A series of root cracking tests were conducted with y-groove restraint cracking specimen of high strength steels with thickness from 30 to 50 mm. Effects of size of specimen, including bead length and plate thickness, and of multi-layer weld on root cracking were mainly studied in the report. The conclusions obtained in the study are as follows: (1) The root cracking tendencies of six high strength steels with their strength levels from 60 to 79 kg/mm2, that is, HT 60 and HT 70 steels, were evaluated and preheating temperatures necessary for the prevention of root cracking were decided. The maximum hardnesses in HAZ of these steels, which were safe to root cracking, ranged from 350 to 400 VHN under the load of 1 kg. Microstructures of HAZ safe to root cracking were decided with an, aid of of CCT diagrams, that is, 5-10% ferrite, 20-30% intermediate structure and 60-70% martensite for HT 60 and 10-25% intermediate structure and 75-90% martensite for HT 70 in their area percentages of constituents. (2) Neither the root crack, which was initiated at the first layer weld, was propagated nor a new crack was initiated with the progress of a multi-layer weld. Cracking tendencies rather decreased in multi-layer welds with a choice of proper interpass temperature. (3) An increase of root cracking was not detected with an increase of bead length and a root crack rather increased with a decrease of bead length at a bead length below 150 mm, because of a rapid cooling of a short bead. Therefore it was suggested that the transverse tensile residual stress in a weld would directly affect the initiation of a root crack and the longitudinal tensile residual stress would indirectly affect the initiation of a root crack. (4) The tendencies of root cracking did not increase with an increase of plate thickness if the welds were cooled by the same cooling process. (5) It was confirmed that the slotted cracking specimen with a size of 200 mm×150 mm was equivalent to one of 300×200 mm. The smaller size is therefore satisfactory for a root cracking test of high strength steel welds.
The important role of hydrogen in root cracking of high strength steel welds had been already discussed in the previous report. In this report the quantitative effect of hydrogen was clarified with HT 60 welds and the effects of lower cooling process below 300°C on root cracking were discussed. The activation energy necessary for root cracking was measured with slotted cracking specimens which were quenched at various low temperatures. Difference in cracking behavior with a change of groove type or root opening was discussed and the repeating number of specimens necessary for a root cracking test was obtained. The conclusions obtained in the study are as follows: (1) There was ascertained an obvious difference in the iniation of root cracks when diffusible hydrogen contents in HT 60 welds varied from 1 to about 3 cc/100 gr (2) Initiation of a root crack in a weld metal was remarkably affected, by chemical compositions of a weld metal, even with a diffusible hydrogen content of lcc/100gr. (3) Propagation of a root crack in an HT 60 weld metal was little affected, even if the diffuible hydrogen content was increased from 3 to 6cc/100gr. (4) It was the cooling process above 300°C that affected the initiation of root cracks in HT 60 welds. In HT 80 welds, however, the cooling process below 300°C also affected the initiation of root cracks. (5) Through the measurement of incubation time for root cracking in slotted cracking specimens which were quenched at various low temperatures, the activation energy necessary for root cracking was estimated at about 10, 500 cal/g. atom, the value of which was in good agreement with the activation energy for the diffusion of hydrogen in a-Fe. (6) Repeating number of specimens which was necessary and sufficient for estimation of cracking tendency was three or four, so far as the slotted cracking test was regarded as "go or no go test". At least eleven specimens were necessary in order that cracking percentage might be estimated in detail. (7) It was understood with consideration of angles of inclination of fusion lines that a root crack occurred exclusively in HAZ with y-groove specimens and in a weld metal with U-groove specimens. (8) It was also explained by the angle of inclination of a fusion line that cracking behavior was varied with the gap of root.
The author measured various angles on the surface of penetration obtained by rapid hammer blow exerted upon the mother plate in the course of welding in flat position for various welding conditions with five kinds of covered mild steel arc welding electrodes, and discussed on the relation between the contact angle of mother drop covered with various slags and weld bead configurations. Results obtained were as follows. (1) The angle of solidification θs' angle between the extension of bead surface and the solidifying surface of weld bead, is almost a constant value along the total length of the solidifying line except at both ends. (2) Angle of solidification at the mid-point between the end and the center of the solidifying line is almost the same under various currents. 9s, mean value of 0s, seems to be nearly the same as the contact angle θo characteristic to the kind of welding electrode. (3) The order of wetting property of mother drops from various electrodes, which is presumed from the angle of solidification against the solidifying surface in manual welding or in submerged arc welding using slags of the same coated electrode, roughly coincides with what presumed from the shapes of alcoholic water drops of various concentrations placed within the actual surface of penetration coated by paraffin. And the order is the same as the one of JIS code number. (4) The ratio of height to width of bead in various welding electrodes increases according to the JIS code number, except D 4320. (5) The effect of up-and down-hill welding or of welding speed on the shape of penetration and weld bead can explanined by means of the changes of direct and indirect digging action of welding arc.