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Tuyeres — an old French term for “nozzles” —are used in many metallurgical vessels, including blast furnaces, which smelt iron, and cupolas, which melt iron, to direct the air blast into the furnace. Because they are in such a hot and chaotic environment inside the furnace, they are prone to premature failure. Tuyeres for blast furnaces and most high-production cupolas are castings made using robust designs from high-conductivity copper. Since pure copper melts at 1984 °F, tuyeres must be water-cooled to survive in the furnace environment, where temperatures exceed 3,000 °F. Even with water cooling, tuyeres suffer from molten iron and slag splashing, overheating, abrasion, and mechanical damage from scrap and coke.
The Blast Furnace Tuyere Story — Much work has been done to improve blast furnace tuyere performance, including design optimization, boosting cooling water, and the use of coatings to prevent burns by liquid metal and slag. Since molten iron burns are common, many different weld metals and coating materials have been tried over the years on copper tuyeres to protect them. The coating materials used are mostly a ceramic or
ceramic blend of alumina, silica or zirconia, applied in some manner, usually thermal spray processes like plasma, oxy-fuel (HVOF), or detonation guns. The problem with this approach is getting good adhesion of the coating on the substrate, especially considering it will have a different coefficient of thermal expansion than copper, and thus be prone to chipping or spalling. To address this problem, back in the 1970’s, Bethlehem Steel ran some trials with calorized blast furnace tuyeres at their Lackawanna, NY plant blast furnaces. These trials were inconclusive, and the idea lay dormant for 20 years until their research department decided to revisit calorizing. This time, laboratory investigation and testing was performed on calorized copper samples. Abrasion, corrosion, and splash testing were all performed with positive results and researchers recommended controlled plant trials at their blast furnaces in Burns Harbor, Indiana. That was 15 years ago, and calorized tuyeres are now the benchmark for most high-production blast furnaces in North America.
What is Calorizing? — The calorizing process was developed by General Electric in the early 20th Century, after extensive research of ways to protect heating elements. GE’s Emery Gilson was the first inventor to be awarded a patent for calorizing, “Process of Treating Metal”, on March 24, 1914. GE engineers researched two basic methods to protect metals: One is to apply a coating to the surface; the other is to modify the material itself. Calorizing is “the other” process.
Calorizing is a pack cementation process, and is also called aluminizing, or alonizing. The calorizing treatment involves loading metal parts into a container, or retort, and surrounding them with a blend of proprietary metal powders, including aluminum. Then, the retort is hermetically sealed and placed in a furnace where it is heated to a specific temperature, and held for a specified length of time. Chemical reactions create a transport mechanism by which aluminum is introduced into the surface of the component.
The Benefits of Calorizing
Regardless of the substrate material, i.e. ferrous or nonferrous, this produces an inter-metallic alloy layer that provides protection against high-temperature corrosion, and erosive wear. In addition, under certain conditions, the process can be used to create an outer growth layer of alumina to be present to provide a barrier to oxidation, and also liquid metal penetration. Calorized layer thickness is controlled by temperature, and time at temperature. After furnace cooling, the retort is opened and the parts removed and cleaned of excess powder.
There are many benefits of this technology. The intermetallic alloy layer formed is not a coating; due to the establishment of a metallurgical bond, the alloy layer cannot be chipped off. This protective layer can only be removed by machining or grinding. And, since diffusion is not a line-of-sight technology like thermal spray, the protective layer can be created on the inside of pipes and on components with a complex shape that are difficult to protect with conventional spray coating methods. The introduction of aluminum into the surface of a base metal seals the base metal. This does not allow oxygen and other gases to diffuse into the microstructure and cause corrosion. In addition, the intermetallic layer can be much harder than the base metal, resulting in improved wear resistance. The aluminum oxide on the surface is an extremely stable material and acts as a ceramic barrier.
When one researches calorizing applications, the main references are to treating ferrous parts, mainly steel. The most common usage has been steel piping for chemical plants, and parts for other corrosive environments such as cement and limestone processing, power generation plants, heat treating furniture, seamless tube mills, metal forming plants, and many more that use components made of different ferrous alloys, including stainless steel. Since the research work by Bethlehem Steel, calorizing copper parts has grown and has expanded beyond blast furnace tuyeres to other applications, like cupola tuyeres, basic oxygen furnace lances, and electric arc furnace panels and burners.
Evolution to the Cupola — Most small iron foundries still in business no longer operate cupolas, but chose to convert to electric melting, for cost, manpower, simplicity or environmental reasons. Those that still operate cupolas are probably cold-blast type and use a simple tuyere to inject the blast air into the furnace. These plain nozzles may be made of refractory or metal. I worked at an iron foundry with a 60-in.-diameter lined cupola, and we cast our own tuyeres! They were gray cast iron tubes that were replaced as required after a few campaigns.
The next step up in complexity is the fabricated tuyere made from steel forms with wound copper coils for the cooling water. In general, these are used on mid-sized cupolas. The largest cupolas are similar to small blast furnaces in output, and require a more robust tuyere design, like the cast high-conductivity copper tuyeres used there. The improvement in tuyere life resulting from calorized tuyeres in the blast furnace has been dramatic, with average life doubling, and in some cases, even more. So the natural evolution of this technology is to transfer it to cupola tuyeres.
The first successful experience with calorized cupola tuyeres was in 2014 at Waupaca Foundry’s Plant 5, in Tell City, IN. Since that time, two other high-production cupola foundries have run calorized tuyeres, and if this trend follows the blast furnace experience, it will become the benchmark for iron foundry cupola tuyeres across North America.
Trevor Shellhammer is a metallurgist and consultant; Shellhammer Consulting, www.shellhammerconsulting.com. Dennis Webb is a plant manager with Alabama Copper & Bronze Inc., www.alabamacopper.com. Andrew Park is the director of Operations for Quantum Ceramalloy Inc., www.q-c-inc.com.