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Technovations International Inc (TII) provides innovative solutions in metal products manufacturing to reduce cost improve product quality and increase productivity.TII has been engaged in providing surface engineering solutions to metal products manufacturing companies over ten years. Recently TII has joined with UniqueCoat Technologies LLC (www.uniquecoat.com) based in Richmond VA to bring a new development in thermal spray technology (called AC-HVAC) to steel plants.  

Activated Combustion High-Velocity Air-Fuel Spraying (AC-HVAF) is a new thermal spray technology, which incorporates the best features of both HVOF and Cold Spray processes. Like HVOF, AC-HVAF utilizes high velocity heated particles. By spraying at temperatures lower than the particles melting point, this process avoids negative effects caused by high temperatures, such as oxidation and deterioration of spray material, thus producing superior coatings of metals and carbides. AC-HVAF allows easy spraying of the most popular commercial Ni, Co, Fe-alloys and cemented carbides with highest coating quality and deposit efficiency. AC-HVAF is a lower cost process and produces higher quality coatings.  

Electric arc furnace (EAF) and Basic Oxygen Furnace (BOF) are under constant pressure to reduce maintenance costs, increase the number of heats per furnace-day and increase the time between shutdowns. A wide variety of components associated with these plants are under severe attack from heat, particulate and acidic gases which are a normal consequence of the steel making process. In particular, water-cooled components as found in the off-gas duct systems such as pans, roofs, boxes and panels, have become the weak link in the production schedule. Combustion gas streams are increasing in velocity while a number of different corrosives condense and attack heat transfer surfaces. This area is also the most difficult (i.e. highest cost) to access when conducting maintenance and inspection. Surface engineering solutions including thermal spray coating and weld overlay have dramatically improved the operation of BOF Hoods by eliminating repetitive maintenance and on-going tube failures. Recently, EAF operators have begun to enjoy the same success to solve reliability problems for water-cooled components in EAF Off gas Systems.  

In continuous casting, the copper mold serves as an effective heat exchanger to quickly remove heat from the steel being cast through it. As the cast shell begins to increase in thickness in the lower half of the mold, it abrades and wears the bottom of the mold. Additionally, the diffusion of the copper substrate from the mold into the surface of the cast product leads to a quality defect called “star cracking.” In most cases, the wear on the copper substrate and the pick-up of copper by the cast product can be eliminated by the use of a mold coating on the bottom portion of the copper mold. Decreasing mold wear is another objective for coating the mold surface. Mold coatings are therefore used on continuous casting copper molds to decrease wear and improve product quality. At present chrome and nickel based coatings continue to be used predominantly for protecting mold coppers from wear. Nickel plating for reconditioning continuous caster mold, allows the nickel to be deposited in various configurations and deposit thicknesses. The deposited nickel enhances caster product quality by greatly reducing cast product contamination and star cracking problems. The heat transfer properties of nickel are similar to those of copper. Therefore coating of the mold require minimal operational changes at the caster, making it an excellent investment for extending mold life and improving product quality.  

The recent development of thermal coating processes, such as the AC-HVAF, to successfully coat caster molds with ceramic, ceramic/metallic, and/or metallic alloy coatings provide an opportunity for casters to improve the performance of their coatings beyond that of the current options. Using the AC-HVAC, we can develop a mold coating that utilizes ceramic/metallic materials to create a coating with excellent wear, erosion, and thermal resistance properties. The coating is able to reduce mold wear, meniscus erosion and cracking, and to improve the quality of the cast product. The mold coatings would also be beneficial to both operators and maintenance personnel, especially when coatings are applied over the entire face of the copper. Because of the thermal resistance of most mold coatings, the rate of cooling in the meniscus can be slowed and controlled more precisely when a coating is applied over it. Often referred to as “soft” cooling, this allows for better development of the mold flux powder layer and tends to create a more uniform hot face temperature profile. These attributes can have positive effects on mold performance and product quality.  

Galvanizing pot hardware, including rolls and supporting bearings, are totally submerged in zinc-aluminum baths under highly stressed conditions. In galvanizing operations, it is well known that galvanizing baths are aggressive media and will react with most bearing materials. The reaction of materials with galvanizing baths obviously has an impact on the performance of pot bearings. As a result, pot bearings experienced not only wear under limited lubrication conditions (molten zinc) but normally react with the galvanizing baths, which aggravated the wear process when the reaction products (intermetallics) act as abrasive debris. Wear of a material reflects its response to the imposing conditions and is often related to the coupling pair and the surrounding media. Sink and pot rolls in galvanizing experience dross (an intermetallic compound of Al, Fe, and Zn) adhesion, which could mark the galvanized sheet. The coatings on rolls allow smooth operation of the line and produce an improved strip surface.  

Knowledge in this technically challenged area has been steadily advanced in recent years through systematic efforts to search for novel materials, designs and coatings to improve the performance of pot bearings. TII and UniqueCoat Technologies are in a position to develop thermal spray coatings for galvanizing pot bearings as well as the surfaces for the sink, stabilizer and equalizer rolls. These coatings extend the in service life of rolls, which reduces the frequency of maintenance shutdown. For the sink roll bearing sleeve we can provide a coating with a molybdenum based alloy which is resistant to corrosion by zinc and aluminum. Our estimate is that the sleeve life with this coating can be increased from the current two weeks to over 8 to 10 weeks.  

Bridle and accumulator rolls in entrance and exit ends of a processing line are coated with tungsten or chromium carbide or oxide ceramic. These coatings eliminate surface damage on the roll and provide proper grip and prevent slippage. A properly textured coating can provide the required characteristics or profile on the strip surface. Coatings of furnace and hot tension bridle rolls also prevent pickup due iron fines or an accidental change in the furnace atmosphere. The coating provides high enough friction to eliminate hydroplaning. Deflector or tower rolls are coated with carbide or ceramic to minimize pickup.  

Coatings of furnace and hot tension bridle rolls also prevent pickup due iron fines or an accidental change in the furnace atmosphere. The coating on pot and stabilizer rolls in a galvanizing pot prevents dross adhesion and provides high enough friction to eliminate hydroplaning.  

Deflector or tower rolls are coated with carbide or ceramic to minimize zinc pickup. Also, deflector and tower roll experiences zinc pickup particularly when the tower is not sufficiently high or running galvaneal products. As you know, the coatings on rolls allow smooth operation of the line and produce an improved strip surface. Coatings also extend the in service life of rolls. This reduces the frequency of maintenance shutdown.  

 

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