Casting Answers & Advice.
Background--This would be a good opportunity to visit the problem of gas in copper alloys. Because of the high solubility of hydrogen (H) in the liquid state of copper alloys, H gas can be absorbed into the metal during melting. Because the solubility of H in the solid state is much lower, the gas must be removed or it will be rejected during solidification, causing gas porosity in the castings. During melting, the main source of H is moisture, primarily from the atmosphere (humidity) and moisture-containing materials. Common sources of moisture include fluxes, dirty tools, ingot and hardeners, combustion by-products, and charge materials containing molding sand, binder and additives. The amount of H absorbed into the melt depends on many factors, including melt temperature, mold conditions, atmosphere and raw materials. The best way to minimize H gas in the melt is to prevent its absorption in the first place. The use of a reducing cover flux, such as graphite, and only clean, dry tools and charge materials can keep the H level below the solid solubility limit and reduce gas porosity in the castings. The molten bath also can be purged with nitrogen or argon to remove dissolved H from the melt.
One common way to prevent the absorption of H into the melt is to saturate the melt with oxygen (0). Under molten conditions, H and 0 form an equilibrium (when the 0 content is high, the H content is low) so keeping the 0 content high maintains levels of H in the melt below the solid solubility. This can be done by using an oxidizing flux or by melting under heavy oxidizing conditions. But even with small amounts of H in the melt, there can be a reaction between the H and 0 that forms steam, resulting in blow holes forming in the casting during solidification. This may be the source of the "holes" you see in your castings.
Recommendations--To prevent the formation of steam, remove the 0 from the melt. Deoxidation of copper alloys by phos-copper additions is a common practice (typically 1-3 oz phos-copper/100 lb metal just prior to pouring). The addition of phosphorus (P) to the melt forms cuprous phosphate ([Cu.sub.2][OP.sub.2][O.sub.5]). The cuprous phosphate easily separates from the melt and is skimmed off the surface. However, during the tapping and pouring process some 0 may be reintroduced into the melt, so there must be enough uncombined P in the melt to combine with this 0 and prevent the steam. Melting and pouring quickly can reduce the absorption of gasses into the liquid metal, and foundries can pour a step-block and fracture it to determine metal quality before pouring. The prevention of gas porosity in the casting requires that H gas is prevented from being absorbed. If this is done by oxidizing the melt, then the 0 must be removed to prevent the formation of steam that can create holes in the casting.
Q We are experiencing pin holes in our steel castings. Can this be related to our deoxidation practice?
Background--The first suggestion for a steel foundry experiencing pinholes would be a thorough review of deoxidation procedures. With continuing emphasis on "clean steel," it is apparent from discussions with many steel casting producers that widely varying deoxidation practices exist, including misconceptions regarding the necessity of a properly deoxidized steel heat. Whether the heat is produced in an electric arc furnace or an electric induction furnace, a properly processed heat of steel must be deoxidized before the metal is sent for pouring.
The primary object of deoxidation is to prevent the reaction between dissolved oxygen and carbon, which produces gaseous products that create pinholes in castings. Any element that is capable of reducing iron oxide (FeO) in molten steel may be considered a deoxidizer. Deoxidizers vary in their ability to reduce FeO, although no deoxidizer is able to reduce all FeO present in molten steel. Deoxidizers other than ferrosilicon or ferromanganese are referred to as "special" deoxidizers, with aluminum (Al) the most widely used of all special deoxidizers.
Recommendations--A review of the melting practices of 26 acid-electric foundries showed that 25 used Al. More than 50% of the foundries that use Al add it to the ladle during the "tap-out" from the furnace. Others add Al to their hand shanks or to both the ladle and hand shanks. Some also reported adding Al to the molds. The majority of those adding Al to the ladle will tie or wedge the pre-weighed Al to a rod and plunge the rod to the bottom of the ladle after all of the metal has been tapped into the ladle. The Al addition must not float upward during the tap Out or lose its power as a deoxidizer by becoming "trapped" in the slag.
Other special deoxidizers such as calcium (Ca) silicon (Si), Ca manganese Si, Ca silicide, and ferrotitanium may be added with the Al. These deoxidizers are added for the development of certain types of inclusions and microstructural changes to meet customers' specifications.
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|Date:||Jul 1, 2000|
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