dr. jörg fischer-bühner fem, schwäbisch gmünd, … · jörg fischer-bühner is head of the...

Jörg Fischer-Bühner is head of the Division of PhysicalMetallurgy and Precious Research of the GermanInstitute of Precious Metals (FEM) in SchwäbischGmünd. He has backed and participated in numerousE.C. financed, research projects in the goldsmith field.

The hardness of sterling silver can be increased toabout 150HV with a correct heat treatment carried outindustrially. This paper will outline the methods torealize the hardening treatment that allows a substantialimprovement of the characteristics of these products,with particular reference to wear resistance.

Dr. Jörg Fischer-BühnerFEM, Schwäbisch Gmünd, GERMANY

167June 2004

Hardening Possibilities of Sterling Silver Alloys

Abstract

Sterling silver alloys are comparably soft in the as-cast state (60-70 HV). A higherstrength is often required e.g. for filigree jewellery items, and has a positive influenceon polishing properties. Higher strength also results in increased resistance againstscratching and unwanted marking. The hardness of sterling silver alloys can beincreased significantly up to ca. 140-160 HV by suitable heat-treatments. Althoughthe process is well-known in principle since decades, it is applied seldomly, becauseit is time-consuming and significant improvements are sometimes not easy toachieve in industrial environment.

More recently, new sterling silver alloys have been developed with grain refinement, fire-stain resistance and improved investment casting properties. Compared to standard925 sterling silver, the copper content is significantly reduced in modern commercialalloys. As a consequence, the as-cast hardness (or soft-annealed hardness) of thesealloys is even lower than for standard sterling silver (down to 50 HV), so that hardeningby subsequent heat treatment possibly will gain larger importance. The influence of the reduction in Cu-content as well as of the different alloyingelements contained in these alloys has not been reviewed so far, however. Furtherissues which are of relevance in this context are: danger of grain coarsening duringhomogenisation, loss of ductility of hardened material, hardening of soldered items,cost efficiency of heat treatments in mass production as well as problems withhardening of stone-in-place castings for which heat treatments that involve rapidquenching processes are not feasible.

The paper is an update of a presentation that has been given at the Santa FeSymposium in 2003. It first reviews the basics of strengthening sterling silver alloys byage-hardening and discusses the range of suitable heat treatment parameters forstandard sterling silver. This is followed by an overview on hardening properties of thenew sterling silver alloys. The paper continues with some results on a more cost-efficient hardening process for investment cast material, i.e. the hardening directly fromthe quenched as-cast state without the high temperature homogenisation annealwhich is usually required before age-hardening. Several case-studies for realisation ofhardening of castings in industrial environment are presented. Finally, some findingson the interplay of age-hardening and soldering treatments are discussed.

Background

The possibility of hardening sterling silver by heat treatment is already mentioned in

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early publications e.g. by E. Raub [1]. The metallurgical principles of 'age-hardening'or 'precipitation hardening' of sterling silver were presented in detail by MarkGrimwade and Aldo Reti at earlier Santa Fe Symposia [2-4]. The usual processconsists of a 3-step heat treatment schedule starting with a homogenisationannealing at high temperature. In order to understand the necessity for this first step,the solidification process and microstructure requires brief consideration.The complete binary equilibrium Ag-Cu-phase diagram is shown in figure 1. Aschematic cooling curve of a binary 930 Ag alloy is shown in figure 2 and is relatedto the Ag-rich section of the phase diagram on which all further discussion can focussince all sterling silver alloys have a Cu-content of max. 7.5%. Solidification starts at ~900°C and theoretically would be finished around 820°C. Inreality, accelerated cooling conditions causes solidification to end at 780°C, which finallyresults in the formation of the so-called eutectic phase (a lamellar alternate arrangementof Ag- and Cu-rich areas) in the solidification microstructure. During further cooling,coarse Cu-rich precipitates form until eventually the casting is quenched. Acorresponding example for a typical solidification microstructure is shown in figure 3.

The age-hardening of sterling silver bases on the formation of very, very fine Cu-richprecipitates formed in the silver matrix, however, whereas the very coarse Cu-richprecipitates which form during solidification are entirely ineffective in terms ofhardening. Therefore, and with reference to the scheme indicated in figure 4, theusual age-hardening heat treatment process for a binary 925-935 Ag alloy (i.e. with7.5-6.5 wt% Cu) is as follows:

- Homogenisation (or so-called solution treatment) above ~ 745°C for ~ one hourto dissolve all the copper in the silver matrix

- Rapid quenching in water, which prevents formation of coarse Cu-richprecipitates during slow cooling and results in a so-called super- saturated state

- Annealing at low temperatures (or so-called ageing), typically 300°C for one hour,which results in precipitation of very fine Cu-rich particles.

The microstructure of as-cast, homogenised & quenched material is shown in figure5. The very fine precipitates which form during ageing can only be identified at veryhigh magnifications, however, usually in a transmission electron microscope.

Figure 1 - Binary equilibrium Silver-Copper - phase diagram

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Figure 2: Schematic cooling curve of a 930 Ag casting relating to the Ag-rich section of the binary Ag-Cu-phase diagram

Figure 3: Solidification microstructure of investment cast sterling silver (flask quenched ~ 15 min after casting)

Figure 4: Usual heat treatment schedule for 925-935 sterling silver alloys relating to the Ag-rich section of the binary Ag-Cu-phase diagram

Figure 5: Microstructure of a sterling silver casting after homogenisation at 800°C / 1 hour, followed by water quenching


Experimental

The first part of this study aimed at reviewing the range of suitable heat treatmentparameters for standard sterling silver, followed by studying the hardening propertiesof the new sterling silver alloys. Compared to standard 925 sterling silver, the coppercontent is significantly reduced in modern commercial alloys, especially firestain-resistant alloys (see table 1 and figure 4), e.g. [5]. Some details on the alloys included in this study are given in Table 1:

Table 1

* two Legor-alloys were included, namely Ag108M and SF928CH

In order to arrive at results without interference with deficiencies of as-cast states(inhomogeneous grain size, porosity etc.), some work was performed with small barsof ~50 g material melted and cast in protective atmosphere and afterwards rolled to~60-70 % (section reduction). Material was then cut into samples suitable for heattreatment and hardness testing. This approach was chosen for identifying thepossible influence of the major alloying constituents, namely Copper and Zinc, aswell as of the grain refiners Iridium and Boron. In addition, hardening with as-cast material was studied using material from trees of~300 g with a standardised test tree set-up (figure 6) which were cast using standardvacuum-assisted investment casting equipment and parameters (flask ~ 500°C,casting temperature 970°C). These experiments also included the firestain resistantcasting alloys with Si, Ge etc.

Figure 6: Standard casting tree (~300 g) for silver investment casting trials

Alloy Cu % Zn % Other

Ag925 7.5 - -Ag935 6.5 - -

Ag925+Ir 7.5 - 0.01% IrAg925+B 7.5 - 0.005% B

Ag930 Zn20 5.0 2 -Ag930/935 Zn20 Ir 4.5 - 5 2 0.01% Ir

925 Legor * ~5 ~2 Si, Ge, B, …927 Apecs G7 4 2.6 Si, Ge

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In order to evaluate the influence of hardening on ductility, as-cast wires of 925Ag and930Ag20ZnIr material were drawn from a diameter of 10 mm down to 2 mm, heat treatedand cut into samples suitable for tensile testing. Yield strength, tensile strength andelongation until failure were recorded during room temperature tensile tests usingconventional equipment.All heat treatments were carried out in protective nitrogen atmosphere withtemperatures/times specified later in the paper. After high temperature solution treatment,samples were removed quickly from the furnace and quenched into water immediately.After ageing at low temperatures samples were cooled on air.All samples were investigated by standard metallographic procedures; hardness wasdetermined on prepared cross sections using a conventional Vickers hardness tester anda load corresponding to HV1.

Review of suitable heat treatment parameters instandard sterling silver

A screening of ageing properties was carried out on rolled sheets of 935Ag. Anunusually high homogenisation temperature of 800°C was chosen to ensurecomplete solution of the copper in the silver matrix during 0.5-1 h annealing. Waterquenching of the samples was followed by ageing at 150-350°C for 0.5-16 h.Selected results of the screening are shown in Figure 7. It is confirmed that• in standard sterling silver an optimum hardening effect occurs for material aged at~ 300°C for 0.5-1 h

At 250°C the same hardness level is achieved only after ~4 h. Prolonged annealingat 300°C as well as annealing at 350°C leads to overageing and loss in strength.Ageing times lower than 30min were not studied in detail since they would be difficultto control in industrial environment as soon as the process is applied to largerquantities of material or samples.

Figure 7: Influence of ageing temperature and time on age-hardening of 935Ag after rolling (~66 %) and homogenisation (800°C / 1 h / water quench)

Homogenisation at too high temperatures leads to grain coarsening, however. Thisis less pronounced for as-cast material, but definitely a severe issue for rolledmaterial, as shown in figure 8. As already discussed by Aldo Reti in [4], graincoarsening can be suppressed in 925Ag if homogenisation is carried out slightlybelow the usual border line value of 745°C, e.g. at ~730°C, so that grain boundariesare effectively pinned by rests of eutectic or Cu-rich phase.

8a) 925Ag, 730°C / 1 h

8b) 925Ag, 800°C / 1 h

Figure 8: Microstructure of 925Ag after rolling (~66 %) andhomogenisation at 730°C and 800°C / 1 h / water quench

9a) cast & homogenised at 650°C / 1 h, water quenched

9b) cast & homogenised at 730°C / 1 h, water quenched

Figure 9: Microstructure of 925Ag after investment casting and homogenisation at 650°C and 730°C / 1 h / water quench


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10a) 925Ag, rolled material (~66 %)

10b) 925Ag, investment cast material

Figure 10: Influence of homogenisation temperature (650-800°C / 1 h / water quench) on age-hardening of 925Ag after a) rolling and b) investment casting

Figure 9 illustrates the change of an as-cast microstructure during homogenisation at650°C and 730°C, respectively. If compared to the as-cast microstructure (figure3) andthe state homogenised at 800°C (figure 5), it is obvious that still large amounts of Cu-rich phases in the matrix are present after homogenisation at 650°C (figure 9a), whereasonly some rests of eutectic phase are present after homogenisation at 730°C (figure 9b).In agreement with the statements in [4], some rests of undissolved Cu-phases afterthe high temperature homogenisation can be tolerated with regard to the success ofthe subsequent ageing process. Figure 10 shows the results of a screening on a)rolled sheets and b) cast material of 925Ag after homogenisation at 650°C-800°C for1 h, water quenching, followed by ageing at 300°C for 0.5-1 h. It is concluded that

• in standard sterling silver alloys significant hardening still occurs if the temperatureof homogenisation is lowered to 730°C (~140 HV1) and even 700°C (~120 HV1)

whereas a drastic reduction or almost no hardening effect occurs afterhomogenisation at 650°C / 1-2 h.


Influence of age-hardening on ductility

Table 2 shows results of tensile testing of wires drawn from 925Ag and 930AgZn20Ir-castings. Data are given for samples in the homogenised state in comparison tosamples with subsequent ageing at 300°C.

Table 2: Influence of hardening on tensile ductility and strength

The results clearly show that the strengthening effect is not encompassed by anembrittlement, but that there is a

• marked reduction in tensile ductility in sterling silver by age-hardening.

The data illustrate also the positive effect of grain refining additions, since in thematerial grain-refined by Ir the tensile elongation is higher for both, the homogenisedand age-hardened condition, and the reduction in ductility by hardening is lesspronounced. Hence, usage of grain refined material is recommended in cases wherethe material or product needs to tolerate relevant plastic deformation after thehardening treatment (e.g. sizing of rings).

Influence of alloy composition on age-hardening

The elements Zn, Si and Ge, as well as grain refiners like B or Ir, are typical additionsof the modern firestain-resistant alloys and all together lead to a significant reductionof the Cu content (compare table 1).

Investment cast samples of 925Ag, 935Ag20ZnIr and of the firestain-resistant alloyswith Si, Ge additions were homogenised at 700°C and 730°C for 1 h, waterquenched, and age-hardened at 300°C for 0.5-1 h.

HV1 0.2% Y.S. UTS Elongation

(N/mm2) (N/mm2) (%)Ag925, 730°C / 1 h 66 112 276 29

Ag925, 730°C / 1 h +300°C / 1 h

142 333 405 9

Ag930Zn20Ir, 700°C / 1 h

70 121 267 40

Ag930Zn20Ir, 700°C / 1 h +300°C / 0.5 h

127 302 372 19

Ag930Zn20Ir, 700°C / 1 h +300°C / 1 h

127 293 347 14

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From the hardness data given in figure 11 it is obvious that :

• the peak hardness is lower in these firestain-resistant alloys, compared tostandard 925Ag.

Since a corresponding lowering of the peak hardness is also observed in the935Ag20ZnIr-sample, this effect can not be attributed to the small Si- and Ge-additions in the firestain-resistant alloys. In a former set of experiments, which hasbeen reported in full detail in [6], it also turned out that there is no major influenceof the grain refiners Ir and B on age-hardening of sterling silver under the presentconditions. Hence the lower peak hardness must be related to the high Zinccontent and the resulting low copper content (4-5 % in these alloys, see table 1).It is finally concluded, that the lower Cu-content in the firestain-resistant alloysresults in a lower volume fraction of precipitations during age-hardening andcorresponding less pronounced hardening effect.

The data that are presented for the firestain-resistant alloys are consistent withdata measured by the corresponding alloy suppliers under same ageingconditions [7]. However, their data consistently show that a slightly higher andoptimum peak hardness of ~120 HV can be obtained during ageing at 260°C / 1.5h (Legor) and 250°C for 1-2 h (Apecs).

Special care needs to be given to the choice of the homogenisation temperatureof the firestain-resistant alloys. Low-melting phases (containing Cu + Ge and/orSi) with a solidus temperature around 700°C are present in the as-cast state, sothat homogenisation should not be carried out above 700°C. In any case, anenhancement of age-hardening by homogenisation at higher temperature is notevident (figure 11). Lowering the homogenisation temperature below 730°C is alsobeneficial with regard to avoidance of grain coarsening after cold-rolling. It isworth noting that in 935Ag (and in contrast to 925Ag), homogenisation at 730°Calready leads to slight grain coarsening (figure 12a).

This is attributed to the fact that for 935Ag complete solution of Cu in the silvermatrix already occurs during homogenisation at 730°C (figure 12b), which is inaccordance with the binary phase diagram. Corresponding observations havebeen obtained for the firestain resistant alloys with even lower Cu-content.

For sake of completeness, as shown in more detail in [6], it is mentioned here, thatthe grain coarsening during high temperature homogenisation can not besuppressed by the addition of the grain refiners Ir or B. The grain refinement by Ir-or B-additions in the as-cast as well recrystallised state after cold-rolling ispronounced , however, and beneficial for ductility as already discussed.


Figure 11: Influence of additions on age-hardening of different sterling silver firestain-resistant alloys after investment casting and homogenisation

(700-730°C / 1 h / water quench)

12a) 935Ag, 730°C / 1 h & water quenching Note slight grain coarsening compared to 8a

12b) 935Ag, 730°C / 1 h & water quenchingNote nearly complete solution of Cu-phases compared to figure 9b

Figure 12: Microstructure of 935Ag after rolling (~66 %) and homogenisation.

Age-hardening directly from the quenched as-caststate after investment casting without in-between hightemperature annealing

The motivation to study this aspect is obvious: silver jewellery often is a massproduct, with considerable demands concerning the cost-efficiency of theproduction process. Moreover, the fear to increase firestain problems by prolongedannealing at high temperatures is always present. However, simply skipping thehomogenisation treatment of castings has often proofed to lead to no age-hardeningeffect during ageing, no matter at which temperature the ageing was carried out. As

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explained earlier, during the conventional slow cooling of the flasks after casting,coarse Cu-precipitates grow in the Ag-matrix and eliminate the potential for age-hardening during a following ageing treatment. Hence, early quenching of the flasks after casting would be a prerequisite, since theformation of coarse precipitates would be reduced. The schematic process schedulefor this modified heat treatment schedule is shown in figure 13. The following resultsshow, however, that the parameters need to be chosen carefully. Trees of ~300 gsize of 925Ag were cast by standard investment casting processes and quenched inwater at different times after casting (2-15 min). The corresponding approximatemetal temperatures at the time of quenching were recorded and are reported inFigure 14 together with hardness data obtained during subsequent ageing at 300°C.

Figure 13: Modified process schedule for hardening sterling silver investment cast materialdirectly from the quenched as-cast state

Figure 14: Age-hardening of 925Ag directly from the as-cast state: influence of quenchingtime and temperature (right column: reference data for cast & homogenised material)

It is concluded, that

• significant hardening of 925Ag to ~ 110 HV1 directly from the as-cast state ispossible, i.e. without the in-between homogenisation anneal at hightemperatures, but only if the flasks are quenched in water within a critical rangeof time after casting.


The suitable time of quenching after casting will certainly depend on individualcasting parameters (especially casting and flask temperatures, tree weight), but isabout 4 min in the presented case. Later quenching lead to marked reductions andfinally elimination of the age-hardening effect. Interestingly, quenching too early doesalso reduce the hardening effect. Obviously, some effective homogenisation occursalready during cooling within the first minutes after solidification, so that hardness inthe as-cast state drops from 80 HV1 (quenched 2 min after casting) down to 60 HV1(4 min), which gives the potential for subsequent age-hardening from the as-caststate. The resulting hardness in 925Ag is markedly lower compared to the hardness whichcan be obtained by the conventional process. Furthermore the process is lesscontrolled and some scattering of properties will occur depending on the position ofitems on the tree due to different cooling conditions over the length of a tree. For alloys with reduced Cu-content the level of hardness that can be obtained withthis modified process is in the range of 95-110 HV, which still means a significantstrength increase compared to ~50 HV in the conventional as-cast state.

Industrial trials on hardening sterling silver

In the following some results of industrial trails on hardening investment cast 925-935Agand 930Zn20Ir, starting from castings of usual industrial size (Figure 15), will bediscussed. Both, the conventional 3-step age-hardening process, as well as theshortened process directly from the quenched as-cast state has been tested out. Thecase studies focussed on the feasibility of the process schedules in industrialenvironment, reproducibility of hardness levels with special attention to the possibleinfluence of sample position on the tree and sample geometry. In the first case presentedhere, an industrial partner supplied several heat treated 935Ag test samples for hardnessmeasurement at FEM, which were randomly taken from different positions of multipletrees. The results obtained during a sequence of trials are shown in Figure 16. After afirst trial the homogenisation temperature was increased to 750°C (on the display of thefurnace) in order to ensure effective homogenisation of the castings, but ageing during300°C still was ineffective for the 0.5 h-anneal, which indicates that heating rate waslower than during lab-scale research. However, age-hardening for 1h at 300°C proved tobe reproducible and without large scatter within the sets of samples (+/- 10-15 HV).Controlled quenching of trees led to a surprisingly good result for hardening directlyfrom the as-cast and quenched state without a marked increase in scatter of properties.

Figure 15: Examples for investment cast material used in industrial trials

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Figure 16: Sequence of age-hardening trials of 935Aginvestment cast material by an industrial partner

Figure 17: Sequence of age-hardening trials of 930Ag20ZnIr investment cast by anindustrial partner; temperature of homogenisation 650°C - 700°C.

Note the minor influence of sample position on the tree

Figure 17 illustrates the results of trials carried out with another partner whichroutinely casts a 930Ag20ZnIr-alloy. Samples from top, middle and bottom positionof a tree were supplied to FEM for heat treatment and hardness testing. The resultsshow that homogenisation at 700°C is effective in eliminating the mutual differencesbetween samples on a tree which may result during cooling of a flask, and that aconstant hardness of 120 HV1 was obtained. Another case study in cooperation with a third industrial partner focused entirely onhardening directly from the as-cast state. The 925Ag-castings were quenched in water~4 min after casting. Again samples from top, middle and bottom position weresupplied to FEM for heat treatment and hardness testing. The results shown in figure18 illustrate that, although a hardening effect occurred, the optimum parameters werenot yet identified in this case. Note also that the influence of the sample position on thetree increases if quenching is not carried out at the optimum time or under bestconditions (Stirring of flasks in the water can have an influence). Figure 19 finally shows further results on hardening directly from the as-cast state, but


in this case for samples with comparably high length (tie-pins). For this particulargeometry, considerable scatter of property could be expected within individualsamples. The 930AgZn20Ir-castings were quenched in water ~4 min after casting.The results shown in figure 19 demonstrate surprisingly low scatter of properties anda medium hardness level of ~100 HV, which is close to the data obtained on lab scalefor the same alloy (low Cu content).Obviously, it needs to be assessed on a case-by-case basis whether hardeningdirectly from the as-cast state is a suitable option that gives satisfying properties, orif the conventional process needs to be chosen for safety. Furthermore, it turned out that for standard sterling silver the annealing atmospherefor homogenisation needs to be carefully controlled in order to avoid extended internaloxidation and increased subsequent problems with firestain. Internal oxidation of thesubsurface layers is also detrimental to the surface hardness and the hardness insurface-near areas. During oxidation, Cu-oxides are formed on and below the surface,so that much less Cu is available for the age-hardening process in these areas. Regarding a good protective annealing atmosphere, in principle the treatment innormal furnaces with a nitrogen flow is sufficient. But a more safe and still cheapsolution is to treat the items in closed stainless steel vessels, again with a continuousnitrogen flow inside the vessel. Quick quenching times after homogenisation need tobe assured in all cases, however.

Figure 18: Sequence of age-hardening trials of an industrial partner on 925Ag-castingsquenched ~4 min after casting; note the influence of sample position on the tree on the

resulting hardness if hardening is carried out without in-between homogenisation

Figure 19: Age-hardening trials of an industrial partner on 930AgZn20Ircastings quenched ~4 min after casting;

Data profiles correspond to HV1-values measured on the side faces of the samples; Note the low scatter of properties over the considerable length of these samples (tie pins)

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Interplay between age-hardening and solderingprocesses

Soldering an already age-hardened piece inevitably leads to a local loss in hardnessin the heat-affected zone. Depending on the diameter and geometry of a particularsample, the loss in hardness may even affect the complete sample due to the veryhigh thermal conductivity of silver and sterling silver alloys. Hence the age-hardeningstep needs to be placed towards the end of the manufacturing cycle and definitelyafter the soldering process.

In principle two options, which are illustrated in figure 20, are available for the integrationof the age-hardening treatment in a process that involves soldering operations:

Figure 20: Options for the sequence of processing steps for integration of age-hardening in a manufacturing process that involves soldering

For solders that melt high enough, the homogenisation treatment can be carried outafter the soldering process (option 1, figure 20), followed by quenching andsubsequent ageing in conventional way. However, solder alloys for silver jewellery canhave solidus temperatures down to ~ 600°C (table 3). Low melting solders are usedespecially in processes with multiple soldering steps in order to avoid partial melting ofprevious joints. For low melting solders, the homogenisation annealing treatment of asoldered item (at the temperatures which are required for subsequent hardening) is notpossible because it would result in melting of the solder and failure of the joint. Theother option (nr. 2, figure 20), which may apply especially for low melting solders, couldbe to carry out the homogenisation anneal right before soldering and to perform theageing afterwards as a final step.

Table 3: Data for common silver solder alloys [8]

Solder alloy (trade name)

Working temperature Melting range

40000 ~ 790 °C 760 - 787 °C

40010 ~ 740 °C 704 - 731 °C

40020 ~ 710 °C 673 - 708 °C

40030 ~ 650 °C 623 - 655 °C


In order to study these two options in more detail, as-cast wires of 925Ag were cold-drawn (from 10 to 3 mm diameter), bent to ring shape (resulting in a total weight of~ 5 g) and soldered manually using different commercial solder alloys with workingtemperatures ranging from 650°C up to 790°C (table 3). After soldering, sampleswere quenched in water to avoid oxidation. Hardness testing was carried out onmetallographic cross sections at the locations marked in figure 21.

Figure 21: Sample geometry used for studying the interplay of age-hardening andsoldering; the figures mark the locations for hardness testing

With reference to option nr. 1 (see figure 20), the hardness profiles obtained for the solderedstate, the subsequently homogenised state and the finally age-hardened state are shownin figure 22, together with the profile of a sample that has been aged directly after soldering. After soldering the cold-worked wire, the hardness drops to typical soft-annealedhardness levels in the vicinity of the joint, but softening due to the heat of the solder isless pronounced in areas far away from the joint (position 5) with the actual leveldepending strongly on the working temperature of the solder alloy. Subsequenthomogenisation yields a constant hardness level of ~ 60 HV1, and age-hardening finallyincreases hardness to ~ 130 HV1 for the given set of parameters. The data obtained areconsistent with data from processes without involvement of soldering processes. Asalready underlined above, this sequence of processing steps is not feasible for lowermelting solders, since lower homogenisation temperatures would be required which donot provide the basis for significant hardening in standard sterling silver, however. The fourth profile in figure 22 corresponds to samples that have been aged directly aftersoldering, i.e. without in between homogenisation anneal (i.e. without steps 2&3 in figure20 / option 1). Interestingly, for the given geometry a good age-hardening potential wasalso observed without any in-between homogenisation treatment. Accompanying trials(not shown in this paper) on investment cast + early quenched material (~ 4 min aftercasting) revealed also similarly good age-hardening properties after soldering, i.e. alsowithout in-between homogenisation treatment. Obviously, the soldering process itselfprovides a certain level of homogenisation, which depends again on the workingtemperature of the solder, soldering time, sample size & geometry etc. It needs to beassessed on a case-by-case basis whether a process schedule without in-betweenhomogenisation yields satisfying properties or not. Obviously, these findings may be ofparticular interest for processes involving low-melting solders and the compromise willbe some scatter in properties for different sample geometries and sizes.

With reference to option nr. 2 (see figure 20), the hardness profiles obtained for the

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homogenised state, the soldered state after preceeding homogenisation, as well as thesubsequently age-hardened state are shown in figure 23. After homogenisation, aconstant hardness level of ~65 HV1 is obtained as expected. Surprisingly, the hardnessprofile after subsequent soldering is very inhomogeneous, with still low hardness nearto the joint, but significantly increased hardness in areas far away from the joint (~80-90HV1; position 5). Obviously, the heat introduced during soldering has diffused into theseareas which has been enough to evoke some local hardening, although soldering timeswere low and samples were quenched immediately after soldering. The final ageingstep yields strong age-hardening in areas near to the joint (~130 HV1), but only minoradditional hardening in the areas far away from the joint (~110 HV1; position 5). For theparticular geometry and parameters, the result is a quite high scatter in propertiesalthough the overall increase in strength certainly provides a considerable benefit overnon-hardened material. Consistent data with similar characteristics were obtained for allsolder alloys (different working temperatures) included in this study.

For both options studied, the ageing treatment directly after soldering yields pronouncedage-hardening near to the joint. This is attributed to an sufficiently homogenisedmicrostructure after soldering, which is consistent with the low hardness after solderingin these areas. In contrast, some uncontrolled ageing already occurs during soldering inareas far away from the joint, which provides much reduced potential for subsequentcontrolled age-hardening.

An obvious conclusion on the interplay of age-hardening with soldering processes is,that the behaviour of the material is quite complex and in reality will depend critically onsample geometry, size, working temperature of the solder and individual skills ofpersonnel. Moreover, in mass production, soldering is mostly carried out by continuousprocesses with little control of the cooling process after soldering. The most consistentand reproducible results still are obtained for material that is homogenised inconventional way after soldering. Probably, alloys with slightly modified alloycomposition and potential for age-hardening after homogenisation at lowertemperatures than 700°C can help in overcoming the obstacles connected with usageof low melting solders.

Figure 22: Hardness profiles for the process steps: soldering, homogenisation 700°C / 1 h / wqageing 300°C / 1 h (option nr. 1, see figure 20), using a solder alloy with working temperature

740°C; sample geometry and locations of measurement correspond to figure 21


Figure 23: Hardness profiles for the process steps: homogenisation at 730°C / 1 h / wq,soldering, ageing at 300°C / 1 h (option nr. 2, see figure 20), using solder alloys with

working temperature 650°C and 740°C respectively; sample geometry and locations ofmeasurement correspond to figure 21

Summary

Generally speaking, the influence of Zn, Si, Ge and grain refiner-additions on the age-hardening of sterling silver is low. The lower Cu-content in the firestain resistant alloysas well as the need for lower homogenisation temperatures (low melting phases) leadto a lower peak hardness in these alloys, 110-120 HV1 after 700°C / 1 h /waterquench, compared to 140 HV1 in 925Ag after 730°C / 1 h / water quench,followed by ageing at 300°C / 1 h.

The obvious advantage of the firestain-resistant alloys is, that the resistance againstfirestain is also maintained during the homogenisation treatment. For alloys withoutSi- or Ge-additions, the annealing atmosphere for homogenisation needs to becarefully controlled in order to avoid extended internal oxidation and increasedproblems with firestain and reduced surface hardness.As expected, the tensile ductility of age-hardened material is significantly lower thanfor soft material. Choice of a grain-refined material is recommended for applicationswhere larger plastic deformation needs to be tolerated by the hardened material.

Significant hardening of investment cast sterling silver up to ~110 HV1 directly fromthe as-cast state is possible, i.e. without the in-between homogenisation anneal athigh temperatures, if the flasks are quenched in water within a narrow and criticalrange of time after investment casting (~4 min). The scatter of properties may beincreased, however, and the optimum parameters as well as the suitability of thisshortened (i.e. more cost-effective) hardening process need to be assessed on acase-by-case basis. The interplay of hardening and soldering processes is complex and demands forindividual solutions depending on the working temperature of the solder alloy, thesample geometry & size, and the acceptance level with regard to scatter inproperties. In any case, significant strength increase by age-hardening is possiblealso for items which require soldering processes.

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Although not discussed in this paper so far, hardening of stone-in-place castings byheat treatment obviously is not advisable, since quenching steps are involved in anycase and will result in damage to the stones.

Acknowledgements

The author is especially grateful to the coworkers from the metallurgical department atFEM for the realisation of all the research work. Fruitful discussions with Dieter Ott andValerio Faccenda are gratefully acknowledged. Furthermore, the author likes to thankthe industrial partners that contributed with supply of material and own trials: Daub,Quinn Scheurle, C. Hafner (all D), Legor (I) and Apecs (Australia).

References

[1] Raub, E., "Die Edelmetalle und ihre Legierungen", J. Springer, 1940

[2] Grimwade, Mark, "The nature of metals and alloys", Proceedings of the 15th Santa Fe

Symposium, 2001, pg. 151

[3] Grimwade, Mark, "Heat treatment of precious metals", Proceedings of the 5th Santa Fe

Symposium, 1991, pg. 241

[4] Reti, Aldo, "Understanding sterling silver", Proceedings of the 11th Santa Fe Symposium, 1997,

pg. 339

[5] Eccles, Anthony, "The evolution of an alloy", Proceedings of the 12th Santa Fe Symposium,

1998, pg. 203

[6] Fischer-Bühner, Jörg; "An update on hardening of sterling silver by heat treatment",

Proceedings of the 17th Santa Fe Symposium, 2003, pg. 29

[7] Basso, Andrea (Legor s.r.l, Italy), and Eccles, Anthony (Apecs, Australia), private

communication, 2003

[8] Wiesner, Klaus (C. Hafner GmbH, Germany), private communication, 2004

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