Thermal spray coating is a technology that meets the most critical challenges of materials requirements in the aerospace, chemical, and automotive industry. Surface engineering and especially thermal spray coating has provided an additional tool to materials engineer to overcome the problems of wear, oxidation and corrosion.
Activated Combustion High-Velocity Air-Fuel Spraying (AC-HVAF) is a new thermal spray technology, which incorporates the best features of both the well-tested high velocity oxy-fuel (HVOF) process and the more recently developed Cold Spray process. 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.
In automotive manufacturing, sheet metals with very high surface quality are required, particularly in exposed panels. Surface coating of the steel sheets with zinc, a process popularly known as galvanizing provides protection against corrosion in severe environments. The equipment used in hot dip galvanizing is also subjected to high temperature oxidation and corrosion during galvanizing of steel. The life of some of this equipment is several weeks. In order to obtain an economically viable life majority of strip galvanizers are using thermal spray coatings on the rolls in annealing, galvanizing, and other processing sections. Galvanizing pot hardware, including rolls and supporting bearings, are totally submerged in zinc-aluminum baths under highly stressed conditions. The molten zinc baths are aggressive and react with most bearing materials. This has an impact on the performance of pot bearings. As a result, pot bearings experienced corrosion and accelerated wear when the intermetallic reaction products act as abrasive debris. In addition Sink and stabilizer rolls experience pick up of dross, which is an intermetallic compound of Al, Fe, and Zn. The hard dross particles could mark the galvanized sheet. By coating the sink and stabilizer rolls with tungsten carbide and other materials, the rolls remain smoother produce an improved strip surface.
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 zinc pickup. Also, deflector and tower roll experiences zinc pickup particularly when the tower is not sufficiently high or running galvaneal products. 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.
Managers of 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. The velocity of combustion gases has increased while a number of corrosives chemicals condense and attack the heat transfer surfaces. This area is also the most difficult 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. EAF operators are relying on thermal spray technology to solve reliability problems for water-cooled components in 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. At present chrome and nickel based coatings are used for protecting copper molds from wear. The deposited nickel also 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, gives an opportunity to coat caster molds with ceramic, ceramic/metallic, and/or metallic alloy coatings to improve the performance of these coatings beyond that of the current options. Using the AC-HVAC, a mold coating that utilizes ceramic/metallic materials, provides a coating with excellent wear, erosion, and thermal resistance properties. The coating is able to reduce mold wear, meniscus erosion and cracking, and improves the quality of the cast product. The mold coatings are beneficial especially when are applied over the entire face of the copper. The thermal resistance of most mold coatings reduces the rate of cooling in the meniscus and enables more precise control. The resulting controlled cooling produces better mold flux powder layer and a more uniform hot face temperature profile. These attributes can have beneficial effects on mold performance and product quality.
Knowledge in this technically challenged area has been steadily advancing through systematic efforts to search for novel materials, designs and coatings to improve the performance of pot bearings. In this effort, 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 several metal products manufacturing companies. TII and UniqueCoat Technologies bring this new thermal spray technology to steel plants and other manufacturers of metal products.
