The authors investigated the change of penetration shape with time of wide range, and the influence of atmospheric condition and slag addition to the molten pool upon the penetration by fixed TIG arc hitting the mild steel plate. After discussing the result obtained, we have come to the following conclusion. 1) The shape of penetration, in general, changes in the undermentioned series with the time lapse ; a) central type b) peripheral type and c) simple type. Such a change can be explained by the change of convectional mode in the molten metal corresponding to the distribution of arc current on the surface of the molten pool. 2) Conical or saucepan-like penetration obtained in Nitrogen atmosphere seems to be a modified central type caused by the characteristic turbulent state of molten pool. 3) Deep penetration of simple type in the hydrogen rich atmosphere or under the very viscous slag such as silica shows the effect of concentrated arc. 4) If the molten metal is added to the molten pool, the central part of penetration surface is re-solidified leaving the peripheral penetration of ring-shape. This fact proves the existence of radial surface flow caused by the differrence in surface tension.
As one of the weldability tests of high tensile strength steel, the measurement of continuous cooling transformation diagram in welding has been widely adopted. Since we had a kind of doubt for the conventional method to make the C.C.T. diagram for welding which had been published by various national or private research institutes including us, we conducted systematic investigation about how to judge the microstructures and how to interpret the dilatation curves. And we have found a certain mistake in the conventional way to make the C.C.T. diagram. As a result of the previous experiment about 80 Kg/mm2 high tensile strength steel, we have found most of the structure which was thought as pre-eutectoid ferrite was bainite structures. And we have already reported that the C.C.T. diagram should be fairly revised as the results. As a continuation of the first report, new C.C.T. diagrams in welding of various high tension steels ranging from 50 Kg/mm2 to 100 Kg/mm2 level were mesured in this report. We sincerely hope that the revision of previous C.C.T. diagram which had been already published in our catalogue or others will attribute to the need of customers and further develop ment of high tensile strength steel in general.