CA1071900A

CA1071900A - Phosphorus steel powder and a method of manufacturing the same

Info

Publication number: CA1071900A
Application number: CA263,946A
Authority: CA
Inventors: Jan R. Tengzelius; Per F. Lindskog; Lars-Erik Svensson
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 1975-10-24
Filing date: 1976-10-22
Publication date: 1980-02-19
Also published as: SE7511916L; DE2648262A1; GB1565984A; AU1898576A; AU511895B2; IT1069565B; DE2648262C2; JPS5284107A; FR2328779B1; BE847546A; FR2328779A1; SE410984B; ES452675A1

Abstract

ABSTRACT OF THE DISCLOSURE
A novel steel powder mixture for use in powder metallurgy dis-closed. Mouldings made from this powder have high toughness and strength.
The steel powder employed is substantially phosphorus free and is intimately mixed with a low-temperature-melting ferrophosphorus powder having a phosphorus content of at least 2.8% by weight. The phosphorus content of the mixture is 0.2 to 1.5% and the ferrophosphorus powder has a maximum particle size of 20 µm.

Description

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-The present invention relates to phosphorus steel powder mixtures to be used in powder metallurgy. In addition to iron and phosphorus these powder mixtures can contain other alloying elements commonly employed in this ~ ;
technique, such as copper, nickel, molybdenum, chromium and carbon.
The use of phosphorus as an alloying element in powder metallurgy has been known since the nineteen forties. Sintered steel alloyed with phosphorus has substantially improved strength characteristics in relation to non-alloyed sintered steel. Al~eady at an early date mixtures of pure iron powder and . ..... . ..
ferrophosphorus powder were used in powder metallurgy. However, the ferro-phosphorus first used had a composition which made it extremely hard and caused a considerable wearing of the tools. This drawback has been reduced to an acceptable degree by using a ferrophosphorus powder having a :Lower content of phosphorus and thereby reduced hardness ~see for example Swedish Patent No.
372~293).
However, sintered articles manufactured by pressing and sintering such steel powder mixtures sometimes have an unacceptable brittleness. This is revealed for example by the fact that a group of sintered test bars made from these mixtures can include bars having extremely reduced mechanical characteristics especially with regard to impact strength and permanent strain after rupture (break elongation). Since the advantage of phosphorus aLloyed sintered steels is high strength in combination with very good strain charac~
teristics, the above brittleness risks are very serious.
This brittleness risk has been found to be present when the ferro-phosphorus is of such composition that there is established a liquid phase at the sintering temperature. At the normally employed sintering temperatures, 1040 C ~nd above, this fact provides that phosphorus contents of more than

2.8~ in the ferrophosphorus give a sintered material having an increased brittleness risk. The fact that ferrophosphorus having a high phosphorus content is used in spite of this drawback is dependent on the favourable ' :

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~7~0 sintering process which is provided by the liquid phase and the favourable distribution of the phosphorus in turn providing for a rapid diffusion thereof which is obtained because of the fact that the ferrophosphorus provides for a liquid phase.
Thus, the obiect of the present imvention is to solve said problems with regard to the brittleness of sintered steel manufactured from a mixture of iron powder and a ferrophosphorus powder having a phosphorus content exceeding 2.8%. The solution of the problem has proved ~o reside in the use of a ferro-phosphorus powder having a small maximum particle size.
Accordingly, the invention provides a phosphorus steel powder for manufacturing sintered articles having high toughness and strengthp comprising a basic powder of a steel substantially free from phosphorus and having good compressibility to which is :Lntimately added a low-temperature-melting ferrophosphorus pow-der having a phosphorus content of at least 2.8% by weight in such an amount that the phosphorus content of the mixture is 0.2 to 1.5%, wherein the ferrophosphorus powder has a maximum part-icle æiæe of 20fum.
Thus, a phosphorus steel powder according to the in-vention for manufacturing sin~ered articles having an extremely small tendency to brittleness ruptures conslsts of steel powder substantially free from phosphorus, mixed with a ferrophosphorus powder characterized by a maximum particle size of 20,um, pre-ferably a maximum particle size oE lO,um. The phosphorus con-tent of the ferrophosphorus powder should be at least 2.8% and in order to reduce the wearing of the tools the phosphorus content should preferably be less than 17%. If the ferrophosphorus pow-der is manufactured by grinding piece goods the~phosphorus con tent should exceed 12% and shall preferably be between 14 and 16%. The phosphorus content of the mixture is between 0.2 and 1.5%.
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- ~o7sL~0(3 In this case there is a great difference between the particle si~e of the powder components in the mixture, leading to an especially great risk of segregation, and thereby of a dis- .
continuous distribution of the alloying elements. In order to reduce the tendency of the mixture to segregate after the mixing operation 50 - 200 g of a light mineral oil per metric ton powder can be added during the mixing operation. Thereby the fine alloy-ing particles are caused to adhere to the coarser iron powder particles.
In order to improve the protection against segregation the iron-ferrophosphorus mixture is heated, with or without the addition of oil, in a reducing atmosphere to a temperature of between 650 and 900C for a period of 15 minutes to 2 hours.
Thereby, the powder Is loosely sintered together - 2a - `

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so that subsequently a cautious disintegration has to be carried out in order to restore the original particle size. The powder provided in this way has iron particles with particles of the fine grained ferrophosphorus powder sintered thereto.
The methods described above in order to avo:id segregation can be performed to a mixture having an increased content of the phosphorus powder.
The concentrate so obtained can be mixed with the iron powder to provide for the desired phosphorus content in the final product.
The advantage o-f a powder mixture according to the in~ention appears from the two following examples, and from Figures 1-4 each of which is a graph illustrating a fea~ure of the invention.
Example 1 A ferrophosphorus powder having a phosphorus content of 15.8 weight-%
was divided in the size classes 0~5 ~m, 5-10 ~ , 10-20 ~ and 20-40 ~m by means of a known wind-sieve device. The different powder fractions were mixed with very pure iron powder having a maximum particle size of 150 ~m. The phosphorus content of the mixture was o.6 weight-% as this content has proved to give a distinct evidence as to the brittleness tendency o-f sintered materi-als. Seven impact strength test bars were manufactured from each mixture at 588 MPa. (megapascals) They were sintered in cracked c~mmonia at 1120C for 60 minutes. The bars were not impaired by any indications of fracture and were treated in a Charpy-apparatus at room temperature. The mean value of the impact strength ~I~ of these seven bars as a function of the particle size of the phosphorus is shown in Figure 1. The standard deviation(~I) ~orthe established values is given in Figure 2~
Example 1 evidently shows that the material manufactured from the mixture having a ferrophosphorus powder particle size of 5 to 10 /um has the greatest toughness. The material having ferrophosphorus particles of greater size than 15 /um does however provide for brittle sintered details.

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Example 2 A ferrophosphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 /um~ 5-10 /um and 10-40 /um by means of a wind-sieve device. The different powder fractions were mixed with very pure iron powder having a maximum particle size of 150 /um. The phosphorus content of the mixtures was o.6 weight-%. Seven tensile test bars were pressed from each mixture at 588 MPa. The bars were sintered in cracked ammonia at 1120 C
for 60 minutes. Thereupon the test bars were loaded to break and were then examined with regard to permanent strain after rupture (break elongation) (S), which is a good indication of the toughness of a material. A tough material -has a high elongation value while a brittle material has a low elongation value. Furthermore, the standard deviation (~I) with regard to the elongation values for the seven bars was calulcated. A high standard deviation means great dev:;ation of the values while a low standard deviation means a small deviation. The result of the test is shown in Figures 3 and 4.
It appears from the above example that a large particle size of the ferrophosphorus provides for a break because of brittleness while a small particle size provides for a break because of insufficient toughness. Both of the two tested characteristics unequivocally indicate this fact.
Thus, the present invention represents a solution of the problems of breaks because of brittleness, which sintered steel manufactured from a mixture of iron powder and ferrophosphorus powder present in cer~ain cases.
The solution resides in the use of a ferrophosphorus powder having a particle size less than 20 ~m~ preferably less than 10 ~m.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A phosphorus steel powder for manufacturing sintered articles having high toughness and strength, comprising a basic powder of a steel substantially free from phosphorus and having good compressibility to which is intimately added a low-temperature-melting ferrophosphorus powder having a phosphorus content of at least 2.8% by weight in such an amount that the phosphorus content of the mixture is 0.2 to 1.5%, wherein the ferrophosphorus powder has a maximum particle size of 20 µm.

2. A phosphorus steel powder as defined in claim 1, said powder further comprising 0.005 to 0.02% of a fluent mineral oil for diminishing the risk of segregation.

3. A phosphorus steel powder as defined in claim 1, wherein the ferrophosphorus particles are by means of sintering substantially adhered to the steel powder particles for obviat-ing segregation.

4. A phosphorus steel powder as defined in claim 1, 2 or 3, in which the ferrophosphorus powder has a phosphorus content of 12 to 17% by weight.

5. A phosphorus steel powder as defined in claim 1, 2 or 3, in which the ferrophosphorus powder has a phosphorus content of 12 to 17% by weight, and in which ferrophosphorus powder has a maximum particle size of 10 µm.

6. A method of manufacturing a phosphorus steel powder, wherein a basic powder of a substantially phosphorus-free steel having good compressibility is intimately mixed with a low-temperature-melting ferrophosphorus powder having a maximum particle size of 20 µm and a phosphorus content of at least 2.8%
by weight, the powders being employed in such amounts that the mixture has a phosphorus content of 0.2 to 1.5% by weight, the ferrophosphorus particles are adhered to the steel powder particles by adding 0.005 to 0.02% light mineral oil, and the mixture is subjected to loose sintering with subsequent cautious desintegration of the cakes thus created.

7. A method as claimed in claim 6, wherein the ferro-phosphorus powder is first mixed with a portion of the steel powder to form a concentrate and the concentrate is subjected to sintering and desintegration, and thereafter the concentrate is mixed with the rest of the steel powder.