The higher the temperature, the coarser the grain size. Annealing temperature is an important factor to obtain a fine grain size. These are reviewed and discussed in terms of their mechanisms, ease of use and their effectiveness. Rushforth, unpublished work, Johnson Matthey plc, 1978, Stewart Grice, Know your defects: The Benefits of understanding Jewelry Manufacturing Problems, in, Greg Normandeau, Applications of the Scanning Electron Microscope for Jewelry Manufacturing, in, Mark Grimwade, The Nature of Metals and Alloys in. Normally, if we wish to examine the macrostructure or microstructures of an alloy, we need a flat polished surface as optical microscopes have a limited depth of focus. As we have seen, cast microstructures may not be optimum for manufacturing or service. 
If there is restricted feed, then shrinkage cavities (porosity) will result. Control of grain (crystal) size in jewellery manufacture is important for several reasons. Left: with Co, Right: without Co (from reference 12). Vedi il carrello per i dettagli. Il venditore si assume la piena responsabilit della messa in vendita dell'oggetto. So working of ingot material serves two purposes: (a) to change the physical shape to that desired (sheet, wire, etc) and (b) to refine the structure. The more you buy the more you save, and each gram is individually bagged.
Its effect in grain refining a 14K gold is shown in Figure 17.
Some of the problems that can arise from lack of control will also be discussed. At low or ambient temperatures, the deformation process under an imposed load is governed mainly by the dislocation slip mechanism within each grain (dislocations are linear crystal defects responsible for deformation on crystal slip planes). [*There are a few exceptions, such as eutectic alloys which also solidify at a fixed temperature like the pure metals]. The terms large and small are, of course, relative. The variation in annealed grain size due to uneven amounts of deformation can be seen in Figure 14 which shows part of a cross-section of a C shaped wire in an annealed 18 carat nickel white gold.
Left: with Ir, Right: without Ir (from reference 12). Why are they important? We also see this deformation in the overall macrostructure: Figure 11 shows one-half of the cross-section of a washer in the process of being upset into a wedding band; the heterogeneity of deformation is evident in its fibrous appearance. Well, it is down to the relation between the grains (crystals) and the grain boundaries the region at the junction of adjacent grains - and their relative influence on mechanical deformation processes. The karat indicates the amount of pure gold in the metal. Figure 17 - Grain refining by cobalt in a 14ct gold. Figure 5- Solidification proceeds inwards from the colder mould walls, Figure 6 - Grain structure of ingots cast into metal moulds at a relatively high pouring temperature. Most metals and alloys are composed of many crystals, or grains as we metallurgists call them; thus, most alloys are polycrystalline. All Rights Reserved. In rolling or extrusion, for example, most deformation occurs at the surface, especially if only small reductions per pass are imposed. Such non-uniform deformation can also have repercussions on the grain structure on subsequent annealing when the process of recrystallization takes place. 
These can be either net shapes, as in investment (lost wax) casting, or stock materials, i.e. This can be a problem for craftsmen during gas torch annealing as there is less control of temperature during annealing and a tendency to overheat the piece. Iridium and ruthenium are insoluble in molten carat golds, so act as nucleation sites.
Grain boundaries are where the atoms sitting on the crystal lattices of adjacent grains do not match across together, creating a narrow region of imperfect crystal, Figure 2. Two-phase alloys: Where an alloy consists of two (or more) phases, there is an effect on grain size after working and annealing. Cold working and annealing: influence on microstructure & grain size.
Two fundamental points to understand are that1: In this presentation, I want to focus on alloy macro- and micro-structures, particularly grain size and shape. 
Annealing involves a process of recrystallization, where the hard deformed grains reform themselves into new undeformed grains by a nucleation and growth process analogous to solidification. If we wish to impart additional hardness and improved strength as well as a more accurate shape and superior surface, then we cold work the material, usually at ambient temperature. To achieve this, planes of atoms in each grain (crystal) must slide over each other, Figure 10, via crystal defects called dislocations. This is generally true for other non-precious engineering components such as sheet steel for car bodies and white goods. We can clearly see several dendrites, each pointing in different directions. For our precious metals, that will be in the face-centred cubic arrangement discussed in another presentation1. This is reflected by a change in the microstructure, where the grains must deform to accommodate the change in shape. The interface between them forms a boundary. Here the temperature is insufficient to promote annealing. There are obvious differences in appearance and these will be explained later. The focus of the presentation will be on gold alloys but all precious metals are considered. Working the alloy leads to a higher level of dislocations (crystal defects) in the matrix phase due to the presence of a hard second phase and this leads, in turn to a finer grain size after recrystallisation during annealing. At these high temperatures, the main deformation mechanisms are phenomena such as creep and fatigue. All Rights Reserved. strength, malleability, hardness). Figure 4 shows some dendrites in a platinum alloy7. 14 carat coloured golds are especially prone to excessive grain growth during annealing, as Grimwade has noted10. 24K .9999+ medical grade Pure Gold Shot, 5 Grains of Round Bullion, Not Scrap. To understand the process of solidification, it helps to understand the atomic structure of liquids and how atoms coalesce to form solid material. I would like to thank the organisers of the Jewellery Technology forum for inviting me to present once again and for their kind hospitality. These, in turn, influence the manufacturing process and the jewellerys behaviour during wear by the customer.There are a number of ways grain size (and shape) can be controlled in precious metal jewellery alloys by casting, working and annealing and by use of alloying additives that refine the grain size during casting and during working and annealing. Uneven deformation can give rise to initiation of cracking from the surface, as Battaini has explained8. As grain boundaries are less perfect than the crystals, they etch preferentially to reveal themselves. All Goldis double weighed with a high precision scale to be between 1.001 - 1.100 grams in weight. In hot working, as the metal deforms, it is at a high enough temperature for it to recrystallize (anneal) during the deformation. As has been mentioned before5,6, pure metals solidify at a fixed temperature; for example gold solidifies at 1064C and silver at 962C. Will call you back. Cold working of metals results in an overall shape change. For jewellery, we focus on the alloys of the precious metals gold, silver, platinum and palladium, all four of which are inherently ductile metals - but what I say is of general validity and applies to most metals. Figure 4 - SEM image of dendrites in Pt-Ru alloy, seen in a shrinkage cavity (from reference 7). This is a comparative method of measuring grain size. Is 10k harder than 14k or 18k? Influence of solidification on grain size and shape. Higher the karat, higher will be the percentage of pure gold in the metal.
These, in turn, influence manufacturability and service performance. This is known as coring. They do so in preferred crystal directions, extending from the cube faces and branching out as the crystal grows. We are a reputed Supplier of 24K Yellow Gold Casting Grain from Cotonou. This is a preferred microstructure. These nuclei grow by adding more atoms from the liquid. We talk about large (or coarse) grains or small (or fine) grain sizes and generally state the desirability of the latter in terms of jewellery production. Most cold-working processes result in uneven deformation through the cross-section. Much of this is best achieved by hot working the material, by hot forging or rolling, extrusion and/or drawing or combinations of methods.
Figure 15 shows schematically the effect of annealing temperature on hardness/strength , ductility and recrystallised grain size. He worked with Johnson Matthey Plc and World Gold Council before setting up his own technology consultancy in 2009. You may also hear of grain sizes referred to in terms of an ASTM numerical value. Why that is so, we can readily explain from the phase diagram6. If too much is added or it is not well dispersed, one can get nests of hard particles at the surface that give rise to comet tailing defects on polishing11. In such alloys, a larger or more uneven grain size may result.
Figure 16 shows the fine grain structure of an annealed 18 carat gold with iridium additions, compared to that without iridium. Other metals have also been shown to act as a grain refiner in gold alloys, such as boron, beryllium, yttrium and the rare earth metals, rhenium, rhodium, nickel, barium and zirconium13-16. This results in a tree-like structure that we call a dendrite. When we pour molten metal into a mould, it begins to solidify inwards from the mould walls as this is the coldest temperature. In this presentation, it is concluded that, for jewellery manufacture, it is desirable to have a fine (small) grain size. If we examine an etched metallographic section of a cast metal under the microscope, such as shown in Figure 3, we can clearly see the dendritic structure.
That is why annealing is often recommended only after substantial cold work, e.g. Christian P.Susz, Recrystallization in 18 carat gold alloys, Valerio Faccenda and Michele Cond, Is Pure Gold really Pure?, in, Dieter Ott, Influence of Small Additions and Impurities on Gold and Jewelry Gold alloys, in, W Truthe, US Patent 2,143,217, January 1939 (assigned to Degussa), C Raub & D Ott, German patent DE2803949A1, August 1979, M Poliero & A Basso, US Patent 2015/03544029A1, December 2015. Anyone involved in the making of jewellery should have an appreciation of the nature of the metals and alloys with which they work and understand how alloying and processing of the metals influences the microstructure and consequently their properties. 
Subsequent mechanical processing of ingot materials enables us to break down coarse, non-uniform structures to more desirable refined structures better suited to the purposes that we require in manufacture and in subsequent service and generally have improved, more consistent properties. On the other hand, engineering components can be subjected to often-complex stresses over long periods at high temperatures; for example, turbine blades and disks in jet engines and boiler tubes in utility power stations. Figure 1- Interrelationship of alloy composition, microstructure and processing history on properties (schematic). When a ceramic (plaster) muold is used, as in investment (lost wax) casting, the cooling rate is markedly slower and equiaxed grains are formed throughout the casting. Here flatness of the surface is not such an issue as in optical light microscopy and we can often see different phases by atomic number contrast, without the need for etching (see figure 22 in reference 3, for example)3,4. ingots, that can be further processed to modify the shape, structure and properties. Figure 7 - Splitting of gold alloy ingot down the centre during rolling (alligatoring), Figure 8 - Grain structure of ingots cast into metal moulds at a relatively low pouring temperature.