This paper, following the previous report that dealt with the method of selecting the heat-treatment conditions of spot welds in anti-corrosive high tensile steels, gives details of the weldability assessment of these steels. The ductility ratio of these steels is about 50% as welded. But it can be improved by tempering to about 70%. Under combinations of these steels and low carbon steels, it can be improved to about 80% by tempering. The objective of this study was to check the reliability of the welded joint of these steels in SHIN KANSEN electric railcars structures. Therefore, a strict test had to be carried out on the joint. A new machine has been developed to test the fatigue strength of spot welds in these steels. The testing data show that the efiect of heat-treatment on the improvement of ductility of spot welds and the relationships between the fatigue strength of spot welds and the welding conditions can be easily analyzed. Meanwhile, the crack initiating mechanism of spot welds in fatigue failure has been clarified. The experimental work has shown that a clean scale-free surface and high electrode force are essential for the uniform tempering of spot welds.
TIG arc spot welding was performed on thin sheets of aluminum with different chemical compositions. Migration of solid-liquid interface during weld solidification was observed with 16 mm cine camera and the actual growth rates of the columnar grains were measured from the films. Then, the observed results were compared with the results calculated with the theory of the heat conduction and an attempt was made to explain the mechanism of the formation of the solidification structures by applying these results to the modern solidification theory. It was found that the growth rate increased as the growth proceeded into the nugget and that the result showed qualitatively good agreement with the result obtained with the theory of the heat conduction. The solidification theory and the construction mechanism of the solidification structure obtained in the unidirectional solidification were also applicable to the case of the weld solidification, when the growth rate was much larger.
Flashing phenomena are observed by using high speed motion pictures of 7500 frame/sec, flashing voltage and current being recorded simultaneously by oscillograph. Some typical phenomena are shown in Photo. Nos. 1-15. Table 1 shows the experimental con-ditions. for the photographs. The marks S, A and N beside the individual frame of the photographs show short circuit, arc and no current-flow respectively. S and N are judged from the oscillogram and A from the neon lamp sign as shown Fig. 3 (b). Explosive production of many small molten particles is observed when a short circuit breaks and the following arc strikes. The cause of the splashing might be the local high pressure rise due to the striking of arc. The molten globules hanging at the end of the test pieces are blown off by the shock pressure rise. Due to the difference in the relative position of the arc striking point and the hanging molten metal, various types of flashing as shown in the photo. Nos. 8-15 are observed. The mechanism of the splashing is illustrated in Figs. 15-19. The electromagnetic force acting on the molten metal bridging between the test pieces causes the metal to move, and results in the flashing of the type shown in photo No. 6 and Fig. 13. The authors presume that the particles in Nos. 12 and 14 frames in photo No. 14 are driven out of the molten metal by rapid pressure rise due to the arc in No. 9 frame. The arc in No. 9 frame vanishes in the next No. 10 frame and consequently the shock pressure rise is presumed to be small. This molten metal has a very high temperature and can splash into many small particles by a comparatively small shock pressure rise because this molten metal is presumed to have very high fluidity. The discontinuous moving of arcing point in frame Nos. 5-6 of photo No. 5 may be explained if we imagine that a short circuit of very small duration causes the vanishing of the first arc and the breaking of the short circuit results in the second arc. We can see from photos Nos. 1-3 that the increasing rate of temperature rise is very high and from photo No. 3 that three parallel arcs are produced by breaking the bridging parts one after another in No. 1 frame. It is observed that flashing and surface appearance of test pieces are influenced by arc current and temperature of test piece end, and fine and smoothing flashing and plain surface of test piece end 'can be obtained by adjusting the secondary no load voltage or inductance of the circuit.
Among the factors affecting the tensile strength of mild steel butt joint brazed with Ag-Cu alloy, it is usually considered that the following are very important for joint strength: (1) Chemical composition of filler metal. (2) Chemical composition of base metal. (3) Joint clearance. (4) Brazing temperature and time. (5) Surface preparation of base metal. Of these factors, there are some investigations made hitherto, regarding the factors (1), (2), (3), (4). However, fundamental investigations of surface preparation of base metal which affects the joint strength are very few. Especially, there are no studies and development as to effect of rouhgness of base metal surface on joint strength. This paper presents some experimental evidences that roughness of base metal surface affects the butt joint strength of meld steel brazed with BAg-8 filler metal. To obtain some experimental data as to the effects of surface preparations, specimens of mild steel (ss4l) were prepared according to the design of Fig. 3. The two abutting surfaces of each specimen were identically finished by emery paper No. 60 120, 240, 400, 600. These specimens were then brazed with BAg-8 filler metal using a resistance furnace operated in hydrogen atmosphere, and machined according to design of Fig. 4. Tensile test of these specimens gave results shown as follows: (1) Tensile strength of specimens polished by emery No. 120 was very high, its value being 43.5 kg/mm2. But the specimens polished by emery No. 600 showed 35.2 kg/mm2. (2) In general, these results indicated that tensile strength of specimens polished by large grain size emery powder was higher than that of ones polished by small grain size emery powder. This difference depends on the actual interface area of filler metal and base metal. Strength of specimen polished by emery paper No. 120 is extremaly larger than that of one polished by emery paper No. 600. (3) Most of specimens were failured at filler metal zone, but a few of them were failured in base materials. (4) Usually, as regards the joint strength of mild steel, clearance of base metal is very important, but this experiment showed that the main factor affecting the joint strength is not joint clearance of base metal but roughness of base metal. Also, studies on spreadabilty of filler metal on base metal with varied roughness were conducted. To obtain some experimental data, mild steel plates (SS41 50φ × 1.6t) polished by emery paper No. 60. 120. 240, 400, 600 and BAg-8 filler metal (wt=0.1 g) were prepared. This experiment was carried out using a resistance furnace and operating it in a hydrogen atmosphere. As the result, spreadabilty of filler metal on the specimens polished by emery paper No. 60 was ex-cellent and that of one polished by No. 120 was second. However, there was a little difference between No. 60 and No. 120 in the spreadabilty of filler metal.
This study examines the corrosion fatigue properties in weld metal and heat-affected zone of butt welded joints of high strength steels in order to investigate reduction of their respective fatigue strengths in saline water. The tested materials are two kinds of 50 kg/mm2 class high strength steel (A-steel is ordinary steel, B-steel atmosphere-resistance steel) and 60 kg/mm2 class high strength steel (C-steel, quenched and tempered steel). From the test results, it is clear that the fatigue strength in 3% saline water on the weldment of Asteel is lower in the heat-affected zone than in the base metal while it is almost even in each zone of the weldment of B- or C-steel. The lower fatigue strength in 35/a saline water in the heat-affected zone of A-steel is due to the high sensitivity of the notches such as corrosion pits in comparison with other steels.