GB1565984A

GB1565984A - Phosphorus steel powder and a method of manufacturing the same

Info

Publication number: GB1565984A
Application number: GB43997/76A
Authority: GB
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 1975-10-24
Filing date: 1976-10-22
Publication date: 1980-04-30
Also published as: DE2648262C2; CA1071900A; DE2648262A1; JPS5284107A; FR2328779B1; SE7511916L; AU1898576A; SE410984B; FR2328779A1; IT1069565B; ES452675A1; BE847546A; AU511895B2

Description

(54) A PHOSPHORUS STEEL POWDER AND A METHOD OF MANUFACTURING THE SAME (71) We, HOGANAS AB, a Company organised under the laws of Sweden, of Fack, 263 01 Hoganas, Sweden do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and by the following statement: 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 common 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.

Already at an early date there were for this object used mixtures of pure iron powder and ferrophosphorus powder. However, the ferrophosphorus 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 details manufactured by pressing and sintering such steel powder mixtures sometimes have an unacceptable brittleness. This is revealed for example by the fact that a population of sinteredtest bars made from these mixtures can comprise individuals having extremely reduced mechanical characteristics especially with regard to impact strength and permanent strain after rupture (break elongation). As the advantage of phosphorus alloyed sintered steels is high strength in combination with very good strain characteristics the above brittleness risks are very serious.

Said brittleness risk has shown up to be present when the ferrophosphorus is of such composition that there is established a liquid phase at the sintering temperature. At the usually used sintering temperatures, 10400C and above that, this fact provides that phosphorus contents of more than 2.8 wt. % 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 sintering process which is provided by the liquid phase and the favourable distribution of the phosphorus in turn providing for a rapid indiffusion thereof which is obtained because of the fact that the ferrophosphorus provides for a liquid phase.

Thus, the object of the present invention 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 wt. %. The solution of the problem has proved to reside in the use of a ferrophosphorus powder having a small maximum particle size.

A phosphorus steel powder according to the invention for manufacturing sintered details having an extremely small tendency to brittleness ruptures consists of iron or steel powder substantially free from phosphorus, mixed with a ferrophosphorus powder characterized by a maximum particle size of 20 ,um, preferably a maximum particle size of 10 pm. The phosphorus content of the ferrophosphorus powder shall exceed 2.8% and in order to reduce the wearing of the tools the phosphorus content shall preferably be less than 17%. If the ferrophosphorus powder is manufactured by grinding a workpiece the phosphorus content should exceed 12% and should preferably be between 14 and 16%.

The phosphorus content of the mixture is between 0.2 and 1.5%.

It is often the case that there is a great difference between the particle sizes of the powder components in the mixture leading to an especially great risk of segregation and thereby of a discontinuous distribution of the powder components. 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 alloying particles can 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 9000C for a period of 15 min to 2 hours. Thereby, the powder is loosely sintered together so that a following 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 avoid segregation can be performed to a mixture having more 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 of a powder mixture according to the invention appears from the two following examples.

Example 1 A ferrophosphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 ym, 5-10 pm, 10-20 ,am and 20-40 ,am by means of a wind or air-sieve device. The different powder fractions were mixed with very pure iron powder having a maximum particle size of 150,am. The phosphorus content of the mixture was 0.6 weight- % as this content has proved to have a distinct tendency to brittleness. Seven impact strength test bars were manufactured from each mixture. They were sintered in cracked ammonia at 11 200C for 60 minutes. The bars were not impaired by any indications of fracture and were treated in a Charpyapparatus at room temperature. The mean value of the impact strength (I) of these seven bars as a function of the particle size of 5 to 10 ,am has the greatest toughness.

1. The standard deviation (cri) for the established values is given in Fig 2.

Example 1 evidently shows that the material manufactured from the mixture having a ferrophosphorus powder particle size of 5 to 10 ,ttm has the greatest toughness.

The material having ferrophosphorus particles of greater size than 15 ym does however provide for brittle sintered details.

Exanzpte 2 A ferrophosphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 pm, 5-10 um and 10-40 ,um by means of a wind or airsieve device. The different powder fractions were mixed with very pure iron powder having the maximal particle size of 150 qrm The phosphorus content of the mixtures was 0.6 weight- %. Seven tensile test bars were pressed from each mixture. The bars were sintered in cracked ammonia at 1 1200C for 60 minutes. Thereupon the test bars were loaded to breaking and were then examined with regard to permanent strain after rupture (break elongation) (3), 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 (a6) with regard to the elongation values for the seven bars was calculated. A high standard deviation means great deviation of the values while a low standard deviation means a small deviation.

The result of the test is shown in Figs 3 and 4.

It appears from the above example that a larger particle size of the ferrophosphorus provides for breakage because of brittleness while a smaller particle size provides for breakage 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 certain cases. The solution resides in the use of a ferrophosphorus powder having a particle size less than 20 ,am, preferably less than 10 calm.

Our co-pending serial no. 1565983 application number 43996/76 of even date herewith claims a phosphorus steel powder having the same composition as the powder of the present application but in which the total content of impurities which are at the sintering temperature more easily oxidised than the main components iron and phosphorus does not exceed 4%. It also claims such a powder when made by the method of the present application and a sintered article when made from the powder. We make no claim herein to what is claimed in that application.

Subject to the foregoing disclaimer, WHAT WE CLAIM IS: 1. A phosphorus steel powder for manu

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. manufactured by grinding a workpiece the phosphorus content should exceed 12% and should preferably be between 14 and 16%. The phosphorus content of the mixture is between 0.2 and 1.5%. It is often the case that there is a great difference between the particle sizes of the powder components in the mixture leading to an especially great risk of segregation and thereby of a discontinuous distribution of the powder components. 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 alloying particles can 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 9000C for a period of 15 min to 2 hours. Thereby, the powder is loosely sintered together so that a following 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 avoid segregation can be performed to a mixture having more 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 of a powder mixture according to the invention appears from the two following examples. Example 1 A ferrophosphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 ym, 5-10 pm, 10-20 ,am and 20-40 ,am by means of a wind or air-sieve device. The different powder fractions were mixed with very pure iron powder having a maximum particle size of 150,am. The phosphorus content of the mixture was 0.6 weight- % as this content has proved to have a distinct tendency to brittleness. Seven impact strength test bars were manufactured from each mixture. They were sintered in cracked ammonia at 11 200C for 60 minutes. The bars were not impaired by any indications of fracture and were treated in a Charpyapparatus at room temperature. The mean value of the impact strength (I) of these seven bars as a function of the particle size of 5 to 10 ,am has the greatest toughness. 1. The standard deviation (cri) for the established values is given in Fig 2. Example 1 evidently shows that the material manufactured from the mixture having a ferrophosphorus powder particle size of 5 to 10 ,ttm has the greatest toughness. The material having ferrophosphorus particles of greater size than 15 ym does however provide for brittle sintered details. Exanzpte 2 A ferrophosphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 pm, 5-10 um and 10-40 ,um by means of a wind or airsieve device. The different powder fractions were mixed with very pure iron powder having the maximal particle size of 150 qrm The phosphorus content of the mixtures was 0.6 weight- %. Seven tensile test bars were pressed from each mixture. The bars were sintered in cracked ammonia at 1 1200C for 60 minutes. Thereupon the test bars were loaded to breaking and were then examined with regard to permanent strain after rupture (break elongation) (3), 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 (a6) with regard to the elongation values for the seven bars was calculated. A high standard deviation means great deviation of the values while a low standard deviation means a small deviation. The result of the test is shown in Figs 3 and 4. It appears from the above example that a larger particle size of the ferrophosphorus provides for breakage because of brittleness while a smaller particle size provides for breakage 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 certain cases. The solution resides in the use of a ferrophosphorus powder having a particle size less than 20 ,am, preferably less than 10 calm. Our co-pending serial no. 1565983 application number 43996/76 of even date herewith claims a phosphorus steel powder having the same composition as the powder of the present application but in which the total content of impurities which are at the sintering temperature more easily oxidised than the main components iron and phosphorus does not exceed 4%. It also claims such a powder when made by the method of the present application and a sintered article when made from the powder. We make no claim herein to what is claimed in that application. Subject to the foregoing disclaimer, WHAT WE CLAIM IS:

1. A phosphorus steel powder for manu

facturing sintered articles or mouldings having high toughness and strength, comprising a basic powder of a steel powder substantially free from phosphorus and having good compressability to which is intimately added a low-temperature-melting ferrophosphorus powder having a phosphorus content of at least 2.8%, 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 ,am, preferably a maximum particle size of 10 slum.

2. A phosphorus steel powder as claimed in claim 1, the ferrophosphorus powder having a phosphorus content between 12 and 17%.

3. A phosphorus steel powder as claimed in claim 1 or claim 2, said powder further comprising 0.0015 to 0.02% of a mineral oil for diminishing the risk of segregation.

4. A phosphorus steel powder as claimed in any preceding claim, wherein the ferrophosphorus particles are adhered to the steel powder particles by means of sintering for obviating segregation.

5. A method of manufacturing a phosphorus steel powder according to any of the preceding claims, wherein a basic powder of steel powder is intimately mixed with ferrophosphorus powder and the ferrophosphorus particles are adhered to the steel powder particles by adding 0.005 to 0.02% mineral oil and/or by means of a loose sintering with subsequent cautious disintegration of the cakes thus created.

6. A method as claimed in claim 5, wherein the ferrophosphorus powder is first mixed with a portion of the steel powder to a concentrate possibly with the addition of 0.005 to 0.02% mineral oil and the concentrate is subjected to the sintering and desintegration, whereupon the concentrate possibly together with lubricants and if the steel powder is non-alloyed possibly together with alloying powder is added to the rest of the steel powder.

7. A method of making a phosphorus steel powder, substantially as hereinbefore described in either one of the Examples.

8. A phosphorus steel powder for manufacturing articles or mouldings having a high toughness and strength, substantially as herein before described.

9. Articles or mouldings having a high toughness and strength made from powder according to any one of claims 1 to 4 or 8 or by a method according to any one of claims 5 to 7.