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.