US3138851A - Lead powder metallurgy - Google Patents

Description

pouring it through a suitable nozzle.

United States Patent 3,136,851 LEAD POWDER METALLURGY Schrade F. Radtke, New Canaan, Conn, and Fritz V.

Lenel, Rensselaer, N.Y., assignors to Lead Industries Association, Inc, New York, N.Y., a corporation of New York No Drawing. Filed Apr. 3, 1961, Ser. No. 99,995 6 Claims. (Cl. 29-182) This invention relates to lead-base alloys having high strength and resistance to creep, and adequately high ductility.

Pure lead is soft and Weak; its rate of creep at room temperature is so high that in many applications lead cannot be used as a structural material in itself. The strength and creep resistance of lead can be somewhat improved by alloying, particularly if the alloying element causes precipitation hardening as in conventional lead-antimony and lead-calcium alloys.

In precipitation hardening, an alloying element is added to the lead, which has higher solid solubility at temperature near its melting point than at room temperature. The alloy is rapidly cooled from the high temperature and then aged.

However, the extent to which lead can be strengthened and made more creep resistant by this type of alloying is limited. For instance, at 86 F. antimonial lead containing 6% Sb, has a creep rate of 1% in one year at a stress of 400 p.s.i.; and of 100% in one year at a stress of 860 psi. The age hardened lead-calcium alloy with 0.03% Ca has creep resistance similar to the antimonial lead alloys.

In contrast, one of the lead base alloys according to the present invention, containing copper, has, at 77 F., a creep rate of 113% in one year at a stress of 4,000 p.s.i.; such alloy also has adequate tensile properties and ductility.

The preferred properties of prior art age hardened lead-base alloys are obtained by choosing a suitable chemical composition for the alloy and then giving the alloy an appropriate mechanical and thermal treatment. For the alloys according to the invention it is also necessary to choose a suitable chemical composition and appropriate mechanical and thermal treatments.

In contrast to most conventional alloys, the alloys according to the invention are fabricated by atomizing them into a fine powder, then compacting the powder, and then extruding the powder compact.

It has also been previously proposed to strengthen lead by atomizing it into a lead powder, then compacting' it and working the compact. The dispersion of the lead oxide present in the powder strengthens the lead but such lead is so brittle as to be impractical for many uses.

It is an object of the invention to provide a lead base alloy article having greater resistance to creep and having also high strength and adequate ductility. Other objects of the invention will be more apparent from the following description and claims.

A preferred method of producing the alloy of the invention is described in the following, for an alloy composition of 95% Pb, 5% Cu by weight.

The alloy is melted in a suitable crucible and heated to a temperature high enough to dissolve all the copper in the liquid lead. This temperature is in the range of 16501830 F. The melting point of lead is 621 F. and the melting point of copper is 1985 F.

The molten alloy is then atomized in the following manner:

A thin stream of the molten alloy is produced by This stream of 3,138,851 Patented June 30, 1964 ice metal is broken up into very fine droplets by impinging a stream of air of suitable velocity upon the stream of molten metal. The droplets of molten alloy freeze into powder particles.

The resultant powder is collected in a suitable container and sifted through a 325 mesh sieve. Only that portion of the powder, finer than 325 mesh (less than 44 microns) is used in the preparation of the alloy.

The powder is compacted in the atomized (as-received) condition, or it may be given a reducing treatment before it is compacted. A suitable reducing treatment consists of heating the powder in a boat in a stream of hydrogen to a temperature of 570 F. for 24 hours. During reducing, the powder sinters into a sponge, but this sponge can be readily broken into powder.

The as-received powder is cold compacted into a compact 1 inch in diameter in a steel die under a pressure of 35,000-40,000 psi. The resultant compact has a density of 98-99% of the density calculated by the rule of mixtures for a completely dense Pb 5% Cu alloy. The compact is extruded by indirect extrusion into a wire 0.156 inch in diameter at a temperature in the range from 250 F. to 450 F.

It should be understood that the above method of consolidating the powder into a solid shape is only one of several methods which may be used to produce a solid shape. For instance, the lead powder may be fed into a rolling mill and rolled into a sheet. Cold rolling the powder into a sheet, and then hot re-rolling the sheet in the same temperature range as used for extrusion, results in rolled sheet having properties similar to those for the extrusions described below.

The preferred alloy composition is 95% Pb, 5% copper. This alloy composition has been tested in the form of extrusions, 0.15 inch in diameter. The tensile properties and the creep rate has been determined for extrusions made from as-received powder at two extru sion temperatures. For reduced powder, only tensile properties of extrusions produced at two extrusion temperatures have been determined.

The data obtained are shown in Tables I to V below.

TABLE I Mechanical Properties of As-Received Lead Powder at Various Extrusion Temperatures Extrusion Tensile Percent Test Powder Tempera- Strength, Elongation ture, F. p.s.i. in 1 inch Pb Room 7,690 5.2

Tempetature Pb 200 7,950 l 6.6 Pb 300 6,660 7.1 Pb 400 5,840 11.6 Pb 500 5,120 15.6

TABLE H Extrusion Tensile Percent Test Powder Tempera- Strength, Elongation turc, F. p.s.i. in 1 inch As-Received Pb-Cu 200 .B, 050 15.0 Reduced Pb-Ou 200 *6, 665 23. 5 As-Reccived Pb-Cu 450 '6, 880 19. 8 Reduced Pb-Cu 450 4, 970 45. 4

3 TABLE 111 Steady-State Creep Rate and Tensile Strength of As- Received 95 Lead-5 Copper Powder Extrusions Extrusion Creep Steady- Creep Tensile Test Temperw Stress, State, Rate, Strength,

ture, F. p.s.i. Percent/ Percent/ p.s.i.

day year TABLE IV Creep Stresses of Various Rolled Lead Alloys Compared to 95 Leaa Copper Powder Extrusions Mechanical Properties of Reduced Lead-Copper Powder Extrusions with Varying Amounts of Copper Weight, Extrusion Tensile Percent Test Percent Tempera- Strength, Elongation Copper ture, F. p.s.i. in 1 inch A 0.5 450 4, 800 29.6 B 2.1 450 4, 700 28.0 C 5.0 450 4, 970 45. 4 D 8. 1 450 6, 700 32. 4

A practical use of this invention is for lead products which require resistance to creep, either at room or at slightly elevated temperatures. For radiation protection, lead may be extruded into bricks or into other products. For example, the extruded products may be used in leadsteel or lead-concrete sandwich structures, which must keep close dimensional tolerances. The design of these bricks or structures has heretofore been dictated by creep properties of the lead, rather than by its radiation protection. Simpler, lighter and lower cost structures are possible with a lead alloy according to the invention having improved creep resistance.

Very similar considerations apply to the use of lead or lead linings for corrosion protection in the chemical industry.

It is believed that the improved strength and creep resistance of the lead alloys according to this invention is due to their structure, which consists of a very fine dis -persion of particles of copper in the lead matrix of the powder particles. In order to produce such a structure it is necessary to cool the lead-copper alloy rapidly from a temperature at which the copper is in solution, i.e. well above the melting point of lead, to the vicinity of room temperature.

The strength and creep resistance of the alloy will depend upon the degree of dispersion of the copper particles in the particles of powder; the finer the dispersion, the stronger and more creep resistant the alloy. The degree of copper dispersion in the particles of powder will in turn depend on the rate of cooling of the alloy from the temperature where the copper is in solution. The finer the particles of powder, the faster the rate of cooling. It is therefore desirable to use quite fine powder.

Work has also been done on powders of diiierent particle size. Even though best results have been obtained with powder all of which is below 44 microns, good results have been obtained with powders of larger particle size.

Work has also been done to determine the range of composition of the 'PbCu alloys, which have these desirable properties. This work indicates that the copper content should run from 0.2% to 12% copper. It would be diificult to get much more than 12% copper into homogeneous liquid solution before atomizing. Alloys with 0.2% Cu have improved strength over pure lead and other lead alloys.

The diiferent properties of products made from asreceived lead-copper powder, reduced lead-copper powder, lead-lead oxide powder; the eiiect of particle size. of the powder; the effect of extrusion temperature, particularly on the properties of strength and creep, will now be briefly discussed.

It will be noted from Table I that, for lead-leadoxide powders, the effect of increasing the extrusion temperature is to'decre'ase the tensile and yield strengths and to 7 increase the ductility.

It will be noted from Table II that for lead-copper powders, the effect of increasing the extrusion temperature is to decrease the tensile and yield strengths and to increase the ductility; this applies to both as-received and reduced powders.

The principal difierences between articles made from lead powder and the lead-copper alloy powder is in their ductility. Even the extrusions from the as-received alloy powder are more ductile than those from the straight lead powder.

Reducing the lead-copper powder, and thereby eliminating most of the oxide, causes the elongation values to more than double as compared with the as-received leadcopper powder. The lower strength values may be due to the lower oxide content of the reduced lead-copper powder, which would eliminate some of the strengthening oxide dispersion and leave only the strengthening effect of the dispersed copper.

It will be noted that the dispersion of lead oxide has been found to strengthen the lead matrix with both leadlead oxide and lead-copper powder. Unfortunately with the lead-lead oxide powders the strengthening of lead matrix by oxide dispersion is accomplished by severe embittlement of the material. The stronger the material, the more brittle it becomes. For this reason tests on the leadlead oxide powder have been limited.

As indicated in Table III, at a given creep stress, the specimens which have been extruded at higher temperatures show lower creep rates, which means that their creep strength is higher while their tensile strength is lower.

The creep stresses used, to obtain appreciable creep rates in a 100 hour test period, are unusually high for lead alloys. These stresses in most cases are above both the yield strength and tensile strength of conventional lead base alloys.

Table IV shows a comparison of creep characteristics for the several more common lead base metals, as compared to the lead-copper powder alloy of the invention. It is easily seen that the creep resistance of the leadcopper powder alloy is much superior to the other lead base metals.

What is claimed is:

1. A worked lead base alloy powder article formed of a compacted powder, said worked alloy powder article. being obtained by atomizing a liquid lead-copper alloy in which the copper is completely dissolved, the said alloy consisting essentially of lead and copper in the range from posing said powder consisting of a fine dispersion of characterized by a ductility greater than that of pure lead,

and by a creep rategreatly lower than that of pure lead.

2. A worked lead base powder article according to claim 1 in which the obtained powder is subjected to treatment with a reducing agent to substantially eliminate any lead oxide present.

3. A worked lead base powder article according to claim 1 which is free from lead oxide except as an incidental impurity.

4. A Worked lead base powder article according to claim 1 in which the copper content is approximately 5% by weight.

5. A Worked lead base powder article according to claim 4 in which the powder particles are of a size less than 44 microns and the powder has been cold-compacted and hot-worked, said article being characterized by a creep rate at 77 F. of less than 113% in one year at a stress of 4000 p.s.i.

6. A worked lead base powder article according to 6 claim 1 in which said article is cold-compacted and hot- Worked at a temperature in the range of from 250 F. to 450 F.

References Cited in the file of this patent UNITED STATES PATENTS 874,866 Reed Dec. 24, 1907 1,761,506 Williams June 3, 1930 2,156,802 Cooper May 2, 1932 2,234,371 Fetz Mar. 11, 1941 2,916,809 Schell et al. Dec. 15, 1959 FOREIGN PATENTS 713,009 Great Britain Aug. 4, 1954 1,004,457 Germany Mar. 14, 1957 1,086,052 Germany July 28, 1960 OTHER REFERENCES Metal Powder Report, Vol. 1, No. 5, January 1947, page 75.

Claims (1)

1. A WORKED LEAD BASE ALLOY POWDER ARTICLE FORMED OF A COMPACTED POWDER, SAID WORKED ALLOY POWDER ARTICLE BEING OBTAINED BY ATOMIZING A LIQUID LEAD-COPPER ALLOY IN CONSISTING ESSENTIALLY OF LEAD AND COPPER IN THE RANGE FROM LEAD 88% AND COPPER 12% TO LEAD 99.8% AND COPPER 0.2%, THE PERCENTAGES BEING BY WEIGHT, THE PARTICLES COMPOSING SAID POWDER CONSISTING OF A FINE DISPERSION OF COPPER PARTICLES IN A MATRIX OF LEAD AND SAID ARTICLE BEING CHARACTERIZED BY A DUCTILITY GREATER THAN THAT OF PURE LEAD, AND BY A CREEP RATE GREATLY LOWER THAN THAT OF PURE LEAD.