The Behavior of Hydrogen in Arc Welding (Report 4):Hydrogen dissolution into submerged arc weld metal
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In submerged arc welding of high tensile strength steels, it is positively demanded to produce weld metal having low crack sensitivity, especially hydrogen induced crack sensitivity.<BR>Agglomerated flux has many advantages, such as generation of more shield gas, high basicity and so on, but it has disadvantage of having a hygroscopic character.<BR>Meanwhile, it is generally believed that fused flux decreases the hydrogen content in weld metal on account of its unhygroscopicity.<BR>This is, however, not true because fused flux contains much hydrogen in flux itself in the form of OH radical, which is derived from the atmosphere of its manufacturing process.<BR>This investigation shows clearly that highly basic agglomerated flux, which generates plenty of CO<SUB>2</SUB> gas in welding, can minimize hydrogen content of the weld metal.<BR>The effects of basicity, CO<SUB>2</SUB> gas, absorbed moisture and dissolved hydrogen in flux on hydrogen content of the weld metal are quantitatively determined by a model of hydrogen dissolution mechanism proposed in this paper.<BR>The main results obtained are summarized as follows;<BR>1) With an increasing basicity of the flux, the diffusible hydrogen in deposited metal decreases and the hydrogen dissolved into slag increases.<BR>2) The diffusible hydrogen in deposited meatl steeply decreases with an increasing carbonate in flux.<BR>3) Agglomerated flux has hygroscopicity, but that is lower than that of the low hydrogen type electrode coating.<BR>4) Absorbed moisture of agglomerated flux completely evaporates at temperatures up to about 200°C.<BR>5) In the case of the agglomerated flux about 20% of the air which has existed in the consumed particles of the flux enter the arc cavity and its ratio decreases to one third through generation of CO<SUB>2</SUB> gas.<BR>6) Only 0.015-0.020% of absorbed moisture in the flux enters the arc cavity, and its ratio decreases to one tenth when the flux generates CO<SUB>2</SUB> gas.<BR>7) Partial pressure of CO<SUB>2</SUB> gas generated from the flux in the welding accounts for two thirds of total pressure in the acr acvity.<BR>8) In the case of the fused flux, main source of the diffusible hydrogen in deposited metal is the hydrogen contained in flux itself in the form of OH radical.
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