US2226520A - Iron article and method of making same - Google Patents

Description

' sPajteiited Dec. 24, 194.0

STATES z orrics Fritz V. Lenci, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich, a corpora,

tion of Delaware No Drawing. Application November 29, 1939, Serial No. 308,703

8 Claims.

This invention relates to porous ferrous articles and is particularly concerned with ferrous articles fabricated from substantially pureiron powder, wherein a'small percentageoi phos- 5 phorus is included to improve the physical characteristics of the articles. I

' It is an object of the invention to provide a sintered iron article of high strength by utilizing phosphorus as a means for increasing the strength and improving other physical characteristics thereof.

In carrying out the above object it is a further object, in some cases to introduce the phosphorus in the form of term-phosphorus.

It is a still further object to maintain the phosphorus in the finished article in percentages between A to 2 percent.

. Another object of the'invention is to provide a method of fabricating iron articles from substantially pure iron powder and a phosphorus containing material wherein the phosphorus ingredient forms an iron-iron-phosphide eutectic during sintering, which eutectic functions as a low melting constituent for bonding the particles of substantially pure iron together.

Further objects and advantages of the present invention will be apparent from the following description. The ever increasing use of sintered iron articles in industry has created the necessity for producing articles of high strength and improved ductility. It has been found that porous iron as fabricated from sintered iron powder is a good substitute for articles heretofore i'abricated from conventional material since iron powder may be briquetted to substantially the exact shape and size desired, considerable savings are possible due to the elimination of costly machining operations. This adaptation of porous iron has extended into many fields. In some instances it has been found that the porous iron of the priorart lacks sufilcient strength and/or ductility to function satisfactorily for certain purposes. Furthermore, in many new uses where porous iron has been introduced an increase in tensile strength and improvement in other physical characteristics thereof would be desirable. 1

I have discovered that the tensile strength of ferrous articles made from metal powder may be increased and other physical properties improved by adding an alloying ingredient to the iron in the form of phosphorus. I have further ascertained that the percentage of phosphorus as to be added is quite critical and that when too much of this ingredient that the sintered article is too brittle to be of any commercial value.

Phosphorus may be utilized as an addition to any of the conventional 'types of substantially a pure iron powders, such as, sponge iron powder, electrolytic iron powder, comminuted iron powder, or iron powder from iron carbonyl, etc. These powders may consist of pure iron, or they may consist of substantially pure iron which may 10 include suitable alloying ingredients, such as: copper, nickel, cinemium, molybdenum, vanadium, etc. It is preferred that the iron-phosphorus briquettes are substantially free from carbon in either the free state or combined form 16 since carbon tends to make the sintered article excessively brittle.

It has been discovered that phosphorus may be advantageously added to powdered iron or mixtures'thereof with other ingredients by the 20 use of term-phosphorus powder. This powder may be obtained with varying percentages" of phosphorus therein as desired. I prefer to use a ferro-phosphorus powder containing .above 16% phosphorus.

Forillustratlve'purposes only, the following example is given for disclosing one method of forming ferrous articles as herein described. Substantially pure soft sponge iron powder is mixed with ferro-phosphorus powder so as to yield phosphorus in the proportion of preferably l /2% by weight of the total powder mixture. This mixture is then briquetted to the desired shape under pressures of between 20000 to'-'75000, pounds per square inch. The briv .quettes are next sintered at a temperature above the melting point of the iron-iron-phosphide eutectic, preferably, between 1950 and 2100 F., the melting point of the eutectic being 1922 F. The sintering is carried out under non-oxidizing 59 conditions for periods ranging from 15 minutes to one hour, whereupon the sintered briquette is cooled under non-oxidizing conditions. cases where carbon free iron powder cannot be obtained, it is preferable to sinter under deas carbonizing conditions whereby the finished sini tered' article is substantially free from carbon and therefore of um ductility.

Tests have shown that articles formed by the process herein disclosed have high tensile so strengths, which vary with the percent of phosphorus therein, are more ductile than conventional articles formed without the phosphorus constituent, and are hard though not brittle when phosphorus is present in the preferred as In A Tensile Permanent M n 3 Percent of P strength deflection (msvem) hardness In each case six samples were tested and the figures noted are an average of these six tests. The permanent deflection was measured by placing a test bar measuring 1% by by A," upon two supports one inch apart with the inch dimension in the horizontal position and then loading the bar in the middle thereof between the supports. It will be noted that the tensile strength increases with an increase in percent of phosphorus up to 1 percent whereupon the tensile strength begins to decrease and it has been found that percentages over 2 percent are undesirable because of rapid decrease in ductility. The permanent deflection may be taken as a measure of ductility, and in this respect test bars including phosphorus in the percentages noted 0 have greater ductility than similar bars having comparable strengths and made without phos phorus.

One theory explaining the increase in tensile strength proposes the use of the iron-iron-phosphide eutectic constituent as a bonding agent, which flows uniformly over the particles of iron thereby coating the same and bonding adJacent particles of iron togethen. upon continued sintering the eutectic difluses partially into the grains of iron thereby strengthening the bond. This eutectic is extremely hard and brittle and when phosphorus is present in percentages above 2%% it is believed that the eutectic constituent is either in globules or in the form of such a heavy coating that the article loses its tensile strength due to the excess of the eutectic constituent. Likewise, it is believed that when the phosphorus content is at the low end of the range that insufflcient eutectic is present to completely coat the particles of iron thereby explaining the reduction in the tensile strength at the beginning of the table. The iron-iron-phosphide eutectic is further desirable in that it presents a low melting constituent, which readily wets the surface of the iron particles thereby bonding them together and improving the physical characteristics thereof.

While the example herein is limited to the use of term-phosphorus it is to be understood that phosphorus may be added in any desired form, the only limiting factor being that the total percentage of phosphorus in the finished article is within the limits noted. v

It is to be further understood that when using stantially free from carbon consisting substantially 01, a composition of iron and phosphorus, said phosphorus being in the proportions of from A to'2 of the total weight of the starting ingredients.

2. A strong porous ferrous article substantially free from carbon fabricated from a sintered mixture consisting substantially of from powder containing phosphorus, said phosphorus being in the proportions of from A; to 2 of the total weight of the starting ingredients.

3. A strong porous iron article substantially free from carbon fabricated from a sintered mixture consisting substantially of iron powder and. ferro-phosphorus powder, said ferro-phosphorus being in such a proportion that the final percentage of phosphorus in the article ranges from A to 2 /g%. v

4. A porous sintered iron article substantially free from carbon characterized by its high strength and good ductility, consisting substantially of an iron matrix bonded together with a eutectic mixture of iron and iron phosphide wherein the phosphorus contents is between 25% and"2.5%.

5. A porous sintered iron article substantially free from carbon characterized by its high strength and consisting substantially of a mixture of iron, and phosphorus'in the order of 1 of the total weight of the ingredients.

.6. A porous sintered iron article substantially free from carbon having a tensile strength above 45,000 pounds per square inch consisting substantially of iron and phosphorus in percentages between .25% and 2.5%.

'7. A method of fabricating porous iron articles substantially free from carbon and characterized by their high strength including the ste s of, briquetting a mixture consisting of substantially pure iron powder with sufficient ierro-phosphorus powder to bring the phosphorus content of'the final article to between V and 2%% by weight, sintering the briquette under suitable conditions at a temperature above the melting point of the iron-iron phosphide eutectic for a time suflicient to cause said eutectic to coat the particles of iron powder and thereby act as a bonding agent therebetween, and then cooling the sintered briquette.

8. As a new article of manufacture, a sintered agglomerated powder of carbon-free iron bonded by an iron-phosphorus eutectic.

- FRITZ V. LENEL.