Avaliação da usinabilidade de aços de livre corte com adições de chumbo e/ou bismuto
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Data
2024-10-14
Autores
Alves, João Paulo Luiz Grisotto
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Universidade Federal do Espírito Santo
Resumo
Free-cutting steels are developed using metallurgical techniques to achieve high machinability through the addition of alloying elements that disrupt the material matrix, enhancing productivity and reducing manufacturing costs. This study aimed to evaluate the machinability of the three free-cutting steels, coded as Steel A (MnS+Pb), Steel B (MnS+PbBi), and Steel C (MnS+Bi). For this purpose, microstructural characterization, hardness tests, and machining tests were conducted, including constant pressure drilling tests (evaluating drilled length, cutting temperature, and chip characteristics) and turning tests (measuring machining force, specific cutting pressure, surface roughness parameters, chip characteristics, and tool wear). In the drilling tests, spindle speed, applied load, and material were varied. In the turning tests feed rate and material were varied. The results indicated that Steel A exhibited a predominantly pearlitic microstructure, while Steels B and C showed ferritic microstructures. These microstructural differences were reflected in the Vickers hardness values, with Steel A showing a hardness 78% and 84% higher than that of steels B and C, respectively. In drilling process, Steel A presented greater machining difficulty, with Steels B and C being the materials with the best machinability responses. Increasing spindle speed or applied load resulted in a higher material removal rate. Lower spindle speed combined with greater applied load resulted in higher temperatures during drilling of free-cutting steels. Increasing applied load contributed to the breakage of the chip, forming shorter chips. Lead and bismuth additions contributed to chip embrittlement, however, only bismuth addition showed tendency to longer and snarled chips formation. In turning process, different microstructures, free cutting elements and hardness promoted changes in the material machinability. At lowest feed rate, FU for Steel C was 105% higher than the others. For f=0.2 mm/rev, FU for Steel A became the superior. And for the greatest feed rate, Steel A showed the better machinability result, presenting a FU value 15.2% lower than Steels B and C. The specific cutting pressure (Ks) showed significant variations depending on the feed rate and materials. Free cutting elements contributed to Ks reduction. The free cutting steels showed similar behavior, increasing roughness with feed rate. Material and feed rate influenced the chip shape during the turning process. In general, chip breaking was facilitated by feed rate increasing. Bi addition contributed to chip embrittlement, making it shorter. Cutting tool used for turning Steel A showed the greatest wear (VBBmáx = 844.523 µm, and crater), the largest build up layer, and evidence of wear mechanisms by diffusion and attrition
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Palavras-chave
Aço de livre corte , Usinabilidade , Furação , Torneamento , Força de usinagem , Pressão específica de corte , Rugosidade , Temperatura , Característica do cavaco