Fatigue tests have been conducted on HT80 as-welded and post weld heat treated weldments under four point bending. Fatigue crack was propagated in the direction of thickness from base metal to weld metal. The difference of hardness on the both sides of crack tip was found to influence the direction of fatigue crack propagation. The results of propagation rates plotted against ΔCTOD, all of the propagation data fell within the same scatter band, and differences of propagation rates due to the changes of microstructure and residual stress were not observed clearly in this relationship.
To get more understandings about hydrogen-assisted cracking features occurred in HAZ of HY80 steel, morphologies of cracking and stress necessary to initiate crack were investigated on the specimens heated under various thermal cycle conditions. Furthermore, the similar experiments were performed on SM50B steel to compare their susceptibilities to hydrogen-assisted cracking in HAZ of both steels. The susceptibility of HY80 steel to hydrogen-assisted cracking in HAZ increased with rise in heating temperature under any thermal cycle condition. In particular, when kept at the peak temperature and at a certain temperature during cooling, it was more raised. However, no dependence of hydrogen-assisted cracking susceptibility in SM50B steel HAZ on the peak temperature was recognized.
This paper deals with the reason and the prevention of the lack of fusion formation in MAG welding. The results obtained are as follows: (1) The lack of fusion which is the plane defect appears in the weld metal near weld bond, and the inside of the lack of fusion consists of oxides of Mn, Si. (2) The lack of fusion is formed when the solid surface appears frequently in molten pool. This solid surface becomes wider with an increase in welding speed, and the humping bead is formed when the solid surface spreads out all over the molten pool. (3) The motive force which forms the solid surface is the arc pressure. Addition of helium into the shielding gas is very effective to prevent the lack of fusion. With addition of helium gas, the arc pressure decreases, and the critical welding speed at which the lack of fusion appears can be shifted to the high speed zone in comparison with argon gas. (4) The arc fluctuation also promotes the formation of solid surface. The arc fluctuation appears when the metal transfer is globular type in high arc voltage and low welding current range.
The flashing current waveforms have been discussed using the steel sheets in Part 1. In Part 2, all the flashing operations were performed using the constant rates of platen acceleration same as Part 1. The microcomputer was used to calculate the secondary current and comsumed power during flashing. The following results are obtained. 1. In the latter flashing operation, the flashing current amount to a constant value and decreases with the increasing of the secondary no load voltage, but the electric power is not affected by them. 2. In regard to the mechanical properties of welded joints, the factors in bridge phase, such as peak or effective current and power, have the positive coefficient of correlation, and those in flash phase have the negative one. 3. It was found that the decreasing of secondary no load voltage increases the values of current and power in brigde phase, and also improves the quality of the welds due to the increasing of absorbed thermal energy in the material to be welded.
Authors reported already that the twin electrode switching TIG welding method was effective to increase welding speed in the welding of thin mild steel and copper plate. This method in which welding current is alternately switched to each electrode by the transister circuits is characterized by preventing the arc blow between the closely spaced two electrodes. In this report, the new method was applied to the welding of 3.2 mm thick copper plate and the following results were obtained. 1) The critical welding speed at which acceptable beads are obtained is about 600 mm/min. This speed is two times as fast as that of one electrode DCSP TIG. 2) The preheating by the leading electrode is effective under the conditions of closely spacing the two electrode and of selecting the welding current distribution ratio of the leading electrode larger than that of the trailing electrode.
An algorithm using a hydrogen diffusion theory, which can calculate the hydrogen concentration distribution in a multipass welded joint under various welding conditions, is introduced and referred to the result of hydrogen measurement in multipass welded joints. In order to obtain the welding conditions such as the necessity of a post-heating employment for the prevention of hydrogen-induced weld metal transeverse cracking, the algorithm is modified and the usefulness of this algorithm is confirmed from a result of restraint-type multipass weld cracking tests. It is also found that the diffusible hydrogen concentration measured by the method such as JIS Z 3116 is approximately 80 % of the initial hydrogen concentration just after welding (Ho), and that the maximum hydrogen concentration in a multipass welded joint (Hmax) seems to be less than Ho.
Through the report 1, it was revealed that humping bead formation was caused by the instability of capillary liquid metal at the weld surface. In this report, effect of beam current on humping bead formation was investigated. And the formation mechanism of unstable capillary liquid metal was discussed. The obtained results were as follows. 1) Humping bead formation ranges were classified into high and low beam current ranges according to the formation mechanism. 2) In high beam current range, the formation of humping bead was cuased by the large amounts of liquid metal that formed the unstable capillary liquid mead above surface. 3) In low beam current range, the formation of humping bead was caused by the undercut that formed unstable caipllary liquid metal. 4) Undercut was formed by the reduction of the wetting between liquid metal and the wall of electron beam gouge. 5) In low beam current range, the increase of beam current was apt to form the humping bead due to the formation of undercut. 6) In high beam current range, welding with a suitable depth of groove prevented from humping bead formation due to the reduction of liquid metal above surface.
The cyclic deformation behaviour near the crack tip in the yield stress heterogeneous field was investigated by cyclic elasto plastic analysis on F.E.M. model. ΔOCTD obtained analytically had a certain linear relationship with experimental ΔCTOD. Crack tip coming near the soft-hard boundary, the increment of ΔCTOD was reduced. The distance from the soft-hard boundary to the point where the increment of ΔCTOD was beginning to be reduced was not related to the yield stress in the hard region. By using the plastic region asymmetric parameter, the fatigue crack inclined angle was plotted on the same curve as experimental one. ΔCTOD was related to the cyclic equivalent plastic strain range distribution in the vicinity of the fatigue crack tip.