Takuro Kobayashi

Welding Current and Nitrogen Content of Weld Metal (Report 1)

Effect of welding current and sampling method on the nitrogen content of weld metals in nitrogen and air atmospheres using a pure iron electrode wire was studied.Three methods i.e. depositing on a mild steel plate, rotating copper disk and water cooled copper block were used for sampling at different current levels.The main results obtained are summarized as follows;1. The nitrogen content of weld metals obtained by the bead on mild steel plate method decreases slightly in a nitrogen atmosphere but largely in an air atmosphere with increasing welding current.2. The rotating copper disk method is not suitable for the study of nitrogen absorption as the weld metal droplets are not so rapidly chilled in water bath and the arc condition is pretty different from that of the conventional arc welding.3. The water cooled copper block method is more suitable than the rotating copper disk method for studying nitrogen absorption through arc space, though it can not be used at the ionhigher currents.4. The head on plate method is useful for studying the absorption of nitrogen by molten droplets and weld pool at low welding current.

Transformation Characteristics and Delayed Cracking in the Heat-Affected Zone of High Tensile Strength Steel

Underbead cracking was investigated in relation to the progress of isothermal transformation in the heat-affected zone of two high tensile strength steels.C.C.T. diagram for one of the two steels was determined using specimens austenitized in hydrogen and in vacuum.Delayed cracking tests were carried out using specimens, into which hydrogen was charged in austenitic and martensitic states.Main results obtained are summarized as follows:1. Underbead cracking is closely related with the progress of isothermal transformation in the welding heat-affected zone.2. Holding time for the prevention of underbead cracking is shortest in the intermediate transformation range.3. Below the Ms point, longer holding time is required to prevent underbead cracking with a lowering of temperature.4. Hydrogen dissolved in steel does not play any significant role in the transformation.5. From delayed cracking tests on two series of high tensile strength steel specimens containing the same amount of hydrogen charged in austenitic and martensitic states, incubation period for cracking in the former series was found to be about ten times longer than in the latter.

The Microstructure of Iron Base Binary Weld Metals (Report 1)

Microscopic survey, measurement of grain size and electron probemicroanalysis of Fe-Al, Fe-Cr, Fe-W, Fe-V, Fe-Si, Fe-Ti Fe-Mo and Fe-Zr weld metals were performed.The results obtained are summarized as follows:(1) The solidification structure of Fe-Al, Fe-Cr, Fe-W, Fe-V or Fe-Si weld metals changes from equiaxed to columnar crystal with increasing contents of alloying elements.(2) The solidification structure of Fe-Ti weld metals changes from equiaxed to columnar crystal and again to equiaxed crystal attended with the segregation of titanium.(3) The solidification structure of Fe-Zr weld metals changes from equiaxed to cell structure and further to cellular dendrite with increasing zirconium content.(4) The segregation of alloying element is observed on sub-boundaries of the cell structure and cellular dendrite in Fe-Zr weld metals.(5) The solidification structure is disarranged during cooling through the γ-loop.(6) Concentration gradients of alloying elements of 10 to l00μ width are observed.near fusion lines.(7) The alloying elements in weld metals appear to diffuse into unmelted base metal from 5 to 10 μ.(8) The macroscopic segregations of alloying elements are not seen in the weld metals.

Research on Welding Residual Stresses, Report 1

The welding residual stresses in both directions, parallel to and perpendicular to the welded edge, were measured by the X-ray diffraction method at six points over the surface of mild steel rectangular plates, which had a weld bead placed along one edge; as shown in Fig. 1.As it was found that the longitudinal stresses in the zone from the point. 3to 5. were in. compression, the zone was heated to 110° to 370°C. Progressively from one end to another by a gas flame. The changes. in residual stresses due to the local heating are shown in Fig. 3 to Fig. 15.The stress near the weld was reduced to a small value, but the one around the heated zone turned to be in large tension. This fact seems to be due to improperly slow heating and cooling, and a more appropriate measure will be introduced in the next report together with the clarification bf the mechanism for the relieving of welding residual stress by local heating.

Effects of Electrodes on Heat Affected Zone and Distortion of Arc Welded Steel Specimens

In the case of alloy steels, the zone adjacent to weld metal is profoundly affected by the heat of arc welding. The hardness of the heat-affected zone is very high, and the so-called hard cracks are apt to occur in this zone.In this research, first, it was revealed that under some definite conditions for welding hard cracks did not occur when austenitic steel electrodes were used, but appeared always in the case of mild steel electrodes.Next, using four types of electrodes, which were made of mild steel (very low carbon) and austenitic steel core rods shielded with a thin or thick coating, single beads were deposited on the surfaces of rectangular specimens of ducol and Mn-Cr-Mo steel. The welding was carried out taking into consideration the characteristics of each electrode. Then the hardness and microstructure of the heat-affected zones were surveyed.Under an approximately identical condition for the uses of thick or thin coated mild steel electrodes and thick or thin coated austenitic steel electrodes, the heat quantities supplied to base metals appear to be arranged from large to small in due order of the above-mentioned. And the maximum hardness of the haet-affected zone of alloy steel having an air hardening property like ducol steel is arranged from low to high in good order of the above description.Order differences among the microstructures were found in accordance with the tendency for the hardness.In the case of alloy steel having a severe air hardening-property like Mn-Cr-Mo steel, the differences among the hardness values and microstructures of the heat-affected zones were not found, but cracks were always induced when mild steel type electrodes were used as previously described.Finally, the grooves of mild steel specimens under restraint (θ-type specimens) were welded using the four types of electrodes, and the amount of welding contraction perpendicular to the weld line was measured.The amount of contraction is arranged from large to small in order of the before mentioned arrangement for four types of electrodes.Judging from these results alone, it is unable to explain the reason why the hard cracks do not occur in the case of austenitic steel type electrodes.The explanation of this reason shall be done in another research conducted by the author.