Dynamic characteristics of the rectifier type d.c. welders with drooping or c.p. characteristics are discussed and it is shown that the short circuit transient current takes the form shown in Fig. 1(a) when the impedance of the interphase connection ZP is low, and the drooping characteristic is attained. When ZP is large, the transient current shows the tendency shown in Fig. 1 (c)-(e). Transient current if in control winding is also shown in Figs. 3 and 4 for ZP =0 and Zp= ∞ respectively.
The authors studied on the various welding characteristics and droplet transferring phenomena of the newly developed flux-cored covered electrode, whose core wire contained a flux of 4 weight percent chiefly composed of carbonates. The results obtained are as follows: 1. The bare flux-cored wire gives greater specific melting rate as compared with ordinary solid wire, and this merit is retained when it is covered with outer flux. 2. The new electrode gives, independently of the coating type, a greater melting rate and a smaller transferring droplet. 3. The feature of the new electrodes was proved to originate from the inner cored flux, because it is not seen in the ordinary electrode whose outer flux is the same as the sum of the outer and inner flux of the new electrode in its component and total quantity. 4. The new electrode gives more stable and less divergent arc, less spatter and accordingly fine and beautiful bead appearence. 5. The new method of high-speed cine-photography clarified the phenomena inside the electrode sheath. It is considered that finer droplet from the new electrode depends not only upon the mechanical action of the evolving gas from inner flux, but also upon its characteristic arc which separates the droplet sticking to the core wire tip in an earlier stage than in the ordinary electrode.
Recently, plasma spraying has been applied to various purposes in the industrial field. Plasma process is applicable to various materials which have been considered impossible to spray by oxy-acetylene flame spraying. However, properties of coating made by ordinary plasma process are not always so good because of porosity or weak bonding between the substrate and the coating. The authors have developed a plasma spraying method which is characterized by the following: 1. Application of shield nozzle to insure sufficient protection of substrate and spraying material from the chemical attack of atmospheric gases. 2. Effective and local preheating of the substrate to promote the metallurgical bonding between the coating and the substrate. 3. Selection of proper size of particle to secure their perfect melting. 4. Determination of the stand-off distance of torch from the substrate to assure the proper flying velocity of particle. This new method was applied to the coating of mild steel with the Ni-Cr-B-Si wear resistant alloy using the plasma spraying torch and shield nozzle as shown in Fig. 1. Various examinations of coating were carried out with the following successfull results; (1) Sprayed layer has a dense and fine grained structure, and a defect is hardly seen. Grain growth and carburizing of substrate, which are observed in sprayed and remelted one, are not observed. (2) Hardness of coating is in range of VHN 850-1100, and is much higher than that of remelted one. (3) Density of coating is as high as 99.2% of cast one, and the porosity is less than 1%. (4) X-ray diffraction analysis of coating has shown that detectable change of alloy did not occured during the spraying. (5) Adhesive strength between the coating and the substrate was more than 40 kg/mm2 as a result of metallurgical bonding between them. (6) Sand wear tests of shield sprayed specimen show that the average weight loss was about 1/2-2/3 of remelted one.
A transition piece used for super-heater tubes of power plants was made by friction weld holding an Inconel tube between an austenitic stainless steel tube and a 2.25 Ci-1Mo steel tube, them mechanical properties at room temperature of the tubular transition piece were examined and thermal cycle and creep rupture tests were carried out for practical use. The test results are as follows: 1. Welded joints (730°C S.R) made by friction weld showed satisfactory tensile and bending properties. 2. Under thermal stress by 230-650°C thermal cyclic heating and internal pressure, the tubular transition piece was ruptured within the 2.25 Cr-1Mo steel tube in every case. 3. In the creep rupture test at 650°C, the tubular transition piece was ruptured within the 2.25 Cr-1Mo steel tube.
Most of machines, vessels and others for high-temperature service are not static in their load or stress in actual service but vary in some regular or irregular manner depending on the operating conditions. In designing and manufacturing these products, therefore, the flow or creep properties and rupture characteristics of used materials and their welds under varying stresses at elevated temperatures are very important. Such varying stresses as mentioned above may be classified into two types; one is the alternating static stress. which has as long period as several minutes or several hours and another is the continuous and dynamic stress which has as short period as less than one second. The latter stress condition was adopted in this study. Both static and tensile pulsating stress rupture tests were carried out on mild steel at 450°C as a fundamental research before the investigations on heat resisting alloys and their welds were made. Pulsating stress rupture tests were conducted in which four values of stress amplitude ratio A (σa/σm) were selected for a given mean stress; stress amplitude ratio A=5, 10, 20 and 30%. The frequency of pulsating stress was 380 rpm. The mean stress versus time for rupture diagram in log-log plots showed that the rupture life decreases as the stress amplitude ratio A increases for the same mean stress. For a given mean stress, therefore, the static creep has longer rupture life than pulsating stress creep in which the rupture life decreases with increase of stress amplitude. But it is not appropriate to consider that decreases in rupture life mentioned above are only caused by the dynamic stress or fatigue behaviour due to alternating stress. In view of fatigue behavior, a stress versus time for rupture diagram was plotted for maximum stress (crest stress) in the stress cycle on log-log scale. It was shown that the rupture life in pulsating stress creep became longer as the stress ratio A increased and the static creep had the shortest life. This is incompatible with fatigue behavior. Rupture characteristics in pulsating stress creep were also investigated from creep behavior, assuming that ruptures under pulsating load were caused by accumlation of static creep damage produced by the instantaneous stress and the dynamic effect did not influence the rupture behavior. Consequently, the rupture strength of pulsating stress creep, regardless of stress ratio A, turned out approximately equal to that of static creep. Ductilities of test specimens, determined by true strain at the fractured section, were compared in both static and pulsating stress creep. But no remarkable difference was found. For the mild steel at 450°C, ruptures under the stress conditions of a relatively small pulsating stress being superimposed on the static stress which is sufficient to produce a creep rupture may be caused by static creep damage, and dynamic stress effects may scarcely affect the rupture behaviors.