The authors studied on the phenomenon of microcracking in single bead obtained from ilmenite electrode. Electrolytic-polishing was used to detect the microcracks on the longitudinal section of a bead-on-plate specimen. Effect of cooling condition after welding, plate thickness and initial plate temperature were mainly studied. Results obtained were as follows. (1) The formation of microcracks in mild steel weld metal, previously noticed by Flanigan, has been observed in single bead from ilmenite electrode under condition of rapid cooling. The length of microcracks varied widely being often 0.3 mm. and above, but most commonly about 0.1 mm. (2) There was a clear trend towards increasing crack numbers with the increase of plate thickness. The specimen of 25 mm. and 32 mm. thick, for instance, contained microcracks when air-cooled after welding at 0°C., but no cracking was found with 19 mm. thick specimen. (3) A change in initial plate tempere from 0°C. up to 40°C had a pronounced effect, decreas-ing the extent of microcracking ; i.e., the preheating at 40°C. was remarkable in preventing micro-racking even with 32 mm. thick specimen. (4) The specimen of 19 mm. thick, welded with ilmenite electrode and water quenched within 1 min. after welding, contained microcracks, but no cracking was found even though 32 mm. thick specimen with low hydrogen electrode.
In almost every research which was done by the tensile stress corrosion test, the sensitivity for cracking was compared in terms of the fracture time of test piece. The fracture time of stress corrosion cracking consists of two processes: the induction period which is the period from the start of test to the initiation of a crack and the propagation period which is the period from the initiation of crack to the fracturet of test piece. There is no relation between induction period and propagation period, but these vary indepe-ndently of each other by the changes in stress value, material and size of test piece. Therefore, because the fracture time is long, it does not follow that the sensitivity to cracking is low. In this report, to make clear these weak points of previous works, we tried to apply the con-tiouous microscopic observation to the stress corrosion test for type 304L stainless steel and its welded metal, and analysed the stress corrosion process: the initiation and propagation of crack. The test pieces used in this report were notched tensile plate type. The induction period was not constant, but became longer with a decreasing stress value. The fracture time of base metal was shorter than that of deposited metal, but the induction period of base metal was longer than that of deposited metal. The average propagating speed in the deposited metal was lower than that in the base metal, because the crack propagation through the deposited metal was hindered by the ferritic phases dotted in the austenite matrix. Moreover, the propagating speed of crack decreased linearly with decreasing stress.
The actual cooling rates in electron-beam welds for carbon steel, stainless steel, aluminium plates and zirconium sheet are compared with the theoretical cooling rates which are induced by the thermal conduction theory. According to this comparison, the actual cooling rates in the elec-tron-beam welds which show unique wedge-shape penetration agree with the theoretical cooling rates by the two-dimensional heat flow theory. Especially at the temperature below 900°C, the actual cooling rates and cooling curves in the electron-beam welds completely agree with those induced from the theoretical equation. With utilization of this theoretical equation, therefore, a nomograph from which is estimated the cooling time from 800°C to 500°C is made for a carbon steel of which the weld metal and heat affected zone are hardened when electron-beam welding is done. Moreover, the method for the estimation of hardness in the electron-beam welds is established with the use of the nomograph and C.C.T. diagram for welding and is discussed for the reliability. As a result of the discussion, it is seen that the above method for the estimation is reliable for carbon steel.
Shrinkage distortion of welded structures is an important subject in welding, together with metallurgical features of welded materials. Theoretical analyses of the former which undergoes thermo-elastic-plastic processes, however, are extremely difficult to perform, so we have very little practical knowledges on them. The authors have conducted a series of studies from the following viewpoint. Shrinkage distortion due to flame-cutting, line heating, welding or flame-straightening should be represented as a weighted sum of shrinkages due to spot heating. First, the fundamental nature was sought for of shrinkage distortion in a rectangular plate, which was heated on one spot. The previous reports presented exact expressions for shrinkages due to heating of a single spot. Approximate formulae, which are self-explanatory and convenient to apply in practice, also were obtained. The author elsewhere found "The Linear Addition Law" concerning the shrinkage distortion due to local heating on arbitrarily-distributed regions. The present report proposes a straight-forward expression for the transverse shrinkage, which results from heating many spots in a rectangular plate. The formula was derived by combining theoretical expressions for single-spot-heating with the law of linear addition. The formula indicates that the transverse shrinkage due to multiple-spot-heating is inversely proportional to the pitch of heated spots; it is independent of the breadth of the plate, unlike the case of single-spot-heating. Line heating and welding are correlated with multiple-spot-heating, with an appropriate assumption of the pitch of heated spots.
In the U.S.A. the modified aluminum alloy filler metals such as 5183, 5356 and 5556 have been developed for welding Al-4-55/, Mg base metals so that they may have their own uses for the spe cified base metals as seen in the examples of 5183 filler metal to 5083 alloy and 5556 filler metal to 5456 alloy, though in some cases they seem to be handled as a group because of their approximately equal Mg contents, while in the U.K. and Canada only NG 6 and Alcan 56S filler metals are available respectively for welding these base metals. This study has been made to compare the properties of welds in JIS A 2 P 7 (A. A. 5083, B. S. N5/6) alloy made with 5083, 5183 (only for cracking test) 5056, Alcan 56S, AWCO NG 6, 5356 and 5556 filler metals. The results obtained are summarized as follows. (1) The chemical analyses of the deposited metals made on 99.85% Al plate showed that Mg loss increased with welding current and seemed to be affected by arc voltage. Mg loss estimated from weld metals in A 2 P 7 alloy made with various filler wires was 5 to 8 % of the Mg content in the wire. Regarding minor alloying elements, Ti seemed to be lost, but no Mn and Cr losses occurred. (2) The fish-bone cracking tests for A 2 P 7 and A 2 P 1 (A. A. 5052) alloys indicated that A 2 P 1 was apparently more susceptible to cracking than A 2 P 7 alloy, but the cracking suscepti-bilities in both alloys were little affected by the filler rods used. Whether Ti is of any significance or not for cracking sensitivity remains to be investigated. (3) There was little difference in the gas contents between the mechanically finished AWCO NG6, as well as the gas contents of their weld metals. Therefore, it is difficult to point out which type of filler metal is more desirable. But the chemically finished filler metals excet NG 6 contained a large amount of gases. This means that care should be taken in performing the chemical clea-ning method. (4) The total gas and hydrogen contents of filler metals reduced to 4.82-6.49 cc/100 g and to 1.35-2.33 cc/100 g in the weld metals respectively. These values of gas contents had no effect on the X-ray grade porosity. (5) The tensile and bend tests, and hardness measurements of the welded joints showed no differences in the results obtained with various filler metals.