Authors have made a study of the effect of the thicker plates urged by the need to solve the following constructional problem of 45, 000 DWT ore and oil carrier which our yard is planning of build.The construction of this ship as it is shown in the Fig. 1-Midship section, there is a large opening in the centre of upper deck. In this case, in way of supporting upper deck sectional area, there arose a question whether we shall adopt the doubling system which is usually applied to ore carrier, or boldly do away with the doubling plate and use extra thick plate instead. To be more in detail, if (a) two lines of doubling plates were passed on one side the plate thickness of upper deck would be sufficient by being 38 mm, but if (b) the doubling plates were limited to a single line then the thickness of plates requires to be 40mm and in case of (c) no doubler the thickness must be 47 mm.In consideration of the work in the spot, the compensation by the doubling involves a diffecult job. On the other hand, in consideration of the quality of steel and its weldability it seems more undesirable as the thickness of the steel plates become larger. As far as the thickness of the plates is concerned, there is not much difference in the effect 'between 38mm and 40 mm, so our consideration of the problem was focussed to whick should we choose, (b) or (c).The steel to be used for the purpose which came to our mind was C class steel of AB Rule, but when the thickness of steel becomes so large it is questionable whether we many obtain a suitable material that comes within the specifications of C class steel and a satisfactory joinability of the same time. So with a view to find out which possesses better joinability and the weldability for service, we made the following tests in order to make a comparative study between (b) and (c).1. Tension Test in room temperature.2. Standard V-Charpy Impact Test in the range of temperature -40°C to +60°C.3. Longitudinal Bead Bending Test (Kommerell Test) in the rang of temperature -60°C to +40°C.From these tests we could reach to the following conclusions.(1) According to the static tension test of both plates, of cf .Fig. 4, the normalizing will increase both yield strength and elongation, but gives hardly any effect to the tensile strength. (cf Fig. 5)(2) In the V-Charpy test, cf Fig. 6, the normalizing will lower 40 ft-1b transition temperature of the steel of both plates, but there was not hardly notceable of the effect to 15 ft-1b transition temperature. (cf Fig. 9) Generally 47mm thick plates, in comparison with 40mm thick plates, the notch sensitivity is somewhat larger, but the impact value in low temperature was sufficiently large when normalized. (cf Table 3 and Figs 7-13)(3) In the Kommerell test, cf Figs 14 and 27, both plates showed much larger bending angle than what is required under Austrian Welding Standard (cf Fig. 27). (cf Fig. 28). When low hydrogen electrode was used, the toughness of the weldments was very high, and even when the other types of electrode were used, if weldment was preheated it regained its toughness very noticeably. (cf Figs. 22-26)
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