13 Sep Understanding the technical chart
The technical chart is a key element that helps to choose and correctly use every alloy, understanding it will allow you to avoid the most common mistakes and getting the best results.
Let’s take a look at the main features indicated in this document.
1 – Product Code
The code identifies in univocal way every product, the presence of letters NS (No Silver) indicates that the product needs the addition of silver to be used.
2 – Fineness
Many of the products for gold and silver are designed and studied to be used in different carats, in this case each carat has a dedicated page. The value indicated in this section indicates to which carat the data in the data sheet refers.
3 – Colour
Indicates which color will be obtained using the product, in this field the colours are generally divided between yellow gold, white gold, red gold and silver, a more precise description is given in the body of the description.
4 – Brief Description
The description indicates the most important features of the alloy, the colour shade that the finished product will have, all the carats in which it can be used and the main processes to which it can be subjected. This section defines the main field of use of the alloy:
Mechanical works: suitable for cold plastic deformations.
Casting: suitable for in-mold casting, after casting the metal will undergo few further deformations.
Universal: suitable for both previous applications.
5 – Suitable Applications
It is an indication, with a score from 0 to 5, of how suitable the alloy is for a specific application.
Plates&sheets: production of semi-finished products obtained by lamination processes, where the malleability of the product is important.
Solid chains: production of chains with solid wire, unless otherwise specified it refers to the production with chain machines, ductility is above all important.
Hollow chains: production of chain with “aggraffato” wire, unless otherwise specified it refers to production with chain machines.
Soldered tube: production of tube with profiling machine, unless otherwise specified it refers to welded tube.
CNC: production of objects machined from solid pieces with the aid of Computerized Numerical Control cutting tools. To score well in this application, the metal must be hard enough and produce small scraps.
Open casting: casting with hand casting or with machinery that does not use a protective atmosphere during casting, the alloys suitable for this application have a high content of deoxidizing elements.
Closed micro-casting: casting with machinery using a protective atmosphere, fewer deoxidizing elements are required than casting in open systems, while fluidity is particularly important.
Stone casting: casting in moulds that contain pre-mounted wax stones. The alloy must have a casting temperature as low as possible in order not to risk damaging the stones and a good reduction in oxidation of the metal, which could change the colour of the stones.
6 – Composition
Indicates the percentage of the main elements of the master alloy.
7 – Density
Indicates the density that the product will have at the time of use (i.e. when it will be alloyed to the desired caratage). The density value is useful for calculating the weight of a finished product, if its volume is known, or for calculating how much carat gold is needed to fill a mould, starting from the weight of the wax tree, in investment casting applications.
8 – Melting range
Unlike pure elements, which have a precise melting temperature, most alloys go from solid to liquid through a mixed phase, where the metal is softened but not completely liquid. For this reason, in the case of alloys of two or more metals, the melting temperature is not indicated, but the melting range. The interval is made up of two values, the lesser, called Solidus and the greater, called Liquidus.
Solidus: is the temperature that marks the boundary between the completely solid alloy and the beginning of melting.
Liquidus: is the temperature at which the alloy is completely liquid.
The first value is useful for calculating the annealing temperature, which is normally carried out at 70-80% of the Solidus temperature.
The second value is useful for calculating the casting temperature of the alloy, which is normally carried out from 50 to 100°C more than the Liquidus temperature.
9 – Hardness
hardness is indicated in Vickers. The minimum value is for alloy subjected to annealing, the maximum value for alloy subjected to age hardening, if available.
10 – Mould casting
In this field are summarized the main measures for mould casting. Mould casting represents the first casting process of many production types, the resulting mould can be rolled and re-melted inside a continuous casting machine or be the starting point to produce wire or sheet. Whatever the final use is, it is important to start from the right order of loading of the raw materials, insert the alloy first in the crucible and cover it with pure metal. The casting temperature varies between 50 and 100 ° C above the Liquidus temperature of the alloy, for lower alloy quantities use higher temperatures and vice versa. After casting, the metal can be immediately cooled, in water or in a solution of water and ethyl alcohol 90/10.
11 – Continuous casting
This section is visible only if available. In continuous casting, the metal is melted inside a crucible at the base of which there is a die capable of solidifying the metal with a specific shape, typically a bar, a plate or a tube. Since the production takes place continuously, it is good practice to carry out a separate pre-melting of all metals, in order to have a homogeneous alloy with which to feed the continuous casting machine, this procedure is particularly important for the lowest carats.
12 – Mechanical works
This section is visible only if available. Mechanical works refers to the classic plastic transformations: rolling and drawing. During these processes, the metal is compacted and acquires strength and hardness, while the workability and the size of the grains that form its internal structure decrease. Overcoming the limit of workability there is a risk of breakage, to avoid this inconvenience annealing process is needed. However, if the annealing process is carried out too early, the material may suffer aesthetical defects. To avoid these defects, it is important to sufficiently reduce the surface of the semi-finished product before annealing. Typically, if the sheet or bar has just been melted, the first reductions should be of 50%, while if it is an intermediate step, it usually ranges from a minimum of 50% to a maximum of 70-80%. The percentages refer to the surface of the bar or sheet.
13 – Annealing
During plastic processing, a point is reached beyond which it is no longer possible to work the metal without the risk of incurring a break, for this reason it is necessary to anneal the metal. Annealing relieves all internal tensions in the metal, increasing its workability and allowing the work to continue. To be effective, annealing must be done correctly. The annealing temperature which is typical of the alloy is indicated in the technical data sheet, this temperature should be respected as much as possible; on the other hand, it is possible to decide to keep the metal under annealing for more or less time depending on the total weight of the semi-finished product, the heavier the product, the longer it must be kept at the annealing temperature. Annealing can also be useful to eliminate residual stresses in objects coming from investment casting.
14 – Age hardening
If the composition of an alloy allows it, it is possible to increase the hardness of the products through the hardening process. This process must be done after all the mechanical transformations have been performed and consists of two steps: The first one is a short annealing, followed by a quick cooling; the second is carried out at low temperatures, typically between 250 and 300 ° C, for a period of time ranging from one to three hours; after this time, not only is there no greater hardening, but there is a risk of reducing the effectiveness of the treatment. Once the treatment is complete, the metal should be allowed to slowly cool down to room temperature.
15 – Casting
Alloys suitable for investment casting generally contain a good quantity of deoxidizing elements, which help to obtain a good surface finish of the cast objects; these elements are very reactive and can “burn” easily, making the alloy less effective. For this reason, investment casting alloys are more delicate than traditional ones and require some attention in their use, for example by always protecting the liquid metal with inert gas and avoiding excessively high temperatures.
16 – Pickling
Pickling is a chemical treatment that consists in dissolving the oxide layer that can form on the surface of the machined pieces with a hot acid solution. Typically, a solution of sulfuric acid (H₂SO₄) and water is used, but more aggressive acids can be used, such as hydrochloric (HCl) or hydrofluoric (HF) acid. Attention, the use of acids requires skill and experience and it is a good idea that the personnel in charge are adequately trained. The dilution of acids must always be done by pouring the acid into water very slowly and never vice versa. Before using an acid, make sure of the most suitable type of material that can be used as a container.
17 – Scraps reuse
Scraps are formed both during mechanical processing (swarfs) and during casting (sprues), reusing them by adding them to the new alloy is a common practice, however it must be kept in mind that every time the metal is melted it can incorporate a certain amount of polluting elements: oxides, small pieces of graphite or refractory material, metals, etc. To reduce the inconveniences due to these pollutants, it is good practice not to exceed the ratio of 50% between scrap metal and new alloy and properly clean the waste.