Cable sheathing of a Pb-Ca-Sn alloy and method of manufacture thereof

Abstract

A method for manufacturing a sheathing of a cable and a sheathing for a cable is provided where the method includes forming the cable sheathing by extrusion and the sheathing is made of a PbCaSn alloy having a composition having from 0.03 to 0.05 weight % Ca and from 0.4 to 0.8 weight % Sn.

Claims

1. A method for manufacturing a cable sheathing, wherein the method comprises the following steps: applying an extruder comprising: a supply of molten PbCaSn alloy, and a die-head adapted to receive the power cable to be coated and to exit the cable and simultaneously form a coating of supplied PbCaSn alloy onto the power cable, wherein the PbCaSn alloy has a composition comprising of from 0.03 to 0.05 weight % Ca and of from 0.4 to 0.8 weight % Sn, based on the total mass of the alloy and where the balance is Pb and unavoidable impurities.

2. A method according to claim 1, wherein the extruder further comprises a stem adapted to solidify the PbCaSn alloy and having an inlet fluidly connected to the supply of molten PbCaSn alloy and an outlet for solidified PbCaSn alloy, and the die-head has a cavity fluidly connected to the outlet for solidified PbCaSn alloy.

3. A method according to claim 1, wherein the PbCaSn alloy has a calcium content of from 0.025 to 0.05 weight %, based on the total mass of the PbCaSn alloy.

4. A method according to claim 1, wherein the PbCaSn alloy has a tin content of from 0.425 to 0.8 weight %, preferably of from 0.45 to 0.8 weight %, more preferably of from 0.5 to 0.8 weight %, and most preferably of from 0.5 to 0.7 weight %, based on the total mass of the PbCaSn alloy.

5. A method according to claim 1, wherein the PbCaSn alloy further comprises one or more of: Ni, Cu, As, Zn, Ag, Sb, Te, or Cd, each element is present in an amount of maximum 0.002 weight %, the balance is Pb and unavoidable impurities.

6. A method according to claim 2, wherein the method further comprises the steps of: supplying the PbCaSn alloy at a temperature of from 350 to 380 C. to the stem of the extruder, and cooling and solidifying the PbCaSn alloy inside the stem of the extruder to a temperature in the range of from 180 to 250 C. when exiting the stem and entering the cavity of the die-head.

7. A method according to claim 6, wherein the temperature of the PbCaSn alloy inside the stem of the extruder is of from 190 to 225 C., preferably of from 190 to 210 C. when exiting the stem and entering the cavity of the die-head.

8. A method according to claim 6, wherein the cooling of the temperature of the PbCaSn alloy inside the stem of the extruder is regulated by a cooling fluid flowing thorough one or more cooling fluid conduits in the wall of the stem, and where the flow volume of the cooling fluid through the cooling fluid conduits is regulated according to output from a temperature sensor reading the temperature the PbCaSn alloy at the exit of the stem.

9. A method according to claim 8, wherein the regulation of the cooling of the temperature of the PbCaSn alloy inside the stem is adapted to reach, from its initial molten state, a temperature of 180 to 250 C., in less than 3 minutes.

10. A method according to claim 1, wherein the extrusion of the sheathing is performed at flow volumes giving a travel time of the PbCaSn alloy through the extruder of less than 7 minutes.

11. A sheathing for a cable, wherein the sheathing is made of a PbSnCa-alloy having a composition comprising of from 0.03 to 0.05 weight % Ca and of from 0.4 to 0.8 weight % Sn, based on the total mass of the alloy and where the balance is Pb and unavoidable impurities.

12. A sheathing according to claim 11, wherein the composition of the PbSnCa-alloy comprises from 0.0325 to 0.05 weight % Ca and/or from 0.425 to 0.8 weight % Sn, where the unit weight % is based on the total mass of the PbCaSn alloy, and wherein said PbCaSn alloy optionally comprises one or more of: Ni, Cu, As, Zn, Ag, Sb, Te, or Cd, each element is present in an amount of maximum 0.002 weight %, and the balance is Pb and unavoidable impurities.

13. A sheathing according to claim 11, wherein the sheathing is made by an extrusion process comprising: a supply of molten PbCaSn alloy, and a die-head adapted to receive the power cable to be coated and to exit the cable and simultaneously form a coating of supplied PbCaSn alloy onto the power cable, wherein the PbCaSn alloy has a composition comprising of from 0.03 to 0.05 weight % Ca and of from 0.4 to 0.8 weight % Sn, based on the total mass of the alloy and where the balance is Pb and unavoidable impurities.

14. A sheathing according to any of claims 11, wherein the cable is a subsea power cable.

15. A method according to claim 3, wherein the PbCaSn alloy has a calcium content of from 0.035 to 0.05 weight %, based on the total mass of the PbCaSn alloy.

16. A method according to claim 15, wherein the PbCaSn alloy has a calcium content of from 0.040 to 0.05 weight %, based on the total mass of the PbCaSn alloy.

17. A method according to claim 16, wherein the PbCaSn alloy has a calcium content of from 0.040 to 0.045 weight %, based on the total mass of the PbCaSn alloy.

18. A method according to claim 4, wherein the PbCaSn alloy has a tin content of from 0.45 to 0.8 weight %, based on the total mass of the PbCaSn alloy.

19. A method according to claim 18, wherein the PbCaSn alloy has a tin content of from 0.5 to 0.8 weight %, based on the total mass of the PbCaSn alloy.

20. A method according to claim 19, wherein the PbCaSn alloy has a tin content of from 0.5 to 0.7 weight %, based on the total mass of the PbCaSn alloy.

Description

LIST OF FIGURES

[0042] FIG. 1 is a facsimile of FIG. 3.84 of ref 2 illustrating a typical extruder for producing a sheathing around a cable.

[0043] FIG. 2 is a facsimile of FIG. 1 of JP 2003-088915 illustrating an example of a screw type continuous extruder.

[0044] FIG. 3 is a graphical presentation of measured average flow stress for a set of samples of PbCaSn alloys having the compositions as given in Table 1.

[0045] FIG. 4 shows an isothermal cut of the PbSnCa phase diagram calculated for a temperature of 302 C. near the pure Pb corner.

[0046] FIGS. 5a) and b) are charts showing measured flow stress for the same samples of PbCaSn alloys as presented in FIG. 4.

[0047] FIG. 6 is a diagram showing measured 0.2% offset yield strength as a function of time of the same PbCaSn alloys having the compositions as given in Table 1.

[0048] FIG. 7 is a diagram showing measured ultimate yield strength as a function of time of the same PbCaSn alloys having the compositions as given in Table 1.

VERIFICATION OF THE INVENTION

[0049] A series of PbCaSn alloys having the different calcium and tin contents, as summarised in Table 1, were prepared and tested for their extrudability and natural aging properties. The test samples consisted of PbCaSn alloy containing from 0.02 to 0.04 weight % Ca and from 0.2 to 0.4 weight % tin. The test samples contained also unavoidable impurities in undetermined minute amounts.

[0050] The extrusion tests were performed in a horizontal continuous lead extruder. Samples where extracted directly after extrusion and tensile tested at a constant strain rate of 0.08%/s.

[0051] It is expected that PbCaSn alloys become harder, i.e. require higher flow stresses to be extruded, with higher calcium contents. This was also observed in the tests as shown graphically in FIG. 5a). The figures present measured flow stress, i.e. the middle value between yield strength and ultimate strength of the alloy samples as a function of strain. The upper curve in FIG. 5a) shows the measured flow stress in MPa for samples 1A, 1B, and 1C. These samples contained relatively small amounts of tin of 0.2 weight %. The figure shows that when the calcium content is increased from 0.015 to 0.039 weight %, the measured flow stress during the extrusion was nearly quadrupled. A similar result was obtained for the samples 2A, 2B, and 2C the samples 2A, 2B, and 2C which had an increased tin content of 0.3 weight %. These results are shown in the middle curve on FIG. 5a), and the

TABLE-US-00001 TABLE 1 Composition of PbCaSn alloy samples Alloy content [weight %]* Sample Ca Sn 1A 0.015 0.20 1B 0.027 0.20 1C 0.039 0.20 2A 0.017 0.30 2B 0.031 0.29 2C 0.040 0.30 3A 0.018 0.41 3B 0.025 0.40 3C 0.037 0.40 *The remainder being Pb and unavoidable impurities
curve shows an increased flow stress with increased calcium content, however much less as pronounced as for the tests samples 1A, 1B, and 1C. The third curve in FIG. 5 a) shows the test results for samples 3A, 3B, and 3C, which all contained about 0.4 weight % tin. In these samples there are no sign of increased flow stress with increased calcium content. Sample 3A with 0.015 weight % Ca has the same low flow stress as sample 3C with 0.04 weight % Ca.

[0052] The same effect is also illustrated in the curves of FIG. 5b). The upper curve shows the flow stress for samples 1C, 2C, and 3C, which all had about 0.04 weight % Ca, as a function of tin content. Th curve show a strong reduction of flow stress with increased tin content. The middle curve displays the flow stress for samples 1B, 2B, and 3B, which all had about 0.03 weight % Ca. It is a clear reduction in flow stress with increased tin content. The lower curve shows the same result for samples 1A, 2A, and 3A, which all had relatively low Ca-contents of about 0.02 weight %. For these alloys, the tin content gave no significant reduction in flow stress.

[0053] The natural aging of the PbCaSn alloy samples was investigated by measuring the 0.2% offset yield strength, R.sub.0.2, at different time intervals from extrusion (time 0) up to about three months (9 000 000 seconds), and further by measuring the ultimate tensile strength, R.sub.m at the same time intervals.

[0054] The results of the R.sub.0.2 measurements are presented in FIG. 6. As seen on the figure, the sample 1C which had a high Ca content and low Sn content has a relatively high R.sub.0.2 at all times. PbCaSn alloy sample 1C is tough to extrude and shows moderate natural aging of about 15-20% increase in R.sub.0.2 over three months but has good strength. The lower bundle of curves in FIG. 4, presents the R.sub.0.2 as a function of time for the samples 1A, 2A, and 3A, which had relatively low Ca contents, and 1B, 2B, and 3B, which had moderately high Ca contents. All these PbCaSn alloy samples had relatively low R.sub.0.2 shortly after extrusion but showed only a moderate natural aging of 25-30% increase in R.sub.0.2 over three months. The curves for PbCaSn alloy samples 2C and 3C show that these samples have the benefit of having, despite containing relatively high levels of Ca, a relatively low R.sub.0.2 at time 0 which is comparable with the R.sub.0.2 of PbCaSn alloy samples having low Ca contents, and by having a significantly larger natural aging of around 40% increase in R.sub.0.2 over three months for sample 2C (0.04Ca and 0.3Sn) and around 50% increase in R.sub.0.2 over three months for sample 3C (0.04Ca and 0.4Sn).

[0055] The results of the Rm measurements are presented in FIG. 7. As seen on the figure, the R.sub.m measurements gave similar results as the R.sub.0.2 measurements and confirmed thus those results. A PbCaSn alloy having relatively high Ca contents of from 0.03 to 0.04 weight % Ca and relatively high amounts of Sn from 0.3 weight % or more, is relatively easy to extrude but hardens by natural aging to a relatively strong and resilient material well suited for use as cable sheathing, and which obtains after natural aging an ultimate tensile strength and a 0.2% offset yield strength of around 25 MPa. This yield strength is about 20% higher than alloy E.

[0056] The extrusion output obtained in the extrusion tests was between 21 and 22 kg/min. As a comparison, a similar extrusion test was performed with an E-alloy (PB021K). In the latter case it was obtained an extrusion output of 18 kg/min.

REFERENCES

[0057] 1. Sivaraman Guruswamy (2000), Engineering Properties and Applications of Lead Alloys, Marcel Decker Inc., pp. 579, ISBN: 0-8247-8247-X [0058] 2. M. Bauser, G. Sauer, and K. Siegert (editors), Extrusion, 2.sup.nd edition, ASM International (2006), p. 128, ISBN-13: 978-0-87170-873-3