1. Raw materials required for preparing cable sheath lead

 

1. Addition of copper In order to avoid the formation of Cu2Sb, an intermetallic compound that seriously affects the extrusion performance of cable sheath lead, the method of directly adding copper to the lead melt is adopted. The copper content of the finished sheath lead is well consistent with the amount added during production. Under normal circumstances, the error between the two is less than 5%.

 

Now the cable factory's requirements for the copper content of sheath lead are gradually increasing, tending to 0.06%. This brings certain difficulties to production. On the one hand, it prolongs the time of the melting process; on the other hand, due to the increase in copper content, it is easy to melt out of the melt, which reduces the copper content in the lead liquid, especially in the presence of Sb. However, it is completely feasible to try to avoid the influence of some unfavorable factors and increase the Cu content. The melting of copper in lead liquid does not mean that copper and lead have been alloyed. The alloying process must be given.

 

2. Addition of antimony The melting point of antimony is 630.5"C and the specific gravity is 6.684. Under hot conditions, it is easily oxidized to produce white smoke of antimony trioxide. Antimony has a greater affinity for oxygen than lead. Therefore, antimony is easily burned during the preparation process. However, antimony has a high solubility in liquid lead and dissolves quickly. Antimony that is broken into blocks is put into the stirring lead liquid and is quickly sucked into the vortex of the liquid flow and never floats to the surface of the liquid. It only takes a few minutes from addition to dissolution, but the stirring must be sufficient. After the antimony is completely dissolved, it needs to continue for a period of time, otherwise a separate antimony phase can still be seen in the metallographic photograph.

 

Antimony loss is large, and its loss degree (also called burning loss) is related to the lead liquid temperature, stirring intensity and time, and the amount of lead oxide remaining on the surface of the lead liquid. The burning loss rate can reach 10-18%. The effect of temperature on the utilization rate of antimony. In order to reduce burning loss, lower temperature and stirring intensity and shorter stirring time should be used under the premise of meeting the production process requirements. This has practical significance for reducing production costs. Calculated based on the finished product containing 0.55% Sb, if the utilization rate of antimony is increased from 85% to 90%, the cost of each ton of sheath lead can be reduced by nearly 3 yuan.

 

3. Addition of tin Tin should be added to the Pb~Sn-Sb alloy. The melting point of tin is 231.9 ℃ and the specific gravity is 7.29. The issues to be paid attention to when adding tin are similar to those when adding antimony. The speed at which it is oxidized is greatly affected by the operating temperature and stirring intensity. Since tin easily reacts with PbO to form PbO • SnO2, the amount of lead oxide remaining on the lead liquid surface will directly affect the utilization rate of tin. The influence of operating temperature on the utilization rate of tin. Since the burning loss of tin increases sharply with the increase of operating temperature, in order to reduce the burning loss, it should be controlled at the lowest possible temperature. Controlled at 400-410 ℃ range. If the temperature is too low, it will increase the difficulty of ingot casting machine operation, the flash burrs and surface slag of the sheathed lead ingot will increase significantly, the pouring hole will be easily blocked, and the labor intensity of the workers will be greatly increased. Excessive stirring can reduce the tin content in the alloy. Therefore, the stirring intensity and time should be controlled during operation. In addition, the reflux of lead liquid should be reduced as much as possible during ingot casting and the duration of the casting process should be shortened as much as possible. Under the condition of constant stirring intensity, the effect of stirring time on the Sn content of the alloy has been tested. It can be seen that with the extension of stirring time, the Sn content in the alloy decreases.

 

When the liquid alloy is placed calmly and insulated, the Sn content changes after several hours. The tin burnout is very small. It can be seen that the burning loss of tin is related to the contact of the fresh liquid surface with the air.

 

2. About the metallographic structure of Pb-Sb-Cu alloy

 

From the perspective of the controlled chemical composition range and the Pb-Sb-Cu ternary phase, the Pb-Sb-Cu sheath lead alloy should be a lead-based metal solid solution. The cast structure of the sheath lead ingot is unbalanced, and there is inevitably dendrite segregation. This segregation will disappear after annealing. Since the sheath lead ingot must be remelted before being used to make cable sheaths, its melting temperature generally reaches 380-400~C, and its cast structure is bound to be destroyed. Therefore, the state of the cast structure should not be used as a basis for judging the quality of the sheath lead ingot.

 

However, based on the practice of sheath lead extrusion processing, cable manufacturers believe that the presence of needle-shaped Cu2Sb in the sheath lead is very harmful and will seriously affect the extrusion performance of the alloy. Therefore, some manufacturers require that there should be no needle-shaped Cu2Sb in the sheath lead when placing orders, and the diameter of the granular Cu2Sb should not be greater than 1 mm on the X250 metallographic photo, that is, the diameter of the granular Cu2Sb should not be greater than 4 × 10-3mm. Whether the quantitative standard of this requirement is reasonable is another matter, but it reflects the practical experience of cable manufacturers from one aspect.

 

If the method of first mixing Sb-Cu intermediate alloy and then adding lead is used to produce Pb-Sb-Cu alloy, there must be a large amount of Cu2Sb in the alloy, of which needle-shaped Cu2Sb accounts for a considerable proportion. This alloy cannot be used in the production of continuous extruders. This has been confirmed by cable manufacturers.

 

The method of directly mixing copper and antimony can greatly reduce the generation of Cu2Sb, but it is impossible not to produce Cu2Sb. At 11% Sb and 0.075% Cu, there is a eutectic point of lead, antimony and antimonide copper, and the eutectic temperature is only 248C. If the alloy melt composition is at a certain point where Sb>11% and Cu>0.075%, the Cu2Sb phase will appear first during the cooling process, followed by the eutectic of Cu2Sb and Sb, and finally the ternary eutectic of Pb, Sb and Cu2Sb, in which a part of Cu2Sb forms a needle-like structure. Therefore, as long as the process conditions are strictly controlled, the formation of needle-like Cu2Sb can be avoided.