Study on Mechanisms of Blowhole Formation by Hydrogen : On Formation of Porosity in Underwater Weld Metal (the 2nd Report)
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The nucleation and the growth of gas bubbles at the solidification front of fused metal and the formation of pores in weld metal were discussed, in ease of a concentration distribution of hydrogen existing before the solidification front. Main results obtained are summarized as follows: (I) When the radius of gas bubble R is larger than critical radius R_c, the gas bubble is able to exist or grow in the fused metal. However, the gas bubble is not able to grow over maximum radius R_<max>. When R is smaller than R_c, the gas bubble becomes smaller and finally disappears. (2) When the thickness of highly concentrated layer of hydrogen before solid(fication front is sufficiently larger than the diameter of nucleus of gas bubble, the critical radius of the gas bubble R_c is given by the following formula: R_c=2γ/(C_0^2/k^2K^2-pd-P_a) (3) The growth rate of gas bubble V_R increases and critical radius R_c decreases with an increasing concentration of hydrogen C_0 and with a decreasing solidification speed V_s. (4) The growth rote of gas bubble V_R decreases with an increasing ambient pressure P_a, for relatively large radius of gas bubble. Contrary to this, it increases with an increasing ambient pressure for smaller radius of gas bubble. Consequently the critical radius of gas bubble R_c decreases with an increasing ambient pressure P P_a. (5) The contour of pore changes from "closed type", "cylindrical type" to "cup type (bubbling type)", as C_0 increases and V_s, decreases. (6) As ambient pressure P_a increases, a smaller nucleus of gas bubble may grow more easily into a pore. Therefore the occurrence of the pore becomes easier with an increasing ambient pressure. However, diameter of pore has a tendency to decrease with an increase of P_a.
- 社団法人溶接学会の論文
- 1987-05-01
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