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Subject on This Issue:
* Steels & Properties
* Heat Treatment


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GENERAL PRACTICE FOR PLASMA NITRIDING AND RENITRIDING

1. General practice
  • Before nitriding, all parts must be degreased to get quality result for their metallographical structure. Gloves must be worn to manipulate the parts.

  • Understand the parts to be treated and define the place not to be nitrided.
    Most of the time, the bottom of the job part to be treated constitutes a treatment masking. Parts can be laid directly on the cathode, standing up or hanged from a tripod.
    • Space out the parts to be treated to enable the passage of the plasma. The gap must be at least 15mm, depending on different furnace.
    • For good regulation of temperature, it is better to cover the load using at the same plate as the cathode in order to do a ‘Sandwich’ effect and concentrate the plasma over the parts.
    • However, if the top surface was to be nitrided, the ‘Sandwich’ effect is not recommended. Doing so will deprive the top surface of a nitriding layer.
    • Be sure to see a part during the treatment through the porthole.

  • Understand how to protect the parts that are not to be treated.
    • Apply a metallic masking or a protective coating of paint

  • To control the thickness of the plasma
    Material with holes or slots cannot be covered by plasma completely in any situation. Therefore, depending on the situation, suitable pressures have to be used.
    • With low pressure, the thickness of the glow is high, so that the plasma cannot penetrate in deep holes
    • With high pressure, the effect is a small glow, that follows the contour
    • Job should always be kept under watch through the potholes, and pressure should be adjusted accordingly. If the glow is not uniform, pressure should be increase
  • Finish requirements
    • A finish between 15 and 30 RMS is recommended for plasma nitriding products.
    • Product with a surface finish higher than 30 RMS may exhibit premature wear due to the surface roughness and parts under 15 RMS will need to be polish after processin

  • Considerations when choosing your case depth.
    • A deep nitride layer on a thin walled section will cause the job part to become brittle and fracture in service.
    • Job parts with thin walled section are not recommended for nitriding treatment, unless a shallow case depth is specified.
  • Appearance of the nitrided product
    • The overall appearance of a nitrided product is chalky gray color. In some cases, parts may be produced with a black surface.
2. Renitriding

The nitrided part can be renitrided to meet the case depth requirement. The surface should be cleaned by sand blasting prior to renitriding. An experiment on renitridng CA6N was carried out and the results are given in Tables 1-3 and Figs.1-5.

3. Remarks

The nitrided part can be renitrided to meet the case depth requirement. The surface should
The conclusions drawn from the above experiment are as follow:

    • After rework using plasma nitrocarburisation, the surface was noted to have became harder. However, this increase is not uniform. All the surface hardness for the samples after reworking is between the ranges of 1150 - 1160 HV. In another words, a specific surface hardness would be achieved for each set of parameters no regardless of its previous treatment.
    • Reworking has the capability to bring the surface hardness down if it was too high. However, this is at the expanse of increasing its ECD.
    • Reworking improves the hardness profile and increases the ECD.
    • Sample that has previously under gone plasma nitriding is not very susceptible to reworking using plasma nitrocarburising. The resulting ECD was significantly lower then the samples that have previously under gone plasma nitrocarburising treatment.
    • Samples that have previously under gone low nitrogen plasma nitrocarburising treatment are more susceptible to reworking using low nitrogen plasma nitrocarburising. Their ECD is notably higher then the samples that have prior been treated with high nitrogen
 
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