Effect of melting variables on the slag-metal reactions in electro-slag-remelting process (ESR) was studied using several manganese bearing steel electrode wires and CaO-SiO2-CaF2 system fluxes. Main results obtained are as follows: 1 The temperature distribution in the molten slag and metal pool was measured. The temperature of salg was the highest in the middle part between electrode tip and molten metal pool. 2 The temperature of slag rose with the increase of melting voltage. 3 Electrode tip to liquid metal surface distance was affected by melting voltage and flux weight. 4 Mn, Si and O content of ingot were considerably affected by melting voltage and flux weight. 5 Using the results obtained, some slag-metal reactions were discussed.
In order to clarify the relationship between the reduction of hydrogen concentration through low temperature postweld heat treatment (LTPWHT) and its treating conditions, the analyses on the diffusion of hydrogen during the treatment were carried out using an analytical computer program in which the influence of the gradient of hydrostatic stress caused by welding residual stress distributions was taken into consideration. As a result, it was found that the ratio of the highest hydrogen concentration at any time during the treatment to the hydrogen concentration at the point right below the final pass of welds just after welding, could be arranged by a parameter (τ+Dptp), where r and Dptp are the parameter of hydrogen diffusion determined from welding conditions and the product obtained by multiplying the diffusion coefficient at a temperature of the postweld heat treatment by its holding time, respectively. The influences of the dimensions of the weld zone, such as plate thickness or groove width, upon its relationship were also clarified. Moreover, the diffusion coefficient of hydrogen for 2 1/4Cr-1Mo steel weldments, which becomes necessary to determine the conditions of the heat treatment for preventing the cracks, was determined in the temperature range of 150°C through 300°C.
A study has been carried out into the mechanical and metallurgical properties of welds of the so called super-ferritics with carbon and nitrogen impurities of less than 200 ppm. The tests were carried out on three kinds of materials, one commercial grade and two kinds of 19Cr alloy with carbon and nitrogen impurities of around and less than 30ppm respectively. Tensile test specimens were so designed as to possess a TIG weld bead at the center of the gage sections. They were aged at various temperatures in order to induce 475°C embrittlement, σ-phase embrittlement, high temperature embrittlement, and an embrittlement due to fine carbide and nitride precipitation at grain boundaries. Tensile properties were obtained using an Instron-type machine at room temperature. As a result, it was found that there exist at least four kinds of embrittlement for the present weld joints: a. 475°C embrittlement b. embrittlement due to fine carbide and nitride precipitations at grain boundaries c. high temperature embrittlement d. grain boundary embrittlement due to phosphorous segregation at grain boundaries by 1350°C aging and the following water-quenching. 475°C embrittlement is not induced by welding process only. However once the embrittlement is realized by aging heat treatment, it is most severe at weld section. TIG arc-strike weld cracking test was conducted to investigate the cracking behaviour of weld nugget, especially the locations and the temperature range, the latter being monitored by use of acoustic emission technique. The cracking occurs at sub-grain boundaries at equiaxed zone of the weld nugget in the temperature range of 400-1000°C during cooling after welding. The cracking increases with the increase of heat input. The mechanism of the cracking is of the combination of matrix hardening due to high temperature embrittlement and carbon (nitrogen) segregation at sub-grain boundaries.
We have developed the world-first 10 channel acoustic emission telemetry system where acoustic emission singals (envelope profile) from incipient crackings are transmitted to signal processing unit by way of FM radio. The system was designed to apply acoustic emission techniqure to the continuous safety surveillance of very large welded structures like oil storage tnaks, line pipes, and chemical reactors where enormous lengths of wiring are required otherwise. The system was applied to a 17 m diameter spherical tank and a 104 kl cone-roof tank where a pulse simulating acoustic emission was generated at certain point on the surfaces and recieved at the various locations of different distances up to 10 m from the signal source. Both arrival-time triangulation and attenuation calibration method were used to locate acoustic emission events. Sound velocity and attenuation characteristics were obtained by using the leading edge of the FM-radioed acoustic emission envelope and its amplitude respectively. The measured values correspond well with the ones reported in the literatures. Further the system was successfully applied to monitor and locate cracks of hydrogen weld cracking as well as fatigure crack propagation of a 3 m diameter spherical pressure vessel.
A new welding method has been developed to prevent arc blow from magnetic interaction between twin electrodes. In the new method the welding current which is supplied from a DC power source is alternately switched by transistor switching ciruits to each electrode. The results obtained in this study are as follows. 1) Arc stability is maintained when the distance between the centers of the twin electrodes is 5-15 mm and the frequency of the pulse current is 10Hz-1kHz. 2) When twin electeodes are used in close proximity to one another, the critical welding speed at which humping beads are induced is from two to three times as fast as that at which theyy are induced when the conventional twin electrode method is used. 3) The critical welding speed of the new method at which acceptable beads are obtained is about 3 m/min for butt welding of mild steel plate in 1.2 mm thickness.
A particular projection welding was investigated for welding dissimilar metals using single phase projection resistance welder. A pair of dissimilar metals, carbon steel disc with ring projection and cast aluminum alloy block, is pressed with each other and welded. Softened by welding current, projection in plastic state flows from the top to the bottom portion along the weld surface of the block under the influence of electrode force. Simultaneously, the high temperature clean surfaces of dissimilar metals are obtained and are pressure-welded. Main results of this study are as follows; (1) Projection shape should be determined as to make the plastic flow maximum at the weld surface during welding process. If projection shape is too sharp, it collapses in its middle position and if too blunt, its adjacent part to the weld surface is not deformed, conforming to the temperature distribution. Consequently, there exists adequate projection sharpness, and it varies by welding time. Projection should have sufficient height not to interfere the plastic flow from its top portion, throughout welding process. (2) High welding current, short welding time and high electrode force are adequate welding conditions. Short welding time is needed to retain the maximum temperature portion in projection adajcent to the welding surface, diminishing the removal of heat from the welding surface by cast aluminum alloy block, and also not to form intermeltalic compounds in the welding surface. Short welding time leads to high welding current. Electrode force raises welding strength of the joint proportionally by increasing the amount of the plastic flow at the welding surface. (3) Thus obtained welding factors determined appropriate are; the ring projection diameter is 22 mm, its crossection is isoscelestrapezoid, height 2 mm, top width 0.5 mm, bottom isotropic angle 70°, and welding current is 70KA, and welding time 3 cycles (50 msec), electrode force 2.5 ton, the weld joint having the tensile strength equivalent to that of base cast aluminum alloy (22 kg/mm2) can be gotten without any intermetallic compounds. (4) Surface conditions, for example, oiled or greased, do not affect the welding strength because of the impurity-elimination action of the plastic flow of projection which, in welding process, also squeezes out molten metal and intermetallic compounds. (5) The welding joint strength decays as its holding temperature rises forming intermetallic compounds. The decay occurs as low temperature as 250°C.
The joint with the thin fusion layer at the interface is accomplished by means of using the pulsed high current. The factors related to the joining mechanism are described and these factors may be chiefly related to the. exothermic phenomenon as follows. The evolution of heat at the interface is found to be connected with heating of resistance caused by the actual constricted contact between two metal surfaces. The electrode force keeps the important role for the enlargement of this constricted contact setting the current path due to the generation of heat through the weld time. Using the electrode force changing up a certain slope, the joint with good mechanical properties is found to be obtained by the energy input smaller than using the constant electrode force.