Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

Transcription

1 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality Paolo Consulting Engineer 8853 SpA Milan, Italy Introduction One of the most important topics in materials and alloys in particular is their microstructure. In regards to alloys, the solid-state crystal structure is what determines their success in engineering applications. A sound knowledge of the relationship between the alloy structure and its physical properties is absolutely necessary in order to understand the alloy s future behavior. A metal alloy is composed of a whole of several crystals and is called a polycrystal, whereas the single crystals are called crystal grains. 1,2,3 Due to their size, an optical microscope is, in most cases, needed to observe crystal grains. During observation with an optical microscope, magnifications are usually between 50x and 1000x. Generally, any structure requiring this magnification range to be examined is called a microstructure. Sometimes, the metal alloys prepared for metallographic examination show features and crystal grains large enough to be seen with the naked eye, or with an optical magnification lower than 50x. In these cases, what is seen is the material s macrostructure. Figures 1 and 2 offer the comparison between macrostructure and microstructure. Figure 1 Macrostructure of a 14K gold alloy visible with the naked eye just after the metallographic preparation. A band-structure due to solidification-induced elemental segregation is also observed. The alloy dendrites are deformed due to a previous rolling process. Figure 2 shows the corresponding microstructure. May

2 Figure 2 The same specimen as in Figure 1 observed at a higher magnification (about 750x). The rolled alloy was also annealed. As a consequence, the net formed by the boundaries of the recrystallized grains is visible. Recrystallization is not visible by investigating the macrostructure only. It is important not to mistake the term microstructure for crystal structure, since the latter refers to the atom positions within the single crystal grains. Optical metallography is one of the techniques used to examine the alloy microstructure. Partly, it can be considered an art, or a special skill, as it requires a good expertise to obtain good-quality results. However, it is relatively simple even for beginners to obtain from it enough information to be used successfully in daily operations. The present work is aimed at showing the advantages of metallographic analysis in facing and solving problems related to precious alloy metalworking, as well as in improving the product quality and optimize production cycles. Some examples are given to that purpose. The procedures of a metallographic analysis are not described here, as a great deal of literature is already available on this subject. 4,5 It is only reminded that the preparation of metallographic specimens generally requires their embedding into resin cool-hardening is better for goldsmith alloys, and a mirror-like finishing of the surface to be examined. Chemical etching is then necessary, aimed to highlight the microstructure. In some cases, relevant information can be achieved just from the optical examination of the mirror-polished surface, without further chemical etching. 2 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

3 Problems During Metalworking of a Nickel White Gold Wire The metalworking of an 18K white gold is considered. A wire with a diameter of 1.5mm is rolled to a C-shaped metal strip (Figure 3). Figure 3 General aspect of the white gold wire after rolling. A C-channel shaped metal strip is obtained. The biggest defects are visible on the raised flanges, where roughness is quite high. After rolling and subsequent annealing, cracks appear in some of these regions. The wire is then annealed at 650 C (1202 F) for 15 minutes. Wires from different manufacturers may give different qualities of the rolled and annealed strip. Important problems, such as excessive strip roughness after rolling and cracks after annealing take place in some cases. The gold fineness turns out to be compliant with specifications in all cases. In order to understand the wires different behavior, it is necessary to analyze them. The observation of the wire surface after rolling shows different roughness levels on various wires. However, this observation is inconclusive. Very well known is the orange peel phenomenon, 6,7,8,9 i.e. a kind of surface roughness having approximately the same size as the crystal grains. Since adjacent grains have different orientations, their tendency to lengthen or crush during deformation gives rise to a rough surface. Unfortunately, Scanning Electron Microscopy (SEM) doesn t allow us to evaluate the grain size either (Figure 4). Consequently, a precise solution to the problem cannot be given. May

4 Figure 4 The scanning electron microscope analysis of the rolled surface does not give precise information about grain size. A detail of the C-channel flange on the rolled wire is visible. From hardness measurements, it turns out that the hardness of a defect-free wire, HV = 200, is only slightly higher than that of the defective one, HV = 190, and slightly lower than that of the third manufacturer s one, HV 300 = 225, which shows the highest number of cracks after annealing. Undoubtedly, these measurements create further confusion, because they are unable to identify the causes of production problems. On the contrary, the microstructural analysis of the material under different conditions would allow a better understanding of what happens and allow us to move towards the problem s solution. Figure 5 shows the microstructure of the defect-free wire s transversal section after rolling and annealing. From the detail in Figure 6, the mean grain size can be estimated to be about 20μm. Figure 5 The metallographic section of the wire shows a fine-grained, homogeneous microstructure with no defects after rolling and annealing. 4 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

5 Figure 6 Detail of Figure 5. The maximum grain size is about 30µm and the minimum a few µm. On an average, the grain size is higher on the right, which corresponds to the flange protrusion. These differences are due to the different work hardening undergone by the various regions during rolling. The detail in Figure 6 helps highlight the relation between the work-hardening level and the mean grain size after annealing. Where the material underwent a slightly lower work hardening as in the side regions on the right of Figure 5 and the outer zones of the two flanges the mean grain size is higher. This is due to the lower number of crystal defects acting as potential grain nucleation sites during annealing. Figure 7 shows the section of an uncompliant wire. May 2007 Figure 7 Metallographic section of the wire characterized by surface roughness and cracks after rolling and annealing. 5

6 Even though Figure 7 refers to a rolled product whose size is a bit different from that in Figure 5, it can be easily seen that remarkable differences exist between the grain size of the two semifinished products. In fact, the material shown in Figure 7 is characterized by a crystal grain whose mean size is about 100μm, i.e. much larger than in Figure 5. This is partly due to the lower work-hardening level undergone by the material to be shaped as in Figure 7. However, the difference can mainly be ascribed to the initial grain size in the wire. As a matter of fact, the orange peel feature was clearly seen before the final annealing and cannot be ascribed to the annealing itself. Let s now consider a third nickel white gold wire that showed cracks after annealing at 650 C (1202 F) for 15 minutes, but no orange peel. In this case too, the metallographic analysis helps us reach a better understanding of what happens. Figure 8 shows the material microstructure before rolling. The grain is quite small and with optimal size. The occurrence of cracks after rolling and annealing suggests a comparison with the microstructure of the compliant wire. The comparison can be made by looking at Figure 9. Figure 8 Microstructure of the third wire, transversal section. The mean grain size is less than 10µm. From this point of view, the wire quality is optimal. Nonetheless, the wire showed cracks after rolling and annealing. 6 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

7 Figure 9 This microstructure of the wire, which does not give rise to drawbacks in metalworking, suggests that the third wire (see Figure 8) has optimal grain size. In fact, the grain size is slightly lower than that of the compliant wire shown in this image. The microstructure comparison did not identify any problem with regards to the third wire. However, the analysis concerned the transversal sections, and could not show the typical structures elongated along the wiredrawing direction, which may be crucial in drawn wires. A microstructure comparison of the longitudinal sections was then performed. The microstructures are visible in Figures 10 and 11. A striking difference is visible, mainly due to longitudinal bands observed in the third wire (Figure 11). These bands typically originate from the previous rod milling and from wiredrawing, which tends to create a fiber-like structure along the drawing direction. These bands are not visible in the transversal section (Figure 8), because their transversal widths moderate. In order to understand these microstructural non-homogeneities, higher magnifications are needed, as in Figure 12. May

8 Figure 10 The microstructure of the longitudinal section of the compliant wire is identical to the transversal section s microstructure (Figure 9). Figure 11 The longitudinal section microstructure of the third wire shows the typical bands due to both the wiredrawing process and material s nonhomogeneity. The bands are parallel to the drawing direction. 8 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

9 Figure 12 This detail of Figure 11 reveals that the dark bands consist of a distribution of dark points. Other elongated dark features are also visible in some regions (see the arrows). Figure 12 suggests that the third alloy is of poor quality. In fact, the little dark points distributed in longitudinal bands may be due to inclusion or oxide dispersion caused by contamination. The elongated dark structures may have the same nature as well. By the way, the metallographic analysis suggests that this alloy is characterized by defects that could be jointly responsible for the cracks observed in the rolled and annealed material. To solve the problem, a further step is needed, that is a Scanning Electron Microscope (SEM) analysis. Figure 13 is a SEM image of the microstructure in Figure 12. May

10 Figure 13 The SEM image of the specimen in Figure 12 shows that the elongated dark structures are due to a locally stronger action of the chemical etching which results in a deeper material removal. The nickel content within these regions is higher than the average nickel content of the alloy. This image clearly shows that the elongated dark structures are due to a locally stronger action of the chemical etching used to highlight the alloy microstructure, resulting in a deeper material removal. This can be ascribed to the higher nickel content of these regions, eaten away by the acid solution more easily than the rest of the material. The chemical composition of the elongated isles can be determined by Energy Dispersive Spectroscopy (EDS) microanalysis, a technique available as a SEM attachment. This technique also identifies the fine cavities as Si-Al and Ti-Si oxides. This means that the alloy underwent contamination during the melting process. By examining the production cycle, it turns out that melting was performed by continuous casting. The alloy was pre-melted and poured in a contaminated cast iron ingot mold. Dross present in the mold was absorbed by the alloy giving rise to the problem. Figure 14 shows details of some of the coarsest Ti-Si inclusions. 10 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

11 May 2007 Figure 14 The numerous, finely dispersed cavities contain inclusions. Some of them can be analyzed by Energy Dispersive Sprectroscopy (EDS). The arrows point to Ti-Si oxides. Figure 15 The EDS microanalysis spectrum of the inclusions in Figure 14 shows that they are composed of Ti-Si oxides. Hence, metallographic analyses demonstrated that the third wire has two phases, with a high nickel content phase. It suggests that the alloys should be analyzed not only to verify their gold content, but also to determine the content of minority elements, i.e. nickel, copper and zinc. The nickel content of the third wire 11

12 is 8%, whereas it is 5% in the other two. It is well known that the higher the nickel content, the more difficult white gold alloy metalworking becomes. 10,11,12,13 Furthermore, their hardness depends on the relative concentrations of zinc and nickel. Finally, the third wire has a higher zinc content and probably Si contamination, which promotes fire cracking, as is indeed observed. As a conclusion, it can be said that metallography allowed the understanding of the cause of the different behavior of the three wires. The second wire had undergone excessive annealing, leading to an excessively large grain size. The third wire has an ideal grain size, but the alloy could not easily support metalworking because its nickel and zinc content was too high and because it was contaminated by silicon. Even though silicon was absent in the last case, metallography reveals a problem in the metalworking process and suggests that the user should change his own metalworking cycle to adapt it to a different alloy. The different nickel white gold alloys don t behave alike. Metallography presses us to study the fundamentals of gold alloy metallurgy for a better choice and understanding of metalworking. Ruptures Due to Residual Stresses Produced During Mechanical Operations Residual stresses are zero resultant tension or compression stresses, present into a component without the application of any external force. 14,15 Generally, they originate from a non-homogeneous plastic deformation during the production process. They are usually confined to some regions and vary in the interior of the component. In the goldsmith industry, crucial residual stresses are those that develop during wire or tube drawing and the most affected alloys are the nickel white gold alloys. In this case, ruptures during heat treatments are a frequently recurring situation. However, the problem significantly affects other goldsmith alloys, since it is often accompanied by ruptures during metalworking. Figure 16 helps demonstrate the relevance of residual stresses. 16 It shows their distribution in a drawn steel bar. Three kinds of residual stresses can be identified, i.e: transversal (T), whose direction is tangential to the wire s circular section (also called circumferential); longitudinal (L), directed along the wire axis; and radial (R), along the radius of the circular section. The most dangerous residual stresses in metalworking are the transversal ones, since they may cause longitudinal ruptures in wires and tubes. Figure 17 shows the longitudinal residual stress distribution in a cold-drawn brass wire. 17 It is interesting to see that they come to a maximum and then decrease with increasing reduction. Indeed, it often happens that a decrease in residual stresses is obtained by going on with mechanical operations. When ruptures occur during wiredrawing, it is important to understand whether they are due to residual stresses, and metallography can help solve the problem. Figure 18 shows longitudinal ruptures in a drawn and annealed nickel white gold tube. Optical metallography allows us to ascertain whether the causes can be ascribed to residual stresses or defects in the TIG welded joint used in the production of the raw tube. 12 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

13 Figure 16 Residual stress distribution in a drawn steel bar. The same distribution is observed in a drawn goldsmith alloy, apart from the absolute stress values. Transversal residual stresses (T) are crucial because they may be responsible for longitudinal ruptures because they are tensile at the bar or wire surface. The reported distribution is still valid for drawn tubes, with longitudinal tensile stresses at the outer surface and compressive ones at the inner surface. Transversal residual stresses are tensile at the outer surface and compressive at the inner tube surface. Radial stresses are negligible. The situation does not change whether the material is drawn with or without a mandrel. May

14 Figure 17 Longitudinal residual stress distribution in a cold-drawn brass wire. Residual stresses decrease with increasing reduction. 14 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

15 Figure 18 An 18K nickel white gold tube (8% Ni) with a diameter of 2mm showing longitudinal ruptures after drawing and belt-furnace annealing. The longitudinal rupture (see the arrows) may be due to transversal residual stresses. Some cross-sections, cut out from the tube by a rotary blade, are visible in the upper part of the image. The examination of the transversal tube section obtained in the cracked region shows that the rupture path is consistent with transversal tensile residual stresses at the surface. In fact, the crack opening is widest at the tube surface (Figure 19). Besides, the previously TIG-welded region, with a slightly larger grain, is still visible (Figure 20). The welded region underwent recrystallization after workhardening induced by rolling and annealing, preserving a different grain size notwithstanding. It can be dismissed that the longitudinal rupture is due to defects in the welded region. The cause of rupture can be attributed to residual stresses. A further confirmation of that is found by observing the crack propagation path. It follows the grain boundaries, which is typical of cracks caused by residual stresses. The detail of Figure 19, given in Figure 21, shows this feature. The analysis can be completed by observing a crack that has not crossed the whole tube section, as in Figure 22. This image confirms the intergranular propagation even at the crack tip, where it is easier to recognize this characteristic. It can also be noticed that the crack propagates along different directions. May

16 Figure 19 Transversal section of the cracked tube. The crack opening is typical of tensile transversal stresses. In fact, it reaches the maximum width at the outer tube surface (see the arrows in the lower part of the photograph). The rupture propagates along the grain boundaries, as shown in Figure 21. At the top right of the photograph there is the region where the TIG weldment was carried out to start the raw tube, which was subsequently drawn and annealed. See Figure 20 for details. 16 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

17 Figure 20 Detail of the upper right part of Figure 19. The dashed line corresponds to the interface between the base material (below) and the previously TIG-welded region (above). The welded region is clearly visible, due to the larger crystal grain size. Figure 21 Detail of the rupture region shown in Figure 19. The crack path is intergranular, as typically occurs in these materials for residual stress-induced ruptures. May

18 Figure 22 The examination of another transversal section where the rupture is not complete makes it possible to recognize the intergranular nature of the crack path. The propagation morphology is more easily identifiable at the crack tip. While the presence of residual stresses can be easily recognized in nickel white gold alloys because of the frequently induced ruptures, in other goldsmith alloys it is more difficult to detect them. However, their presence can cause other drawbacks, such as stress corrosion cracking. 18,19,20 The example below demonstrates how metallography can help identify residual stresses in an 18K 5N gold tube produced by continuous casting and subsequently drawn. The work-hardened tube is cut into washers (see Figure 23), worked to obtain wedding rings afterwards. Figure 23 Washers are cut out from the tube to be transformed into wedding rings. Neither the tube nor the washers show ruptures. 18 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

19 The tube does not show cracks nor does it seem subject to residual stresses. The washers cut out from the tube do not reveal any deformation or ruptures either. However, extensive radial cracks appear on the transversal metallographic section, as shown in Figure 24. See Figure 25 for details. Figure 24 General aspect of the washer metallographic section after chemical ecthing. The crack was absent before etching. The acid solution, composed of HCl and CrO 3, produces the crack, promoted by residual stresses present in the material. The high residual stresses present in the material lead to rupture under the action of the acid solution used to highlight the crystal grains. The solution etches the material along the grain boundaries and promotes the crack initiation. The detail of Figure 25 shows the intergranular crack path. Figure 25 Detail of Figure 24. The crack propagates along the grain boundaries, which is typical of residual stress-induced corrosion cracking. May

20 The example demonstrates that optical metallography can reveal the presence of residual stresses even in this material in the post-drawing work-hardened condition. The knowledge of that can be of valuable relevance in the subsequent working steps, in order to avoid inconveniences. In conclusion, metallographic analysis turns out to be a helpful aid in the study of ruptures due to residual stresses. To prevent them, thermal treatments can be performed at temperatures where recrystallization has not yet started, but recovery takes place. It is known that residual stresses are eliminated or weakened during recovery. 21 Alternatively, it is possible to optimize new production cycles that do not cause residual stresses that are to high, but allow an appropriate work-hardening level, nonetheless. As seen in the comments for Figure 17, residual stresses do not always decrease with a decrease in the extent of mechanical working. Sometimes the opposite occurs. Material Fiber-Like Structure and Possible Ruptures during Mechanical Working During mechanical working, the crystal grains lengthen along the direction of maximum material slip. This directional pattern of crystals is known as the grain-flow pattern. This pattern is responsible for the familiar fiber structure of semi-finished, wrought metal products. As an example, the usual wedding ring production can be considered. In Figure 26, the steps of this process are shown. Figure 26 The production phases of raw wedding rings starting from a rolled material are summarized. During working, the material undergoes a remarkable plastic deformation and is characterized by a fibrous microstructure. (From Johnson Matthey). 20 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

21 Figure 27 Metallographic section of the washer in Figure 26 after phase 3. The metallographic sections of the washer are characterized by a noticeable fiberlike structure. Figure 28 is a detail of one of the two sections. Figure 28 Example of fiber structure that characterizes the washer section of Figure 27. The material is a nickel white gold alloy. It should be pointed out that this type of anisotropy, sometimes called mechanical fibering, affects properties that are fracture initiated: ductility, fracture toughness, and fatigue strength. This anisotropy in properties is greatest between the working (along fibers) direction and the transverse direction. 22 As a consequence, any small crack propagating from the surface to the material interior along the fiber direction can be dangerous and lead to rupture. Stresses applied perpendicularly to the crack could be sufficient to initiate the cracking process. The fiber structure can be studied by optical metallography, both with images like Figure 28, and with the aid of image elaboration as in Figures 29 and 30. May

22 Figure 29 (Left) Detail of the left part of Figure 28. Image elaboration helps highlight the material fiber-like structure. Such a structure is particularly dangerous at the upper left edge. A crack in this region can easily propagate towards the interior. Figure 30 (Right) Detail of the right part of Figure 28. In this case, the lower right edge is the most critical, as small cracks, if any, propagate towards the interior when subject to stresses perpendicular to the fiber plane. Once the fiber morphology is known, the working cycle can be adjusted to eliminate the most critical regions of maximum plastic slip, that is by modifying both the geometry and the extent of plastic deformations. Obviously, a valuable aid to eliminate any ruptures is to introduce annealing between a plastic deformation step and the subsequent one. In this case, optical metallography allows us to identify the annealing parameters. In fact, the fiber structure can be broken as early as the first recrystallization phases. It is then advisable to carry out an annealing process such that it does not cause excessive grain growth (see Figure 31). 22 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

23 Figure 31 Optical metallography allows us to identify the right recrystallization thermal treatment able to break the fiber structure due to plastic deformation. The formation of crystal grains with a mean size less than 25µm is visible in this image. The new crystal grains are able to recreate a good mechanical workability. The break of the fiber structure lowers the possibility that surface or internal cracks propagate into the material along fibers. The fiber structure is not always clear, as in Figures 28 to 31, but can be revealed thanks to deformation bands located in single-crystal grains and observable by optical metallography. An example is given in Figure 32, where an 18K 2N yellow gold is shown. Parallel dark lines of variable width, and with a preferential orientation, are visible within some grains. During metalworking, the mechanical strength of the material is lower along the deformation bands. Consequently, cracks with such an orientation may form. Figure 33 shows the material of Figure 32 after annealing. The annealing process eliminates the deformation bands, thanks to recrystallization, and makes it possible to better discern the cracks. They are not visible on the tube s outer surface and are due to excessive work-hardening during drawing. The metallographic analysis allows us to identify the problem and suggests the solution, which consists in adjusting the drawing operations. Cracks along fibers are visible in a work-hardened 18K 5N red gold alloy (Figure 34). In order to avoid ruptures, the degree of plastic deformation should be changed. Fiber structures are also produced by rolling. While in the case of sheet rolling, the resulting fiber structure features are known, rod milling gives rise to a much more complex situation. Figure 35 shows the flow of fibers in the transversal section of a bar. The material deformation is rather complex and develops on X-crossed planes. Plastic flows on X-crossed planes are observed along the longitudinal sec- May

24 tion as well. In conclusion, the metallographic analysis makes it possible to assess the plastic deformations that the material undergoes during work hardening. This, in turn, allows us to optimize processes, in order to prevent ruptures and reduce the number of non-compliances of the finished product. Figure 32 Transversal section of a tube made of an 18K 2N yellow gold alloy. The alloy is work-hardened by drawing. Deformation bands are visible as dark lines within grains. They are preferentially oriented along the drawing direction horizontally in the photograph. Deformation bands offer a low resistance in case of applied radial or longitudinal stresses and can act as crack initiation sites. 24 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

25 Figure 33 The deformation bands within the grains of the work-hardened alloy of Figure 32 disappear after annealing. Internal cracks oriented as the previous deformation bands are visible (arrows). This feature cannot be revealed by observations of the tube s outer surface. May 2007 Figure 34 Plastically deformed and annealed 18K 5N red gold alloy. The arrows point to two cracks propagated along the material s fibers. The material was annealed, so the fiber-like structure is only partially visible. Fibers spread from the lower left corner towards the right. 25

26 Figure 35 Rod-milled white gold transversal section. Even in rod-milled materials, fibers can be well characterized by optical metallography. The fiber distribution on the transversal section is typically X-shaped, independently of the alloy chemical composition. Excess deformation is present at the edges. A similar fiber distribution is observed on the longitudinal section. Set-Up of Thermal Treatment Cycles and Grain Size The advantages offered by optical metallography in the set-up of annealing cycles have already been discussed. It is now important to remember that the efficacy of an annealing treatment cannot be assessed simply by hardness measurements. Hardness changes after annealing are well known. However, a hardness value is not representative of a single grain size in the case of annealed materials. On the other hand, grain size after annealing strongly affects the outcome of the subsequent mechanical operations. Figure 36 shows the microstructure of a 925 silver bar obtained by continuous casting, roll milled to an 80% reduction, annealed at 745 C (1373 F) for one hour and water cooled. Its Vickers hardness is equal to 61 HV 5/30. If we change the annealing temperature to 645 C (1193 F) and reduce the time to 10 minutes, the same bar shows a remarkably different microstructure, 26 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

27 characterized by fine grains (see Figure 37). Its Vickers hardness is now equal to 68 HV 5/3. Even though hardness measurements can be related to different microstructures, they give no information about the crystal grains dimension. Furthermore, it is recognized that the mechanical workability of a work-hardened alloy can be re-established during the recovery phase and, consequently, an excessive material recrystallization is not necessary. As an example, Figures 38 and 39 can be considered. They show the different microstructures of two samples of the same alloy (a 95Pt5Cu alloy), after the same plastic deformation followed by annealing treatments at 850 C (1562 F) and 750 C (1382 F). At first sight, annealing at 850 C is more effective, even at breaking the fiber structure. However, the alloy annealed at 750 C is better, as it can be easily subjected to further processing without any orange peel effect, which occurs after annealing at 850 C. Hence, the most appropriate annealing is the one that gives rise to an initial recrystallization with no excessive grain growth. It is worthwhile noticing that the hardness of the alloy in Figure 38 is about 40 HV lower than that of the annealed alloy in Figure 39. If the criterion for setting up the annealing cycle is based only on hardness measurements, wrong conclusions are drawn. In conclusion, these two examples demonstrate that the logical process for the setup of the annealing treatments is always the same, independently of strong differences among materials. Optical metallography is irreplaceable in regards to the set-up of annealing cycles, as it offers an accurate check of the crystal grain size. It is worthwhile to remember that specific standards for mean grain size measurements are available, such as ASTM E (2004)e2: Standard Test Methods for Determining Average Grain Size. Figure 36 Microstructure of a 92.5Ag alloy rod-milled to a reduction of 80%, annealed at 745 C (1373 F) for one hour and subsequently subjected to rapid water cooling. May

28 Figure 37 The same 92.5 Ag alloy as in Figure 36, rod-milled to a reduction of 80% and then annealed at 645 C (1193 F) for 10 minutes. The grain size is clearly smaller and the mechanical workability is re-established. Hardness is only 7 HV higher than that of the alloy in Figure 36. Figure 38 95Pt5Cu alloy annealed at 850 C (1562 F) after work-hardening. The annealing process gives rise to a noticeable recrystallization, where new grains interrupt the fiber structure. 28 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

29 Figure 39 The same alloy as in Figure 38 after annealing at 750 C (1382 F). Recrystallization is less defined, but is ideal for further plastic working operations. Furthermore, fibers are broken by the growing grains. Various Applications of Optical Metallography Besides the above-mentioned cases, optical metallography can be successfully applied in several other circumstances in which the set-up of working cycles is crucial, for example in TIG and laser welding and in brazing (Figures 40 to 42) Figure 40 Transversal section of a TIG-welded raw tube made of 2N yellow gold. Optical metallography allows us to verify that the welded region is defect-free and to set up the process parameters, such as the electrical current, the torch potential difference and the displacement velocity of the piece to be welded. May

30 Figure 41 The metallographic examination of brazed joints leads to the detection of any defects, such as shrinkage porosity and gas porosity. Then, it is possible to modify the process parameters to avoid them. The image refers to the section of the brazing region between two 95Pd alloy components. Figure 42 Laser melted region of an 18K nickel white gold. Laser welds often show internal defects like gas porosity or cracks. Optical metallography allows us to set up the welding process in order to avoid them. Other processes find a helpful aid in optical metallography, for example, the production of bi-metallic wires (Figure 43). In this case, the internal continuity of the various components and the absence of cracks and anomalous inter-diffusion can be verified. Optical metallography finds a wide range of applications even in particular cases, always proving to be a valuable support in the set-up of operating methodologies. 30 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

31 Figure 43 Metallograpic transversal and longitudinal sections of a bi-metallic wire. The analysis of these sections demonstrates that the performed production operations are successful. Neither voids nor ruptures are present in the inner component, that is a brazing alloy. It is also confirmed that no anomalous diffusion between the two alloys occurs, which may be very dangerous. May

32 Optical Metallography and Lost-Wax Casting The contribution that optical metallography gives to the set-up of the lost-wax casting process, and the study of its typical defects, is intentionally left as the last topic in this work. A considerable amount of literature on that is indeed available. 23,24,25 What is worthwhile remembering here are the main aspects that metallographic analysis helps explore, that is the study of porosity and its causes (Figure 44), anomalous inclusions (Figure 45) and alloy contaminants responsible for alloy embrittlement (Figure 46). Optical metallography can be used together with Scanning Electron Microscopy (SEM), which is often essential to definitely understand the problems. However, optical metallography should be used first to get a general idea of the situation. SEM can then be used later. Figure 44 Porosity is a typical defect in jewelry obtained by lost-wax casting. Metallographic analysis is fundamental to distinguish between gas and shrinkage porosity. The image shows gas porosity. 32 Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

33 Figure 45 Ruthenium clusters in jewelry obtained by lost-wax casting. Clusters are very hard and prevent a good polishing of the surface. In this case, Scanning Electron Microscopy and EDS microanalysis are necessary to identify the clusters chemical nature. However, metallography is the technique that detects the problem first. May

34 Figure 46 95Pd alloy. Optical metallography detects a phase extraneous to the matrix and located at the grain boundaries (see the arrows). This alloy had undergone Si contamination that caused intergranular embrittlement. Optical metallography reveals the anomalous phase at grain boundaries and electron microanalysis can subsequently identify the chemical element responsible for that. Conclusion The examples above reported show that metallographic analysis is a valuable support to a goldsmith s production. It allows us to optimize the production process, avoid working mistakes and discover the causes of serious defects. Thanks to metallography, product quality can be improved and production costs due to non-compliances reduced. References 1. R.W. Cahn, Physical Metallurgy, third ed. (North-Holland, 1983). 2. R. E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, third ed. (Boston: PWS-Kent Publishing Company, 1991). 3. M.F.Grimwade, Solidification of Metals, Gold Technology 2 (June 1990): Metallography and Microstructures ASM Handbook Vol.9, (ASM International, edited by G. F. Vander Voort, 2004). 5. D. Ott, U. Schindler, Metallography of Gold and Gold Alloys, Gold Technology 33 (Winter 2001): M. F. Grimwade, Working and Annealing, Gold Technology 2 (June 1990): Metallography in Jewelry Fabrication: How to Avoid Problems and Improve Quality

35 7. D. Ott, C. J. Raub, Grain Size of Gold and Gold Alloys, Gold Bulletin 14, (2) 1981: M. F. Grimwade, Causes and Prevention of Defects in Wrought Alloys, Gold Technology 36 (Winter 2002): F. Klotz, S. Grice, Live and Let Die (Struck), Gold Technology 36 (Winter 2002): G. Normandeau and D. Ueno, Fire Cracking in White Gold Jewelry Articles in The Santa Fe Symposium on Jewelry Manufacturing Technology 2002, ed. Eddie Bell (Albuquerque: Met-Chem research, 2002). 11. Allen S. McDonald and George H. Sistare The Metallurgy of Some Carat Gold Jewellery Alloys. Part II Nickel Containing White Gold Alloys, Gold Bulletin, 11 (4) 1978: Christian P. Susz and Michel H. Linker, 18 Carat Gold Jewellery Alloys, Gold Bulletin, 13 (1) 1980: Greg Normandeau, White Golds: A Review of Commercial Material Characteristics & Alloy Design Alternatives, Gold Bulletin, 25 (3), 1992: George E. Dieter, Mechanical Metallurgy, third ed. (Boston: McGraw-Hill, 1986). 15. William F. Hosford, Robert M. Caddell, Metal Forming, Mechanics and Metallurgy, (Englewood Cliffs: Prentice-Hall, 1983). 16. S. Kalpakjian, Manufacturing Processes for Engineering Materials, (Addison- Wesley Publishing Company, 1991). 17. W. Lincius and G. Sachs, Mitt Dtsch. Materialprüfungsanst., vol.16, 1932: W. S. Rapson, Tarnish Resistance, Corrosion and Stress Corrosion Cracking of Gold Alloys, Gold Bulletin, 29 (2), 1996: Stewart Grice, Failures in 14KT Nickel-White Gold Tiffany Head Settings in The Santa Fe Symposium on Jewelry Manufacturing Technology 2002, ed. Eddie Bell (Albuquerque: Met-Chem research, 2002). 20. Jennifer M. M. Dugmore and Charles D. DesForges, Stress Corrosion in Gold Alloys, Gold Bulletin, (1), 1979: Mark. Grimwade, Working, Annealing and Deformation Processes in The Santa Fe Symposium on Jewelry Manufacturing Technology 2003, ed. Eddie Bell (Albuquerque: Met-Chem research, 2003). 22. Metalworking: Bulk Forming, ASM Handbook Vol.14A, (ASM International, edited by S. Lee Semiatin, 2005). 23. Dieter Ott: Handbook of Casting and Other Defects in Gold Jewellery Manufacture (World Gold Council Publication, 1997). 24. Dieter Ott, Christoph J. Raub and William S. Rapson, Investment Casting of Gold Jewellery, Porosity in Castings, its Causes and Prevention, Gold Bulletin, 18 (3) 1985: Dieter Ott, Analysis of Some Common Casting Defects, Gold Technology 13 (July 1994). May