It is generally said that residual stress existing in weldment must be removed for its unfavourable effects on brittle fracture, delayed cracking or stress corrosion cracking etc.. Postweld heat treatment is usually applied to remove the residual stress. Some problems are met when the heat treatment is applied, namely toughness decrement, reheat cracking especially in weldment of high tensile steel. This report describes, concerning mainly with high tensile steel weldment, proposal of testing method to evaluate the cracking susceptibilities and some characteristics of the cracking. The results obtained are summerized. as follows; 1) Reheat cracking susceptibilities can be evaluated quantitatively, easily and precisely by newly developed testing methods. Same crack is obtained as in the actual weldment. 2) This crack is typical grain boundary crack along prior austenite grain boundary. At the tip of crack, voids are observed and some of them coalesce to become a small crack. These voids exist on grain boundary which is perpendicular to stress axis and they are round type just like ones observed in creep phenomenon. 3) The behavior of the cracking is considerably affected by the existence of notch. When the weld reinforcement is removed, the crack can not be found at the surface but small cracks are observed at the interior of plate and so it is difficult to detect the crack with NDT. There is no effect of heating rate in generally used range on the susceptibility of cracks and cracks begin to grow at the temperature of about 550°C. 4) There is a tendency that cracking susceptibility decreases with higher preheat temperature or lower strength electrode. 5) Alloying elements, V, Cu, Cr, Mo, Nb and C are unfavourable for the reheat cracking. But only Mn is favourable. Quantitative effects of these elements on the cracking are obtained.
The spattering phenomenon of molten Iron-Nitrogen system under the conditionof arc melting has been observed using a high speed camera. It was found that the spattering phenomenon was much related to the nitrogen content of molten iron under the arc. The nitrogen content of arc melted iron (260 ppm O) increases with increasing the nitrogen partial pressure up to PN2=0.004 atm, while above 0.004 atm, the nitrogen content becomes nearly constant around 650 ppm N, and the spattering phenomenon begins to occur from above 0.004 atm nitrogen partial pressures which correspond to the solubility of nitrogen in the molten pure iron above I atm nitrogen partial pressure of non-arc melting. From the photographic analysis for spattering particles, the internal pressure of a bubble in the molten iron during the arc melting was calculated from the initial velocity of a spattering particle, which was also a function of supersaturation of nitrogen.
The authors have newly developed a Narrow-Gap One-side Horizontal CO2 Arc Welding Process (NOW-H Process) for on-site welding of column to column joints of steel structures of building. In this paper, mechanism of the formation of reverse-side bead is discussed experimentally to establish the basis of the NOW-H Process. Formation of reverse-side bead in the horizontal position welding is different from that of the flat position, as already mentioned in the Report 2. In the horizontal position welding, concavity of the reverse-side bead is apt to occur at the root of upper side plate, and also the shape of 1st pass is prone to become convexity, resulting in the lack of fusion in the medium of 1st and 2nd layer. Accordingly, in order to prevent these weld defects and obtain a steady reverse-side bead, adequate backing methods are needed. Effects of diameter of wire, root-opening, arc voltage and other factors on the formation of reverseside head are described.
Relation between Charoy Impact properties and microstructure of weld metal with submerged arc welding were investigated. And the the way for improvement of Charpy impact properties by the control of microstructure on weld metal were found out. Results can be summarrized as follow. (1) The growth of the coarsened ferrite grains and the lath like structure on the microstructure deteriorate the Charpy impact properties, but the uniform growth of the fime ferrite grains improves the Charpy impact greatly. (2) The uniform growth of the fine ferrite grain structures depends on the temperature range of γ→α transformation on the weld metal, and on Mo and Ti additional weld metals, the uniform growth of them are promted, the very high Charpy impact properties can be gotten.
Fatigue strength of welded joint in air is remarkably low compared with dressed welded joint or base metal, because of stress concentration at the toe. But, in the case of corrosion fatigue of welded joint, it is not clear, which factor is the most predominant, the notch effect, corrosion or the discontinuity of structure. In this experiment the corrosion fatigue of three kinds of steel welded joints was lower than the fatigue strength in air only at lower stress and longer life, but not at higher stress and shorter life. And then, this reason was experimentally deduced that the notch effect at the toe was the main factor to decide the corrosion fatigue strength of welded joints at higher stress but the corrosion effect was predominant at lower stress.
In previous reports, it was made clear that knife line attack phenomenon in stabilized austenitic stainless steels was closely related with the precipitation of M23C6 type carbide at the austenitic grain boundaries in HAZ near the weld interface during sensitizing-heat treatment. However, knife line attack phenomenon in SUS 321 could not be explained fully by the precipitation of M23C6 at the austenite grain boundaries. In this report, the further investigation on the mechanism of knife line attack phenomenon in austenitic stainless steels, especially in SUS 321 was carried out, in order to clearify this phenomenon in detail. From experimntal results, the following facts were identified on the mechanism of knife line attack phenomenon. (1) In SUS 347, the precipitation of M23C6 type carbide at austenite grain boundaries in HAZ near the weld interface by sensitizing heat treatment after welding is the main cause of knife line attack. (2) In SUS 321, δ ferrite phases precititate at the region heated higher than about 1100°C in HAZ, and these phases occupy about 10 volume % at this region. (3) After sensitizing heat treatment, the corrosion resisting property of δ ferrite phases in HAZ deteriorates and these phases accelerate the intercrystalline corrosion at this region. (4) The new mechanism of knife line attack in SUS 321 is that δ ferrite phases at austenite grain boundaries accelerate the intercrystalline corrosion, in addition to the deteriorating effect of M23C6 typpe carbide on the intercrystalline corrosion resistance.
Weld thermal cycles during underwater wet welding (bead welding in the flat position) using a plasma arc as a source of heat were measured. On this measurement, the cooling process and the effects of welding parameters on the time required for cooling from 800°C to 700°C, 800°C to 500°C and 800°C to 300°C were investigated. The experiments were carried out at a depth of 20cm in city water by using 12mm, 19mm and 25mm thick steel plates as the base metal. The results of this investigation are summarized as follows: (1) The cooling process during underwater wet welding is characterized by a maximum cooling rate under cooling due to the transition of boiling and by discontinuous curves of cooling due to the instability of boiling. (2) In a quasi-stationary state of underwater wet welding, (2-1) The cooling time for each range of temperature near the center of bond is prolonged by some 20% as initial temperature of the water environment rises from 1.5°C to 30°C. (2-2) A reduction in thickness of the base metal hardly prolongs the cooling time. (2-3) The cooling time is nearly in proportion to relatively low input of weld heat. The prolongation ratio of cooling time to weld heat input, however, decreases gradually as input becomes still higher.
The effect of disturbance from tack of weld part and root gap on bead appearance, depth of penetration, rate of blowhole and mechanical properties of deposited metal were studied. The results are summarized as follows: (1) The intensity of arc current and its range in which good bead and good penetration coexist increases with increasing of the throat thickness of tack weld and decreases with increasing of root gap. (2) In the region where good bead and good penetration coexist, both the rate of blowhole and the absorbed energy of deposited metal decrease with increasing of the throat thickness of tack weld and increase with increasing of root gap.
In the first part of this paper, the fundamental characteristics of arc behavior and bead formation were made clear in non gas-shielded arc welding using flux cored wire. Non gas-shielded arc welding is characterized by the following two properties in comparison with the other arc welding using solid wire. (1) The digging action by arc force on molten pool is very weak. (2) The arc is soft and formed in accordance with the surface shapes of mother plate or molten pool. As a result of above mentioned property (1), the penetration in this method comes to be shallow one with wide bead-width, especially at low welding speed in which molten pool volume increases just under the arc. The arc behavior such as property (2) is clearly observed at high speed V-groove welding where the arc is formed separately on the both faces of groove. Such a separation of heat source strongly depends on arc voltage and groove angle and influences the bead formation and results in the lack of fusion at groove-bottom in the extreme case of high welding speed. In the second, a study was made on the melting phenomena and bead formation in horizontal fillet welding on the basis of above mentioned characteristics. The bead formation in high speed horizontal fillet welding is similar to that in downward V-groove welding, at low speed range, the effect of gravitational force on bead formation, however, appears remarkably as follows : (1) The leg-length on vertical plate is considerably greater than that on horizontal plate, because the metal-flow in the direction of the vertical plate by plasma stream comes to be strong with hanging of molten pool and contributes to the melting process. (2) Because of weak arc force in the present process, the bead surface shape is quantitatively close to the result of surface tensional analysis under gravity assuming two dimensional molten pool. Therefor, if the leg-lengths and the quantity of deposit metal are given, bead shape in horizontal fillet weld might be derived from the two dimensional analysis used in this study.