Method of producing a mechanical component with superior fatigue strength

A method of producing a mechanical component with superior fatigue strength comprises the steps of casting melted ferrous alloy which includes carbon within the range of 0.3% to 0.45% by weight, silicon within the range of 1.3% to 2.0% by weight, chromium within the range of 5.0% to 6.0% by weight, molybdenum within the range of 1.0% to 1.5% by weight, vanadium within the range of 0.8% to 1.2% by weight, manganese not more than 0.5% by weight, and iron of the major remainder, into an untreated mechanical component by the use of a casting mold formed through a lost-wax process; annealing the untreated mechanical component to produce an unfinished mechanical component; machining roughly the unfinished mechanical component having been annealed; quenching and tempering in sequence the unfinished mechanical component having been roughly machined; and finishing the unfinished mechanical component having been quenched and tempered to obtain a finished mechanical component.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing a mechanical 
component with superior fatigue strength, such as a rocker arm used in a 
valve operating mechanism of an engine, through a process of casting 
ferrous alloy into an untreated mechanical component. 
2. Description of the Prior Art 
In the field of an overhead-camshaft engine, there has been proposed to use 
a roller rocker arm assembly which comprises a swingable rocker arm having 
a roller mounted thereon for transmitting rotary movements of each of cams 
provided on a camshaft to an intake or exhaust valve with the object of 
reducing friction resistance in a valve operating mechanism and thereby 
improving fuel consumption. In the valve operating mechanism which 
contains the roller rocker arm assembly and is provided in the 
overhead-camshaft engine, one end portion of the rocker arm constituting 
the roller rocker arm assembly is supported by a supporting portion 
provided on a cylinder head, such as a concave acceptor formed at the top 
of a hydraulic lash adjustor (HLA) fixed to the cylinder head, the other 
end portion of the rocker arm comes into contact with the top of a stem of 
the intake or exhaust valve, and the roller which is mounted on the middle 
portion of the rocker arm comes into contact with the cam provided on the 
camshaft. When the engine is operating, the roller of the roller rocker 
arm assembly is pushed down and rotated by the cam in accordance with the 
rotation of the camshaft and thereby the rocker arm is swung with a 
fulcrum positioned at the supporting portion provided on the cylinder head 
to transmit rotary movements of the cam to the intake or exhaust valve. 
The rocker arm which constitutes the roller rocker arm assembly used as 
mentioned above in the valve operating mechanism of the overhead-camshaft 
engine is preferably required to have, as its mechanical properties, 
superior fatigue strength in its entirety, superior abrasion resistance 
particularly at the end portion thereof supported by the concave acceptor 
formed at the top of the HLA and another end portion thereof caused to 
come into contact with the top of the stem of the intake or exhaust valve, 
and superior accuracy in its dimension. Consequently, it has been also 
proposed to make the rocker arm of cement steel or ferrous alloy with 
superior abrasion resistance, such as standardized by the Japanese 
Industrial Standard JIS - G 4404 to be referred to as SKD61, through a 
lost-wax casting process. 
However, when the rocker arm is made of cement steel through the lost-wax 
casting process, it is feared that melted cement steel cannot flow 
smoothly in a casting mold formed through a lost-wax process in process of 
casting so that a rocker arm obtained through a casting process is 
provided therein with a number of ingot pipings and thereby deteriorated 
in mechanical strength. For the purpose of preventing the rocker arm from 
having the ingot pipings, it is considered to carry out dead head in the 
casting process. However, in the case of the casting process for obtaining 
relative small casting works such as the rocker arms, generally, a 
plurality of casting molds are provided to be supplied with melted 
metallic material for casting simultaneously so as to produce a plurality 
of casting works at the same time, and therefore it is practically 
difficult to make the dead head effective actually to each of the casting 
molds. 
Further, when the rocker arm is made of ferrous alloy such as standardized 
to be referred to as SKD61 through the lost-wax casting process, it is 
feared that a rocker arm obtained through casting process is provided with 
silicon-inclusions, oxygen-inclusions or the like caused to arise in grain 
boundaries contained in a part of a matrix structure of the ferrous alloy 
close to the surface thereof and thereby comes to fatigue failure or 
fatigue fracture resulted from the silicon-inclusions, oxygen-inclusions 
or the like positioned to be close to the surface when external force acts 
on the rocker arm. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of producing a mechanical component with superior fatigue strength, which 
avoids the aforementioned disadvantages and problems encountered with the 
prior art. 
Another object of the present invention is to provide a method of producing 
a mechanical component with superior fatigue strength, by which a 
mechanical component which has superior fatigue strength and superior 
abrasion resistance and is prevented from being provided therein with 
ingot pipings and various inclusions arising in grain boundaries contained 
in a part of a matrix structure close to the surface thereof, is made of 
ferrous alloy with superior abrasion resistance through a lost-wax casting 
process. 
A further object of the present invention is to provide a method of 
producing a rocker arm used in a valve operating mechanism of an engine, 
by which a rocker arm with superior fatigue strength and superior abrasion 
resistance is made of ferrous alloy with superior abrasion resistance 
through a lost-wax casting process without being provided therein with 
ingot pipings and various inclusions arising in grain boundaries contained 
in a part of a matrix structure close to the surface thereof. 
According to the present invention, there is provided a method of producing 
a mechanical component with superior fatigue strength, the method 
comprising the steps of casting melted ferrous alloy which includes carbon 
within the range of 0.3% to 0.45% by weight, silicon within the range of 
1.3% to 2.0% by weight, chromium within the range of 5.0% to 6.0% by 
weight, molybdenum within the range of 1.0% to 1.5% by weight, vanadium 
within the range of 0.8% to 1.2% by weight, manganese not more than 0.5% 
by weight, and iron of the major remainder, into an untreated mechanical 
component by the use of a casting mold formed through a lost-wax process; 
annealing the untreated mechanical component obtained by casting to 
produce an unfinished mechanical component; machining roughly the 
unfinished mechanical component having been annealed; quenching and 
tempering in sequence the unfinished mechanical component having been 
roughly machined; and finishing the unfinished mechanical component having 
been quenched and tempered in sequence to obtain a finished mechanical 
component. 
In the method of the present invention as described above, the reason why 
the ferrous alloy used for casting the untreated mechanical component is 
selected to include carbon within the range of 0.3% to 0.45% by weight, 
silicon within the range of 1.3% to 2.0% by weight, chromium within the 
range of 5.0% to 6.0% by weight, molybdenum within the range of 1.0% to 
1.5% by weight, vanadium within the range of 0.8% to 1.2% by weight, 
manganese not more than 0.5% by weight, and iron of the major remainder is 
explained as follows. 
Carbon contributes to causing the melted ferrous alloy to flow smoothly in 
the casting mold in the process of casting and to improvement in hardness 
of the unfinished mechanical component having been subjected to the heat 
treatments. As a result of an experiment achieved by the inventors for 
determining a desirable carbon content in the ferrous alloy in 
consideration of the above mentioned character of carbon, it has been 
ascertained that the melted ferrous alloy does not flow smoothly in the 
casting mold so that the untreated mechanical component obtained by 
casting is provided therein with a number of ingot pipings and thereby 
deteriorated in mechanical strength and further the unfinished mechanical 
component having been quenched and tempered in sequence comes to be 
relatively low in hardness and inferior in abrasion resistance when the 
carbon content is less than 0.3% by weight, and it has been also 
ascertained that carbides which are crystallized in the ferrous alloy in 
the process of casting are coarsened so that the untreated mechanical 
component obtained by casting is reduced in toughness and insufficient in 
fatigue strength when the carbon content is more than 0.45% by weight. 
Accordingly, the carbon content in the ferrous alloy has been determined 
within the range of 0.3% by weight to 0.45% by weight. 
Silicon acts as deoxdizing agent to the melted ferrous alloy and 
contributes to causing the melted ferrous alloy to flow smoothly in the 
casting mold in the process of casting and reinforcing matrix structure of 
the ferrous alloy in the untreated mechanical component. As a result of an 
experiment achieved by the inventors for determining a desirable silicon 
content in the ferrous alloy in consideration of the above mentioned 
character of silicon, it has been ascertained that the melted ferrous 
alloy is not effectively deoxidized and does not flow smoothly in the 
casting mold in the process of casting so that the untreated mechanical 
component obtained by casting is provided therein with oxygen-inclusions 
caused to arise in grain boundaries contained in a part of the matrix 
structure close to the surface thereof and thereby is reduced in fatigue 
strength when the silicon content is less than 1.3% by weight, and it has 
been also ascertained that grains in the matrix structure of the ferrous 
alloy in the untreated mechanical component obtained by casting are 
coarsened so that the untreated mechanical component is reduced in both 
toughness and hardness when the silicon content is more than 2.0 % by 
weight. Accordingly, the silicon content in the ferrous alloy has been 
determined within the range of 1.3% by weight to 2.0% by weight. 
Chromium contributes to improvement in hardenability of the unfinished 
mechanical component and acts with carbon to produce carbides by which the 
untreated mechanical component obtained by casing is increased in abrasion 
resistance. As a result of an experiment achieved by the inventors for 
determining a desirable chromium content in the ferrous alloy in 
consideration of the above mentioned character of chromium, it has been 
ascertained that the untreated mechanical component obtained by casting is 
not sufficient in hardness when the chromium content is less than 5.0% by 
weight and carbides contained in the untreated mechanical component are 
increased so that the untreated mechanical component is reduced in 
toughness when the chromium content is more than 6.0% by weight. 
Accordingly, the chromium content in the ferrous alloy has been determined 
within the range of 5.0% by weight to 6.0% by weight. 
Molybdenum produces carbides by which the untreated mechanical component 
obtained by casting is increased in abrasion resistance and increases 
tempering-crack resistance of the unfinished mechanical component so as to 
improve mechanical strength of the unfinished mechanical composition at 
high temperature. As a result of an experiment achieved by the inventors 
for determining a desirable molybdenum content in the ferrous alloy in 
consideration of the above mentioned character of molybdenum, it has been 
ascertained that carbides are not produced sufficiently in the untreated 
mechanical component obtained by casting so that the untreated mechanical 
component is reduced in abrasion resistance and the unfinished mechanical 
component is not increased in temper-brittleness resistance when the 
molybdenum content is less than 1.0% by weight, and it has been also 
ascertained that the untreated mechanical component is reduced in both 
machinability and toughness when the molybdenum content is more than 1.5% 
by weight. Accordingly, the molybdenum content in the ferrous alloy has 
been determined within the range of 1.0% by weight to 1.5% by weight. 
Vanadium produces carbides and contributes to making grains minute in the 
matrix structure of the ferrous alloy in the untreated mechanical 
component obtained by casting so that the untreated mechanical component 
is increased in toughness, and further contributes to improvement in 
tempering-crack resistance of the unfinished mechanical component. As a 
result of an experiment achieved by the inventors for determining a 
desirable vanadium content in the ferrous alloy in consideration of the 
above mentioned character of vanadium, it has been ascertained that the 
untreated mechanical component is not actually increased in toughness and 
the unfinished mechanical component is not actually improved in 
tempering-crack resistance when the vanadium content is less than 0.8% by 
weight and carbides which are crystallized in the matrix structure of the 
ferrous alloy in the untreated mechanical component are coarsened and 
enlarged so that the untreated mechanical component is reduced in 
toughness when the vanadium content is more than 1.2% by weight. 
Accordingly, the vanadium content in the ferrous alloy has been determined 
within the range of 0.8% by weight to 1.2% by weight. 
Manganese which is contained in the ferrous alloy as an impurity element 
contributes to improvements in both abrasion resistance and mechanical 
strength of the untreated mechanical component obtained by casting. As a 
result of an experiment achieved by the inventors for determining a 
desirable manganese content in the ferrous alloy in consideration of the 
above mentioned character of manganese, it has been ascertained that the 
untreated mechanical component is reduced in toughness when the manganese 
content is more than 0.5% by weight. Accordingly, the manganese content in 
the ferrous alloy has been determined to be not more than 0.5% by weight. 
In accordance with the method of the present invention, the untreated 
mechanical component is obtained by casting the ferrous alloy which 
includes silicon within the range of 1.3% to 2.0% by weight and other 
elements in such a manner as described above by the use of the casting 
mold formed through the lost-wax process. The untreated mechanical 
component thus obtained is prevented from being provided therein with 
ingot pipings and various inclusions arising in grain boundaries contained 
in a part of the matrix structure close to the surface thereof. The 
untreated mechanical component is subjected to the annealing process so as 
to be the unfinished mechanical component. Then, the unfinished mechanical 
component is subjected in sequence to the machining process, quenching and 
tempering process and finishing process so as to produce a finished 
mechanical component. 
The finished mechanical component obtained in the manner as described above 
in accordance with the present invention, is not provided therein with 
ingot pipings nor various inclusions, such as silicon-inclusions, 
oxygen-inclusions or the like, arising in grain boundaries contained in a 
part of the matrix structure of the ferrous alloy close to the surface 
thereof, and thereby is superior in both abrasion resistance and fatigue 
strength. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following detailed description 
which is to be read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, one embodiment of method of producing a mechanical component with 
superior fatigue strength according to the present invention, which is 
applied to production of a rocker arm, and the rocker arm produced through 
the embodiment will be described in detail. 
In this embodiment, first, ferrous alloy which includes carbon (C) within 
the range of 0.3% to 0.45% by weight, silicon (Si) within the range of 
1.3% to 2.0% by weight, chromium (Cr) within the range of 5.0% to 6.0% by 
weight, molybdenum (Mo) within the range of 1.0% to 1.5% by weight, 
vanadium (V) within the range of 0.8% to 1.2% by weight, manganese (Mn) 
not more than 0.5% by weight, and iron of the major remainder, is 
prepared. By way of example, three samples of the ferrous alloy X1, X2 and 
X3 having respective compositions as shown in Table 1 mentioned below are 
provided. 
TABLE 1 
______________________________________ 
Components 
Samples of Ferrous Alloy 
(% by weight) 
X1 X2 X3 
______________________________________ 
C 0.35 0.341 0.34 
Si 1.31 1.57 1.99 
Cr 5.1 5.3 5.2 
Mo 1.16 1.2 1.15 
V 0.97 0.98 0.95 
Mn 0.31 0.31 0.3 
P 0.015 0.016 0.018 
(Phosphorus) 
S 0.011 0.011 0.009 
(Sulphur) 
Fe remainder remainder remainder 
______________________________________ 
Next, a casting mold which has been formed through a lost-wax process is 
prepared. In the lost-wax process for producing the casting mold, a wax 
model which is to be shaped coincidentally with the rocker arm is formed 
by pouring melted wax at temperature within the range of 68.degree. C. to 
80.degree. C. in a metallic mold and cooling the wax in the metallic mold. 
The wax model shaped to be coincide with the rocker arm is steeped in 
investing solution with which powder of refractory material and caking 
agent are mixed, and then fire resistant sand is sprinkled over the wax 
model. Subsequently, the wax model which is covered by the fire resistant 
sand after being steeped in the investing solution is dried in a drying 
oven at about 26.degree. C. The process from the step of steeping the wax 
model in the investing solution to the step of drying the wax model 
covered by the fire resistant sand is repeated several times to form a 
laminated shell covering the wax model. 
After the laminated shell has been formed, the wax model covered by the 
laminated shell is heated at, for example, about 140.degree. C. so that 
the wax model is melted away and the laminated shell is left 
independently. The laminated shell thus obtained is covered by refractory 
material and then the refractory material is sintered to form a casting 
mold. The sintering of the refractory material covering the laminated 
shell is carried out in a sintering furnace which is divided into the 
first zone heated at about 450.degree. C., the second zone heated at about 
600.degree. C., the third zone heated at about 900.degree. C. and the 
fourth zone heated at about 1050.degree. C. through which the laminated 
shell covered by the refractory material is moved to pass successively. 
Then, one of the samples of the ferrous alloy X1, X2 and X3, each of which 
has a melting point at about 1520.degree. C., is melted and deoxdizing 
agent is added to the melted ferrous alloy for deoxidizing the latter. The 
melted ferrous alloy which has been deoxidized is poured into the casting 
mold, which is formed through the lost-wax process as mentioned above, at 
temperature within the range of 1,620.degree. C. to 1,690.degree. C. In 
such a process, the casting mold is maintained at temperature within the 
range of 900.degree. C. to 1,000.degree. C. Since the casting mold is 
maintained at such high temperature as that within the range of 
900.degree. C. to 1,000.degree. C., the ferrous alloy is required to have 
its melting point at relatively high temperature for keeping superior 
fluidity in the casting mold, in a different manner from ordinary iron 
casting. Each of the samples of the ferrous alloy X1, X2 and X3 satisfies 
such requirement. Incidentally, the melting point of ferrous alloy depends 
on carbon content therein in such a manner that the smaller the carbon 
content is, the higher the melting point is. 
After that, the melted ferrous alloy in the casting mold is cooled down to 
solidify into an untreated rocker arm. The untreated rocker arm is taken 
out of the casting case to be subjected to burring for the exterior 
thereof and then steeped in alkali aqueous solution to be subjected to 
so-called caustic treatment. 
The untreated rocker arm having subjected to the caustic treatment is 
further subjected to shot blasting so that undesirable extraneous matter 
is removed from the exterior of the untreated rocker arm, and then is 
heated at about 860.degree. C. for about about 3.5 hours continuously so 
as to be annealed and thereby turned into an unfinished rocker arm. The 
unfinished rocker arm thus annealed is roughly machined to be provided 
with, for example, a pair of opening with which a roller is to be engaged 
and then is heated in a vacuum furnace at about 1020.degree. C. for about 
40 minutes so as to be quenched. After that, The unfinished rocker arm 
having been subjected to quenching is maintained in a vacuum furnace at 
about 560.degree. C. for about 100 minutes and then cooled in the ambient 
atmosphere of nitrogen gas so as to be subjected to tempering. 
The unfinished rocker arm having been subjected to tempering as mentioned 
above is finished up to be tuned into a completed rocker arm. 
FIG. 1 shows one example of rocker arms produced through the embodiment of 
method according to the present invention in such a manner as described 
above. The rocker arm shown in FIG. 1 has an engaging end portion 11 
provided to be supported by a concave acceptor formed at the top of a HLA 
or the like, a contacting end portion 12 for coming into contact with the 
top of a stem of an intake or exhaust valve, and a pair of openings 13a 
and 13b for supporting the shaft of a roller. The roller is mounted on the 
rocker arm with its shaft inserted at both end portions thereof into the 
openings 13a and 13b respectively, as shown in dot-dash lines in FIG. 1, 
so that a roller rocker arm assembly is obtained. 
The untreated rocker arm which is obtained in process of producing the 
rocker arm in accordance with the method of the present invention is 
prevented from being provided therein with ingot pipings and various 
inclusions arising in grain boundaries contained in a part of the matrix 
structure of the ferrous alloy close to the surface thereof. 
FIGS. 2 and 3, FIGS. 4 and 5, and FIGS. 6 and 7 show microphotographs of 
four hundred magnifications which represent internal structures at the 
inner portion and the surface portion of each of unfinished rocker arms 
which was made respectively of the samples of the ferrous alloy X1, X2 and 
X3 in accordance with the embodiment of method of the present invention 
aforementioned. In the microphotographs of FIGS. 2, 4 and 6, the internal 
structures of the inner portions of the rocker arms made of the samples of 
the ferrous alloy X1, X2 and X3, respectively, are shown, and no ingot 
piping is observed in each of the microphotographs of FIGS. 2, 4 and 6. In 
the microphotographs of FIGS. 3, 5 and 7, the internal structures of the 
surface portions of the rocker arms made of the samples of the ferrous 
alloy X1, X2 and X3, respectively, are shown, and no inclusion is observed 
in grain boundaries contained in the matrix structure of the ferrous alloy 
close to the surface of the rocker arm in each of the microphotographs of 
FIGS. 3, 5 and 7. 
The rocker arm obtained based on each of the unfinished rocker arms made of 
the samples of the ferrous alloy X1, X2 and X3, respectively, in 
accordance with the embodiment of method of the present invention, is not 
provided therein with ingot pipings nor various inclusions, such as 
silicon-inclusions, oxygen-inclusions or the like, arising in grain 
boundaries contained in a part of the matrix structure close to the 
surface thereof, and thereby is superior in both abrasion resistance and 
fatigue strength. 
It has been recognized that the rocker arms made of the samples of the 
ferrous alloy X1, X2 and X3 in accordance with the embodiment of method of 
the present invention are provided with hardness of HV=542, HV=530 and 
HV=530, respectively. 
Now, comparison between the rocker arm made of the sample of the ferrous 
alloy X2 in accordance with the embodiment of method of the present 
invention (hereinafter, referred to as a rocker arm A2) and a rocker arm 
produced through a method other than the method according to the present 
invention to be shaped into the same figure as the rocker arm A2 
(hereinafter, referred to as a rocker arm Sa) will be described below. 
The rocker arm Sa was made of ferrous alloy including carbon of 0.36% by 
weight, silicon of 0.53% by weight, chromium of 5.11% by weight, 
molybdenum 1.21% by weight, vanadium of 0.96% by weight, manganese of 
0.31% by weight, phosphorous of 0.002% by weight, sulphur of 0.009% by 
weight and iron of the remainder, through the method other than the method 
according to the present invention, by which treatments similar to those 
in the method according to the present invention were provided. 
FIGS. 8 and 9 show microphotographs of four hundred magnifications which 
represent respectively internal structures at the inner portion and the 
surface portion of an unfinished rocker arm which was obtained in process 
of producing the rocker arm Sa and subjected to annealing (hereinafter, 
referred to as an unfinished rocker arm USa). In the microphotograph of 
FIG. 8 showing the internal structures at the inner portions of the 
unfinished rocker arm USa, no ingot piping is observed. However, in the 
microphotograph of FIG. 9 showing the internal structure at the surface 
portion of the unfinished rocker arm USa, silicon-inclusions or 
oxygen-inclusions which extend from the left side (the side of the 
surface) to the right side (the inside) are clearly observed. 
For comparison in performance between the rocker arm A2 and the rocker arm 
Sa, a roller rocker arm assembly RA2 was obtained by mounting a roller on 
the rocker arm A2 in such a manner that both end portions of the shaft of 
the roller are inserted into a couple of opening on the rocker arm A2, a 
roller rocker arm assembly RSa was also obtained by mounting a roller on 
the rocker arm Sa in such a manner that both end portions of the shaft of 
the roller are inserted into a couple of opening on the rocker arm Sa, and 
then each of the roller rocker arm assemblies RA2 and RSa was subjected to 
a fatigue test. 
In the fatigue test for each of the roller rocker arm assemblies RA2 and 
RSa, as shown in FIG. 10, an engaging end portion provided to be supported 
by a concave acceptor formed at the top of a HLA or the like and a 
contacting end portion for coming into contact with the top of a stem of 
an intake or exhaust valve of each of the rocker arms A2 and Sa were 
supported by a concave acceptor 21 fixed on the base 20 and a slant 
supporting member 22 fixed also on the base 20, respectively. Then, a 
pushing device 23 provided over each of the roller rocker arm assemblies 
RA2 and RSa was caused to push the roller down intermittently with press 
load Y at the rate of repetition of fifteen times per second. The press 
load Y was selected to be twice as large as press load assumed to act on 
the roller in the case where the roller rocker arm assembly RA2 or RSa is 
actually used. 
As a result of such fatigue test, the rocker arm Sa with which the roller 
rocker arm assemblies RSa was constituted was broken down when the pushing 
device 23 pushed the roller repeatedly by 2.4.times.10.sup.6 times, and to 
the contrary, the rocker arm A2 with which the roller rocker arm 
assemblies RA2 was constituted did not have any trouble after the pushing 
device 23 pushed the roller repeatedly by 1.times.10.sup.7 times. As 
apparent from this result, it was recognized that the rocker arm A2 was 
much more superior in fatigue strength than the rocker arm Sa. 
Although, in the aforementioned embodiment, the method of producing a 
mechanical component with superior fatigue strength according to the 
present invention is applied to production of a rocker arm, it is to be 
understood that the method according to the present invention can be also 
applied to production of various mechanical components required to have 
superior fatigue strength other than the rocker arm, such as a tappet used 
in a Diesel engine or the like.