(1) In broad sense, the defenition of inclusions in steel must be extended to gaseons substances such as hydrides or nitrides, not only for solids.
(2) One of the main causes of flakes or white spots in forged steels is considered to be the destructive force of free hydrogen arrested by doxidiging agents, which evolves suddenly at Ar3 point in the case of cooling.
(3) In order to obtain clean steels free of gaseous inclusions in the melting, it is necessary to have.
a) long vigorous boil, owing to the quality of furnace charges, and b) minimum use of reducing agents.
(4) Ni-Cr-Mo steels are suitable manufactural clean steels with toughness in large ingots, as main additional elements such as nickel and molybdenum have little effect to kill the bath and are easy to get quality steel free of gaseous defects. It is advisable, however, to select 0·3% molybdenum steel in stead of usual 0·6% one in view of the diffusibility of gases for avoiding hair cracks.

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From an investigation of the gas permeability of steel up to 600°C, it is shown that Steel is capable to pass various gases easily through the grain boundary, and further the space amounts to about 0·02mmmm² in the area of 1mm². The specimens treated with are 13-pieces of common carbon steel and 3-stainless pieces. The gas permeability of steel up to 600°C subjects to the logarithmic decreament against the temperature rise owing to the space diminution around the grains from the grain expansion caused when heated, and there exist five points in the temperature range up to 600°C where gas is almost unable to pass through, the points were shown to be 150°C, 220°C, 300°C, 410°C and 570°C thereabout respectively for all specimens of various carbon content.
Now, it is brought to light that these gas stoppages, except at 220°C and 570°C, due to the space diminution being produced among grain boundry as both the stresses given before to the grains and the stresses caused from the grain expansion by h ating, relieve in results mutually acted at these points, thus giving a dense and compact structure at these variation points. The variation point of gas passage at about 220°C and 570°C is shown to be the space diminution produced from the grain expansion at Ao-point of steel and the recrystallization of grains respectively.
By what the gas stoppage caused, were well explained from the results obtained by this work. For an example, the blue-shortness of steel is well understood as an erack or a lack of freedom in the grain deformation when a hard external force applied to it at about 300°C, because it is put in the non space state at this temperature as above described. Further, as the effect of cold drawing, it is conclused that the space is extremely diminished especially at about 150°C, and twisting stress affords generally a space increase though it produces a gas stoppage at about 410°C where the twisting stress relieves. Next, the cause of self hardening of a springmaterial worked hard at room temperature when it is cooled from about 200°C, the nature of hardening is not yet clear up to date, can be well explained as the result of retaining yet the dense structure at 150°C when cooled down to the room temperature, and further a increament of strength at 150°C in shock and drop-hardness test can also be well cleared with this consideration. Now, from the results obtained the object of the annealing process of steel will be so understood as it is nothing but to relieve the stresses by passing through these relieving points at 150°C, 300°C, 410°C, and completed by passing these points repeatedly.
Further, in the cooling period a gas passes more easily than in the heating as well as gas stoppage in the cooling is also little, because stresses have been relieved at most part by heating through these variation points at first, Thus, ofter the relieving of stresses it accompanies the space increament and consequently let the gas pass easily after relieving, but the gas stoppage always occurs of cause during eacn relieving period.
The behavior on the gas permeability of Stainless steel up to 600°C in both the heating and cooling in well similar to that of common steel described above, for the number of gas variation points and those occuring temperatures. The permeability for stainless steel simore or less smaller than common steel.

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The author made various heat-treatments on Ni-Cr steel (0·3% C, 3·3% Ni and 0·8% Cr), and investigated the influence of heat-treatment on the form of stress-strain diagram in the vicinity of yielding point and the degree of work-hardening (or the difference between the tensile strength and the yielding point) by accurately measuring the stress-strain diagram up to 5% strain. The results of the investigation are outlined below:-
A. Investigation by quenching and tempering
(1) When the tempering temperature is raised to about 40°C below Ac1 (715°C), carbide begins to aggregate in crystallographic planes, and simultaneously the form of the stress-strain diagram varies and the yielding point lowers; contrarily the degree of work-hardening suddenly increases.
(2) When the tempering temperature approaches Ac1, the aggregated carbide makes beautiful laminar layers forming a special structure which is named "laminar sorbite" by the author, and the yielding point steadily lowers, whilst the degree of work-hardening considerably increases.
(3) In accordance with the reduction of the cooling speed in quenching, the yielding point lowers, but the degree of work-hardening contrarily increases, whilst the cooling speed in tempering has no effect.
B. Investigation by double quenching.
(1) Double quenching is a heat treatment, in which the secondary quenching is carried out between the stages of the primary quenching and the tempering. In accordance with the temperature of the secondary quenching, two kinds of completely different mechanical properties are given to the material.
(2) When the secondary quenching temperature is in the vicinity of Ac1, i.e., 680°-730°C, the structure is of laminar sorbite with a low yielding point and a condiderably high degree of workhardening.
(3) When the secondary quenching temperature is below Ac3 (775°C), i.e., 740°-770°C, a special structure named "granular pearlite" by the author appears, and the yielding continues over 2% and the yielding point is comparatively high but the degree of work-hardening is extremely small.

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