Abstract
The ASTM SA-516 steel has applications in compact steam boilers of low and medium temperature and pressure. Deterioration can show when exposed to high temperatures due to fuel gases burning. Failures detected by inspections done in a frequency required by standard on the boilers. Among the main failure mechanisms found in materials used in boilers, creep, corrosion, when subjected to the action of the combustion gases or steam oxidation conditions, thermal fatigue and others can be mentioned. This work evaluates the effect of thermal fatigue in the metallurgical aspects of steel ASTM SA-516 grade 60. After the initial characterization of the steel checked that it heeds the specifications required by the standard, showing a microstructure of ferrite and perlite. The tests in heat transfer conditions produce temperatures between 750 to 850°C on the surface, starting with an oxidizing flame produced by oxygen and acetylene gases. The cooling was by water, being the samples were submitted to several cycles of heating and cooling, causing thermal fatigue. After the tests, occurs the characterized of the samples through optical microscopy techniques, Scanning Electronic Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Vickers hardness and macrostructure analysis. ferrite, perlite colonies and no tempered martensite were the microstructures detected on the Heat-Affected Zone (HAZ). On substrate’s region was not detected anisotropy or significant variations. When analyzing austenitic grain size was observed a refinement showing 12 ASTM grain size considered extra fine. The higher microhardness values occurred at the surface. The surface has had been affected by heat due to heating in intercritical and austenitization temperature and the cooling by water.
Highlights
In thick smoke tube boilers characterized by furnaces located in the inside the surface, the circulation of the combustion gases takes place inside the tubes of the tube beam until the outlet through the chimney
The results obtained in the initial characterization are of paramount importance and ensured that ASTM SA 516 grade 60 steel met the required requirements
This work evaluated the metallurgical aspects of ASTM SA-516 Gr 60 steel subjected to thermal fatigue, steel commonly used in the manufacture of boilers, after tests in different thermal fatigue cycles, evaluated the results of each cycle and compared them with each other and with the material without being subjected to such an analysis, so it is possible to conclude
Summary
In thick smoke tube boilers characterized by furnaces located in the inside the surface, the circulation of the combustion gases takes place inside the tubes of the tube beam until the outlet through the chimney. SA-516 grade 60 steel is used in pressure vessels and boilers for medium and low service temperatures. This steel has tensile strength between 415 to 550 MPa, low carbon content (≤ 0.21%) and with elements in its chemical composition that stand out, such as Manganese (0.55-0.98%) and Silicon (0.13-0.45%), the metallurgical structure of this steel, which assesses the austenitic grain size, must have a larger grain size equal to number 5 (ASTM 2004). The microstructure that contains ferrite and perlite with a low content of inclusions is typical for low carbon steel used in boilers (Gong et al, 2010), (Duarte et al, 2017).
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