By means of microscopic observation as well as thermal, X-ray, magnetic and dilatometric analyses, the constitution of thernary alloys of Fe-Al-Si system was thoroughly investigated, and the equi ibrium diagram of the whole system was determined. In this system, there exist six phases of ternary compound, corresponding approximately to the chemical formula, Fe₃Al₂Si₂, Fe₆Al₁₂Si₅, Fe₅Al₉Si₅, FeAl₃Si₂, Fe₆Al₁₅Si₅ and FeAl₄Si, respectively, all being formed by a ternary peritectic reaction There occur the following ninteen nonvariant reactions:-
The changes of A₂ points in α-phase and the binary peritectoid reaction, α+ρ(FeSi) σ(Fe₃Si₂), were also studied.

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The cutting efficiency of high-cobalt highspeed steels was examined on sample tools which had been prepared by proper melting, forging and heat treatment. The result of the experiment revealed the following points:
1) As concerns the cutting efficiency of the highspeed steel, the most economical composition contains about 15% Co, and about 20% W, which also meets the requirement for the saving of valuable resources such as Co and W and corresponds to the analysis of the Japanese current specification.
2) The use of vanadium is not necessary and so better to be excluded from the specification.
3) The carbon content should be determined in relation to the Co. The former should be decreased when the latter enhanced. It is not to suitable to specialy the constant amount of C as in the present specification.
4) The low-C highspeed steel is most recommended for hignspeed cutting tools. of the weld-on or brazed tipped types and precision cutting types.
5) It is most proper to decrease the Co content for saving the valuable metal. However, the present study suggests an alternative whether to save Co or to enhance its content together with the increase of the C, thus improving the cutting efficiency and bettering the ease in hot working.

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Up to the present, various methods for the estimation of slag inclusions in steel have been proposed from which some presumptions on the quality of steel would be deduced. Among the many, only an official method was dited by Jernkontoret, Sweden in 1936. The 19th Sectional Committee of the Japan Society for the Promotion of Scientific Research has endeavoured since the beginning of 1937 to establish the most adequate method to designate the results of the prescribed microscopic examination of slag inclusions in steel and to contribute to the appreciation of steel quality. Some parts of our recommended method resemble to that of Jernkontoret's, but the characteristic features are as follows-
a) Types of slag included are classified into two classes, namely A and B, where A (sulphides, silicates) designates those of easily elongated by some plastic deformation of the steel containing, and in the same case, B (oxides) being left untouched.
b) Relative quantity of slag inclusions in steel either in segregated or dispersed distribution has to be estimated under the microscope of magnification at 100 diameters and expressed as a numerical figure (which called Inclusion Number) comparing to the strictly prepared Inclusion Chart.
c) Inclusion Chart is composed of 40 photomicrographs of 80mm in diameter, some 20 of them (10 figures every two types of inclusion) indicate the Inclusion Number 1 to 5 arranged in the order asa=2N-1 where a designates the relative quantity of slag inclusions and N Inclusion Number. The other 20 figures are shown as a standard to measure the mean thickness of inclusions in 3, 6, 9, 12 and 15μ of Inclusion Number 3 as the examples.
d) Mean specific content of inclusions obtained by the following calculation is named Index-Number of Cleanness and adopted as a basis of the appreciation of steel qualities together with the mean thickness of the inclusions examined.
Σ(a.F.)/ΣF=Index Number of Cleanness.
where F designates the number of fields so examined having the same inclusion number and ΣF the total sum of the fields actually examined.
e) Counting of numbers of inclusions under the microscope and also measurements of their sizes are not decidedly done.
Our recommended method is the outcome of about two years' jealous investigation cooperated with many specialists in Japan, and the present author certainly convinces that this method will make an enormous contribution to the study of steel-making and of further treatmonts of steel.

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