| Abstract|| |
Context: Remelting previously cast base metal alloy can adversely affect the mechanical properties of the alloy and necessitates addition of new alloy.
Aims: To study the effect of remelting different combinations of new and used cobalt-chromium (Co-Cr) alloy on its mechanical properties and microstructure.
Materials and Methods: Using induction casting, 24 tensile test specimens were prepared for eight different combinations of new and used Co-Cr alloy. The test specimens were assessed for yield strength and percentage elongation. Microhardness was evaluated using Vickers's hardness tester. The tensile testing was carried out on a 50 kN servo-hydraulic universal testing machine. Microstructure analysis was done using an optical photomicroscope on the fractured samples after acid etching.
Statistical Analysis: The mean values (±standard deviation) and coefficient of variation were calculated. Student's 't' test was used for statistical analysis. Statistical significance was assumed at P=.05.
Results: The mean yield strength of eight different combination groups were as follows: group A: 849 MPa, group B 1 : 834 MPa, group B 2 : 915 MPa, group B 3 : 897 MPa, group C 1 : 874 MPa, group C 2 : 859 MPa, group D 1 : 845 MPa, and group D 2 : 834 MPa. The mean percentage elongation for the different groups were as follows: group A: 7%, group B 1 : 7%, group B 2 : 8%, group B 3 : 7%, group C 1 : 8%, group C 2 : 7%, group D 1 : 7%, and group D 2 : 8%. The mean hardness values were as follows: group A: 373 VHN, group B 1 : 373 VHN, group B 2 : 346 VHN, group B 3 : 346 VHN, group C 1 : 364 VHN, group C 2 : 343 VHN, group D 1 : 376 VHN, and group D 2 : 373 VHN.
Conclusion: Repeated remelting of base metal alloy for dental casting without addition of new alloy can affect the mechanical properties of the alloy. Microstructure analysis shows deterioration upon remelting. However, the addition of 25% and 50% (by weight) of new alloy to the remelted alloy can bring about improvement both in mechanical properties and in microstructure.
Keywords: Cobalt-chromium alloy, microstructure base alloy, remelting
|How to cite this article:|
Gupta S, Mehta AS. The effect of remelting various combinations of new and used cobalt-chromium alloy on the mechanical properties and microstructure of the alloy. Indian J Dent Res 2012;23:341-7
Success with cast partial denture frameworks requires close attention to both clinical and laboratory procedures. Over the years the philosophy behind the designing of cast partial dentures has undergone little change. However, there have been drastic changes in the fabrication procedure with respect to investment materials, casting techniques and, especially, casting alloys. Until a few years back, gold alloys remained the material of choice for fabricating cast partial denture frameworks, but the unchecked escalation in the cost of gold has shifted the focus towards comparatively cheaper base metal alloys. Cobalt-chromium (Co-Cr) alloys were first used to construct cast partial denture frameworks in 1933.  Many technical difficulties were initially encountered and the use of Co-Cr in dentistry was limited till improvements in alloy manufacture and the techniques for their management resulted in greater use of these alloys. Co-Cr alloys exhibit good strength and rigidity, they are lightweight and resistant to corrosion, and they have low cost, all of which make them ideal for use in the fabrication of cast partial dentures. In the current economic conditions, the dentist and the technician need to be cost conscious when fabricating cast frameworks.
|How to cite this URL:|
Gupta S, Mehta AS. The effect of remelting various combinations of new and used cobalt-chromium alloy on the mechanical properties and microstructure of the alloy. Indian J Dent Res [serial online] 2012 [cited 2019 Mar 23];23:341-7. Available from: http://www.ijdr.in/text.asp?2012/23/3/341/102220
Studies by Harcourt HJ,  Harcourt HJand Cotterill WF  and Lewis AJ  have shown that when base metal alloys are reused for castings it is necessary to add new alloy to remelted alloy for maintaining castability and to prevent deterioration in the mechanical properties of the alloy. In this era of evidence-based dentistry, arbitrary addition of new alloy to the old is not tenable; we need to establish the optimum ratio of new to old alloy.
The aim of this study was to study the influence of addition of new alloy to used Co-Cr alloy on the mechanical properties and microstructure of the material. The objectives of the study were to evaluate the effect of remelting various combinations of new and used Co-Cr alloy on the mechanical properties and microstructure of the material and to establish the optimum ratio of new to used Co-Cr alloy for subsequent castings.
| Materials and Methods|| |
Description of alloy groups
For the purpose of study we chose eight different combinations of new and used Co-Cr alloy (S-U Duranium, Schuller, Germany); this alloy consists of Co (62%-65%), Cr (27%-29%), and Mo (5%-6%) [Table 1].
Twenty-four tensile test specimens were cast, three for each of the eight different combinations of new and used Co-Cr alloy; casting was in accordance with ADA specification No. 14 for dental Co-Cr casting alloy. In accordance with the specifications, the tensile bar was 13/8 inch long and 0.09±0.01 inches in diameter; the junction between the grips on either end formed a semicircle of about 1 / 4 th inch radius [Figure 1]. A split silicone mold (RTV, Dow Corning, USA) was fabricated by duplicating, lathe-cut sample of the tensile bar specimen. Hard inlay-wax (S-U Inlay-Wax, Schuler, Germany) was melted and injected with a micropipette to obtain the wax patterns for the bars. Three patterns were casted for each group using the combination of new and used alloy specified for the group. The wax patterns were sprued with 3-mm preformed sprue wire (S- U Wax Wire, turquoise, Schuler, Germany) applied in a semicircular form end-to-end in the grip portion. The patterns were attached to the crucible former and sprayed with debubblizer (True Blue GT Products); this was followed by investing in phosphate-bonded investment material (Kromco-Vest, Matech, Inc. USA) according to the manufacturer's instructions. Using an induction casting machine (Aseg, Galloni, Italy), the casting was carried out after the investment was sufficiently set, following the desired ratio of alloy for that particular group. Eight different combinations of alloy were used for casting.
Excess alloy for reusing was separately cast following standard procedures, since the remnants of the 1 st melt alloy following group A casting was insufficient for subsequent castings of groups B 1 , C 1 , and D 1 .
As shown in [Figure 2], 80 g of new alloy was casted under similar conditions as was used for casting group A bar specimens. The excess casted alloy was thoroughly cleaned by sandblasting, cut into small pieces, and divided into three parts by weight according to the desired ratio of the new and used alloy for groups B 1 , C 1 , and D 1 castings. Similarly, 2 nd melt (B 1 ) excess alloy, required for the groups B 2 , C 2 , and D 2 was obtained by a taking 80 g of fresh alloy and casting it twice. Group B 3 casting was obtained by remelting new Co-Cr alloy thrice, following the standard casting protocol as described above. For all the subgroups castings, irrespective of the ratio of new and used alloy, the total weight of alloy per casting was kept at 30 g for each group for the purpose of standardization.
|Figure 2: Casting done for obtaining excess remelted (used) alloy for mixing with respective groups.|
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Care was taken not to overheat the metal, and the temperature of casting for each group was noted. The castings were allowed to cool down to room temperature, after which they were sandblasted for divesting. All cast samples were finished and electrolytically polished using conventional techniques. The finished bars were grouped and labeled with a permanent marker for identification.
Testing of tensile properties
Before being subjected to testing, the tensile bars were evaluated for internal defects by industrial radiography (CMA-402-Andrex, Escort India). The tensile testing was carried out on 50 kN servo-hydraulic universal testing machine (Biss, India). The samples with their grip portion were held by the fixture and were subjected to tensile load through computerized data feeding at a crosshead speed of 0.5 mm/minute. Load vs strain graphs were obtained for each sample. The graphs were than subjected to computerized postprocessing to obtain values for yield strength, percentage elongation, and modulus of elasticity. The mean value for each group was then derived.
Microhardness analysis was done using the Vickers's hardness tester (Stiefelmayer-Reicherter, Germany). Vickers's hardness number was recorded for each sample at four distant points on the smooth polished surface of the grip portion. The mean was then derived.
Following tensile testing, the grip portion of the fractured bar, one from each of the eight groups, were mounted in self-cure acrylic resin (Acrylan ®) to obtain a resin block with the flat metal surface of the grip portion exposed. The mounted metal samples were ground flat on a belt emery grinding machine. All the specimens were further smoothened manually with emery paper, sequentially from grades 0 to IV. Final polishing was done on a motor-driven revolving disk covered with a polishing cloth, using diamond polishing paste and spray (HFline) to achieve a smooth, reflecting, scratch-free surface. The polished specimens were etched with freshly prepared solution of 90 ml hydrochloric acid and 15 ml of 30% hydrogen peroxide for 30-60 seconds, after which they were observed under an optical photomicroscope (Nikon, Japan) at 50× and 200×.
| Results|| |
All data concerning the mechanical properties are shown in [Table 2] and [Table 3].
|Table 2: Comparative evaluation of the mean change in the values of mechanical properties control group (A) and groups without addition of new alloy (B1, B2, and B3)|
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|Table 3: Evaluation of the mean changes in the mechanical properties for various groups as compared with control group (A). Groups having no addition of new alloy are B 1 , B 2 , and B 3 and the groups having various combination of new and used alloy are C 1 , C 2 , D 1 , and D 2|
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The comparative values of yield strength between control group A (1 st melt alloy) and group B 3 (4 th melt alloy) is −101.28*** (P>.01) shows statistically significant difference. This indicates that repeated remelting of alloy B 1 (2 nd melt), B 2 (3 rd melt), and B 3 (4 th melt) without addition of any new alloy decreases the yield strength as compared to 100% new alloy. On comparison of control group (group A) with group C 1 (75% by weight 1 st melt alloy + 25% by weight new alloy) and D 1 (50% 1 st melt alloy + 50% new alloy), the values of yield strength obtained are −0.35 (P>.05), and −19.27 (P>.01), respectively. On comparison with C 2 (75% 2 nd melt alloy + 25% new alloy), D 2 (50% 2 nd melt alloy + 50% new alloy) having mean yield strength values of −51.28 (P>.01) and 51.28 (P>.01), respectively, there is again statistically significant reduction in yield strength as compared to the control group.
Modulus of elasticity
The differences between the various groups with regard to the modulus of elasticity is "is statistically nonsignificant" clinically insignificant [Table 2] and [Table 3].
On comparing the mean values of percentage elongation of the various groups with the control group we found "we found statistically nonsignificant changes"clinical insignificant changes.
On comparing hardness values of group A (1 st melt) with group B 2 (3 rd melt alloy), and group B 3 (4 th melt alloy), statistically significant changes are seen. However, on comparison of group A (1 st melt; 100% new alloy) with groups C 1 (75% 1 st melt + 25% new alloy), D 1 (50% 1 st melt + 50% new alloy), C 2 (75% 2 nd melt + 25% new alloy), and D2 (50% 2 nd melt + 50% new alloy), the hardness value shows no significant changes between the groups. Thus, with repeated remelting there is significant decrease in values of hardness, but on remixing new alloy there is recovery of hardness value.
| Discussion|| |
It is an established fact that most dental laboratories reuse base metal alloys for subsequent castings. We conducted a telephonic survey of ten commercial dental laboratories in the city of Mumbai, India, and found that almost all of them reused alloy four to five times before discarding it. However, these dental laboratories reused the alloy in combination with new alloy, adding an equal proportion of the reused alloy each time.
In the literature there are differences of opinion regarding the addition of new alloy to reused alloy for subsequent castings. Hesby and Co-workers  advocated that the metal can be reused for at least four generations without addition of any new alloy. In contrast, HarcourtHJ 2 suggested addition of new alloy of at least equal weight to old alloy for subsequent castings. However, most of the previous studies by HarcourtHJ,  Harcourt HJ and Cotterill WF,  and Lewis  on remelting of Co-Cr alloys concluded that with subsequent remelting there was change in alloy composition, and this affected the mechanical properties of the subsequent castings. In this study, the mean yield strength values shows a gradual decrease from group A (100% new alloy; 807.67 Mpa), to group B 1 (2 nd melt alloy; 719.29 Mpa), B 2 (3 rd melt alloy; 705.95 Mpa), and B 3 (4 th melt alloy; 706.39 Mpa) [Table 2]. These findings are in accordance with those of Lewis AJ,  who stated that successive remelting of alloy results in the loss-to a greater or lesser degree-of a number of trace elements either by vaporization or overheating and that this is directly related to the relative difficulty experienced in the remelting of previously cast alloy.
Harcourt and Cotterill  after conducting 13 remelts of Co-Cr alloy concluded that there is a definitive change in composition and physical properties with remelting, which eventually would lead to failure of frameworks during service. He advocated addition of new alloy in the proportion of at least an equal weight of old alloy to maintain the castability and mechanical properties of the alloy.
In our study, addition of new alloy to the old alloy was done in two ratios [Table 1]. In group C) we added 25% by weight of new alloy to the reused alloy and in group D we added 50% by weight of new alloy to reused alloy. Addition of new alloy to the 1 st melt alloy in group C 1 (75% 1 st melt alloy + 25% new alloy) and group D 1 (50% 1 st melt alloy + 50% new alloy) restored the value of yield strength closer to that of group A (1 st melt alloy). However, the difference in the mean value of yield strength of groups C 1 and D 1 is statistically nonsignificant [Table 3]. Thus, it appears that even with 25% addition of new alloy to the 1 st melt alloy, adequate mechanical properties and castability of the used alloy is ensured.
Similarly, addition of new alloy to the 2 nd melt (B 1 ) alloy in the proportion of C 2 (25% by weight new alloy) and D 2 (50% by weight new alloy) caused equal improvement in the yield strength values [Figure 3]. Thus, it is clear that repeated remelting of alloy causes a decrease in the yield strength, but with remixing of new alloy [25% by weight (groups C 1 and C 2 ) or 50% by weight (groups D 1 and D 2 )] there is recovery of yield strength values.
|Figure 3: Mean yield strength (MPa) for various combinations of new and used cobalt-chromium alloys.|
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According to Craig,  the minimum yield strength from dental casting alloy must be at least 415 Mpa to withstand the permanent deformation when used as partial denture clasp. The value of yield strength obtained in this study for all the groups was above 700 Mpa. Hence, according to our study, addition of new alloy of 25% by weight (instead of the standard practice of addition of 50% by weight) could be safely considered up to the 3 rd melt casting. However, with increase in the number of remelts, it may be advisable to increase the proportion of new alloy. Addition of new alloy compensates for the changes in composition and ensures ease of casting by maintaining the castability of alloy.
The mean values of modulus of elasticity obtained in the study are not in accordance with the ADA specification, i.e., 223.5 × 103 Mpa. It is said that the modulus of elasticity is an inherent property of an alloy and is related to the composition of alloy and is not affected by remelting. The reason the values of modulus of elasticity obtained in this study diverged from the expected values could be that the extensometer could not be applied due to the relatively small size of testing area. Hence, the true strain values were not available for calculating the stress/strain ratio. However, the values show statistically nonsignificant difference throughout.
The percentage elongation is important as an indicator of the relative brittleness or ductility of the restoration. The mean elongation values obtained in this study for various groups-with and without combination of new alloys-shows statistically nonsignificant difference up to the 4 th melt [Table 2] and [Table 3]. Hence, addition of 25% alloy by weight to the reused alloy will maintain the ductility of the clasp, at least up to the 4 th melt (group B 3 ) studied. Also, the effect of microporosity on the elongation value in this study was taken care by taking industrial radiographs of the testing specimen. All testing specimens were found to be clear of such defects.
Hardness is the resistance of metal to deformation. The present analysis shows that the hardness values show a drop from 1 st melt (373 VHN) to B 3 (4 th melt; 346 VHN), which is statistically significant [Table 2] and [Table 3]. On addition of new alloy in the ratio of 25% by weight (C 1 ) to 1 st melt alloy brought about improvement in hardness values. The difference between C 1 (25% of new alloy to 1 st melt) and D 1 (50% of alloy to 1 st melt ) is statistically nonsignificant. Thus addition of 25% new alloy to the 1 st melt alloy can be considered as adequate. Addition of new alloy in 25% and 50% by weight to 2 nd melt alloy restores the hardness, and the statistical analysis suggests that the difference between C 2 (75% 2 nd melt + 25% new alloy) and D 2 (50% 2 nd melt + 50% new alloy) group is nonsignificant. Thus, 25% new alloy can be effectively used up to 3 rd melt casting.
The microstructure of Co-Cr alloy (Duranium) used in the study basically shows cored dendrites in solid solution as the matrix (gamma phase). This phase is responsible for toughness and ductility of alloy. The second predominant phase responsible for tensile strength of alloy is in the form of carbide precipitates labeled as MC, M 23 C 6 , and M 6 C types, where M could be molybdenum, chromium, or carbon. According to Harcourt 2 , continuous spherical carbides uniformly distributed results in high yield strength and low ductility. Asgar K and Peyton FA  stated that spherical and discontinuous carbides pattern free from eutectoid composition and microporosities possess greater ductility and elongation.
The microstructure picture for control group A (1 st melt alloy) shows primary and secondary dendrites in a solid solution (gamma phase) [Figure 4]. However, for groups B 1 (2 nd melt alloy), B 2 (3 rd melt alloy), and B 3 (4 th melt alloy) [Figure 5], where remelting was done but no new alloy was added, the dendrites became coarser, disordered, and closely placed, with overall reduction in the gamma phase. However, on addition of new alloy in 25% by weight to the used alloy as in group C 1 (75%1 st melt alloy + 25% new alloy) and C 2 (75% 2 nd melt + 25% new alloy), the dendrites become less coarse and there was also an increase in solid solution (gamma phase) compared to group B 2 and B 3 , suggesting that addition of new alloy has improved the mechanical properties [Figure 6]. Group D 1 (50% 1 st melt alloy + 50% new alloy) and D 2 (50% 2 nd melt alloy + 50% new alloy) predominantly show less of dendritic pattern and overall precipitation of carbides in the gamma phase [Figure 7].
|Figure 4: Microstructure of group A (1st melt alloy) bar specimen (50×) showing primary and secondary dendrites in a gamma phase matrix.|
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|Figure 5: Microstructure of group B 3 (4 th melt alloy) bar specimen (50×) showing closely packed, relatively coarse and irregular dendrites with decreased content of gamma phase matrix (solid solution) as compared to control group.|
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|Figure 6: Microstructure of group C 1 (75% 1 st melt alloy + 25% new alloy) bar specimen (50×) showing dendrites with decrease which are less coarser than in the group without addition of new alloy (B 1, B 2, and B 3) in a gamma phase matrix.|
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|Figure 7: Microstructure of group D 2 (50% 2 nd melt alloy + 50% new alloy) bar specimen (50×) showing little dendrites, with precipitation of carbides in gamma phase matrix.|
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The microstructure of the samples is consistent with the changes in the mean values of mechanical properties obtained for the different groups in this study. For groups without addition of new alloy, there is a steady decrease in the yield strength and hardness value with each reuse there is a constant dip in the yield strength and hardness value, with slight increase in the percentage elongation. Also, for groups C 1 , C 2 , D 1 , and D 2 , where new alloy is added in 25% and 50% combination to the 1 st melt and 2 nd melt alloy, respectively, there is recovery of mechanical properties to levels comparable to that of the control group. Thus, it can be stated from this study that with remelting of alloy the microstructure also changes. These changes could be because of loss of certain minor elements as a result of repeated remelting since there is no control of atmosphere over the crucible. On addition of new alloy, the microstructure shows strengthening of the primary phase, which is responsible for toughness and crack resistance. Thus, addition of new alloy compensates for the loss caused by remelting. Examination of microstructure shows that with repeated remelting there is formation of carbides of molybdenum, chromium, and carbon and that the dendrites become coarser, irregular, and closely packed. On the other hand, addition of new alloy causes the formation of spherical, continuous, and uniformly spaced carbides within the solid solution, which is important for fracture resistance.
Microstructure is a basic parameter that controls the mechanical properties of an alloy and is directly related to the mechanical properties in the sense that if there is any change in the mechanical values a definitive change will occur in the microstructure.
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|2.||Harcourt HJ. Remelting of cobalt-chromium alloys. Br Dent J 1962;112:198-204. |
|3.||Harcourt HJ, Cotterill WF. Induction melting of Cobalt-Chromium alloys. Br Dent J 1965;118:323 -9. |
|4.||Lewis AJ. Changes in the composition of a nickel -base partial denture casting alloy upon fusion and casting. Aust Dent J.1975;20:14-8. |
|5.||Hesby DA, Kobes P, Garver DG, Pelleu GB Physical Properties of a repeatedly used non-precious metal alloy. J Prosthet Dent 1980;44:291-3. |
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|7.||Baran GR. Cast and wrought base-metal alloys. In: Craig RGm, Powers JM, Editors. Restorative dental materials; St. Louis: Missouri,Mosby, Inc: 2002. p. 480-513. |
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Department of Prosthodontics, ITS-CDSR, Muradnagar, Ghaziabad, Uttar Pradesh - 201 206
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3]