Low-Pressure Nitriding is a commercially proven technology that provides advantages over some of the commercial alternatives. This article describes the process and its adaptability to unique customer requirements.

Fig. 1. Low-pressure nitriding process


Nitriding is now a well-known thermo-chemical process. There are various ways of adding nitrogen by diffusion to harden the superficial layer of the part being treated:
  • Nitriding in salt baths: This process provides high-quality results but has been used less frequently in recent days because of its environmental limitations, workplace health and safety and the post-process cleaning required.
  • Atmospheric pressure gaseous nitriding: While it is fairly easy to implement, it is difficult to control and its penetration is minimal.
  • Ionic nitriding: This method makes it easier to reserve zones from nitriding and treat stainless grades, but the plasma-assisted process requires a long preparation of the load and the density is usually very low compared to the volume.
  • Low-pressure nitriding: This has been used commercially for about 15 years and provides productive solutions that respect the environment and meet the latest manufacturing requirements.


For more information: The process described in this article is the BMI patented Allnit® process. Nicholas Weiss, Marketing Manager, can provide additional information on this patented process and on BMI. His contact information is: ph. +33(0)4.74.94.61.62 ; fax: +33(0)4.74.94.10.06 ; e-mail nweiss@bmi-fours.com; web www.bmi-fours.com


Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: Low-Pressure nitriding, nitriding atmosphere, ionic nitriding, gas nitriding, diffusion, post-oxidation

Fig. 2. Heating and cooling representation

Low-Pressure Nitriding

Introduction

Low-pressure nitriding is a specialized treatment conducted in a gaseous form at low pressure. The gas mixture used consists of the following:
  • Ammonia (NH3): This nitriding gas provides the nitrogen atoms N* that are necessary for nitriding by dissociation.
  • Nitrogen (N2): This inert gas is used to dilute the nitriding power of the treatment gas.
  • Nitrous oxide (N2O): This strongly oxidizing gas accelerates the dissociation/adsorption of the ammonia.
  • Carbon containing gas: Such as a hydrocarbon or carbon oxide, can be added if nitrocarburizing is required.

Treatment takes place at a reduced pressure. The vacuum is maintained by a group of vacuum pumps, and the injection of the treatment gas is controlled by mass-flow meters. Working temperatures are in the normal range for nitriding: 400 to 600°C (752 to 1112°F).

Fig. 3. A low-pressure nitriding furnace

Cycle Type

Every type of steel benefits from a unique cycle (Fig. 1) based on the different criteria being sought.

1st phase: Furnace evacuation until a vacuum of about 10-2 mbar is attained; this phase takes about 10 - 15 minutes.

2nd phase: Heating the load by inert gas convection (Fig. 2).

3rd phase: Pre-oxidation by introducing nitrous oxide until the required pressure is reached. This is a surface activation stage. This stage can continue after the load has reached the required temperature level.

4th phase: Nitriding takes place. It is possible to sequence the nitriding by adjusting the nitrogen dilution in the nitriding mixture. This optimizes nitriding kinetics while limiting nitrides and/or carbonitrides precipitating with the iron or alloy components on the surface, and thereby controls the growth of any white layer.

5th phase: Post-oxidation (optional). When corrosion resistance is required, an oxidation finishing stage can be used.

6th phase: Cooling the load by inert gas convection (Fig. 2). For obvious safety reasons a final evacuation is done before opening the doors for unloading as there may still be significant traces of ammonia in the furnace.

Fig. 4. Semi-bulk load of rings in furnace

Range of Furnaces Used

Process flexibility is achieved when a low-pressure nitriding capable furnace (Fig. 3) is used as an option for all tempering furnaces, whether in vertical or horizontal configuration. These installations are therefore capable of low-pressure nitriding, traditional tempering and a full tempering cycle followed by nitriding.

Examples of Uses of the Low-Pressure Nitriding Process

The low-pressure nitriding process can be used for all the standard applications of gaseous nitriding. It is especially useful for treating general mechanical parts: transmission and friction parts, cutting tools, press and extrusion tools, plastic injection equipment...

Furthermore, the greater penetration of the nitriding into cavities combined with the excellent chemical uniformity of the installation makes it possible to prepare loads that are very dense and/or semi-bulk. A few standard uses are shown on the following page.

Very Deep Nitriding of Gearings

Grade: NF-EN: 34CrMo4 (35CD4)

WERKSTOFF No.: 1.7220

AISI: 4135 - 4137

UNE: F.8331 - F.8231

Results

Nitriding temperature: 540°C

Nitriding duration: 35 hours

Case depth: 0.3 - 0.4 mm

White layer: 10 - 20 μm

Superficial hardness > 500 HV

Flash Nitriding of Bearing Rings

Grade: NF-EN: HS6-5-2-HC (Z85WDCV06-05-04-02)

WERKSTOFF No.: 1.3343

AISI: M2

UNE: F.5603

Results

Nitriding temperature: 540°C

Nitriding duration: 2 hours

Case depth: 0.3 - 05 mm

White layer: 0 μm

Superficial hardness > 500 HV

Nitriding Aluminium Extrusion Dies

Grade: NF-EN: X40CrMoV5 (Z38CDV5)

WERKSTOFF No.: 1.2343

AISI: H11

UNE: F.5317

Results

Nitriding temperature: 540°C

Nitriding duration: 6 hours 30 minutes

Case depth: 0.3 - 15 mm

White layer: 3 - 5 μm

Superficial hardness > 500 HV

Fig. 6. Typical resistance times in salt-spray test

Post-Oxidation of Low-Pressure Nitriding

As with any nitriding process, low-pressure nitriding can be completed by a post-oxidation treatment aimed at producing a black superficial layer (Fig. 5). The production method has the twin objectives of providing a surface that is resistant to both mechanical and chemical damage improving both friction characteristics and corrosion tolerance.

This treatment can be used with any grade of steel but is most often used for parts made with carbon steel or low alloy content steel. These grades are selected because of their low cost but often have limited performances and are used in situations where they are often subject to friction/sliding constraints while needing a level of resistance against corrosion that far exceeds their basic characteristics (Fig. 6).