Experiments are carried out to investigate the effects of processing parameters on mechanical properties of bell metal samples and the results are discussed in the subsequent sections.
4.3.1 Cast bell metal
Effect of composition
Figure 4.1 shows the effect of compositions on the hardness and fracture strength of cast bell metal samples under compressive load. From Fig. 4.1, it has been observed that with the 5%
increase in the percentage of Sn, the mechanical properties of cast bell metal samples increased by almost 50%. Park and Joo (2016) [78], from their study, concluded that the mechanical properties of bell metal depend on the amount of secondary phase i.e., the amount of δ phase present in the microstructure. Therefore, it can be inferred that with an increase in Sn% in the composition, the amount of δ phase increases and hence the mechanical properties increases.
Figure 4.1 Effect of composition on (a) Hardness; (b) Fracture strength
4.3.2 Quenched bell metal
Effects of the quenching medium on microstructure
Figure 4.2 shows the microstructure of oil and water quenched bell metal samples having composition 78:22 (Cu: Sn). From Fig. 4.2, it is observed that the microstructure of the oil quenched bell metal samples has a dendritic structure and inter-dendritic zones. The microstructure of the water quenched sample is in the same line with the various work published by different authors [21, 23, 35, 72, 78] and has already been discussed in the previous chapter. No published benchmark results have been found to compare the microstructure of oil quenched samples. The formation of different microstructures can be attributed to the different thermal properties of the quenching medium. The compositions of the different zones of oil and water quenched samples are also studied and represented in Table 4.2. By comparing the compositions with the Cu-Sn alloy phase diagram published by Acharya and Mukunda (1988) [64], it has been found that the dendritic structure of the oil quenched sample is in the α phase. Whereas, the inter-dendritic zone of the oil quenched samples is in the β phase. The effects of these phases on the mechanical properties of bell metal have been discussed in the next sections.
Figure 4.2 Microstructure of different samples: (a) Oil quenched; (b) Water quenched Table 4.2 Compositions of different zones of heat-treated samples
Samples Zone Cu:Sn (wt. basis) Cu:Sn (atomic wt. basis) Oil quenched Dendritic Zone 85.7: 14.3 90.9: 9.1
Inter-dendritic Zone 76.4: 23.6 85.6: 14.4 Water quenched Dendritic Zone 85.9:14.1 91.3: 8.7
Inter-dendritic Zone 76.6: 23.4 85.9: 14.1
Effects of composition on mechanical properties
Figure 4.3 shows the effect of compositions on the hardness and fracture strength of bell metal samples quenched in different quenching mediums from a temperature of 700ºC. From Fig.
4.3, it has been observed that with the increase in the percentage of Sn, the mechanical properties, viz., hardness and fracture strength under compressive load, increases irrespective of the quenching medium. For a 5% increment of Sn in the composition, the mechanical properties of the oil and water quenched samples have increased by almost 30% and 80%
respectively. The increase in mechanical property with an increase in Sn% is attributed to the change in the amount of phases present in the respective samples as observed from the microstructure. Park and Joo (2016) [78], from their study, have concluded that the mechanical properties of bell metal are controlled by the secondary phases present in the microstructure. Therefore, it can be said that with an increase in Sn% in the composition, the amount of secondary phase i.e., the β phase increases and hence the mechanical properties improve. It has also been observed that the mechanical properties of oil quenched samples are relatively less than water quenched samples for all Sn percentages considered in this study.
In general, a decrease in hardness value enhances the workability of material at room temperature to carry out the required cold works for finishing the products. So, it can be inferred that oil quenched the bell metal products will be easier to do finishing work at room temperature due to reduced hardness.
Figure 4.3 Effect of quenching medium on (a) Hardness; (b) Fracture strength Again from the field survey report and literature review (section 1.5.3 and 1.6.3.a), it has been observed that water has been commonly used as a quenching medium in bell metal
production centers from ancient time. The criterion, it seems, of the production of items was to retain higher strength of the product. This is the practice followed not only in ancient times, but also has been continuing till date. Because of the practice, it has been observed that the artisans used to have a tough time to provide finishing of the products, which is normally carried out at room temperature. Many a time, the products also fail during the finishing exercise, as evident from the field survey (see Fig. 1.2).
Effects of quenching temperature on mechanical properties
Figure 4.4 shows the effect of quenching temperature on the mechanical properties of the bell metal sample having the composition of 78:22 (Cu: Sn) quenched in oil and water. From Fig.
4.4, it has been observed that as the quenching temperature increases, the mechanical properties of oil and water quenched samples increase. Again from Fig. 4.4, it has been observed that the percentage of increase in mechanical properties with an increase in temperature is higher for the water quenched sample compared to the oil quenched sample.
Further, it has also been observed that the temperature of 700ºC plays a very significant role in controlling the mechanical properties of bell metal. The mechanical properties of the oil and water quenched sample have increased by 8% and 25% respectively on increasing the quenching temperature by 50ºC above 700ºC. The increase in mechanical properties with an increase in quenching temperature is attributed to the arrest of a higher amount of the β phase.
As the quenching temperature increases, the higher amount β phase arrest in the quenched sample as the BCC structured β phase [69, 202].
Figure 4.4 Effect of quenching temperature on (a) Hardness; (b) Fracture strength Again from the field survey and literature review (section 1.5 and 1.6.3), it has been observed that the bell metal products are manufactured by quenching from a temperature above 700ºC. So, it can be inferred that in the bell metal items production centers the water
quenching is done from a temperature above 700ºC to achieve the higher strength of the products. Practical constraints related to the processing of bell metal at room temperature having high hardness have already been discussed in the previous section.
The present study has helped in understanding the benefit of oil quenching. It is recommended that the products, which require finishing at room temperature, should better be oil quenched.