LEAD-ANTIMONY ALLOY SUITABLE FOR MAKING BATTERY GRIDS

Abstract: An extremely rapid heat-treating process for strengthening a worked lead-antimony alloy, which alloy significantly retains its physical properties after further mechanical working such as expansion of the alloy into a battery grid, the alloy comprising about 0.5 % to 6 % antimony, about 0.002 % to 1 % arsenic, about 0.0005 % to 1 % tellurium, the balance essentially lead.

Description: LEAD-ANTIMONY ALLOY SUITABLE FOR MAKING BATTERY ^ GRIDS BACKGROUND OF THE INVENTION This invention relates to an improved lead- antimony alloy which is strengthened by an extremely rapid heat treatment method and which is capable of being processed into battery grids on a continuous production line. Lead-acid storage batteries have been used for many years as starter batteries for internal combustion engines. Pure lead is a soft material however,, arid extensive research has developed a number of alloys to provide specific physical properties desired by the battery manufacturers to improve the battery and battery making process. In general the conventional method of preparing grids by casting is relatively inefficient and it is now preferred to use an automated continuous method which produces grids by expanding or punching a wrought lead alloy strip as described in U.S. Pat. No. 4,443,918. For example, expanded plates can be obtained by continuously supplying a lead alloy strip, expanding it, pasting the thus produced mesh-like strip, drying it and cutting it to form individual grids. U.S. Pat. Nos. 3,945,097 and 4,271,586 describes methods and machines for making expanded battery plates, the patents being incorporated herein by reference. Antimony is a common alloying material desirable in battery grids and amounts up to about 11% have been employed to improve the strength and castability of the lead. Unfortunately, antimony, aside from being relatively expensive, increases the water loss of the battery and is of limited use in a maintenance free battery and continuous battery making process. U.S. Patent Nos. 4,629,516 and 4,753,688 which are assigned to the same assignee as the present invention and which are hereby incorporated by reference, disclose a number of lead-antimony alloys useful for making battery grids. The patent notes however that the low antimony alloys require long times to heat treat the alloy for increasing their strength and a new alloy and short-term heat treatment were developed to enable continuous grid production. While the alloys and heat treatment methods of the prior art continuous battery making processes produce grids suitable for use in batteries, mechanical working of the alloy during the process after the heat- treatment step, particularly in the strip expansion step, may recrystallize the alloy under certain circumstances and decrease its physical properties. It is an object of the present invention to provide high strength antimonial lead strips or battery grids which exhibit enhanced resistance to loss of physical properties, in particular, strength and hardness due to mechanical working in the process of making the grids. It is a further object of the present invention to provide a continuous process for providing battery grids from a lead alloy strip. Other objects will be readily apparent from the following description. SUMMARY OF THE INVENTION It has now been found that the physical properties of the lead-antimony-arsenic alloys of U.S. Patent Nos. 4,629,516, and 4,753,688, which alloys have been worked and heat treated according to the patents, may be significantly retained after further mechanical working by incorporating an effective amount of tellurium in the alloys. Broadly stated, the process comprises working the alloy and rapidly heat treating (which includes quenching) the alloy for a sufficient time at an elevated temperature to activate a strengthening mechanism in the alloy, the time of the heat treatment step being substantially less than that used to conventionally heat treat lead-antimony alloys. The alloy comprises, by weight, about 0»5%-6% antimony about 0.002-1% arsenic, up to about 1%, or more . tellurium, up to about 0.5% tin and the balance being; essentially lead. The alloy may be worked, e.g., reduced, by an amount greater than about 15%, preferably greater than about 50% and most preferably greater, than 80% or 90% and is preferably reduced by rolling in several successive stages of substantially equal percentage reductions. DETAILED DESCRIPTION OF THE INVENTION The lead-antimony-arsenic-tellurium alloys which may be strengthened by the process of the : invention comprise, by weight, antimony in an amount greater than about 0.5%, e.g., about 0.5-6%, preferably about 0.75-3% and most preferably 1-2.5%, arsenic in an amount of about 0.002% to 1%, preferably 0.05% tp 0.25%, and most preferably 0.1% to 0.2% and tellurium- in an amount of about 0.0005% to 1%, preferably 0.003%.to 0.5% and most preferably 0.007% to 0.14%. Tellurium in combination with the antimony and arsenic, has been found to be essential to provide enhanced maintenance * of the physical properties of the worked, rapidly heat- treated alloy after further working of " the alloy to form, for example, battery grids by expansion of-strips of the heat-treated alloy. Working and heat treatment of the new alloy is as described in the above-referenced.U.S. Patent Nos. 4,629,516, and 4,753,688, with the importance of the processing steps and parameters being to produce an alloy having a miσrostructure such as Fig. 2 of the patents, i.e., a completely recrystallized structure having antimony-rich bands still present and the volume fraction of the antimony-rich regions being approximately the same as the as-rolled alloy of Fig. l therein. In general, less than 50%, usually less than 25%, and typically less than 10%, e.g., 5% or 1% or less of the soluble antimony (black regions-antimony-rich eutectic phase bands) of the as-rolled alloy is dissolved. Working of the alloys may be performed using conventional procedures well-known in the art, and by working or rolling, extrusion, etc. is meant mechanical plastic deformation of the metal and includes cold and hot working. In general, the alloy is cast into a billet and reduced to the desired size strip by passing it through successive rolls, wherein each roll in succession further reduces the thickness of the alloy. Constant reduction rolling schedules in the same rolling direction are preferred whereby, for example, a 0.75 inch thick billet is reduced to a 0.04 inch thick strip by passing it through 11 rolls wherein each roll in succession reduced the thickness of the billet by about 25%. Other rolling schedules can suitably be employed. Heat treatment of the alloy is performed under time and temperature conditions which do not result in a conventional solution treatment effect. The heat treatment of the present invention, which includes the quenching step, when applied to worked lead-antimony alloys containing a correlated amount of arsenic, antimony and tellurium, activates a strengthening reaction and a maintenance of the alloy's strength.by means not yet clear. Solution heat treatment as defined in ASTM Designation: E 44-83, means heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution and then cooling rapidly enough to hold these constituents in solution. The heat treatment of the present invention comprises only requiring the alloy to be heated to the desired temperature. In general, heating the alloy at the desired temperature does not dissolve any appreciable amount of soluble antimony, e.g., less than 50%, usually less than 25% and typically less than about 10%, e.g., 5% or 1% or less. The temperature of the heat treatment is between about 180*C. and the alloy liquidus temperature, preferably, 200*C. to 252*C, and most preferably 220 β C. to 245"C. The time required to bring the alloy to the desired temperature varies according to the thickness of the alloy and the temperature and method of heating, with thinner strips of alloy, higher temperatures and/or higher heat transfer heating means requiring shorter times. It is preferred that the alloy be brought substantially completely to the desired temperature to realize the full effect of the heat treatment on,Aό strengthening of the alloy. In a preferred embodiment, employing a molten salt bath at a temperature of about 230 β C. for about 15 seconds provided excellent strengthening, and retention of the strength after further working, for a 0.040 inch thick strip of alloy. An equivalent heating time for a muffle furnace would be about 2.5 minutes. For an alloy about 0.25 inch.>thick, over the broad range of heating working, temperatures, a heating time using a salt bath is less .than about 2 minutes, and even 1 minute and for a muffle furnace, less than about 8 minutes. As noted above, heating times will vary depending on the temperature and<>the. thickness of the alloy and, in general, for a strip of alloy about 0.025 inch to 0.1 inch thick, a heating time using a salt bath is about 1-3 seconds, pre erably'5 to 30 seconds to less than about 1 minute, and for a muffle furnace, about 1 minute, preferably 2 minutes and most preferably less than about 5 minutes. Longer times may be employed, if desired, although the longer times will not typically result in any substantial increased operating efficiencies. Other heating means can suitably be employed such as oil, induction heating, resistance heating, infra-red, and the like. Resistance or infra-red heating, for example, would provide almost instantaneous heating thus requiring very short heating times of 30 seconds or less, although longer times could be employed if desired. Any method and machine may be employed for making the worked alloy and/or battery plates and U.S. Patent Nos. 3,310,438; 3,621,543; 3,945,097; 4,035,556; 4,271,586; 4,358,518; and 4,443,918 show representative methods and machines, the disclosures of the patents being hereby incorporated by reference. U.S. Patent No. 4,271,586 shows, for example, a ribbon of lead being fed into an inline expander, followed by pasting, drying, cutting and accumulating into stacks. U.S. Patent No. 4,035,556 discloses forming of finished storage battery grids from rolled sheet material by (a) slitting and expanding to form an open grid, (b) punching out an open grid, (c) forming an interlocked type of grid and (d) combinations of (a) or (b) with (c) . It will be appreciated by those skilled in the art that heat treatment of the alloy may be performed at any convenient interval during preparation or manufacture of the alloy or battery grid. • Preferably, the alloy can be continuously cast, worked into strip, heat treated, expanded or punched into the grid, pasted, and assembled directly into the battery. The worked strip can also be coiled for storage and then heat treated and made into grids or it can be heat treated, coiled and stored for processing at a later time. The alloy can also be heat treated after preparation of the grid. Regardless of the method of heat treating and preparing of the grid, it is important that the alloy be worked before the heat treatment. The following examples will further illustrate the present invention. It will be understood that throughout this specification and claims, all parts and percentages are by weight and all temperatures in degrees Centigrade unless otherwise specified. EXAMPLE I A series of alloys having varying tellurium levels were prepared in a heated graphite crucible by melting a base lead, antimony, arsenic, tin alloy to a final composition containing 1.3% antimony, 0.16% arsenic and 0.23% tin, balance essentially lead, Tellurium metal was added to the crucible at ^ tbj__i levels indicated in TABLE I. Four alloys were prepared by casting into a graphite book mold at 400 * C.-500'C. to produce a cast block approximately 10 inch x 3. inch x 0.75 inch. The castings were milled to remove, surface defects and then rolled at room temperature to 0.045 inch in eight-twelve passes taking about a 20-30% reduction per pass. Samples for chemical analyses were cut from the resultant strip. Blanks 4 inqh x 0.5 inch for machining to test bars were cut from the strip in the rolling (longitudinal) direction. J β. Tensilkut Machine was used to cut the test bars to a 1 inch gage length and 0.25 inch width. Heat .treatment was performed by immersion of the bars in a molteji salt bath at 230"C. for 15 seconds and quenched by plunging into room temperature water immediately upon removal from the salt bath. The samples were then stored at room temperature for 10 days. Tensile tests representing the ultimate tensile strength (UTS) were performed on an Instron Machine using a crosshead speed of 0.2 inch/minute and microhardness testing was employed to obtain Vickers Hardness Numbers (VHN) at the grip section (unstrained) and the gage length sections (strained) of the specimen. TABLE I The results in TABLE I clearly show that the tellurium containing alloys retain their heat treated hardness after being pulled in tension to failure (27- 40% elongation to fracture) . The results also indicate the increase in strength of the heat treated alloy versus the as-rolled alloys. Similarly prepared alloys which contained no tellurium were tested in an identical manner. None of the alloys exhibited the permanent hardness retention in the strained region of the tensile specimens as clearly shown in TABLE I, with the hardness decreasing to values near the as-rolled condition. While this invention has been disclosed in terms of specific embodiments thereof it is not intended to be limited thereto and it will be understood that modifications may be made in the improved process of this invention without departing from the scope of the invention defined by the appended claims.

Claims: C L A I M S 1. A process for strengthening a lead-antimony alloy which alloy significantly retains its physical properties] after mechanical working, the alloy comprising, by weight, about 0.5%-6% antimony, about 0.002%-l% arsenic, about 0.0005% to 1% tellurium, the balance essentially lead, comprising: (a) working the alloy by rolling o produce a structure containing soluble antimony in the form of antimony-rich eutectic phase bands; (b) heating the alloy at an elevated-temperature for a sufficient time to provide an alloy having a recrystallized structure which strengthens on aging and which structure contains .greater .than 50% of the soluble antimony in the- antimony-rich eutectic phase bands and less than 50% of the soluble antimony being dissolved in the lead; and (c) quenching the alloy. 2. The process of claim 1, wherein the alloy * is worked by reducing it an amount of greater than about 15%. 3. The process of claim 1, wherein the alloy is heated for a time whereby the amount of soluble antimony dissolved is less than about 25%. 4. The process of claim 1, wherein the alloy is heated at a temperature between about 180"C. and the. alloy liquidus temperature for less than about 2.5 minutes, * 5. The process of claim 4, wherein the temperature is between about 200*C. to 252 β C. and the time is less than about 1 minute. 6. The process of claim 5, wherein the heating means is a molten salt bath. . 7. The process of claim 6, wherein the heating time is less than about 30 seconds. 8. The process of claim 7, wherein the alloy is worked by reducing it an amount greater than about 80%. 9. The process of claim 1, wherein the rolling is performed by successive multiple reductions. 10. An alloy prepared in accordance with the method of claim 1. 11. An alloy prepared in accordance with the method of claim 4. 12. An alloy prepared in accordance with the method of claim 9. 13. An alloy comprising by weight, about 0.5%-6% antimony, about 0.002%-l% arsenic, 0.0005% to 1% tellurium, the balance essentially lead, the alloy being characterized by having a recrystallized structure containing greater than 50% of the soluble antimony in an antimony-rich eutectic •phase in the form of bands. 14. The alloy of claim 13, wherein greater than 75% of the soluble antimony is in the form of antimony-rich eutectic phase bands. 15. The alloy of claim 13, wherein greater than 90% of the soluble antimony is in the form of antimony-rich eutectic phase bands. 16. A battery grid prepared from the alloy of claim 13. 17. A battery grip prepared from the alloy of claim 14. 18. A battery grid prepared from the alloy of claim 15. 19. A process for preparing battery grids from a lead- antimony alloy comprising, by weight, about 0.05%-6% antimony, about 0.002%-l% arsenic, about 0.0005% to 1% tellurium the balance essentially lead, comprising: (a) working the alloy by rolling to a desired size strip, the alloy characterized by having a structure containing soluble antimony in the form of antimony-rich eutectic phase bands; (b) heating the strip at an elevated temperature for sufficient time to provide an alloy having a recrystallized structure which strengthens on aging and which structure contains greater than 50% of the soluble antimony in the antimony-rich eutectic phase bands and less than 50% of the soluble antimony being dissolved in the lead; (c) quenching the strip; and (d) forming the strip into a battery grid. 20. The process for preparing battery grids of claim 19, wherein the alloy is worked in step (a) by reducing i in an amount greater than about 50%. 21. The process for preparing battery grids of claim 20, wherein the strip is heated in step (b) at a temperature between about 180*C. and the alloy liquidus temperature. 22. The process for preparing battery grids of claim 21, wherein the time of heating in step (b) is less than about 1 minute. 23. The process for preparing battery grids of claim 22, wherein the time of heating in step (b) is less than about 30 seconds. 24. The process for preparing battery grids of claim 23, wherein the alloy is worked in step (b) by reducing it in an amount greater than about 80%. 25. The process for preparing battery grids of claim 24, wherein the strip is heated in step (b) at a temperature between about 200 d C. to 252 * C. using a molten salt bath. 26. The process for preparing battery grids of claim 25, wherein the alloy is worked by rolling in successive multiple reductions. 27. The process for preparing battery grids of claim 19, wherein step (d) is performed before steps (b) and (c) . 28. A battery grid prepared in accordance with the method of claim 19.

Inventors: HAYDUK DOUGLAS, HAYDUK, DOUGLAS
Applicants: ASARCO INC, ASARCO INCORPORATED
Application year: 1990
Main classification: C22F1
IPC codes: C22C9, C22F1, H01M4