Aço cromo- molibdênio ASTM A387 grau 22 classe 2 e sua soldabilidade aplicado em fabricação de conversores-LD

Abstract

ASTM A387 Grade 22 Class 2 normalized and tempered steel has been widely used in the manufacture of Converters, due to its chromium and molybdenum content. This combination provides greater resistance to high temperatures, as well as to the corrosive action of hydrogen and creep, which are the main factors limiting the useful life of this equipment. Although DIN 16MO3 and ASTM A516 steel have previously been used, companies have shown a growing preference for ASTM A387 Grade 22 Class 2 steel, due to its superior performance compared to the aforementioned steels, as well as being economically viable due to its long service life. Metallurgical industries are demanding a steel that performs well at high temperatures, and ASTM A 387 Grade 22 Class 2 steel meets this requirement well. And with good resistance to corrosion and oxidation, it is a good choice. The limiting factor in the life of these converters is associated with the deterioration phenomenon known as creep, characterized as deformation at high temperatures through the application of stresses below the yield strength of the material. Metallurgical transformations associated with these high temperatures accelerate the creep process, reducing the useful life of the equipment. Factors, mainly economic, motivate the study of extending the life of this equipment, since in addition to the material and labor costs related to repairs or replacements, there are also costs related to loss of profit during the associated maintenance stoppage. These LD-converters are used in the production of liquid steel and are subjected to the stresses and thermal and mechanical cycles inherent in the steelmaking process. It is important to carry out creep tests and predict the useful life of the steels, taking into account real operating conditions. Life predictions were made based on the deformation curves of each material in the secondary stage of creep, considering deformation ranges between 1.0 and 1.5%, as applied in LD-converter projects, and ASTM A387 GR.22 CL.2 steel had the lowest creep rate. One of the flaws in converters during operation is creep deformation, which leads to the suppression of the gap between the vessel and the trunnion ring of the LD-converter, which is why it is important to choose a steel that works well at high temperatures. Weldability must be closely monitored, due to the amount of heat deposited in the metal, and the welding procedure must be carefully followed, especially in terms of pre-heating, interpass, post-heating, heat treatment and final inspection temperatures.