The mechanism of phase transformation, matrix toughness, and temper embrittlement of a high silicon type tow carbon martensite alloy steel - 24SiMnNi_2CrMoA were investigated through fine microstructure inspection by high magnitude transmission electron micropscopy, X - ray diffraction, electrolysed residue analysis, etc.. Results show that this steel has relatively slow critical cooling rate (V_e = 3 ℃/Sec) of phase transformation. The Martensite transformation orientation relationships chiefly obey KURDJUMOV - SACHS and NISHIYAMA - WASSERMAN relations but some other approximate relations also appear. In all cases, the habit plane is {111} r. The excellent combination of high strength and toughness of this steel is on account of its composite matrix, viz. the low carbon Martensite laths are mingled with continuous retained Austensite film of 5 ～ 8% volume fraction in the matrix which is hardly discovered in the ordinary low carbon middle alloy steels. The temper embrittlement of this steel is chiefly due to two correlated factors, viz. the decompositon of retained Austenite and the precipitation of carbide from Martensite. The "350 ℃ Temper Embrittlement" of this steel can be pushed up and confined within a narrow temperature range, viz. 400 ～ 600 ℃, through isothermal quenching.
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