Estudou-se a solidificação do aço rápido AISI M2, investigando-se os efeitos do teor de carbono (0,75; 0,95 e 1,15%), do teor de vanádio (1,0; 1,5; 2,0; 2,5%), de adições de nitrogênio (0,05%), antimônio (0,20 e 0,40%) e metais de terras raras (0,05 e 0,10%), bem como da velocidade de resfriamento (0,15; 1,5 e 15°Cs), sobre o espaçamento interdendrítico e a morfologia dos eutéticos 'M IND.2'c, mc e 'M IND.6'c. Utilizou-se a técnica de fusão sob cortina de argônio em forno de indução ao ar para a manutenção do teor de residuais, como o oxigênio e o nitrogênio, em baixos níveis. A solidificação dos corpos-de-prova foi acompanhada por métodos de análise térmica convencional e diferencial. A análise das microestruturas foi efetuada com o auxílio de ataques metalográficos seletivos, microscopia ótica, metalografia quantitativa computadorizada e manual, microscopias eletrônicas de varredura e de transmissão e microanálise por micro-sonda. Observou-se a diminuição do espaçamento interdendrítico com os aumentos do teor de carbono e da velocidade de resfriamento, bem como com as adições de N, Sb e metais de terras raras. A variação do teor de vanádio não exerceu influência sobre o espaçamento interdendrítico. Aquelas adições promoveram, ainda, uma distribuição mais homogênea das poças eutéticas na microestrutura e uma maior uniformidade do tamanho das mesmas.

The effects of the carbon and vanadium contents ando f the cooling rate, as well as of nitrogem, antimonium and rre earth metlas (REM) additions on the dendrite arm spacing ando n the morphology of the M2C, MC and M6C eutectics were investigated. The heats were carried out in a high-frequency induction furnace with a continuous flow of argon over the melt surface in order to control the oxygen and nitrogen contents in a low level. The solidification of the samples was monitored by conventional and differential thermal analysis. The characterization of the microstructure was performed through selective etching, optical microscopy, quantitative metallography, SEM, TEM and microprobe analysis. Dendrite arm spacing decrease were promoted by increasing carbon content and cooling rate, and by additions of N, Sb and REM. The vanadium content had no influence on dendrite arm spacing. In addition, the eutectic pools of the N, Sb and REM-containing samples were more homogeneously distributed along the microstructure and presented less size variations. There was a competition between M2C and M6C eutectics formation. M2C eutectic was favored by high V content or high cooling rate, as well as by the presence of N and Sb. M6C eutectic was favored by low V contento r low cooling rate, as well as by the presence of REM. This competition was attributed to the metastability of the M2C eutectic. The morphology of the M6C eutectic was always complex-regular. Two types of M2C eutectic morphology were noticed: irregular, favored by high vanadium contento r low cooling rate, and complex-regular, favored by low vanadium contento r high cooling rate, as well as by the additions of N, Sb and REM. MC eutectic presented three types of morphology: divorced, irregular and complex regular. The formation of each type was determined by thedifference between MC eutectic temperature and peritectic temperature (the smaller this difference, the higher the cooperation degree of the eutectic phases). Antimonium and REM additions promoted the formation of a MC/M2C duplex eutectic which showed connected MC and M2C carbides. In the solidifications under 0,15°C/s, REM additions promoted the formation of a MC/M6C Duplex eutectic. The formation of these duplex eutectics was attributed to the decrease of the MC eutectic temperature.