Method for producing ferrous sintered alloy having quenched structure

The present invention relates to a method for producing, without quench-hardening process, a ferrous sintered alloy having satisfactory strength which is equal to that of the conventional ferrous sintered quenched material, and the method comprises the steps of: preparing a powder mixture by adding, in weight ratios, 1 to 2% of copper powder, 1 to 3% of Ni powder, and graphite to a ferrous alloy powder consisting of 3 to 5% of Ni, 0.4 to 0.7% of Mo, and the remainder Fe, the quantity of said graphite being determined such that the C-content after sintering is 0.2 to 0.7%; compacting said powder mixture in a tool to form a green compact; sintering said green compact in a non-oxidizing atmosphere at a temperature in the range of 1130.degree. to 1230.degree. C.; and cooling the sintered product in the sintering furnace at a rate of 5.degree. C./min. to 20.degree. C./min.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to powder metallurgy and particularly to a 
method for producing a ferrous sintered alloy having an excellent 
strength, which alloy can be made without any quench-hardening treatment. 
2. Prior Art 
Because of the characteristic advantage in the cost efficiency of ferrous 
sintered alloys manufactured in accordance with the powder metallurgical 
method, the ferrous sintered alloy parts are widely employed in the fields 
of, for example, automobiles, machine tools, household electrical 
appliances and so forth. Even in the circumstances like this, however, the 
reduction of production cost is required also in sintered metal parts in 
order to cope with the recent tendency toward the lowering of prices of a 
variety of industrial products. 
In order to meet the above needs, inexpensive iron powder or the like 
materials are being developed. There is, however, a problem that 
characteristics of materials are degraded. Furthermore, the cost reduction 
is intended by employing continuous and automated manufacturing process or 
robotized process, but any satisfactory result has not yet been attained 
in this respect. 
In powder metallurgical parts of which high strength is required, treatment 
of hardening is applied to the obtained parts after the compacting and 
sintering step. If a product having better characteristics as compared 
with those of conventional ferrous sintered products is obtained without 
employing the quench-hardening step, it may be possible to reduce the 
production costs largely. In addition to this, it is also possible to 
avoid the decrease in dimensional precision which is caused to occur in 
the quenching step. As a measure to obtain high-strength parts without 
employing the quenching step, there is proposed a method for producing 
sintered parts in which an alloy powder of a good hardening property is 
used and the sintered material is subjecting to martensitic transformation 
at the cooling rate of sintering. However, the cooling rate in an ordinary 
sintering furnace is 5.degree. to 20.degree. C./min., and for obtaining a 
martensitic structure at this cooling temperature, the amount of alloy 
elements must be increased inevitably, which results in the lowering of 
compressibility markedly. As a consequence, the strength of obtained 
material is lower than that of a conventional ferrous sintered quenched 
material. 
Meanwhile, the material which is prepared by adding the powder of Ni, Cu, 
or Mo to improve the hardenability, to a pure iron powder or diffusion 
bonded powder which is prepared from the these raw materials, is excellent 
in compressibility. However, because the alloy components in the sintered 
products of these materials are uneven, only a part of the micro structure 
is changed into a martensitic structure. In this case, however, for 
changing the micro structure into the martensitic structure as much as 85% 
or more, it is necessary to sinter the raw material at a temperature above 
1250.degree. C. so as to diffuse the added elements. Thus, problems are 
brought about not only in that the cost for sintering process is raised 
with the economical disadvantage but also in that the dimensional accuracy 
is not satisfactory. 
BRIEF SUMMARY OF THE INVENTION 
The present invention has been accomplished in view of the above-described 
status of art. 
It is, therefore, the object of the present invention to provide a novel 
method for producing, without quenching, a ferrous sintered alloy having 
strength which can be compared with those of the conventional ferrous 
sintered quenched materials. 
In order to attain the above object, a variety of studies has been carried 
out for improving the hardenability and minimizing the lowering of 
compressibility of ferrous sintered alloy to be produced. 
As a result, the present inventors have found out that an improved ferrous 
sintered alloy can be prepared by adding the powder of single element to 
improve hardenability to an alloy powder which has the compressibility 
equal to that of the conventional ferrous sintered material. In this 
invention, the lowering of strength due to the lowering of compressibility 
is suppressed and 85% or more of micro structure is transformed into 
martensitic phase with the remainder of bainitic phase at a cooling rate 
of 5.degree. to 20.degree. C./min. in an ordinary sintering furnace. More 
specifically, the method for producing a ferrous sintered alloy according 
to the present invention is characterized by the steps of compacting in a 
tool a mixed powder prepared by adding 1 to 2% of copper powder, 1 to 3% 
of Ni powder, and graphite in such an amount that the C-content after 
sintering is 0.2 to 0.7% to an alloy powder composition consisting of 3 to 
5% of Ni, 0.4 to 0.7% of Mo, and the remainder Fe to form a green compact; 
sintering the green compact in a non-oxidizing atmosphere at a temperature 
within the range of 1130.degree. to 1230.degree. C.; and cooling the 
sintered product in the sintering furnace at a rate in the range of from 
5.degree. C./min. to 20.degree. C./min. It is to be noted that the 
percentage (%) herein used are "percent by weight" unless otherwise 
expressed.

DETAILED DESCRIPTION OF THE INVENTION 
When an alloy powder is used as the principal component of a mixed powder 
and an element or elements improving hardenability are added singly, it is 
easier to obtain a sintered alloy having a high compressibility and a high 
density in comparison with the use of a wholly alloyed powder. However, 
when the content of alloy elements in a ferrous alloy powder is less than 
the prescribed amount or when Ni powder, Mo powder, and Cu powder are 
added to pure iron powder so as to obtain prescribed composition of a 
sintered alloy, it is difficult to obtain a sintered alloy having the 
aimed quenched structure. 
As the alloy elements to be added to the ferrous alloy powder, Ni and Mo 
are preferable which are effective in improving the hardenability and 
which hardly worsen the compressibility. The contents of them to be added 
depend upon the hardenability and the compressibility of material. With 3 
to 5% of Ni and 0.4 to 0.7% of Mo, a product which has a higher 
compressibility as compared with the product of the conventional alloy 
powder of 6.7 g/cm.sup.3 or more in green density at 6 t/cm.sup.2 in 
compacting pressure, can be obtained. When the quantity of the alloy 
elements exceeds the above described range, the compressibility and the 
strength of material become worse. On the contrary, when the content of 
the alloy elements is less than the above described lower limit, it is not 
possible to transform 85% or more of micro structure into martensitic 
phase even when the powder of a single element for improving hardenability 
is added, so that the strength of material is lowered. 
When only the graphite is added to this alloy powder, only the bainitic 
structure is obtained. It is necessary to improve further the 
hardenability in order to form more than 85% of martensitic phase, so that 
it is required to add the element for improving hardenability. Such 
elements are exemplified by Cu, Ni, Mn, and Cr. In view of the sintering 
property, Cu and Ni are effective to improve the hardenability. If the 
amount of Cu added is less than 1%, its effect is not recognized. On the 
other hand, if it exceeds 2%, the impact resistance is lowered. 
Accordingly, the addition quantity of Cu is specified within the range of 
1 to 2%. Furthermore, Ni has an effect to suppress the embrittlement due 
to Cu in addition to the effect to improve hardenability. When the 
addition quantity of Ni is less than 1%, its effect cannot be recognized, 
while if it exceeds 3%, the martensitic phase is rather decreased because 
of the existence of austenitic phase in which Ni is concentrated and there 
occurs a tendency of the lowering of strength, so that it is preferred to 
specify the value of Ni within the range of 1 to 3%. 
A cooling rate in the sintering step is determined by a CCT (continuous 
cooling transformation) diagram of the material. It is specified as a 
value 5.degree. C./min. or higher so that the martensite phase occupies 
85% or more of the grain structure in an area ratio. If the cooling rate 
exceeds 20.degree. C./min., an additional cooling device is required which 
increases the cost for sintering, so that the appropriate value of cooling 
rate is preferred in the range of from 5.degree. C./min. to 20.degree. 
C./min. 
Although there are several measures to add carbon (C) such as the addition 
in the form of graphite and with the use of carburizing gas in a sintering 
atmosphere, it is necessary for adding the carbon as graphite in order to 
obtain a uniform martensitic structure throughout the material. The 
quantity of graphite to be added is determined such that the C-content 
after sintering is within the range of from 0.2 to 0.7%. If the C-content 
after sintering is less than 0.2%, it is impossible to obtain 85% of 
martensite in the area ratio of the sintered alloy, while if it exceeds 
0.7%, remaining austenite phase increases, and further cementite 
precipitates along the grain boundaries resulting in the lowering of 
strength. Therefore, the content of C after sintering must be within the 
range of from 0.2 to 0.7%. 
Because the sintering operation is carried out after the addition of 
graphite, the content of C after the sintering decreases to some extent in 
comparison with the quantity before the sintering. The actual addition 
quantity of graphite was 0.4 to 0.8% in the case of the reducing 
atmosphere with dissociated ammonia gas that was carried out by the 
present inventors. The decrease of C-content depends upon the used powder, 
sintering conditions and so forth, it is necessary to confirm the 
preferable quantity by means of experiments, with calculating the addition 
quantity in view of an aimed content of C. 
In 100% of sintered structure other than the pores, if 85% to 97% of 
structure is a martensitic phase and the remainder is a bainitic 
structure, the strength of the sintered alloy is equal to that of 
conventional ferrous sintered material which was prepared with quenching 
treatment. In addition, because 3% or more bainitic structure is 
dispersed, the material excels in toughness. However, when the area ratio 
of bainitic structure exceeds 15%, the strength of the resulting ferrous 
sintered alloy decreases. Accordingly, the area ratio is kept within the 
range of 3% to 15%. 
Moreover, when a sintered material is maintained at a temperature within 
the range of 150.degree. C. to 300.degree. C., martensitic structure is 
converted into tempered martensite to raise the toughness, so that the 
strength of the material is further improved. Furthermore, because the 
structure is stabilized, it is possible to produce the effect to suppress 
changes, particularly the change in dimensions with the passage of time. 
As the measure to maintain the temperature in the range of 150.degree. C. 
to 300.degree. C., there is a method in which the material after sintering 
is once cooled to the room temperature, and it is then heated again in a 
tempering furnace. In another method, the sintered material is cooled not 
to room temperature but to about 100.degree. C. and it is transferred into 
a tempering furnace to be reheated, so that the saving of energy 
consumption can be attained. Moreover, in certain sintering heat patterns, 
the temperature of materials is directly changed into the range of 
150.degree. C. to 300.degree. C. without cooling the sintering furnace 
below 100.degree. C., thus the isothermal transformation is accelerated, 
remained austenite is transformed into bainite, and the martensite is 
tempered to give high toughness. According to the method described above, 
it is possible to achieve the cost reduction with the cutting down of 
process steps. 
The time for maintaining materials in the above described temperature range 
is preferably from the maximum thickness (mm).times.0.05 to 0.10 hours or 
so. 
In summary, as described above, the present invention is characterized by 
the steps of: compacting a powder mixture prepared by incorporating 
specified quantities of Ni powder, Cu powder, and graphite powder with a 
ferrous alloy powder containing Ni and Mo in a specific composition to 
form a green compact; sintering the thus obtained green compact at a 
temperature in the range of 1130.degree. to 1230.degree. C.; and cooling 
the sintered product in the sintering furnace at a specific cooling rate, 
thereby obtaining a ferrous sintered alloy which is excellent in strength 
and which has a specific quenched structure. 
The present invention will now be described in more detail with reference 
to several examples, in which the percentages and ratios of the component 
materials are those on weight bases unless otherwise indicated. 
EXAMPLE 1 
To each of Fe--Ni--Mo alloy powders of 11 kind chemical compositions shown 
in Table 1, was added 1% of copper powder and Ni powder. The quantity of 
Ni powder was so determined that the total Ni content was 6%. Then, 
graphite powder was added in the amount that the C-content after sintering 
was adjusted to 0.5%. Furthermore, 0.8% of zinc stearate powder was added 
as a lubricant, and they were mixed together for 30 minutes. 
The respective powder mixtures were subjected to compacting at 600 MPa. The 
densities of obtained green compacts are shown in FIGS. 1 and 3. 
Furthermore, the area ratios of martensitic phase in micro structures and 
the bending strengths of sintered products which were obtained by 
sintering in dissociated ammonia gas at 1200.degree. C. for 60 minutes and 
then cooled at a rate of 10.degree. C./min., are shown in FIGS. 2 and 4. 
As is apparent from the above described results, the compressibilities of 
Sample No. 4, No. 5, No. 7, No. 8, and No. 10 of the sintered materials of 
the present invention are superior to those of the sintered materials 
prepared from the conventional alloy powders. Furthermore, because the 
area ratios of martensitic phase in the Samples of the present invention 
are high, sintered metal products of high strength can be produced. 
TABLE 1 
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Sample No. Ni (%) Mo (%) 
______________________________________ 
1 0 0.5 
2 1 0.5 
3 2 0.5 
4 3 0.5 
5 4 0.5 
6 4 0.2 
7 4 0.4 
8 4 0.7 
9 4 0.8 
10 5 0.5 
11 6 0.5 
______________________________________ 
Note: Underlined Sample Nos. are test examples according to the present 
invention and the others are comparative examples. 
EXAMPLE 2 
To the Fe--Ni--Mo alloy powder of Sample No. 5 in Example 1 was added 1% of 
copper powder, graphite powder and Ni powder. The quantity of Ni powder 
was so determined that the total Ni content was 6%. The quantity of 
graphite powder was such that the C-content after sintering was 0.5%. 
Furthermore, by adding 0.8% of zinc stearate powder as a lubricant, they 
were mixed together for 30 minutes. 
The mixed powder was then subjected to compacting at 600 MPa to form green 
compacts, and they were sintered in dissociated ammonia gas at 
1200.degree. C. for 60 minutes. The sintered products were cooled at 
varied cooling rates of 3.degree., 6.degree., 10.degree., and 25.degree. 
C./min. so as to obtain the products with varied area ratios of 
martensite. The bending strengths and impact values of the thus obtained 
products were measured, the results of which are shown in FIG. 5. 
As will be understood in view of FIG. 5, if the area ratio of martensite is 
higher, the bending strength are also higher, however, even when the area 
ratio is raised to 85% or more, the bending strength is not increased so 
much. On the other hand, the impact value is lowered with the raise of 
area ratio of martensite. Because in the sintered material according to 
the present invention, the portion other than martensitic phase is 
composed of bainitic phase, it is excellent in strength and toughness. In 
a material with 80% area ratio of martensite, however, perlitic phase is 
observed, the bending strength of which is low. Consequently, in the 
sintered material according to the present invention, the area ratio of 
martensitic phase is in the range of 85% to 97%, and the part other than 
the martensitic phase is composed of bainitic phase, which material has 
excellent strength and toughness. 
EXAMPLE 3 
To the Fe--Ni--Mo alloy powder of Sample No. 5 used in Example 1 were added 
Ni powder, Cu powder, and graphite powder in weight ratios as shown in 
Table 2, and they were mixed together for 30 minutes. The obtained 
admixtures were subjected to compacting at 600 MPa to form green compacts. 
The green compacts were then sintered at 1200.degree. C. in dissociated 
ammonia gas for 60 minutes, and they were cooled at a rate of 10.degree. 
C./min. Concerning the sintered products, the bending strengths and the 
impact values were measured. 
For comparison purpose, a diffusion bonded powder (Sample No. 23) 
consisting of 4% Ni, 1.5% Cu, 0.5% Mo, and the remainder Fe was used for 
compacting and sintering under similar conditions and the determination of 
properties were done just like the above. By the way, after Sample No. 13 
was cooled to a room temperature, it was heated again at 180.degree. C. 
for 60 min. and properties were determined again as Sample No. 14. The 
results in the evaluations are shown in Table 2. 
TABLE 2 
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C-Content 
Area Ratio 
Powder Composition (%) 
after 
Bending 
of 
Sample 
Ni Cu Graphite 
Sintering 
Strength 
Martensite 
Impact Value 
No. Powder 
Powder 
Powder 
(%) (MPa) 
(%) (J/cm.sup.2) 
__________________________________________________________________________ 
12 0 1 0.6 0.50 1255 
47 12 
13 2 1 0.6 0.50 1671 
95 18 
14 2 1 0.6 0.50 1809 
95 20 
15 4 1 0.6 0.50 1200 
78 19 
16 2 0 0.6 0.50 1272 
83 16 
17 2 3 0.6 0.50 1600 
99 10 
18 2 1 0.4 0.28 1558 
85 19 
19 2 1 0.3 0.18 1000 
60 20 
20 2 1 0.7 0.56 1569 
90 19 
21 2 1 0.8 0.66 1346 
86 19 
22 2 1 0.9 0.74 993 
68 20 
23 (*) 0.6 0.50 1132 
40 27 
__________________________________________________________________________ 
Note: 
Sample No. 14 was prepared by taking out the sintered sample No. 13 and i 
was then heated at 180.degree. C. 
(*): In Sample No. 23, diffusion bonded powder of Fe4%Ni-0.5%Mo 1.5%Cu wa 
used 
As is understood from the results in Table 2, when Ni-content is low, the 
area ratio of martensite decreases so that the strength is lowered. 
Furthermore, because the effect to suppress the brittleness with Cu is 
lowered, the impact value is lowered. On the contrary, when Ni-content is 
excess, the area ratio of austenitic phase increases with the lowering of 
strength. 
When the content of Cu is low, the area ratio of martensite decreases with 
the lowering of strength. If the content of Cu is too high, the impact 
value is low. 
When the C-content is low, the area ratio of martensite is low to form a 
perlitic phase, so that the strength becomes low. If C-content is too 
high, cementite precipitates in the grain boundary, so that the strength 
is also lowered. The sintered material prepared according to the present 
invention has higher bending strength and higher impact value as compared 
with those of the conventional ferrous sintered materials. In addition, 
when the material of the invention is heated again at 180.degree. C., the 
mechanical properties thereof are much improved. 
As described above, the method according to the present invention is 
suitable for producing a ferrous sintered alloy having a specific quenched 
structure. The method comprises the steps of preparing a powder by 
incorporating specified quantities of Ni powder, Cu powder, and graphite 
powder with a ferrous alloy powder containing Ni band Mo of a specific 
composition, compacting the obtained mixture to form green compacts, 
sintering the green compact at a temperature in the range of 1130.degree. 
to 1230.degree. C., and then cooling the obtained sintered product in a 
sintering furnace at a specified cooling rate, thereby forming a ferrous 
sintered alloy having a quenched structure. The ferrous sintered alloy of 
the invention has good compressibility and excellent mechanical strength 
without applying any specific quench-hardening step. 
Accordingly, there is an advantage to produce a variety of mechanical 
elements at low cost and the field of uses of the sintered material can be 
enlarged. 
It will be appreciated by those of ordinary skill in the art that the 
present invention can be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. 
The presently disclosed embodiments are therefore considered in all 
respects to be illustrative and not restrictive of the invention.