Bronze foundry men have had more experience in handling alloys of this type than in any other single group of copper-base casting alloy, and foundry practice for these materials is fairly we!! understood.
Multiple gating is highly desirable to avoid turbulence and overheating of local portions of the mold. Risers should be p1aced where they have access to hot metal. Sprues should be small in diameter and should be kept filled during pouring. Molds should be effective1y vented.
With the semi-red brasses. the cast ability i5 somewhat better. The higher zinc content may present a smoke. and fume pmb1em in the foundry. although this is usually not serious. The greater amount of zinc in these alloys may also necessitate more careful skimming before pouring to avoid dross being poured .into the mold cavity with the metal.
Phosphor-c:opper deoxidizer should be used with these alloys to help control the gas content and to improve their cast ability. Under usual conditions, the recommended addition is two ounces per hundred pounds of metal.
This group of alloys may be the easiest of all for the non-ferrous foundry man to handle. Good foundry practice will enable any foundry to produce salable red brass and semi-red brass castings.
Monday, June 09, 2008
TIN BRONZE, LEADED TIN AND BRONZE
Because of their good mechanical properties and corrosion resistance, these alloys are often used in castings subject to hydrostatic or air pressures, as in valves, pumps, etc, The quality of such castings must be unusually high; they must be free of internal porosity, shrinkage, or other defects.
Good foundry practice is of critical importance in the casting of tin bronzes and leaded tin bronzes. All of these alloys have a wide freezing range of from 200" to 300" F. To produce sound castings in any of the tin bronzes, gates and risers should be designed so that the closest possible approximation is made to true directional solidification.
Gating must be non-turbulent and the metal flow distributed through as many gates as possible to avoid hot spots. The mold should be filled quickly. Risers should be placed so that they do not unnecessarily delay the freezing of heavy sections. Such delay results in a coarse. open structure that will leak under pressure. Risers should preferably feed hot metal directly into the casting without forcing it to cross through any thin section of the casting.
The use of chills with these alloys is highly desirable, and it is often necessary to use them liberally if complex castings are to be made sound.
It is very difficult to feed tin bronze into a parallel section. one inch or more thick. by a riser alone.. Tin bronzes begin freezing at the sand-metal interface and freeze progressively toward the center. leaving a pasty mixture of liquid and solid in the center. The flow of molten metal from the riser into a high, thick section will usually be obstructed by the already solidified portion of the wall. The liberal use' of chills in the drag of such a casting, or riser hot-topping compounds and riser insulators, may be helpful in producing sound sections.
Because of the solidification pattern of tin bronzes, ga5CS may aggravate the shrinkage condition by enlarging cavities until they are interconnected and can cause leakage. Following the melting practices listed in Part I of this Section should help minimize this condition.
The tin bronzes improve in cast ability and soundness when phosphor copper is used as a deoxidizer.
Good foundry practice is of critical importance in the casting of tin bronzes and leaded tin bronzes. All of these alloys have a wide freezing range of from 200" to 300" F. To produce sound castings in any of the tin bronzes, gates and risers should be designed so that the closest possible approximation is made to true directional solidification.
Gating must be non-turbulent and the metal flow distributed through as many gates as possible to avoid hot spots. The mold should be filled quickly. Risers should be placed so that they do not unnecessarily delay the freezing of heavy sections. Such delay results in a coarse. open structure that will leak under pressure. Risers should preferably feed hot metal directly into the casting without forcing it to cross through any thin section of the casting.
The use of chills with these alloys is highly desirable, and it is often necessary to use them liberally if complex castings are to be made sound.
It is very difficult to feed tin bronze into a parallel section. one inch or more thick. by a riser alone.. Tin bronzes begin freezing at the sand-metal interface and freeze progressively toward the center. leaving a pasty mixture of liquid and solid in the center. The flow of molten metal from the riser into a high, thick section will usually be obstructed by the already solidified portion of the wall. The liberal use' of chills in the drag of such a casting, or riser hot-topping compounds and riser insulators, may be helpful in producing sound sections.
Because of the solidification pattern of tin bronzes, ga5CS may aggravate the shrinkage condition by enlarging cavities until they are interconnected and can cause leakage. Following the melting practices listed in Part I of this Section should help minimize this condition.
The tin bronzes improve in cast ability and soundness when phosphor copper is used as a deoxidizer.
Sunday, June 08, 2008
Recommended Pouring Practice
1 At all times the alloy should be kept as quiet and free from agitation as possible. It should never be stirred prior to pouring.
2 The lip of the crucible or ladle should be brought as close as possible to the top Qf the sprue to avoid turbulent pouring, which creates dross.
3 The stream of molten metal should be of such size
that it can be controlled without splashing.
4 The metal should be poured into the sprue without
interruption. A steady stream is desirable.
2 The lip of the crucible or ladle should be brought as close as possible to the top Qf the sprue to avoid turbulent pouring, which creates dross.
3 The stream of molten metal should be of such size
that it can be controlled without splashing.
4 The metal should be poured into the sprue without
interruption. A steady stream is desirable.
brass pouring
The pouring practice used for copper base casting alloys does not vary much from that used for other metals:
Improper pouring may produce excess oxidation and if the oxides an: washed into the castings, they may show up during machining operations. When casting aluminum bronzes it is most important to pour in a non-turbu1ent fashion because the oxides fanned are tenacious and strong and can cause irreparable damage in the casting.
Alloys with high zinc content have a pronounced tendency to form oxides during pouring. It is important that the sprue be kept full and that the stream of metal from the crucible or ladle be uninterrupted while the mold is being filled. Splashing, dumping, or other such practices must be avoided.
Improper pouring may produce excess oxidation and if the oxides an: washed into the castings, they may show up during machining operations. When casting aluminum bronzes it is most important to pour in a non-turbu1ent fashion because the oxides fanned are tenacious and strong and can cause irreparable damage in the casting.
Alloys with high zinc content have a pronounced tendency to form oxides during pouring. It is important that the sprue be kept full and that the stream of metal from the crucible or ladle be uninterrupted while the mold is being filled. Splashing, dumping, or other such practices must be avoided.
Recommended Metal Melting Practice
1 Charge only clean, dry ingot or back scrap. The
presence of moisture or organic materials may cause gas in the metal after it is molten.
2 Use a clean crucible or a furnace in good condition to help avoid chemical contamination of tile alloy.
3 Melt as quickly as possible in an oxidizing atmosphere. This mean; there should be a slight amount of oxygen in the products of combustion. A sharp, green flame over tbe furnace in fuel-fired melting usually indicates this condition.
4 Do not overheat the metal. Superheat only as much
as needed to take the metal from the furnace, handle it, and pour it into the mold at the best pouring temperature.
5 After the metal is taken from the furnace, skim it carefully without stirring or agitation. Measure the temperature, and when it has reached a suitable point, add the necessary deoxidizer and pour the castings.
presence of moisture or organic materials may cause gas in the metal after it is molten.
2 Use a clean crucible or a furnace in good condition to help avoid chemical contamination of tile alloy.
3 Melt as quickly as possible in an oxidizing atmosphere. This mean; there should be a slight amount of oxygen in the products of combustion. A sharp, green flame over tbe furnace in fuel-fired melting usually indicates this condition.
4 Do not overheat the metal. Superheat only as much
as needed to take the metal from the furnace, handle it, and pour it into the mold at the best pouring temperature.
5 After the metal is taken from the furnace, skim it carefully without stirring or agitation. Measure the temperature, and when it has reached a suitable point, add the necessary deoxidizer and pour the castings.
control at metal temperature
There is a direct relationship between temperature and the amount of gas a copper-base alloy will dissolve while in the molten state. The higher the temperature, the more gas will be absorbed. Thus it is essential( that metal b~ taken from the furnace as soon as it has reached the minimum temperature necessary for pouring molds.A pyrometer should be used to determine the melting temperature of copper-base alloys. Generally, the melt should only be taken to the temperature which will permit removal from the furnace., skimming, deoxidizing, other necessary handling, and pouring at the required pouring temperature. This means controlled superheating, which is most important in producing quality copper alloys for quality castings
creation of metal oxides
While some oxidation is desirable for controlling the solution of gases in molten metal, excesses of oxygen will combine with the molten alloy to form undesirable oxides. or dross.
It is quite common to find the oxides of copper, zinc, tin and lead in the slag of the 85-5-5-5 type alloy, for example. Silicon bronze alloys are apparently protected against harmful oxidation by a film of some silicon oxide that forms on the bath. Aluminum bronzes easily form oxides of aluminum, which' complicate the handling of these alloys. The yellow brasses or manganese bronzes frequently form large quantities of zinc oxide.
In a reducing melting atmosphere, such oxidation is negligible, but when an oxidizing melting technique is used, it is necessary to closely control the melting atmosphere so that only a slight amount of free oxygen is present in the products of combustion. Some foundry men use cover fluxes to prevent the formation of undesirab1e oxides. While it is true that certain chemicals will partially protect the metal from exposure to air, improper usage of such covers may exclude too much oxygen and thus Permit hydrogen gas content to build up.
Deoxidizers are helpful in reducing the amount of metal oxide present in the bath except with silicon, manganese and aluminum bronze. Since oxides will impair the castability of the alloy, the deoxidizer should be added immediately after the molten rnetal has been skimmed of dross. It will then inhibit the additional pickup' of oxygen during pouring operations.
One of the most harmful common practices in brass and bronze foundries is stirring or agitating the molten metal' while it is .in the furnace or in the crucible on the foundry floor. Stirring will thoroughly mix the oxides into the metal, and when the casting is finally poured, it will contain .an unusual amount of dross. Of course, when adding deoxidizers, gentle mixing is required, but excessive or violent stirring must be avoided.
It is quite common to find the oxides of copper, zinc, tin and lead in the slag of the 85-5-5-5 type alloy, for example. Silicon bronze alloys are apparently protected against harmful oxidation by a film of some silicon oxide that forms on the bath. Aluminum bronzes easily form oxides of aluminum, which' complicate the handling of these alloys. The yellow brasses or manganese bronzes frequently form large quantities of zinc oxide.
In a reducing melting atmosphere, such oxidation is negligible, but when an oxidizing melting technique is used, it is necessary to closely control the melting atmosphere so that only a slight amount of free oxygen is present in the products of combustion. Some foundry men use cover fluxes to prevent the formation of undesirab1e oxides. While it is true that certain chemicals will partially protect the metal from exposure to air, improper usage of such covers may exclude too much oxygen and thus Permit hydrogen gas content to build up.
Deoxidizers are helpful in reducing the amount of metal oxide present in the bath except with silicon, manganese and aluminum bronze. Since oxides will impair the castability of the alloy, the deoxidizer should be added immediately after the molten rnetal has been skimmed of dross. It will then inhibit the additional pickup' of oxygen during pouring operations.
One of the most harmful common practices in brass and bronze foundries is stirring or agitating the molten metal' while it is .in the furnace or in the crucible on the foundry floor. Stirring will thoroughly mix the oxides into the metal, and when the casting is finally poured, it will contain .an unusual amount of dross. Of course, when adding deoxidizers, gentle mixing is required, but excessive or violent stirring must be avoided.
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