ResumoOs aços ferramenta para trabalho a quente são empregados como matrizes para conformação de metais em alta temperatura. Recentemente, novos aços ferramenta, com menor teor de silício, têm sido utilizados, gerando substancial melhoria de desempenho das matrizes. O presente trabalho discute resultados da caracterização mecânica e microestrutural de tais aços, também apresentando algumas análises de casos. As propriedades mecânicas foram avaliadas quanto à dureza e tenacidade em impacto, em função da temperatura de revenimento. Para caracterização microestrutural, foi utilizada a técnica de microscopia eletrônica de transmissão, com o objetivo de caracterizar os carbonetos secundários, os principais responsáveis pelo endurecimento dos materiais. São observadas diferenças significativas dos carbonetos secundários em função do teor de silício, estando relacionadas, diretamente, às propriedades mecânicas avaliadas. A redução do teor de silício diminui a presença de carbonetos finos e agulhados; isto pode explicar a menor tenacidade dos aços ferramenta de alto teor de silício, sendo proposto um possível mecanismo. Desta forma, os resultados mecânicos e microestruturais justificam o aumento de desempenho desta nova classe de aços ferramenta para trabalho a quente, com menor teor de silício. Palavras-chave: Aço-ferramenta; Silício; Tenacidade; Desempenho.
LOW SILICON HOT WORK TOOL STEELS: CHARACTERIZATION AND APPLICATIONS
AbstractHot work tool steels are mainly used as dies for hot forming processes. New grades have been recently developed, with lower silicon content, leading to a substantial performance increase. The present paper describes the mechanical properties and microstructural characterization of such hot work steels, as well as case studies. Hardness and toughness were determined for a wide range of tempering temperatures; concerning microstructural characterization, transmission electron microcopy was used for observing secondary carbides -the main responsible for the hot strength of these steels. Important differences were observed on the secondary carbides, which are considered responsible for the differences on the mechanical properties. The reduction in silicon content reduces the amount of fine needle shaped carbides; this can explain the lower toughness of high silicon content grades and a possible mechanism is proposed. Therefore, the microstructural and mechanical results enable the understanding of the higher performance of this new class of hot work tool steels, with lower silicon content.
This article focuses on heat treating of the most important H-series and low-alloy hot-work tool steels, namely, normalizing, annealing, stress relieving, preheating, austenitizing, quenching, tempering, and surface hardening. It describes the heat-treating procedure for hot-work tools using examples. The article provides information on the North American Die-Casting Association's requirements for steel grades and heat treatment of dies made of hot-work tool steels. It also describes the chemical compositions and mechanical and metallurgical properties of hot-work tool steels.
The objective of this paper is to evaluate the crack propagation rate of the SAE 4320 steel carburized layer. For the simulation of a carburized layer, samples made of SAE 43XX were employed, varying the content of carbon from 0.20 to 1.00 %. Specimens were copper layer electroplated, and then, they were heat treated in a cycle of carburizing, quenching, and tempering in five different temperatures to expose them to the thermal effects without diffusion of carbon. The results of the microhardness for the steels and for the analyzed conditions are presented in this work. The curve of microhardness has the same profile of a carburized layer for the SAE 4320 heat treated in the same conditions. The crack growth rates as a function of delta K for three tempering temperatures are plotted and the curves are shown. These results show that when the hardness is high (at 200 ºC tempering temperature), there is a scattering of the curves and for the case of lower hardness (at 600 ºC tempering temperature), the curves are closer. With increasing of tempering temperature there is a decreasing of the hardness and a significant effect of the metallurgical condition of the resistance of fatigue crack growth. Furthermore, with decreasing of the carbon content there is a very significant increase on the resistance of the fatigue crack growth. As a result of that, in the case of a carburized layer there is a raise of the fatigue crack growth resistance when the crack grows into the steel, from the surface (higher hardness) to the core (lower hardness).
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