Hydrogen permeation in stationary arc-melted nickel 200
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I.
INTRODUCTION
THE factors affecting hydrogen absorption,
desorption, and the final diffusible hydrogen content in arc welding have been extensively studied. However, hydrogen redistribution at the solid/liquid interface cannot be measured directly, and contradictory views have been expressed concerning this particular phenomenon. PoLhodnya [1] and Salter and Milner [2] have suggested that hydrogen is rejected into the liquid from solidifying weld metal (as the solid/liquid interface advances) and this produces weld metal porosity. On the other hand, Howden [3] has indicated that little or no hydrogen is rejected from the solidifying weld metal. This view was based on an analysis of hydrogen permeation experiments, employing stationary arc-melted nickel, mild steel, and stainless steel. In these results, the weld pool acted as a hydrogen pump so that the equilibrium back pressure (produced by hydrogen diffusion into the base metal) was several times as high as the partial pressure of hydrogen in the shielding gas. In the present work, the combination of a specially designed hydrogen permeation testing setup with detailed computer simulation of hydrogen transfer is used to evaluate the hydrogen content of solid nickel at the solid/liquid interface in a nickel weld pool. II.
EXPERIMENTAL PROCEDURE
Figure 1 shows a schematic representation of the hydrogen permeability testing apparatus This equipment was used to examine (1) the influence of arc current on the steady-state back pressure in a nickel weld pool (for a constant hydrogen partial pressure in the arc); (2) the temperature distribution on the back surface of the specimen immediately under the weld pool (for different arc current levels and hydrogen partial pressures in the arc); and (3) the effects of arc current and of hydrogen partial pressure changes on weld pool geometry. H. LI, Graduate Student, T.H. NORTH, WIC/NSERC Professor, I.D. SOMMERVILLE, Associate Professor, and A. McLEAN, Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON M5S 1A4, Canada. Manuscript submitted March 8, 1989. METALLURGICAL TRANSACTIONS B
A computer simulation of hydrogen permeation during arc melting was developed based on the above measurements and evaluated the hydrogen content in solid nickel adjacent to the fusion line. During testing, an 80-mmdiameter • 3-mm-thick Nickel 200 disc was melted using a stationary arc. The chemical composition of the Nickel 200 material employed is shown in Table I. A thoriated tungsten electrode was used throughout, and the Nickel 200 disc was supported on a water-cooled copper block. The top surface of the copper block and the surface of the Nickel 200 disc were ground flat to improve the effectiveness of the O-ring seal. The cavity under the Nickel 200 disc was evacuated to a pressure of 0.1 torr prior to melting. During arc melting, hydrogen permeating through the disc was measured as a buildup in back pressure. This back pressure was measured using a mercury manometer. A thermal co
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