When one bead is welded on mild steel with a martensitic hardfacing coated electrode, no macrocrack occurs. But when overlapped beads are welded with it, transverse cracks occur in the tempered zone of the first bead. The principal cause of the cracking is hydrogen embrittlement under the stress resulting from contraction of tempered zone and expansion due to martensite transformation. This cracking is cold cracking which occurs at temperatures less than 100°C, and is delayed cracking, which occurs mainly along the grain boundary in which chromium carbides precipitate. Residual hydrogen in the first and second bead cause the transverse cracking. Both preheating and postheating resulted in crack-free welds. This is rather due to discharge of hydrogen resulting from relaxed cooling rate than to stress relaxation or softening of temperaed zone.
In order to improve the intensity of propane-oxygen flame two kinds of tips were designed. One was a preheating tip, in which the gas mixture was heated before combustion. The other was an outside injecting tip, which had two oxygen streams and one propane stream issuing concentrically from the tip. The propane stream is injected by the central high speed oxygen stream and mixed at a point outside of the tip. The experimental results are summarized as follows: (1) Certain effect of the preheating tip was recognized and the time to start the melting of plate became shorter than that by usual tip for oxyacetylene flame. It was deduced that the preheating temperature by this tip was about 100°C. (2) Flame cone by the outside injecting tip was a little larger than that by the usual tip, but the outer flame was much smaller. (3) The time to start melting by the outside injecting tip became a half of that by the usual one. (4) The flame by the outside injecting tip had a stronger oxidizing effect on weld metal than one by the usual tip. (5) The welding rod containing 0.2-0.7% aluminum in addition to silicon and manganese was suitable for a propane-oxygen flame welding using this tip.
Surface treatment of the interface to be welded is one of the most important factors affecting the weld characteristics especially in the pressure welding such as solid phase welding. The purpose of this study is to investigate the influences of the surface treatment technique and interval of time between the surface finishing and welding upon the mechanical properties of the pressure welds of aluminium alloys, copper and carbon steels. The following results were obtained. 1. Filing away surface treating gave higher weld strength characteristics than wire and emery polishing, and was as effective as the fricative scratch treating of the intimate surface each other. 2. Increase of exposure time between the surface finishing and welding decreased the ductility rather than the tensile strength of the welds. 3. Maximum strength of the welds of cold working material such as aluminium and copper was obtained by making at the recrystallization temperature of the metal. 4. When the welding temperature was chosen at low temperature of 200-300°C, the pressure weldability of alclad 24S-T3 was better than that of 24S-T3, therefore it is expected that welding temperature can be lowered by the method with the incert metal.
As a basic study of brazing, the relation between the micrographic structure of bonded layer and the heating temperature was established for metal combination of Ag-Sn, Cu-Zn, etc., near soft soldering; and action of phases was studied. The microstructure of bonded layer was determined by X-ray microanalyses, etc. The following results were obtained: 1) In combinations having equillibrium diagrams of peritectic reaction, according to the rising temperature, the first phase appeared at bonded layer which was the intermetallic compound of the highest concentration of low melting metal (e.g. γ phase of Ag-Sri, η phase of Cu-Sn). The other phases may be formed extremely near the bonded layer, but they were not observed by the microscope. The phases of extremely large solid diffusion were formed at comparatively low temperature adjoining the above-mentioned phases. 2) Ni-rich solid solution of Ni-Sn appeared after intermetallic compounds of low melting temperature. Other combinations were similar. 3) β phase of Ni-Sn and ε phase of Cu-Sn were formed by solid diffusion at 920°C, 302°C which were lower than melting temperature of these phases. β, γ phases of Ag-Zn and β, γ phases of Cu-Sn were formed similarly. Activation energies of these phases were obtained from calculated diffusion coefficient. Activation energies were respectively 43 kcal/mol (β phase of Ni-Sn), 41 kcal/mol (β phase of Cu-Zn), 19 kcal/mol (γ phase of Cu-Zn).
In recent year the tower for transmission line has been growing bigger and bigger. The X section members connected together with angle steels seem to be effective as the main post members of large size towers. Since the steel makers are not interested in rolling X section steel, the X section members have to be built up by welding. The results of pilot test clarified that the intermittent weld might be an adequate operation, because the continuous weld brought about distortions and torsional displacements to the products. The weld cracking comes, however, into question in fabricating the X section members with the intermittent method. The experiments were carried out mainly to make clear the intensity of restraint (K) on the weld joints in X section model specimens and to explain the correlation between K of the X section members and that of the usual cracking test specimens (Tekken, Lehigh and RRC). And yet the intensity of restraint K is defined as the force per unit weld length when the root gap of weld joint narrows elastically as much as 1 mm. The results are summarized as follows: (1) The gradient of the linear line (θ) in the loading process equals to that in unloading process in P-S test. Then, the intensity of restraint K seems not to change even if the restraint bead suffers from a considerable plastic deformation. (2) It became evident quantitatively that K suffered remarkable effects from the number of beads, weld pitch, weld length and.the angle steel size. (3) The correlation between K of the X section members and those of various cracking test specimens could be established.
To know the roles of the preheating flame acting in the oxygen cutting, a series of examinations has been performed cutting the test pieces under several surface conditions without the preheating flame. It is proved that the steel plate of which the scale has previously been removed mechanically or thermally can be esaily cut without the preheating flame, while an untreated and mill-scaled one can not. Therefore, it must be considered that the flame has some functions to exclude the mill scale from preventing the oxygenation of the base material. Possible mechanisms of scale removing effect are considered and discussed as follows: 1) Stripping or cracking of scale 2) Melting of scale 3) Changing of chemical material of scale