Thermal Spray Coating For Steel Processing

Technovations International Inc

Thermal Spray Coating for Steel Processing Ram K. Iyengar, Technovations International Inc, Littleton, MA USA 978-952-2955, [email protected] Thermal spray coating Thermal spray coatings on the rolls in annealing, galvanizing, and other steelmaking and processing sections can ensure an economically viable life at a Steel Plant. Thermal spray coating provides an additional tool to materials engineer to overcome the problems of wear, oxidation and corrosion. There are several operations in steel processing that can benefit from the surface engineering solutions because of the following characteristics of thermal pray coating: • • • • •

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Thermal spray coatings of a wide range of materials from pure metals to cermets have satisfactory adherence between the substrate and the coating. High erosion resistance against molten metal is the most important property for the use of thermal spray coating in galvanizing and aluminizing lines. Thermal spray coating with hard cermets such as tungsten carbide has shown an excellent load bearing capacity with an absence of wear under high load. Thermal spray coatings have strong adhesion to the substrate and do not require grinding to finish. The hardness of thermal sprayed tungsten carbide-cobalt chromium coating is similar to hard chrome plating. The coatings are 100 times more abrasive wear resistant than hard chrome. In comparison with 316 stainless steel, thermal spray coating offer relatively good corrosion protection. Thermal spray coating can be functionally graded to provide thermal shock resistance when there is a large difference in the coefficient of thermal expansion between the topcoat and the substrate.

• Powder particles are heated to targeted temperature and accelerated to velocity approaching that of the gaseous jet. The powder particles are plastically flattened on the surface and form a coating. This process provides coating with low porosity and high adhesion. Additionally, when the particles are not melted the process practically does not oxidize spray powders. This creates perfect conditions for coating sintering during service under elevated temperature in zinc bath. Thermal spray coating processes are based on heating powdered materials to appropriate temperature and propelling them at supersonic velocity against the roll surfaces to produce coatings with desired properties. These processes include: high velocity oxygen fuel, activated combustion high velocity air, plasma, arc etc. These processes operate at different temperatures with particle velocities reaching up to Mach 3. The particle 1

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velocity-temperature regimes of these processes are shown in the chart below.

Figure 1. Temperature-velocity characteristics of thermal spray processes Where Thermal Spray Coating can be used in a steel plant Bridle and accumulator rolls in entrance and exit ends of a steel processing line when coated with tungsten carbide coating eliminate surface damage on the roll and provide proper grip and prevent slippage. The surface coating is properly textured to provide the required characteristics or profile on the strip surface. Surface coatings of furnace and hot tension bridle rolls prevent pickup due iron fines or from oxidation of the rolls due to an accidental change in the furnace atmosphere. The coating provides high friction to eliminate hydroplaning. Deflector or tower rolls when coated with carbide coating minimize zinc pickup. These rolls 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. A wide variety of components associated with Electric arc furnace (EAF) and Basic Oxygen Furnace (BOF) are under severe attack from heat, particulate and acidic gases. Water-cooled components as found in the off-gas duct systems such as pans, roofs, boxes and panels, are subjected to high velocity combustion gases that contain a number of corrosives chemicals that 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 2

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dramatically improved the operation of BOF Hoods by eliminating repetitive maintenance and ongoing tube failures. EAF operators are relying on thermal spray technology to solve reliability problems for water-cooled components in off gas systems to reduce maintenance costs, increase the number of heats per furnace-day and increase the time between shutdowns. In continuous casting the cast shell in the lower half of the mold abrades and wears the bottom of the mold. Diffusion of the copper substrate from the mold into the surface of the cast product leads to a quality defect called "star cracking." 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. Chrome and nickel based coatings protect copper molds from wear, and also enhances caster product quality by greatly reducing cast product contamination and star cracking problems. 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. Galvanized and aluminized steel sheets require very high surface quality, particularly in exposed panels. In continuous galvanizing and aluminizing, the steel strip is dipped in the molten bath through a series of rolls, which control the speed and tension of the strip and guide the steel strip through the molten metal bath. The rolls operating in the molten ZnAl alloy are subjected to severe corrosive environment and require frequent change and repair. A typical galvanizing shop experiences downtimes on the order of every two weeks to change the rolls. It takes upwards of 3 days for a complete turn around to resume normal operation. The molten zinc aluminum alloy baths are aggressive and react with most pot hardware materials. 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. Poor quality of sink and stabilizer rolls can lead to defects in the strip due to: • Roll not turning, • Sink roll groove marks appearing on the galvanized strip. • Dross build up on rolls. By coating the sink and stabilizer rolls with molybdenum boride, tungsten carbide and other materials, the rolls remain smoother produce an improved strip surface. Dissolution of iron from the incoming strip and iron powder on the strip produce dross particles in the vicinity of the V region where the strip enters the bath. This is the major source of dross. Iron in the steel strip and powder on the strip reacts with zinc and aluminum in the molten bath to produce dross as per the following chemical reaction: 3

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2Fe + 5Al Fe2Al5 (Top Dross) In some cases when galvanizing conditions are changed to increase the aluminum content, the bottom dross is converted into top dross. 2FeZn7 (Bottom Dross) +5Al Fe2Al5 (Top Dross) + 14Zn These dross particles already present in the Zn-Al alloy bath contact the sink and stabilizer rolls. The top dross particles suspended in the bath stick and build the rolls. The dross on the roll is picked up by the steel strip and causes a defect. This is a major quality problem for some galvanizers. The roll surface having good resistance to chemical attack by the molten alloy has the following characteristics: • Free of Al and Ti, • Contain sufficient Cr and Ni, • Have an increased amount of Co, and • Maximum content of Mo and Nb. Surface modification by thermal spray coating with ceramic-metals (CERMETS) such as cemented carbides or borides can also reduce dross build-up on the strip. Thermal spray surface coatings prevent wetting and dissolution of the roll material by the molten alloy. The carbide coating significantly decreases the attack rate of Zn, and provides excellent resistance against molten zinc. Double carbides of η-phase (Co3W3C and Co6W6C) in the WC/Co coating is found to be more effective to improve the durability, where η-phase reduces free metal Co with high reactivity. However, the effect of the η-phase is limited to relatively lower temperatures below 890 °F. Galvanizing lines with Al-45wt.%Zn alloy operate at temperature above 1150 °F. At these temperatures thermal spray coating with carbides are not effective. Boride based thermal spray coating are effective for temperatures above 900 °F. The MoB based coating show excellent durability without dissolution and mechanical damage when in contact with molten Zn-Al alloys. Normally a standard blasted roll is changed within three weeks. With boride coating the same roll is used after changing the bearings for several weeks. Plan for Further Action for steel plants Technovations International Inc (TII) ([email protected]) would like to partner with the steel plants to demonstrate how thermal spray coating can increase the 4

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availability of the plant equipment, improve product quality and reduce operating cost. Technovations International, Inc. (TII) provides innovative technical, managerial and marketing solutions to manufacturing companies. TII has been engaged in providing engineering, research and innovation development services to the manufacturers of high performance materials and their products in the USA. TII has highly qualified and experienced Associates with many years of manufacturing, quality management, engineering and research experience in Materials Engineering and Science as applied to the manufacture of high performance and specialty metals and their products. TII provides technical assistance for the start up and operation of facilities based on innovative process technology or equipment, process optimization, manufacturing efficiency improvement and total quality control. In addition TII provides total thermal solution, energy management, and surface engineering technologies for metal products manufacturing plants. TII also undertakes contract research, consulting assignments and development of innovative metallurgical processes, metal fabrication and surface engineering techniques for reducing high temperature oxidation, corrosion and wear of metal components. TII can thermal spray coat sink stabilizer rolls for plants using high aluminum-Zn alloy with the novel boride based coating. TII will define the thermal spray procedure, monitor the process for quality control and assist the steel plant to evaluate its performance. TII can assist the steel plant to identify the source of iron powder on steel strip. This may be due to oxidation of the rolls in the furnaces prior to galvanizing. The problem can be in high temperature zones. These rolls can be thermal spray coated with tungsten carbide or a high temperature oxidation resistant superalloy. TII can assist the steel plant in developing a pre-treatment of steel strip to reduce reaction with molten Zn-Al alloy in the V region. TII can work with the steel plant in identifying which areas in the furnace are prone to oxidation and scale pick up. TII will develop the thermal spray coating parameters (alloy, thickness, properties), monitor the coating process and assist the steel plant in evaluation. TII can assist the steel plant to identify the wear prone area in BOF EAF hoods. TII will identify the surface engineering technique and define its parameters, monitor the process and assist the steel plant in its evaluation. TII can assist the steel plant to identify the wear prone area in mold area. TII will identify the surface engineering technique and define its parameters, monitor the process and assist the steel plant in its evaluation.

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