With the development of "Inert Gas Arc Welding", the superposition of high frequency pulses to the welding current has become an indispensable practice. But, on the other hand, there is an urgent necessity to arrest the disturbing radio noise from the apparatus. The contradiction has not yet been overcome complety. The authers studied the problem from the standpoint to find the most effective pulse shape and timing and to reduce the necessary pulse energy for maintaining the arc. The authors obtained following results: 1) The effect of the frequency of the pulses: The lower the frequency the better the arc maintaining and starting action. 2) The effect of pulse duration: The longer the duration the better the action, and the direct current component is very effective. 3) The polarity: The polarity effect is not evident. 4) The timing: The effective timing period of the pulse is not limited to the close neighbourhood of the zero of the arc current, but practically has an allowance about ±10°electrical angle. This may seem contradicatory to the customary theory. But, the existence of the relatively broad allowance makes the apparatus very feasible. From these results, the authors confirmed the possibility of obtaining an apparatus superior to convensional apparatus from the standpoint of noise reduction.
It is the purpose of a series of reports to propose the method predicting the cooling time from 800°C to 500°C near fusion line, through a simple procedure from welding conditions for the various usual welding processes. In this report, first, the shape of cooling curve, the nature of cooling rate and cooling time were described. Furthermore, effects of kinds of covered electrode and electric current on the cooling time were observed. The main results are as follows : 1. The theoritical formulas for the temperature distribution and the cooling rate (R) near fusion line of base steels were summarized. 2. The cooling rate R can be expressed generally by the following formula, R=dT/dS=-k(T-TM0)n, k : Const. The value of n is about 1.2 for the specimen used to obtain the C-C-T diagram and about 1.8 for the neighbourhood of fusion line by welding. 3. For the prediction of the microstructure and hardness in the heat-affected zone near fusion line from the C-C-T diagram, the use of cooling time from 800°C to 500°C is more reasonable than the use of cooling rate. 4. The cooling time is nearly equal on different portions in the neighbourhood of fusion line, and if the welding conditions are decided, the cooling time near fusion line can be estimated. 5. When the cooling time from 800°C to 500°C is presented as a function of the arc energy per unit bead length, namely, J= 60EI/v Joule/cm, the relation between the cooling time and J is not changed approximately by kinds of covered eledtrode. 6. The cooling time in case of use of direct current is slightly larger than that of alternate current, for the same value of J=60EI/v.
Following the previous study, mechanical and impact properties of the microstructures in weld heat affected zone in structural steels have been studied by means of the synthetic apparatus. Firstly, the effects of the maximum heating temperatures, 500, 650, 750, 900, 1100, 1300 and 1400°C. on the tensile and impact properties of a Mn-Si high strength steels were investigated. The maximum values of tensile and yield strengths were obtained approximately at about 1300°C. of peak temperatures ; but the elongation, the reduction in area and the impact value were found to show more remarkable decrease at higher temperatures over 1300°C. Similarly, from the tests carried on the effects of tempering temperatures, 450, 550 and 650°C. on the tensile and impact properties of five steels, it was found that the tensile and yield strengths were decreased with an increase of tempering temperature, while the elongation, the reduction in area and the impact value were generally increased on the contrary. However, the yield strength was slightly increased by tempering at 450°C. Next, the effects of thermal cycles on tensile properties of five steels were studied. The ductilities as revealed with the values of elongation and reduction in area were decreased with an increase of cooling rates at 540°C., and were closely related to the microstructures of transformation products. From these relations, the critical cooling rate and hardness to preserve a satisfactory ductility were determined for each steel. Finally, from the test results of the small size bead bend test, it was ascertained that elongation or reduction in area of the synthetic round bar tensile specimen were a good indication of the bend ductility of steels.
The welded parts of 14 mm steel plates (Wel-ten 50) and 20 mm steel plates (Wel-ten 55) were postheated respectively when about 70 sec or 40 sec elapsed after welding with a torch travelling at intervals of the definite distance, about 14 cm or 8 cm, behind the electrode tip. Furthermore, 14 mm steel plate (Wel-ten 50) was investigated by varying the postheating time. Those results are summarized as follows : 1) The microstructures of the heat-affected zone in the neighbour hood of fusion line, in deposited and postheated specimen, are estimated practically from the temperature-time fields divided by the critical cooling curves. The peak temperature range of. postheating which causes the spheroidization of carbide and of postheating which produces new structure by austenitization were clarified. 2) The flatness of deposited metal, the width of heat-affected zone and the austenitic grain size change remarkably with preheating as mentioned in the report 4. In the case of postheating, when the distance between the electrode-tip and torch-tip is smaller, such effects of preheating also appear in some degree. If the distance is shortened extremely, the welding arc becomes unstable, the slag is scattered and the pits appear on the surface of bead. 3) The maximum hardness of the heat-affected zone for the Wel-ten 55, 20 mm steel plate becomes remarkably lower with postheating. If the welded joint is postheated at about 650-750°C, the spheroidization of carbide occurs remarkably and the hardness becomes lower.
In the basic weld metal, the roll of deoxidizer is very important to obtain sound weld metal. Attempts have been made to obtain weld metals by deoxidation with metallic manganese. Manganese is very weak deoxidizer and its deoxidized products are large particles characterized by its peculiar dendric forms. As results of investigation with analysis of residues obtained by alcoholic-iodine method and its electron diffaaction pattern, its deoxidized products are solid solution of ferrous oxide and manganous oxide.
Titanium is very strong deoxidizer compared with manganese. In this report titanium have been used ass ingle deoxidizer in the straight lime basic type electrode. The weld metal was attained by usual hand welding. Inclusions thus obtained have been direct observed by electron microscope in carbon extracted replica, and investigated by its electron diffraction pattern. The weld metals were also dissolved by alcoholic iodine solution and its residues were analysed by usual chemical analysis. As result of investigation the lesser deoxidized weld metal by titanium contains inclusions which largely consists of iron titanium oxide (FeS Ti2O3), but the fully deoxidiged metal usually contains inclusions of alpha titanium (α-Ti2O3).
Bead Cracks of Cr-Ni Austenitic electrodes being utilized for low alloy high strength steel welding mostly belong to hot cracks. A study is made applying FISCO welding test in order to Confirm the influence of Chemical Composition of weld metal for hot crack. The results is shown as follows. 1. The chance of hot crack decrease remarkably when following value are given in weld metal P<0.015, Mn/Si>3.15, Mn/S>110 2. Containing of Cb increase hot Crack of weld metal. 3. Special Ni-Cr-Mn austenitic electrode made trially shows excellent effect against hot crack. 4. FISCO test has reliable reproductivity as the test of hot crack of weld metals.