Phosphorus steel powder and a method of manufacturing the same
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     The present invention relates to phosphorous steel powder mixtures to be used within the powder metallurgy. In addition to iron and phosphorus these powder mixtures can contain other alloying elements common within this technique, such as copper, nickel, molybdenum, chromium and carbon. 
     2. DESCRIPTION OF THE PRIOR ART 
     The use of phosphorus as an alloying element within the powder metallurgy has been known since the 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 Pat. 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 sintered test 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, 1040° C and above that, 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 favorable sintering process which is provided by the liquid phase and the favorable 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. 
     SUMMARY OF THE INVENTION 
     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%. 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 phosphorus powder characterized by a maximum particle size of 20 μm, preferably a maximum particle size of 10 μm. 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 be less than 17%. If the ferrophosphorus powder is manufactured by grinding piece goods the phosphorus content shall exceed 12% and shall preferably be between 14 and 16%. The phosphorus content of the preferred mixture is between 0.2 and 1.5%. 
     In this case 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 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 alloying particles are brought 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 reducing atmosphere to a temperature of between 650° and 900° C 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 an increased content of the phosphorous powder. The concentrate so obtained can be mixed with the iron powder to provide for the desired phosphorus content in the final product. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plot of impact strength along the ordinate, vs. particle size of the phosphorus; 
     FIG. 2 is a plot of the standard deviation, plotted along the ordinate, for the plot of FIG. 1, vs. particle size of the phosphorus; 
     FIG. 3 is a plot of break elongation, plotted along the ordinate, vs. phosphorus particle size; and 
     FIG. 4 is a plot of the standard deviation, plotted along the ordinate, for the plot of FIG. 3, vs. phosphorus particle size. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 μm, 5-10 μm, 10-20 μm and 20-40 μm by means of a wind-sieve device. The different powder fractions were mixed with very pure iron powder having the maximal particle size of 150 μm. The phosphorus content of the mixture was 0.6 weight-% as this content has proved to give a distinct evidence as to the brittleness tendency of sintered materials. Seven impact strength test bars were manufactured from each mixture at 588 MPa. They were sintered in cracked ammonia at 1120° C 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 FIG. 1. The standard deviation (σ I ) 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 μm has the greatest toughness. The material having ferrophosphorus particles of greater size than 15 μm does however provide for brittle sintered details. 
     EXAMPLE 2 
     A ferrophorsphorus powder having a phosphorus content of 15.8 weight-% was divided in the size classes 0-5 μm, 5-10 μm and 10-40 μm by means of a wind-sieve device. The different powder fractions were mixed with very pure iron powder having the maximal particle size of 150 μm. The phosphorus content of the mixtures was 0.6 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 premanent strain after rupture (break elongation) (o), 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 (σ.sub.δ) 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 great 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 certain cases. The solution resides in the use of a ferrophosphorus powder having a particle size less than 20 μm, preferably less than 10 μm. We claim: