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


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SURFACE IMPROVEMENT OF HSS STEELS FOR TiN/TiCN COATING

Hard coatings of TiN and TiCN produced using plasma-assisted CVD process (PACVD) can remarkably improve the tool life. This process can provide high quality hard coatings at a temperature down to 450˚C. In the duplex treatment process, part can be first nitrided and then coated with TiN or TiCN to increase coating adhesion and wear resistance.

High-speed steels (HSS) are high carbon and high alloy steels. To improve tool life further, they are plasma coated with TiN or TiCN. However we noticed that that the coating did not last as long as it should be after coating. Thus an effort was made to improve the adhesion of the coating by improving surface condition through proper cleaning. For the coating to be long lasting, the steel surface that is being coated needs to be smooth and uncontaminated with foreign particles.

Traditional cleaning process

Traditionally, the parts are polished using silicon carbide paper to derust and to create bright and smooth surface for coating. After polishing, cleaning solvent is used to degrease and clean the parts. It was found that the cleaning process is unable to provide a very clean and smooth surface. The problems are:

  • During polishing, the silicon carbide particles are embedded into the steel surface. The carbide particles will hinder the coating adhesion.
  • Some parts are complex with crevices and small holes that are difficult to clean simply by the current cleaning process.
Alternative cleaning process

Systematic tests were carried out to identify a suitable cleaning process for the plasma coating of HSS parts.

Alternatives to sandpaper polishing:

  • Chemical polishing
  • Electrolytic polishing
  • Mechanical polishing

Chemical polishing does not require electricity and the equipment required is not complicated. Electrolytic polishing is costly in terms of equipment and operation. Mechanical polishing also requires the purchase of equipment.

Chemical polishing

  • It was found that chemical polishing cannot be done on high speed steel because of smut formation. High speed steel has a high content of carbon. This carbon is diffused out when immersed in the chemical polishing solution. Smut is difficult to be removed unless we do anodic etching.
  • Local companies can only do chemical polishing on mild steel (low carbon steel) or stainless steel. They are unable to do for high carbon steel.

Electrolytic polishing

  • Key Notes
    • Metals can be removed without smut formation by electropolishing in hot concentrated phosphoric-chromic acid solutions at high current densities. This usually leaves the steel in a passive condition. Anodic cleaning followed by anodic etching or acid dipping may activate the surface. (Reference: Electroplating by J.B. Mohler Pg 35)

    • Electropolishing is especially adaptable to high-carbon steel; it removes highly stresses metal and metal debris from the surface without forming smut. Proprietary mixtures of sulfuric and phosphoric acids are used. (Reference: Electroplating by Lowenheim Pg 87)

    • Steel is more difficult to polish to the same degree of perfection as other metals, owing to variations. (Reference: Metal Finishing Guidebook and Directory Issue 98 Pg 448)
  • Chemical solutions for electrolytic polishing:

    The following chemicals used for the test.

    Phosphoric acid (% by wt):                         55-85                                          
    Trialkaline metal phosphate (% by wt):        1-15                                          
    Alkali metal phosphate (% by wt):               0.5 min
    Temperature, ˚F (˚C):                                 160-180 (70-80)
    Current density, A/ft2 (A/dm2):                   400-800 (43-86)

    Sulfuric acid, 15%
    Phosphoric acid, 63%
    Chromic acid, 10%
    Current density, 50-1000 A/ft2
    Temperature, 125˚F
    This solution has a finite life.

Mechanical polishing

  • Buffing is the processing of a metal surface to give a specific or desired finish. The range is from semi bright to mirror bright or high luster. A buffing operation includes the buff wheel, the buffing compound and the buffing machine. It is not a process for removing a lot of metal, which is good because we do not want any big dimension changes. The buffing machines can range from manual machines, semiautomatic machines and fully automatic machines (rotary automatic machines and straight line machines). Automatic machines do not require any operator to hold the workpiece.
  • Tumbling is the polishing of metal by the rotation of the barrel. As the barrel rotates, the tumbling media and parts within climb to what is referred to as the turnover point. At this point, gravity overcomes the cohesive tendencies of the mass, and a portion of the media mass slides in a retrograde movement to the lower area of the barrel. Most of the abrading or other work being performed on parts within the barrel takes place within this slide zone, which may involve as little as 10-20% of the media mass at any moment. Barrel finishing is capable of producing some unique and desirable surface finishes and is highly efficient in compound and media usage. For polishing we should choose centrifugal barrel. However the equipment cost is high and we need to load the workpiece manually. The use of diamond wheel for grinding instead of using silicon carbide paper. In this case foreign particles won’t be embedded into the surface.
Remarks on cleaning processes
  • Both electrolytic polishing and mechanical polishing were chosen for cleaning HSS parts prior to plasma coating. The feedback from customer indicated that the tool life is remarkably improved.

  • Ultrasonic cleaning was also used to clean complex parts and small parts. Ultrasonic cleaning is the introduction of high frequency sound waves into a liquid, usually between 20 and 80 kHz. It has the ability to clean in narrow crevices and small holes that would not be easily accessible by a spray or other methods of cleaning.
 
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