are very stable and inert. Second, the layer depth created by nitriding ensures the core material remains unaffected if the surface layer is worn away.

Minimal Distortion

Unlike quench hardening methods, nitriding does not require rapidly cooling the part. As a result, nitriding causes very little distortion or residual stress, making it suitable for high precision components. The dimensional changes caused by nitriding are negligible.

Types of Nitriding

There are several main methods and variations of the nitriding process used in industry:

• Gas Nitriding– In gas nitriding, the nitrogen source comes from ammonia gas dissociated into nitrogen and hydrogen. The ammonia gas is less expensive than pure nitrogen gas. Components are heated to 500-590°C in a sealed retort with controlled ammonia flow.
• Plasma Nitriding- Plasma nitriding uses plasma discharge generated by a high voltage power supply to create activated nitrogen ions. This allows faster diffusion at lower temperatures of 350-590°C. Plasma nitriding offers precise control over the case depth.
• Salt Bath Nitriding- In salt bath nitriding, the nitrogen source comes from the dissociation of cyanide salts like NaCN. Parts are submerged in the molten cyanide salt bath maintained at 580-590°C. Salt bath nitriding can achieve very high case hardness up to 1500 HV.
• Fluidized Bed Nitriding- Fluidized bed nitriding involves the dissociation of ammonia gas in a bed of alumina powder fluidized by gas flow. This method provides excellent temperature uniformity during treatment.
• Other Variations- Other variations include nitrocarburizing which adds carbon to form carbonitrides, post-oxidizing to form black oxide layers, and low temperature nitriding between 350-380°C. Hybrid processes like plasma nitrocarburizing are also used.

Materials Suitable for Nitriding

Nitriding can be applied to a range of ferrous and non-ferrous alloys:

Low Carbon Steels

Low alloy steels with carbon less than 0.25% are commonly nitrided to increase surface hardness and wear resistance. The low carbon content reduces the formation of unstable iron nitrides. Popular examples include 1018, 4140, and 4340 steels.

Tool Steels

Tool steels including H13, P20, and D2 are ideal for nitriding due to their high hardenability and alloy content. Nitriding increases hardness, strength, and life in tooling for hot or cold work applications.

Stainless Steels

Martensitic and precipitation hardening stainless steels like 410, 416, 420, and 17-4PH can be effectively nitrided to improve wear and corrosion resistance. A surface hardness of 1000-1400 HV can be achieved.

Aluminum Alloys

Certain aluminum alloys containing silicon and magnesium can be nitrided through a process called nitrocarburizing. This diffuses nitrogen and carbon simultaneously into the alloy.

Titanium Alloys

Titanium alloys including Ti-6Al-4V can be nitrided through plasma methods. This creates a high hardness protective surface layer without affecting the core ductility and fracture toughness.

Other Alloys

Other materials like nickel-based superalloys, tool steels, and cobalt alloys can also be nitrided. Both ferrous and non-ferrous alloys with sufficient alloying additions to stabilize nitrides can benefit.

Equipment and Consumables for Nitriding

Nitriding processes utilize specialized furnaces, gas supplies, and temperature monitoring equipment:

• Nitriding Furnaces
  • Box or cabinet style furnaces with gas tight retorts and insulation for gas and plasma nitriding up to 1000°F (590°C).
  • Molten salt bath furnaces for salt bath nitriding around 1100°F (590°C).
  • Fluidized bed furnaces containing porous ceramic media for fluidized bed nitriding.
• Nitrogen Gas
  • High purity nitrogen gas as the source for gas nitriding.
  • Ammonia gas that dissociates into nitrogen and hydrogen for gas nitriding.
• Power Supplies
  • DC power supplies up to 1000V and current rating over 10,000 amps for plasma nitriding.
• Temperature Monitoring
  • Thermocouples to monitor furnace temperature uniformity.
  • Pyrometers to measure surface temperature of components.
• Consumables
  • Cyanide salts for salt bath nitriding.
  • Alumina powder for fluidized bed nitriding.
  • Quench oil, cleaning solutions, tooling, fixtures, etc.

Proper equipment is essential for controlling the nitriding atmosphere, temperature, and duration to achieve repeatable case depth and properties.

Nitriding Process

The key steps in a typical nitriding process are:

1. Cleaning
   1. Remove dirt, oil, grease, oxides, and other contaminants from the component surface through degreasing, alkaline cleaning, or acid pickling.
2. Loading
   1. Load components carefully into fixtures or baskets to avoid contamination and ensure proper exposure.
3. Heating and Holding
   1. Heat at a rate of 400-800°F/hr (220-440°C/hr) to reach nitriding temperature.
   2. Hold at nitriding temperature to allow the workpieces to reach thermal equilibrium.
4. Nitriding
   1. Expose workpieces to nitrogen-rich environment at nitriding temperature for the required time to achieve case depth.
   2. Ammonia gas crack into nitrogen and hydrogen. Plasma generates nitrogen ions. Cyanide salts release nitrogen.
5. Quenching
   1. Rapid cooling such as in oil to preserve the nitrided structure. Not required for some processes.
6. Unloading and Cleaning
   1. Unload workpieces and remove any residues from quench oil or salts by washing.
   2. Apply post-treatment like grinding or polishing.

Careful process control is needed to obtain desired case depth, minimize distortion, and ensure nitride phase formation.

Industrial Applications of Nitriding

Some common industrial uses of nitriding in mechanical components include:

Gears

Nitrided alloy steel gears have higher surface hardness and fatigue strength. Used in transmission gears, cam gears, ring gears, etc.

Bearings

Nitriding of bearing races, rolling elements and bearing surfaces improves wear resistance and durability under cyclic loading.

Crankshafts

Nitriding increases the fatigue strength of crankshaft fillets and journals. Used in automotive and marine engines.

Pistons

Nitrided cast aluminum pistons increases scuffing resistance and adhesive wear resistance against cylinder walls.

Valves

Improved wear in intake and exhaust valves of internal combustion engines with nitrided valve faces and stems.

Cutting Tools

Coated high speed steel and carbide cutting tools treated with nitriding have better metal removal rates and life.

Dies and Molds

Nitrided tool steel molds and dies for casting, forging and stamping show enhanced durability and performance.

Other Uses

Common in pressure vessels, shafts, cylinders, cams, fasteners, actuators, and fluid power components.

The increase in surface hardness, wear resistance, fatigue strength, and corrosion resistance make nitriding ideal for critical mechanical parts.

Nitriding in Jewelry and Accessories

While less common than industrial uses, nitriding has some niche applications in jewelry and fashion accessories:

Improved Surface Finish

Nitriding can provide a smooth uniform surface finish on jewelry metal components like rings, bracelets, and watch cases. This reduces the need for secondary polishing.

Enhanced Corrosion Resistance

The nitrided layer improves corrosion resistance for jewelry pieces exposed to moisture like rings, bracelets, chains, and metal watch straps.

Decorative Black Coatings

Selective blackening of etched surfaces by nitriding stainless steel or titanium can provide decorative patterns and accents on jewelry.

Hardened Surfaces

Improved wear resistance for high use jewelry items like men’s rings and metal watch bands through surface hardening.

Cost Savings

In certain metals, nitriding may provide performance benefits over plating or PVD coatings for a lower cost.

Customer Perception

Some luxury brands use nitriding for the perceived benefits of the technology and surface finish.

While not as prevalent as in manufacturing industries, nitriding can provide functional and aesthetic advantages for select jewelry applications. Proper control is needed to maintain the appearance of jewelry metals.

Comparison to Other Surface Hardening Processes

Nitriding differs from other common surface hardening processes in several ways:

Carburizing

Carburizing diffuses carbon rather than nitrogen into the steel surface. It creates a harder but less stable martensitic case. Nitriding provides better corrosion resistance.

Nitrocarburizing

Nitrocarburizing adds both nitrogen and carbon simultaneously. The combined carbonitride case can offer advantages over either alone.

Induction Hardening

Induction hardening rapidly heats and quenches the surface through electromagnetic induction. Nitriding provides deeper case depth and less distortion.

Flame Hardening

With flame hardening, oxy-fuel torches quickly heat the surface before quenching. Nitriding can harden complex geometries more effectively.

Hard Film Deposition

Hard film deposition techniques like PVD, CVD, and thermally sprayed coatings deposit a thin ceramic coating on the surface. Nitriding diffuses nitrogen into the substrate metal itself for better adhesion and fatigue resistance. However, coatings can provide added benefits like insulation or high temperature resistance.