An investigation has been made to study the weldability of various IN Steels. A preliminary test, in which forged and heat treated mild and low alloy IN Steels (IN-1, IN-4 and IN-5) were used as test materials, was followed by the test, where rolled and press-quenched mild and low alloy IN Steels (IN-6, IN-10 and IN-12) and as-rolled IN Steels (IN-A and IN-B) were tested.
Through this investigation, the following conclusions were obtained.
1. Generally, IN Steel has superior weldability compared to other steels of similar chemical composition and strength. This may be due to its ultra-fine grain size.
2. The welded joint of various IN Steels showed good ductility and mechanical properties.
3. The results of maximum hardness test on the HAZ of non-preheated mild IN Steel showed little hazard of weld cracking. Furthermore, the HAZ of low alloy IN Steel has lower hardness than that of comparable commercial steel.
4. A slight raise in transition temperature was obserbed in Charpy impact test on HAZ of IN Steel.
5. NDT of mild and low alloy IN Steels obtained by the NRL drop weight test were betwen-40°C and -75°C.
6. In crack sensitivity tests on low alloy IN Steel (IN-12), no crack was observed in weld metal and HAZ when the specimens were prebeated above 100°C.

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With utilization of the theoretical epuation which shows the penetration depth in electron-beam welding, the theoretical equation for the general estimation of optimum weld heat input is induced for the specimens of which thickness and material are known, when there are welded by I-shape butt type and single pass method. Moreover the experimental equation for the estimation of ptimum weld heat input is made by rearranging a number of the actual optimum weld heat input which have been obtained in Sciaky Co. Ltd., U. S. A. and in N. R. I. M., Japan.
After the theoretical and experimental equations are compared and discussed, the equation for the general estimation of optimum weld heat input is decided respectably.
Therefore, when the thickness and material of specimens are known before welding, with utili-zation of the above equation, the accurate estimation for optimum weld heat input which gives the fully penetrated bead by single pass for I-shape butt type joint is become possible hereafter.

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This report describes the characteristics of the weld penetration shape by high-voltage electron beam. Included in this study were an investigation of the effect of welding conditions on the penetration shape for stainless steel and aluminum and the relationship between the species of metals and weld penetration.
Results from the studies on the weld penetration shape are summarized as follows.
1) The penetratson shape for stainless steel are peculiar having a bowl-shaped part in the weld penetration. Increasing the accelerating voltage the penetration becomes narrower and deeper. The penetration depth is nearly proportional to the welding input and has no relation with the accelerating voltage in our experimental conditions. If the beam happens to pierce the specimen (over-penetration), the bead width suddenly decreases and the bowlshaped part disappears.
2) In the case of aluminum, triangular-shaped penetration with relatively wide bead is formed differs from the stainlesss teel welding. The penetration increases proportionally with increasing welding input in the range over 400 watts. The penetration ratio for aluminum are no more than 2 and it indicates a shallow and wide weld bead.
3) Investigation of weld penetration shapes on various metals shows thst shapes on steel material are narrow and deep but are nearly triangular and shallow on other metals. The prediction of the penetration depth is generally possible from the physical values, especially, thermal diffusivity and heat required for melting the metal.

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The effects of exposure at 550°C for times up to 5, 000hr on the notch ductility and the microstructure change of welded joints of Ni-Cr-Mo type HT60 tempered steel were investigated. Test steel, 100 mm thick, was produced experimentally for nuclear pressure vessels and welded joints were made by the electroslag welding and the CO₂ arc welding.
The results of the study indicated that the base metal, the weld metal and the heat-affected zone (HAZ) of base metal exposed at 550°C for times up to 5, 000 hr tended to embrittle owing to the grain growth of ferrite and the cohering of carbide toward grain boundaries. This process of change in microstructure may be divided into the following three kinds of stage. There are in different combination of the three kind of stage due to the structure before exposure.
(I) stage ; Tempering of martensite and spheroidizing of carbide
(II) stage ; Formation of fine ferrite
(III) stage ; Grain growth of ferrite and, cohering and coasening of carbide toward grain boundaries
The chang in microstructure for the base metal occur with (III) stage only ; the HAZ with electroslag welding (II) → (III) stages ; the HAZ with CO₂ are welding (I) → (II) → (III) stages, respecitvely.

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