As one of factors which have great influence on the brittle fracture initiation characteristics of steels, the prestrain can be touted.
Previously, the effect of prestrain on the notch toughness of steels has been investigated by using V notch Charpy impact test specimens, but quantitative analysis on the effect of prestrain has not been made.
In this paper, in order to evaluate quantitatively the effect of prestrain on the brittle fracture initiation characteristics of steels, mild steel, 60, 80 and 100 kg/mm² high strength steels and 9% Ni steel which had been prestrained by 2-8% at room temperature were investigated by using the deep notch test.
It can be concluded that the effect of prestrain is relatively great for mild steel and 100 kg/mm² high strength steel, but small for 60 and 80 kg/mm² high strength steels and 9% Ni steel.

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It was shown in previous reports that fatigue strength of steel welded joint was improved by hot galvanizing and that this effect was due to the heat treatment in hot galvanizing process, that is, this effect came from heating in zinc bath and rapid cooling in water. Rapid heating and cooling induce residual stress and aging effect on steel, even if the heating temperature is under A₁ point. Therefore in this report the influence of the residual stress and aging of steel welded joint cooled in water after heating at various temperatures or after hot galvanizing on the fatigue strength is investigated.
The results are summarized as follows:
(1) When the steel welded joint is rapidly cooled in water after heating at various temperatures from 400°C to A₁ point, the residual stress on the surface of the welded joint is converted from tension of as welded joint into compression. Rapid cooling in water after hot galvanizing has the same effect.
(2) The larger the compressive residual stress of the surface of the welded joint, the more improved is the fatigue strength of the joint.
(3) The higher the heating temperature up to A₁ point and the cooling rate, the greater becomes the aging effect of steel and its welded joint.
(4) Steel welded joint increases in the fatigue strength with age hardening.
(5) The improvement of fatigue strength of steel welded joint by hot galvanizing is attributed to the compressive residual stress and aging of steel welded joint.

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In the 1 st report, fundamental formulas and diagrams for welding penetration have been obtained using idealised models of a moving point heat source, and practical application to shallow penetration of fillet weld was discussed.
In this report, the fundamental penetration theory is applied practically to deep fillet weld as follows:
(1) Assuming geometrically a half ellipse of fused zone in a fillet weld cross-section, the actual penetration (d, d₀. and p; see Fgi. 4) of a fillet weld is represented by the following approximate formulas as a function of theoretical penetration (d) derived from the point-source theory:
Nominal penetration ......d/h=1/2{√(2d/h)⁴+1-1}
Root penetration ..........d₀/h=1/√2{(2d/h)²-1}
Efiffective penetration ......p/h=(2d/h)⁴-1/(2d/h)⁴+1
where, h is a given fillet size, and d is obtainable from the given welding condition and material. (see report I)
(2) Reduction rate of fillet size (h_e/h) due to effective penetration (p) is also given by the following approximate formula:
h_e/h=2/(2d/h)⁴+1
where, h_e is the reduced fillet size, and h is the required nominal fillet size.
(3) Utilities of the formulas are experimentally confirmed by submerged arc welding of Tee-joints.
(4) The theory of both-sides penetration of Tee joint welded simultaneously which was presented in the 1st report is also confirmed experimentally for the deep fillet welds by submerged arc welding.

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The burst test by application of internal static pressure of a welded steel pipe made of carbon steel and niobium-containing high tension steel plate was carried out to know the initiation properties of brittle fracture of the pipe.
The following method was found suitable for initiating a fracturee in the pipe and obtaining the transition temperature from ductile fracture to brittle one of the pipe. The pressure ram was located between two pillars welded vertically at the outer surface of the pipe so as to force the pillars apart when the ram operated statically. The fracture was initiated at the welding toe under pillar.
The absorbed energy in Charpy impact test of the high tension steel plate containing niobium, of which the welded pipe was made, was small. But the transition temperature of facture surface in Charpy test of the plate was comparatively low.
The pipe about 6 mm thick and about 250 mm in diameter was burst at the temperature down to -70°C.
The transition temperature from ductile fracture to brittle one, which was obtained from the burst test of pipe, was comparatively low in spite of the absorbed energy in Charpy test being small.
The initiation temperature obtained from the burst test of pipe agreed with the critical temperature, up to which the steel plate could be used without brittle fracture initiation, as calculated from the transition temperature of facture surface in Charpy impact test according to WES (tentative specification of Japan Welding Engineering Society "Evaluation criteria for brittle fracture characteristics of structural steels for low temperature application").

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The rates of the Si and Si-Mn deoxidations in arc melting were studied in order to investigate their behaviour in deoxidation reactions in arc welding. The temperature of the melting pool ranges between the melting point of iron and about 2000°C. However, according to our previous paper on deoxidation equilibriums in arc melting, it was shown that deoxidation reaction in arc melting could be treated as a reaction at some mean temperature.
On this basis, the rates of the Si and Si-Mn deoxidations were measured using the apparatus and melting procedure which were almost identical to the ones reported in the previous paper. The results of the analysis of the experiments were as follows:
(1) Rate of deoxidation is shown by the following equation:
d[%O]/dt=I/V k₀([%O]-[%O]_e)
where [%O] is oxygen content at time t; [%O]_e is oxygen content at apparent equilibrium; I is current; V is volume of molten pool. The rate constant, k₀, is independent of the melting condition (pressure of Ar Atmosphere, current and voltage), but depends upon the content of elements (Si, Mn, O) in the specimen and also the appartus used.
(2) The above equation was compared with the reaction model, which assumes that the separation process of the deoxidation product is the rate determining step and also that the rate depends upon the strength of the stirring in the pool. Then, it is probable that the strength of the stirring (stirring speed at the place where the deoxidation product is separated) is proportional to the current. However, this conclusion has to be further investigated.
(3) In the Si deoxidation process, the rate constant, k₀, is constant in low Si range (Si<0.4%), but increases as the content of Si increases in high Si range (Si>0.4%). In the Si-Mn deoxidation, k₀ increases as the content of Si decreases and the content of Mn increases. In addition, k₀ increases as the oxygen content in the specimen increases.

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