Cavitation occurs when the static pressure in a flowing liquid drops to or below the vapour pressure of the liquid, for example at constrictions or at high flow velocities. This forms vapour bubbles. When the pressure rises again, the bubbles collapse abruptly. If this happens close to a wall, the resulting micro-jet erodes material from the surface. In foundries, this mainly affects cooling water pumps and pipework.
Casting units are usually heated, but can more rarely come in unheated versions. Today, casting is
less frequently carried out via a stopper direct from the vessel. Instead, the process of
casting via a siphon outlet with a stopper has become more popular. Casting equipment is available with capacities ranging from 0.8 t to approx. 20 t. The heating equipment consists of the conventional channel inductor and for special applications also
a crucible inductor with a capacity of up to 300 kg. The vessel or boiler has an inlet
and outlet siphon, which can be fitted by means of a flange. By increasing the gas
pressure above the bath level, the bath level within the vessel is lowered, and it rises
in the outlet siphon to the specified casting height, which is also maintained throughout the casting process. The casting process is controlled and regulated with the aid
of laser or camera equipment. The ‘teach-in’ principle stores up to 99 predefined and
manually initiated casting programmes, which can then be run off box by box.
The total emptying of the vessel is carried out with the aid of hydraulic cylinders
through the inlet. Casting units are coupled to the mould system, and in this way
are always brought into the correct casting position.
Casting materials that can be efficiently melted in crucible furnaces are listed below, although not
in any order of priority:
Grey cast iron, steel, copper, brass, aluminium, zinc, magnesium, gold, platinum,
tin and bronze, nickel and silver.
Casting finishing refers to the separation of the rough castings from the ingates and feeders, the
removal of casting burrs and mould material residues and the cleaning of the
castings by treatment with blasting materials.
A calorie is a formerly common unit of measure for heat, with the abbreviation ‘cal’. 1 cal is
the amount of heat required, at normal pressure, to increase the temperature of
1 g of water from 14.5 °C to 15.5 °C. 1,000 cal = 1 kcal. In the international system of weights and measures, the calorie has been replaced by the Joule. 1 cal =
4.1868 J (old conversion ratio, 1 kWh = 860 kcal for cooling system calculations).
Cables are water-cooled in induction furnaces and used as cooling water return lines. In
the case of low performances, cables can also be used as water feed lines. The
copper conductors have 35 or 50 mm2 copper cross-sections. For medium frequency the individual conductors have a paint insulation, although this offers no
benefits for mains frequency. The electrical connections can take the form of flat
connections or clamp-ring connections.
Bridge formation is a phenomenon that cannot entirely be avoided in induction furnaces. If there is no proper heat-conducting contact between the liquid melt and the material above, the crucible is said to have a bridge. This phenomenon can occur for example due to material becoming jammed above the melt or a ceramic cover over the melt due to superheating and break-up of the ramming mix. To avoid this happening, the furnace should only be recharged with so much material so that a smooth surface can still be seen in the crucible. If a bridge has nevertheless formed, the furnace should be tilted, and the bridge broken up with extreme care. In MF furnaces, the bridge can sometimes be melted at low power. Serious damage and injury to personnel has in the past been caused by the uncontrolled melting and breaking up of the bridge in the basic position.
Baled scrap should be free of water, oils and greases, and without hollow components or
organic inclusions. Baled scrap is used in induction and cupola furnaces. The
dimensions must be suitable for the furnace dimensions, i.e. the maximum
length/diagonal should be no more than 60% of the furnace diameter.
German silver (also called nickel silver) is a copper-nickel-zinc alloy sold, among other things, under the trade name Alpaca. Despite its name, German silver contains no actual silver at all – the name refers only to its bright, silvery-white appearance, which resembles that of real silver.
Because of its silvery color, good corrosion resistance and favorable cold-working properties, German silver is used wherever a silver-like appearance or specific spring and conductivity properties are needed without the cost of real silver. Typical applications include cutlery and flatware, often as a base metal for subsequent silver plating, musical instruments such as flutes, saxophones and valves for brass instruments, decorative fittings and hardware, and electrical contacts, springs and connectors, where the copper-nickel-zinc composition combines corrosion resistance with useful mechanical and electrical properties.
Alpaca is the trade name for standardized German silver wrought alloys such as CuNi12Zn24 and CuNi18Zn20 under EN 1652[1], which cover a composition range of roughly 60-66% copper (Cu), 11-19% nickel (Ni), the remainder zinc (Zn), with lead (Pb) limited to max. 0.03% in both grades.[1]
Element
CuNi12Zn24
CuNi18Zn20
Copper (Cu)
63.0-66.0%
60.0-63.0%
Nickel (Ni)
11.0-13.0%
17.0-19.0%
Lead (Pb)
max. 0.03%
max. 0.03%
Zinc (Zn)
remainder
remainder
Density
approx. 8.7 g/cm³
Standards
Wrought copper-nickel-zinc alloys of the German silver type are covered by European copper alloy standards such as EN 1652 (plate, sheet and strip)[1] and EN 12163 (rod), under designations like CuNi12Zn24 or CuNi18Zn20, with the compositions given in the table above.
Frequently Asked Questions
Does German silver contain any silver? No. German silver, also sold as Alpaca, is a copper-nickel-zinc alloy. The name comes from its silvery appearance, not from an actual silver content.
What is German silver used for? German silver is used for cutlery and flatware, musical instruments, decorative fittings, and electrical contacts and springs, wherever a silvery appearance and good corrosion resistance are required.
What is the difference between German silver and Alpaca? There is no material difference: Alpaca is simply the trade name under which German silver, standardized as e.g. CuNi12Zn24 or CuNi18Zn20 under EN 1652 with roughly 60-66% Cu, 11-19% Ni, max. 0.03% Pb and the remainder zinc, is sold commercially.[1]
German silver is a typical example of a compound alloy, produced through a defined alloying process. For more background on alloys in general, see our article on the term alloy.
References
European Committee for Standardization: EN 1652:1997, Copper and copper alloys – Plate, sheet, strip and circles for general purposes, Table 2 (Composition of copper alloys), p. 13; German national edition consulted: DIN EN 1652:1998-03. Grades CW403J (CuNi12Zn24) and CW409J (CuNi18Zn20).
