Study on Mechanism of Blowhole Formation by Hydrogen

On Formation of Porosity in Underwater Weld Metal (The 2nd Report)

Abstract

The nucleation and the growth of gas bubbles at the solidification front of fused metal and the growth of blowholes in weld metal were discussed, in case the concentration distribution of hydrogen exists in front of the solidification front.
Main results obtained are summarized as follows:
(1) When a radius of gas bubble R is larger than critical radius Rc, the gas bubble is able to exist or grow in the fused metal. However, the gas bubble is not able to grow over the maximum radius Rmax. When R is smaller than Rc, the gas bubble becomes smaller and finally disappears.
(2) When the thickness of highly concentrated area of hydrogen in front of solidification front is larger enough than the diameter of nucleus of gas bubble, the critical radius of the gas bubble Rc is given by following formula:
R_c=2r(Co²/k²K²-pd-P_a)
(3) The growth rate of gas bubble VR increases and critical radius Rc decreases with an increasing concentration of hydrogen Co^* and with a decreasing solidification speed V_s.
(4) The growth rate of gas bubble V_R decreases with an increasing ambient pressure P_a in relatively larger radius of gas bubble. Nevertheless it increases with an increasing ambient pressure in smaller radius of gas bubble. Consequently the critical radius of gas bubble R_c decreases with an increasing ambient pressure P_a.
(5) The contour of blowhole changes from "closed type", "cylindrical type" to "bubbling type", as C_o^* increases and V_s decreases.
(6) As ambient pressure Pa increases, a smaller nuclear of gas bubble may grow more easily to be a blowhole. Therefore the occurrence of the blowhole becomes easier with an increasing ambient pressure. However, diameter of blowhole has a tendency to decrease with an increas of P_a.