Process of manufacturing high-strength sintered members

To permit an economical manufacture of high-strength sintered members for use in valve timing mechanisms of internal combustion engine by powder metallurgy with liquid-phase sintering, an iron-base powder mixture is provided, which contains 13 to 18% by weight chromium or 3 to 6% by weight molybdenum as a carbide-forming alloying element in the iron alloy powder and also contains 1.5 to 2.6% carbon and 0.4 to 1.0% by weight phosphorus. A corresponding molten iron alloy is atomized into an entraining gas or water jet and is subsequently mixed with the remaining components of the powder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process of manufacturing high-strength sintered 
members having a hard-wearing surface, particularly for manufacturing such 
parts for use in the valve timing mechanisms of internal combustion 
engines, where at least that portion of said sintered member which is 
formed with said hard-wearing surface is formed in that a 
carbon-containing powder mixture, which comprises an iron alloy that 
contains at least one carbide-forming allowing element of the group VIa of 
the periodic system, is compacted to form a compact, which is then 
subjected to liquid-phase sintering. 
2. Description of the Prior Art 
In order to provide cams for use on a camshaft or other members of valve 
timing mechanisms of internal combustion engines, which cams or other 
members meet stringent requirements regarding their wear resistance and 
fatigue strength, it is known (EP-A-303 809) to make such members in that 
a powder mixture is compacted which comprises an iron alloy powder that 
contains carbide-forming alloying elements of the groups Vb and VIb of the 
periodic system, and graphite powder in the amount which is required for a 
formation of carbides. The compacts are then sintered at a temperature 
which slightly exceeds the solidus temperature and the compact which has 
thus been subjected to liquid-phase sintering is compacted to at least 99% 
of its theoretical density by isostatic hot pressing. A major disadvantage 
of that known process resides in that the isostatic hot pressing of the 
presintered compacts involves a considerable expenditure but such 
isostatic pressing is essential to ensure a uniform distribution of the 
carbides at the required density. Whereas sintering at a sintering 
temperature slightly above the solidus temperature permits a uniform 
distribution of carbides, this will be possible only with a comparatively 
high void ratio. Besides, it is not possible to use lower-melting alloying 
elements owing to the high temperatures which are required for hot 
pressing and such lower-melting elements would melt at the pressing 
temperatures and would then emerge through the still existing pores during 
the pressing operation. 
It is finally known from DE-A-3 907 886) that the cams of a camshaft may be 
made to have a hard-wearing external layer and a cam body by a 
powder-metallurgical process, which comprises liquid-phase sintering and 
in which the law compacts, which differ in their shrinking behavior, are 
fitted onto a steel shaft, so that a strong bond is obtained between the 
hard-wearing layer and the cam body and between the cam body and the steel 
shaft when the sintering has been performed. In that case the hard-wearing 
external layer is constituted by an iron-carbon-nickel-chromium-molybdenum 
alloy. But that alloy will not withstand high loads because, for instance, 
nickel cannot form carbides, which would be essential for a high wear 
resistance, and nickel-containing materials tend to form austenite so that 
the fatigue strength is reduced. 
SUMMARY OF THE INVENTION 
It is an object of the invention to make high-strength sintered members, 
particularly for use in the valve timing mechanisms of internal combustion 
engines, by a process which includes liquid-phase sintering whereas 
isostatic hot pressing is not required. 
In a process of the kind described first hereinbefore that object is 
accomplished in accordance with the invention in that the powder mixture 
contains 13 to 18% by weight chromium or 3 to 6% by weight of at least one 
component of the group consisting of molybdenum or of molybdenum and 
tungsten or contains corresponding amounts of said components, as a 
carbide-forming alloying constituent of that iron alloy powder and also 
contains 1.5 to 2.6% by weight added carbon and 0.4 to 1.0% by weight 
phosphorus, the iron alloy powder is produced in that a molten iron alloy 
is atomized into an entraining gas or water jet, and said iron alloy 
powder is subsequently mixed with the other components of the powder. 
The relatively high carbon content which is employed ensures a satisfactory 
formation of carbides and the formation of a liquid phase in a 
sufficiently large amount during the sintering. This is not only due to 
the carbon content but also to the fact that the addition of phosphorus 
results in a much lower sintering temperature so that a uniform 
distribution of carbides can be expected. The large proportion of the 
liquid phase will also ensure that the sintered member has the required 
density without a need for a subsequent hot pressing. 
Particularly desirable conditions will be established if 1.0 to 2.3% by 
weight tin and 15 to 20% by weight copper are incorporated in the powder 
mixture because the copper will bind a part of the carbon so that here 
will be no risk of a formation of cementite, which would decrease the 
fatique strength by a formation of cementite caused by the higher carbon 
content. Besides, the bronze phase which is composed of copper and tin 
will act as a lubricant so that the sliding friction of the sintered 
member will be reduced; the bronze phase will also tend to fill the pores 
during the sintering operation. 
To ensure that the distribution of carbides has a uniformity which will 
promote the wear resistance it is essential that the carbide-forming 
elements are also uniformly distributed in the powder mixture. To that end 
the carbide-forming element is included as an alloying element in the iron 
alloy powder, which is produced in that the molten alloy is atomized into 
an entraining jet of gas or water. If chromium is used as a 
carbide-forming alloying element, 0.7 up to 1.5% by silicon, preferably in 
the form of ferrosilicon, must be added to the molten iron alloy as a 
killing agent and to improve the atomization of the molten alloy. If 
molybdenum is used as a carbide-forming alloying element, silicon will 
desirably be replaced by up to 0.4% by weight manganese. 
In order to ensure that the iron powder can easily be compacted and that it 
has a sufficiently large particle surface area for the sintering 
operation, the iron alloy powder is required to consist of dendritic 
particles and at least 70% by weight of the powder particles are required 
to have an individual particle mean diameter of less than 50 .mu.m whereas 
the remaining particles of the powder should have an individual particle 
mean diameter not in excess of 100 .mu.m. In a powder having such a 
particle size composition an optimization can be achieved by the fact that 
the use of extremely fine powders will improve the sintering conditions 
because the interfacial area between individual powder particles will be 
increased and the remaining pores will be decreased in size and that a 
decreasing particle size will increase the cost of producing the powder. 
The carbon content may be provided by a powder that consists of natural 
graphite or electrographite and has an individual particle mean diameter 
not in excess of 5 .mu.m so that the carbon can be provided in the fine 
distribution which is required for the formation of carbides. The 
phosphorus, which together with the carbon is essential for the result to 
be produced in accordance with the invention, may be added to the molten 
iron alloy as ferrophosphorus and in that case will be atomized together 
with the molten iron alloy into the entraining gas or water jet. 
Alternatively the phosphorus may be admixed as a ferrophosphorus powder to 
the iron alloy powder, in the latter case each particle should have a mean 
diameter below 10 .mu.m. The admixing of a ferrophosphorus powder will 
result in a faster diffusion of the phosphorus into the iron matrix so 
that a formation of larger secondary pores by diffusing phosphorus will be 
prevented. 
The copper powder may desirably consist of electrolytic copper in the form 
of dendritic particles having an individual particle mean diameter not in 
excess of 5 .mu.m so that the copper together with the tin, which is 
required to have an individual particle mean diameter not in excess of 20 
.mu.m, will form a uniformly distributed bronze phase and segregation will 
be avoided. 
The molybdenum may be replaced by tungsten as a carbide-forming element and 
in that case a molybdenum-containing iron alloy powder may be replaced at 
a ratio of 1:2 by an iron alloy powder that contains 6 to 12% by weight 
tungsten. The content of tungsten used as an alloying element must not 
exceed 12% by weight so that compacts having a sufficiently high green 
strength will be obtained. But in addition to the iron alloy powder which 
contains up to 6% by weight molybdenum, the powder mixture may contain 1 
to 2% by weight tungsten powder so that the wear resistance will be 
improved further by tungsten carbides. 
It has been stated hereinbefore that a uniform distribution of the 
component powders in the powder mixture is of high importance. For this 
reason the copper and tin powders and optionally also the phosphorus 
powder may be admixed first to the iron alloy powder and the resulting 
mixture may then be mixed with the carbon powder to provide a master 
mixture, to which a conventional lubricant powder may be admixed. If 
mixing is effected in that sequence, a segregation particularly of the 
very fine carbon powder will effectively be prevented, as is required for 
a uniform distribution of carbides. 
The resulting powder mixture is then optionally granulated and under a 
compacting pressure between 700 and 800 MPa is compacted to form compacts 
having a density between 6.5 and 6.6 g/cm.sup.3 and is subsequently 
annealed so that the lubricant usually consisting of wax is removed from 
the compact and the oxygen content is decreased below a limit of 1800 ppm. 
That annealing can preferably be effected by a presintering, which is 
carried out at temperatures between 850.degree. and 950.degree. C. and 
which will increase the green strength. The density of the compact should 
not be in excess of 6.7 g/cm.sup.3 because the carbon monoxide produced by 
the sintering otherwise could not escape and would form blisters. A 
density of the compact below 6.4 g/cm.sup.3 will adversely affect the 
green strength.

EXAMPLE 1 
For a powder-metallurgical manufacture of the cams of a camshaft, an iron 
alloy powder which contained 6% by weight molybdenum and had been atomized 
into an entraining water jet was prepared. The powder had an individual 
particle mean diameter not in excess of 75 .mu.m. 70% by weight of said 
particles had an individual particle mean diameter below 50 .mu.m. After 
the powder had been atomized and had been reduced under a 
hydrogen-nitrogen atmosphere, the iron alloy powder was found still to 
contain about 5000 ppm oxygen. 0.45% by weight phosphorus in the form of a 
very fine ferrophosphorus powder, which contained 16% phosphorus and had 
an individual particle mean diameter below 10 .mu.m, was admixed to that 
iron alloy powder in a double-cone mixer for a mixing time of about 5 
minutes. In a succeeding mixing operation for 5 minutes, 1.85% by weight 
carbon was admixed in the form of a finely ground natural graphite powder 
having an individual particle mean diameter below 5 .mu.m. Before the 
succeeding compacting operation, 0.5% by weight wax as a lubricant for 
assisting the compacting was admixed to the stock mixture and the 
resulting powder mixture was then compacted under a pressure of 700 MPa to 
form compacts having a density of 6.5 g/cm.sup.3. Said compacts were 
reduced for 2 hours at a temperature of 950.degree. C. under a protective 
gas atmosphere of hydrogen and nitrogen at a ratio of 1:3. Thereafter it 
was found that the oxygen content amounted to 1500 ppm and the carbon 
content to 1.6% by weight. For liquid-phase sintering, the thus pretreated 
compacts were heated in a vacuum furnace at a sintering temperature of 
1075.degree. C. for 2 hours. 
The sintering in a vacuum furance might be replaced by a sintering in a 
belt conveyor furnace under a protective gas atmosphere composed of 
hydrogen and nitrogen at a ratio of 1:5 with high economy. 
The sintered compacts exhibited a shrinkage of about 7% and had 98% of 
their theoretical density. Hardness measurements revealed a hardness of 
HRC 42.+-.2. The molybdenum carbides were found to be very uniformly 
distributed in the iron matrix. The carbides were spherical and had 
diameters between 3 and 7 .mu.m so that a very high wear resistance was 
ensured. The remaining pores were also spherical and were not in excess of 
50 .mu.m in diameter so that a high fatigue strength was ensured. 
The hardening treatment succeeding the sintering operation may be performed 
in various ways, namely, by a hardening in the vacuum furnace used also 
for the sintering or in a belt conveyor furnace under a controlled 
atmosphere or by oil hardening. The hardened compacts had a hardness of 
HRC 63.+-.1, which after a tempering treatment at 550.degree. C. for 2 
hours had decreased to HRC 51.+-.1. The cams thus made had a high wear 
resistance and a high fatigue strength and also had a high retentivity of 
hardness. 
EXAMPLE 2 
For a manufacture of cams, a dendritic iron alloy powder was provided, 
which contained 18.0% chromium and had been atomized into an entraining 
water jet and which contained 0.9 to 1.1% silicon to improve the 
atomizing. Just as in the preceding example the stated percentages by 
weight are based on the total powder mixture. The particle size was the 
same as in Example 1. After a reduction under an atmosphere of nitrogen 
and hydrogen the oxygen content was found to amount to 2400 ppm. 
17.0% by weight electrolytic copper having an individual particle mean 
diameter below 5 .mu.m, 1.2% by weight tin powder having an individual 
particle mean diameter below 20 .mu.m, 2.5% by weight dendritic 
ferrophosphorus powder containing 16% phosphorus and having an individual 
particle mean diameter below 10 .mu.m, 2.6% by weight of a very fine 
graphite powder, 0.5% by weight wax as a compacting aid and 0.8% 
molybdenum powder to improve the through hardening were added to that iron 
alloy powder. Mixing was again effected in steps. The ferrophosphorus 
powder, copper, tin and molybdenum powders were first admixed to the iron 
alloy powder before the graphite powder and subsequently the wax powder 
were admixed. That powder mixture was compacted under a pressure of 800 
MPa to make compacts having a density of 6.6 g/cm.sup.3. The precompacted 
compacts were reduced at a temperature of 950.degree. C. under a 
protective gas atmosphere of hydrogen and nitrogen at a ratio of 1:15 for 
2 hours and were subsequently found to contain 1750 ppm oxygen and 2.5% by 
weight carbon. The compacts were subsequently sintered in a vacuum furnace 
at a temperature of 1080.degree. C. for two hours. Just as in Example 1 a 
pressure of 4.times.10.sup.-2 millibars was maintained in the vacuum 
furnace. Alternatively, sintering may be effected in a conveyor belt 
furnace under a protective gas atmosphere composed of hydrogen and 
nitrogen at a ratio of 3:10. The sintered members exhibited a shrinkage of 
about 5.5 to 6.0% and had a density of 97 to 98% of the theoretical 
density. A hardness of HRC 39.0.+-.1 was measured. Owing to the spherical 
chromium carbides having a size of 5 to 10 .mu.m the members had a very 
high wear resistance. The uniform distribution of the bronze phase 
composed of the copper and tin resulted in an excellent running-in 
behavior and in a low sliding friction. A segregation of copper was not 
detected. Hardening was effected in a vacuum furnace or in a conveyor belt 
furnace at 1040.degree. C. for one hour and increased the hardness to HRC 
54.+-.1. After a tempering at 550.degree. C. for 2 hours, a hardness of 
HRC 50.+-.1 was measured.