Temperature cooling rate is calculated for points near bead line in thick plate when linear heat source is applied instantaneously along all the range of bead whose length is l. Fig. 4 shows CR (cooling rate passing a definite temperature Ts) along the bead, and we see that CR at bead end is 2 times the bead center of infinite length. For the point whose distance from the bead end is 2 times rm, CR is 0.56 times the bead end. Here rm means the perpendicular distance from the bead, where the maximum temperature is equal to Ts, and rm is shown in equ. (29). Equ. (4) gives the relation of CR and temperature Ts, input heat q0. Fig. 7 shows the effect of bead length, and we see that for l>2rm, CR for the bead end is practically equal to that for l=∞, and for l<1.5rm, CR is equal to that of the point heat whose heat is equal to q0⋅l. For a moving point heat source, cooling rate is also calculated for starting and stopping ends of bead and those values are compared with that of infinite velocity. From Fig. 8 we see the ratio tends to unity for X>10.
The improvement of through thickness ductility in steel plates is an important aspect to prevent the lamellar tearing in heavy welded structures. Many studies on the relationship between the cracking susceptibility and material characteristics, such as morphology of inclusion and chemical composition of steel, have been reported, but little is known about the effect of steelmaking factors on the susceptibility. Tests are carried out using the steels which are manufactured experimentally under the various conditions, and the effects of the ways to minimize oxides and sulfides on the cracking are discussed. Semi-killed steel has a high lamellar tearing susceptibility, and the crack is originated from large shape of FeO-MnO-Al2O3-SiO2 oxide by predominantly shear mode. Si-killed steel with a little amount of Al has a low lamellar tearing susceptibility. For these two steels S content up to 0.026 % doesn't affect on the cracking susceptibility. For Al-killed steels with more than 0.01 %S the inclusion displayed a significant role against the cracking is found to be sulfide, and the improvement on this type of lamellar tear is readily acquired from using the de-sulferized raw materials. For Al-killed steels with less than the level, the oxides clustered at the sub-surface of steel plates are detrimental inclusions, and the ingotmaking factors, such as the shape of molds and degassing or pouring process, are the most important ingredient. The through thickness tensile specimen imposed synthetically weld thermal stress cycle on reveals that the decohesion of MnS and Al2O3 from the matrix is apt to occur at temperatures of about 300°C during cooling.
The torsional deformation mechanism at brazed joints was investigated in detail. Butt joints of two types of carbon steel brazed with pure copper were used in torsion test. The torsional deformation at filler metal was measured with optical microscope. It was cleared from test results that the discontinuity of shear strain existed at the brazed joint. This discontinuity depends on the mechanical properties of the base and the filler metals, and joint design. Furthermore, the deformation mechanism of the filler metal was discussed in the dislocation theory. It is not expected under shear stress that the triaxial stress as under tention takes place in the filler metal of the brazed joint. Therefore, the brazed joint in torsion is strengthened by the effect of interface between base and filler metals, such as the effect of grain boundaries in the case of plastic deformation of polycrystalline metals. Consequentry, it is suggested that the local deformation of filler metal has remarcable effect on the joint strength under dynamic loading.
Althouth it is well-known that the weld metal by CO2 welding has better toughness and static mechanical properties than those by coated electrode manual arc welding, CO2 welding gives more rough andirregular bead shape in comparison with other welding processes using flux. Therefore, it is not clear, whether CO2 welded joint with higher toughness and ductility may give the higher fatigue strength than coated electrode welded joint, because fatigue strength of welded joint is strongly influenced by geometry at the toe of bead. This study was carried out to compare fatigue strength of full- and semi-automatic CO2 welded joint with that of coated electrode manual arc welded joint of previous report. The obtained results were as follows; 1) Fatigue strength by full-and semi-automatic CO2 welded joint was almost the same level. 2) Full-and semi-automatic welded joint had a little lower fatigue strength than that of coated electrode welded joint; This was attributed to the sharp toe geometry. 3) Fatigue design strength may be proposed as 14kg/mm2 for CO2 flat positoned full-and semi-automatic, and vertical positioned semiautomatic welded joints.
In order to get higher strength and toughness of steel weld metal, the effects of single or double addition of alloying elements such as Al, Ni, Cr, Mo or B on the strength and toughness of 0.03-0.05 %C as-cast steel specimens in stead of steel weld metal have been investigated. This carbon content made the toughness of as-cast steel specimens containing 0.35 %Si and 1.5 % Mn highest in previous report. The results obtained as follows; 1: No single addition of Al, Ni, Cr, Mo or B improve the toughness of as-cast low carbon steel. 2: Double addition of 0.34 %Ni and 0.09 %Mo is effective on the improvement of mechanical properties of as-cast steel. In this case, the tensile strength is 46 kg/mm, and the impact energy transition temperature is -40°C.