Note: Descriptions are shown in the official language in which they were submitted.
1~:97~438
The present invention relates to phosphorus steel powder mixtures to
be used in powder metallurgy. In addition to iron and phospho~s 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 ~ i
non-alloyed sintered steel. Already at an early date mixtures of pure iron
powder and ferrophosphorus powder were used in powder metallurgy. 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 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 t~e advantage of phosphorus
alloyed sintered steels is high strength in combination with very good strain
characteristics, 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
of 1040C and 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
sintering process which is provided by the liquid phase and the favourable
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distribution of the phosphorus in turn providing for a rapid difusion thereof
which is obtained because of the fact that the ferrophosphorus provides for a
liquid phase~
ThusJ the object o~ the present in~ention is to solve said problems
with regard to the brittleness of sintered steel manufactured from a mixture
of iTOn 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 low content of impurities, especially
împurities sensitive to oxidation. A further improvement can be obtained if
the ferrophosphoxus powder also has a small maximum particle size.
Accordingly, the invention provides a phosphorus steel powder for
manufacturing sintered articles having high toughness, consisting of a steel
powder substantially free from phosphorus and having good compressibility,
which is intimately mixtured with ferrophosphorus powder having a phosphorus
content exceeding 2.8 percent by weight in such an amount that the phosphorus
content of the mixture is 0.2 to 1.5 %, wherein the total content of impurities
which at the sintering temperature, are more easily oxidized than the main
components iron and phosphorus does not exceed 4 %, and the ferrophosphvrus
powder has a maximum particle size of 20 ~ m.
Thus, a phosphorus steel powdeT according to the invention for
manufacturing sintered articles having an extremely small ~endency to
brittleness ruptures consists of steel powder substantially free rom
phosphorus~ mixed with a phosphorus powder containing in all less than 4 %,
preferably less than 3 %, of impurities which, at the sintering temperature,
are more easily oxidized than the main components iron and phosphorus.
~Irthermore, the particles of the ferrophosphorus powder shall have a
maximum size of 20 ~m, preferably a maximum size of 10 ~m. The phosphorus
content of the ferrophosphorus powdar shall exceed 2.8 % and in order to
reduce the wearing of the tools the phosphorus content should preferably be
less than 17 %~ If the ferrophosphorus powder is manufactured by grinding
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piece goods the phosphorus content should exceed 12 % and shall preferably
be between 14 and 16 %~ The phosphorus content of the mixture is from 0.2
to 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 af~er
the mixing operation, 50 - 200 g of a light mineral oil per metric ton of
powder can be added during the mixing operation~ Thereby the fine alloying
particles are caused to adhere to the coarser iron powder particles. :
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In order to improve the protection against segregation, the iron-
ferrophosphorus mixture is heated, with or without the addition of oil, in a
roducing atmosphere to a temperature of between 650 and 900C for a period
of 15 minutes to 2 hours. rhereby, the powder is loosely sintered together
so that subsequently a cautious disintegration has to be carried out in order
to restore the original particle size. rhe 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 on a mixture having an increased content of the phosphorus powder.
rhe concentrate so obtained can be mixed with the iron powder to provide for
the desired phosphorus content in the final product.
The critical contents of the impurities appear from the following
examples, and from Figures 1 - 4 inclusive, each of which is a graphical
representation of a feature of the invention.
Example 1
Three melts of iron-phosphorus including 15.5 - 16.5 % phosphorus
and controlled contents of silicon of 0.02, 0.17, 0.75 and 4.81 % and
additional impurity contents of ~ 0.01 % were manufactured and were allowed
to solidiy. rhereupon, they were ground to a powder from which two size
classes were taken out, 0 - 10 ~m and 10 - 40 ~m. These phosphorous powders
were mixed with extremely pure iron powder so that the mixtures got a
phosphorus content of 0.6 %, whereupon the mixture was compressed to make
impact strength test bars without indications of fracture having a size of
55 x 10 z 10 mm. The bars were sintered in cracked ammonia at 1120C for
1 hour. The impact strength was tested at room temperature by means of a
Charpy pendulum hammer. rhe result is shown in Figure 1 wherein the impact
strength (1) relates to -the mean value including the standard deviation for
seven bars.
rrhe curves clearly show the advantage of the phosphorus powder
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having partly a small particle size and partly a low silicon content. The
silicon content should be less than 0.5 %, preferably less than 0.2 %, for
giving the impact strength a stable high value. Ilowever, the silicon content
should not be too low and should exceed 0.05 %. Preferably it should exceed
0.1 %. ,~
Example 2
Iron-phosphorus alloying powder having aluminium as the only impurity
element was manufactured in the same way as in the preceding Example.
Three different contents of aluminium were used: 0.015, 0.03, 0.8 and 4.8 %.
Also powders have two different particle sizes, namely 0 - 10~um and 10 - 40
,um, were manufactured. The further treatment and the return of the results `~
are the same as according to Example 1, see Figure 2.
The same conclusion concerning the particle size can be drawn from
this example as from Example 1. Also, according to this example, the
toughness is better when the impurity contents are low. A suitable maximum
content of aluminium in the iron-phosphorus-alloying powder is 3 %, preferably
2 %, and a suitahle minimum aluminium content is 0.02 %.
Example 3
The same tests as according to the above Example were conducted with ;~
iron-phosphorus-alloys, this time having manganese as the only impurity
element with~a content of 0.01, 0.07, 0.68 and 5.0 %. The phosphorus
content varied between 17.2 and 17.5 %. The result appears from Figure 3.
Once more the Example shows the importance of a small particle size
of the iron-phosphorus alloying powder. Furthermore, the manganese content
should be less than 0.25 %, preferably less ~han 0.15 %, and higher than
0.03 %, preferably higher than 0.05 %.
Example 4
The same tests as according to the above examples were conducted.
The phosphorus con~ent of the iron-phosphorus powders was 16.7 - 17.6 %
while the only impurity element this time was titanium in the amounts of
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0.01, 0.02, 1.0 and 4.4 %. The result appears from Figure 4.
Also this Example shows, even if not as striking as the previous
examples, that the particle size of the iron-phosphol~ls-powder should be low.
Also the content of titanium should be relatively low, less than 3 %,
preferably less than 2 %. If the content of titanium is lo~ered too much,
the hrittleness phenomenon appears again, for which reason this content
should exceed 0.02 %J preferably exceed 0.05 %. rhe following Example shows
this fact even more clearly.
Example 5
An iron-phosphorus alloy was manufactured by melting extremely pure
raw materials Cthe same as used according to the previous Examples). No
artificial impurity elements were added. The alloy was of the following
compositio~: 17.4 % P, 0.02 % Si, ~ 0.03 % Al, 0.01 % Mg, 0.01 % Ti, balance
Fe. The alloy was crushed, ground and screened to a powder having a particle
size partly less than 10 um, partly between 10 - 40 ,um. The iron-phosphorus
powder was mixed with the same pure iron powder as according to previous
examples to a phosphorus content of 0.6 ~. Impact strength test bars were
pressed from the powder mixture, and the bars were sintered in cracked
ammonia at 1120C for a period of 1 hour. The impact strength of the
sintered bars was tested according to Charpy. ~hen the particle size of the
iron-phosphorus powder was less than 10 ,um the mean value of the impact
strength ~or seven test bars was 1.6 kpm (15.7 J) and the standard deviation
was 0.8 kpm ~7.8 J). The corresponding values for the case of the added iron-
phosphorus powder having a particle size between 10 and 40 um were 0.6 kpm
C5.9 J) and 0.4 kpm (3.9 J), respectively.
This example evidently shows that the brittleness risk in connection
~ith phosphorous sintered steel manufactured from a mixture of iron-phosphorus
powder and iron powder is great when using extremely pure iron-phosphorus
material. Therefore, the total content of impurities which are more easily
oxidized than iron and phosphorus at the sintering temperature should exceed
0.1 %.
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Thus~ the present invention represents a solution of the problem of
brittleness ruptures sometimes appearing in sintered steel manufactured from
a mixture of iron powder and ferrophosphorus powder. The solution resides in
the :Eact that the Eerrophosphorus powder shall have a content of impurities
oxidizable at the sintering conditions which is as low as possible. The total
content of these impurities is 4 % and these limits have been defined for
allowing contents of certain, especially sensitive impurities.
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