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Spraying and Fusing Techniques for Overlaying Hard Coatings of  Nickel - Cobalt Alloys and Tungsten Carbide

Uniform coatings of nickel and cobalt alloys can be deposited on stainless steels and carbon steels by a process of spraying metal powders followed by fusing. The coating alloys are in the form of fine powders suitable for oxy-acetylene flame spraying. The powders become semi molten when they pass through the flame and can be sprayed onto a clean, grit blasted base metal surface. The coating can be built up to a maximum thickness of three millimeters. The coating and the base metal are subsequently heated either in air or in a vacuum furnace to temperatures ranging from 1040 to 1120°C. At this temperature, the coating melts partially and reacts with the base metal to form a true metallurgical bond. The diffusion layer is only a few microns thick, as opposed to the deep penetration of conventional welding. 

Several grades of alloy powders are available to give coatings of hardness ranging from 40RC to 60RC. The coating alloys contain chromium which gives them good corrosion resistance. They also contain carbides and bromides which are very hard and give good wear resistance. 

The process of spraying deposits typically 0.005” thickness per pass, and by repeated passé, the coating thickness is built up. Cylindrical parts are rotated in a lathe whilst spraying. In this case, the coating will be very uniform and accurately controlled. After fusing, the coating requires very little machining (less than 0.3mm) to give a clean surface. By comparison, welded coatings are very uneven and require to be machined at least 1mm to 1.5mm to produce uniformity. 

The process of spraying is ideal for coating inner surfaces of tubes. Coating components of less than 3” section can be fused with a torch in air, but fusing of coatings on irregular shapes is best done in a vacuum furnace. 

Typical Coating Alloys: 

Nickel Based Powders (Commonly known by their registered trade marks Colmonoy*) 

Chemical Composition

Cobalt Based Powders:  Charecterised for their high resistance to Corrosion. (Known by their registered trade marks, Stellite or Wallex *)

  Chemical Composition

Vacuum Fusing 

Atmospheric fusing causes oxidation of the coating to a depth of about 0.75mm and may cause micro porosity in thick coatings. The oxidisation leads to loss of hardness and corrosion resistance. The oxidised layer has to be removed by machining. Vacuum fusing avoids oxidisation of the coating. Vacuum fusing provides excellent fusion of the coating to the base metal and gives a clean metallurgical bond. There will be very little dilution of the coating from the base metal. 

The fusing is carried out at around 1020 - 1040° C for Nickel alloy coatings and 1080 - 1100°C for Cobalt alloy coatings. This is much lower than the base metal melting point. After fusion, the parts are cooled at a controlled rate. The uniform heating and cooling reduces distortion of the component and reduces stresses in the coating and the component. 

The excellent quality of furnace fused coatings has made the process popular for components used in the petroleum and chemical industries where high quality is essential. 

Metallurgical Structure of the Coating Alloy 

The matrix is either Nickel / Chrome or Cobalt / Chrome alloy. It has a good oxidisation and corrosion resistance. Chromium Carbide and Chromium Boride particles are uniformly distributed in the matrix. These particles give a high wear resistance as shown in the micrographs below.

The matrix is a eutectic with a melting point of  around 1040°C (Nickel alloys) and 1120°C (Cobalt alloys). Because the melting point of the coating is much lower than that of steels, it wets the substrate and forms a metallurgical braze / bond without melting the base metal. Hence there will be virtually no dilution with the base metal. The bond layer will be a few microns. By comparison, welding melts the base metal and causes significant dilution with the base metal and causes significant dilution with the base metal. Multi pass welding is necessary to reduce the effect of base metal dilution.

Corrosion Resistance

These alloys have been used for hard facing pump shafts, plungers, valves etc. They have excellent corrosion resistance against chemicals present in crude oil and petrochemicals. Molten salts, alkali carbonates and hydroxides can attack the coating if allowed to remain on the surfaces exposed to air. Corrosion data sheets can be obtained from the manufacturers of coating alloy powders for any specific application.

Trade Marks and Copy Rights:

Stellite is the registered trade mark of Deloro Stellite.

Wallex and Colmonoy are the registered trade marks of Wall Colmony

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Last modified: October 30, 2001