Solidification structures of the AISI M2 type high speed tool steel were studied under various solidification conditions by metallographic and thermal analysis. Small laboratory melts were unidirectionally solidified to simplify the freezing process. Also cooling curves were obtained on 300kg ingots to investigate the differences in the solidification structure and freezing process between the unidirectionally frozen ingots and practical ingots.
Solidification structures oj Ihe A 1S I Jl;12 /y/le high speed too/ slee/have been sludied under lite various solidification conditions by means oj Ihe melal/ogra/lh ic observation and thermal analysis. Tlte m elts ill a laboratory scale were llllidirectionally solidified Jor Ih e sake oj simplibing the Jreezing /JYocess. A lso Ihe cooling curves were oblained on 300 kg illgotS to know Ihe differences in Ihe solidificatioll struclure and Ihe Jreedng process between the Ull'idirectionally fro zen ingots and the commercial-service ingots. The results oblained are summari zed as Jollows: (I) The secondary arm spacing oj dendrite, S Il (fl), in the columnar cryslals oj llnidiredionally fro;::en ingols, was influenced only by the average cooling rate, R (O C min) , during Ihe solidificalion, exhibiting tlte relationshi/I, S11 = 100 R-O.28, Jor Ihe Jl I 2 slee/. (2) The /JYogression oj Ihe liquidus isolherm in 300 kg ingot varied linearly with Ihe square root oj Ihe time , bul not Jor the case oj solidu s isolherm. (3) In 300 kg ingols. Ih e grain diameter oj elfuiaxed cl)'s lals, I (II), varied w ith the az .... age cooling role, R , as observed in the case rif Ihe dendrite arlll ' /lacing. showing th e relationshi/I oj 1= 220 R-O .2.. These results indicate Ih at the mechanism oj Ihe growlh oj grobular grain is similar to that oj Ihe secondalY dendrile arm. (4) The morphology oj Ihe dendriles changed from Ihe equiaxed 10 Ihe cel/ular /y/le wilh decreasing liquidu s isolherm drift role, V(cm/min), and wilh increasing temperature groriiellt, Ge C/cm), across the solidified la)'er.
toughness and mechanical properties at high temperature. The results obtained were as follows: (1) The critical temperature and Ms point were lowered with the carbon content and the begining curve of the isothermal transformation was removed to right side with the car
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