The authors undertook a study on the reheat cracking testing method of steel by way of the stress relieving test, and the method for evaluating the sensitivity of steel to the reheat cracking. The results of the study obtained are summaraized as follows: 1) The reheat cracking testing method by use of the weld thermal-restraint stress and strain-cycle simulator has the advantages of permitting the reheat cracking test to be made continuously in the heating and the holding processes. Therefore, this method can be seen the stress at the time of occurrence of cracks in steel, the cracking temperature and the proceeding of the stress relieving until lead to cracks. 2) Judging from the results of reheat cracking test by use of the weld-reinforced test pieces, it has been evident that the cracking position, propagation proceses of cracks, geometry of the cracked fracture surface, the temperature at which the steel test piece was cracked, etc. which are the characteristic phenomena of the reheat crackings coincide with the phenomena obtained in others studies in the past. For that reason, the present studying method of us is justifiable 3) It has been clear, furthermore, that the sensitivity of the test piece to the reheat cracking may be evaluated according to the quantity of plastic deformation, Δlp', as measured at the time of oc-currence of cracking.
The current path area at the weld interface is measured to elucidate the current path in the plate. The results obtained are as follows. (1) Length between the start point of current flow and the center of electrodes Si decreases with increased welding speed and increased welding current. (2) Length between the end point of current flow and the center of electrodes Ei at Imin increases as welding speed and welding current increase. (3) With increased welding speed, the length of current path area Li at Imin increases slightly above 1.2 mm in plate thickness and decreases slightly below 0.5 mm in plate thickness. (4) Si, Ei and Li decrease with decreased electrode force. (5) The width of current path area at the weld interface Wi decreases with increased welding speed and decreased electrode force. (6) In high speed seam welding, current path area of the weld interface falls behind and is smaller than that of the contact surface. (7) Welding current is guessed to flow uniformly in the direction of welding line and width within the current path area. (8) The average current density at the weld interface δi becomes smaller than the current density δc, given by the equation (2) above the welding speed of 6.5, 4, and 2.5 m/min for plate thickness of 0.5, 1.2 and 1.6 mm respectively.
In this study, the criterion of post-weld heat treatment (PWHT) cracking was considered in accordance with the features of PWHT cracking and precipitation behaviors of γ' phase were also examined. The results obtained in this study are as follows; 1) Typical PWHT crackings of Ni-base heat resistant alloys usually initiate at coarse grain boundaries adjacent to the weld interface where residual stress concentrates. Consequently, the deformability of material in stress concentrated region is presumed to be closely related to the PWHT cracking susceptibility. 2) γ' phase precipitates very rapidly in the temperature range from 800°C to 900°C. Each shortest incubation time of γ' phase precipitation in Heats 702A and 722A is about 8 sec at 900°C and 80 sec at 800°C respectively. 3) From the kinetics of aging, coarsening of γ' phase is obeyed with the theory of Ostwald ripening and controlled by the diffusion process of Al in matrix.
Niobium and/or vanadium were added to the 0.09C-0.2Si-1.5Mn system so as to study the toughness of submerged arc weld metal. The plates which show the highest absorbed energy of weld metal contain from 0.02 to 0.04% noibium and less than 0.09% vanadium. When less niobium and vanadium in plate are contained, the refining effect is insufficient so that absorbed energy of weld metal becomes reduced. When niobium and vanadium are contained more than the optimum content, a precipitation hardening effect is produced, thus impairing the, toughness of weld metal. The effect of precipitation hardening is more ermarkable with niobium than with vanadium.
The results of U-tensile tests of the welded joints of the carbon steel sheets containing more than 0.055 per cent carbon are reported in IIW Doc. III-481-73. The purpose of this investigation is to examine the characteristics of the cross-tensile test of the spot welded joints of three kinds of low carbon steel sheets containing less than 0.06 per cent carbon. The specimens to be welded are of 1.0 mm thickness and welded under the conditions of class A recomended from RWMA except for welding duration and the following results are obtained. (1) There are several differences of the fracture characteristics at the cross-tensile test of spot welded joints of some carbon steel sheets, which correspond to the qualities of sheet according to JIS G 3141. (2) The fracture modes, flat or button fracture, of spot welded joints are mainly dicided on the kinds of carbon steel sheets and the welding duration. (3) The flat fracture of the spot welded joint results from the formation of the martensitic structure or the plana growth of nugget during the welding.
The deformation mechanism of filler metal at brazed joint was discussed in terms of the dislocation theory. The deformation of filler metal at joint depends on the pile-up process of dislocations against joint interface, because the joint interface is very strong as an obstacle. The deformation of filler metal at the joint is devided characteristically into the five stages from the low stress up to a high stress: (A) Many dislocations piled-up rapidly against joint interfaces. The stress-strain curve is parabolic, and the work hardening ratio is very high. (B) Plastic flow is restricted by the piled-up groupes of dislocations. The stress-strain curve is straight. (C) Filler metal flows plastically. The stress-strain curve is parabolic. At the transition stress (B) to (C), the stress-strain curve shows an apparent yield point. (D) When the stress becomes higher than the yield stress of the base metal, the work hardening ratio becomes smaller than before. (E) Cracks initiate and propagate until failure of the joint occures.
The brittle fracture tests were performed to examine how the decrease of the residual stress due to the stress-relieving and the resultant quality change would influence on the brittle fracture initiation characteristics and a full consideration was taken as to whether or not the stress-relief treatment is suitable for welded steel structures to ensure their reliability. While the brittle fracture initiation characteristics after the stress-relieving were improved by relieving the residual stress due to welding, the notch toughness of welded joints was reduced by the stress-relief treatment. Consequently, the merit and the demerit were offset, and it was not much expected that the brittle fracture initiation characteristics were effectively improved by the stress-relief heat treatment.