Patent Application
20130263980
The present invention relates to a technology for improving a fatigue strength of a cast iron material, in particular, a spherical graphite cast iron.
A conventional automobile transmission gear has been manufactured by carburizing and hardening a steel material after the steel material was gear cut. However, there was a problem of deformation of a member due to heat treatment strain.
By contrast, a spherical graphite cast iron can be readily manufactured. However, it has a disadvantage that it can not be used in an automobile transmission gear because of a low fatigue strength. Accordingly, it is desired for a cast iron material which was not carburized and not hardened so as to have a fatigue strength being the same as that of a carburized and hardened steel material.
A spherical graphite cast iron has a high mechanical strength in cast irons. As a technology for improving a fatigue strength of a spherical graphite cast iron, there is an austempering treatment applying to a spherical graphite cast iron containing, by weight ratio, 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu.
The bending fatigue strength at 107 cycles of a spherical graphite cast iron having such the composition is shown in
The fatigue strength of “about 200 MPa” can not be used in an automobile transmission gear.
As an another prior art, a technology is proposed, according to which a spherical graphite cast iron is cast to improve the fatigue strength thereof by means of adding an additive to a molten metal of a flake graphite cast iron (see Patent Document 1).
However, such the prior art intends to improve the fatigue strength by improving a casting step and can not improve the fatigue strength of a material after a cast iron material was mechanically machined.
Patent Document 1: Japanese Patent Application Non-examined Publication No. 2005-8913
The present invention was proposed in view of problems of above-described prior arts, and intends to provide a method for improving a fatigue strength, which can improve the fatigue strength of a cast iron material, in particular, a spherical graphite cast iron to a value the same as that of a carbon steel that was carburized and hardened.
A method for improving a fatigue strength of a cast iron material of the present invention, contains the steps of
Performing a first shot peening treatment with shots having the hardness of 600 Hv or more and a particle size (φ) of 0.5 to 0.8 mm (1 step),
performing a second shot peening treatment with shots having the hardness of 600 Hv or more and a particle size (φ) of 0.1 to 0.3 mm (2 step), and
for each on spherical graphite cast iron on which quenching and tempering heat treatment or austempering heat treatment has been performed and tensile strength made to be 1200 MPa or more, the spherical graphite cast containing the following elements in the following mass percentages: C=2.0-4.0%, Si=1.5-4.5%, Mn=2.0% or less, P=0.08% or less, S=0.03% or less, Mg=0.02-0.1%, and Cu=1.8-4.0% Cu.
Upon applying the present invention, it is preferred that, after performing the first to third shot peening treatments, a shot peening treatment is performed with shots composed of tin or molybdenum to perform metal lubrication.
In result of an experiment being carried by the inventor, in a case that the first to third shot peening treatments are performed with respect to a spherical graphite cast iron that contains, by weight ratio, 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu, quenching and tempering heat treatment has been performed to the spherical graphite cast iron and that the tensile strength made to be 1200 MPa or more, the fatigue strength of 350 MPa or more can be obtained, which strength is the bending fatigue strength being the same level as that of carburized and hardened steel material.
Also, in result of an experiment being carried by the inventor, in a case that the first to third shot peening treatments are performed with respect to a spherical graphite cast iron that contains, by weight ratio, 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu, an austempering heat treatment has been performed to the spherical graphite cast iron and the tensile strength made to be 1200 MPa or more, the fatigue strength of 350 MPa or more can be obtained, which strength is the bending fatigue strength being the same level as that of carburized and hardened steel material.
According to the present invention, a compressive residual stress distribution about 600 MPa can be imparted for a range of 100 μm from a surface by performing the first to third shot peening treatments, generations of fine cracks on a surface of a spherical graphite cast iron and development of the cracks are retarded, and therefore, the fatigue strength is improved.
According to the present invention, by subjecting a predetermined machine process (for example, a gear-cutting process for an automobile transmission gear) to a spherical graphite cast iron, which contains, by weight ratio, 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu, quenching and tempering heat treatment or austempering heat treatment has been performed and the tensile strength made to be 1200 MPa or more, and after, by performing the first to third shot peening treatments to the spherical graphite cast iron, the bending fatigue strength being the same level as that of a carburized and hardened steel material can be obtained, without performing a carburizing and hardening treatment.
Further, since it is not necessary to carry out a heat treatment after machine processing, the heat treatment strain can be prevented.
Hereinafter, with reference to accompanying drawings, an embodiment of the present invention will be described.
At first, with reference to
In
Then, a shot peening treatment is performed (conducted) with shots having hardness of 600 Hv or more and a particle size φ of 0.5 to 0.8 mm (step S1: a step for performing a first shot peening treatment: first step).
Next, a shot peening treatment is performed with shots having hardness of 600 Hv or more and a particle size φ of 0.1 to 0.3 mm (step S2: a step for performing a second shot peening treatment: second step).
Then, a shot peening treatment is performed with shots having hardness of 600 Hv or more and a particle size φ of 0.1 mm or less (step S3: a step for performing a third shot peening treatment: third step).
Thereafter, with tin or molybdenum shots having an appropriate hardness and particle size, a shot peening treatment is performed (step S4: a step for performing a fourth shot peening treatment: fourth step).
According to the step S4, on a surface of a workpiece on which the first to third shot peening treatments were performed, metal lubrication can be performed.
In addition, the step S4 may be omitted.
According to said step S4, an effect is advantageously imparted that a surface being flattened by the third shot peening treatment is further metal lubricated.
Said step S4 is not an indispensable step and can be omitted in order to reduce steps and necessary time period of a whole process.
From a test sample being performed the first to third shot peening treatments (1 to 3 steps) thereon, a fatigue test sample shown in
In an illustrated Embodiment, a shape of a test piece which is entirety shown by a character 12 comprises, for example, in a round bar 3 having an outer diameter of 12 mm, a recess 5 being a grooved in a sectional shape of character V and extending around an entire periphery in a circumference direction. At a bottom 5a of a recess 5, a diameter of a round bar 3 is 8 mm. Here, a test piece 12 shown in
With such the test piece 13, a rotating bending fatigue test was performed.
As below-mentioned in Experimental Example 1, the fatigue strength of a spherical graphite cast iron to which the shot peening treatments of steps S1 to S3 of
The inventors have carried out experiments (Experimental Example 1 to Experimental Example 6) such as shown below with a spherical graphite cast iron, which contains 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu, by weight ratio.
By performing the quenching and tempering heat treatment to the above-mentioned spherical graphite cast iron, the tensile strength is made to be 1200 MPa or more.
Results of a tensile test of a test sample, in which samples the quenching and tempering heat treatment applies to the spherical graphite cast iron (the quenching and tempering heat treated spherical graphite cast iron), are shown with a characteristic curve FCD in
In
Next, with shots having hardness of 600 Hv or more and a particle size (φ) of 0.5 to 0.8 mm, a first shot peening treatment was performed. Results of the first shot peening treatment are shown as a residual stress distribution curve A in
According to a residual stress distribution curve A, until a depth of 150 μm from a test piece surface (0 μm), a residual stress has a nearly even numerical value of −800 (MPa) while slightly increasing.
In
On a test piece differing to said test piece from which a residual stress distribution curve A in
In the residual stress distribution curve B, in an area (region) until a depth of 50 μm from a test piece surface (0 μm), a compressive residual stress rapidly increases, and in an area in a depth of 50 μm or more, a compressive residual stress slowly increases.
On a test piece further differing to said test piece from which a residual stress distribution curve A in
In a residual stress distribution curve C, in an area until a depth of 25 μm from a test piece surface (0 μm), a compressive residual stress rapidly increases, and in an area deeper from a surface than a depth of 25 μm, a compressive residual stress slowly increases.
A residual stress distribution thereof is shown in
In
On the other hand, a residual stress distribution of a test piece after the first to third shot peening treatments have been performed is shown with a residual stress distribution curve Sa.
As obvious in
Refering to
A residual stress is such large as 1000 MPa at a surface 0 μm and as about 700 MPa in an area from 25 μm to 100 μm. Also in an area (region) more inside than 100 μm, a test piece on which the first to third shot peening treatments have been performed has an increased compressive residual stress, compared with compressive residual stress of a test piece on which the first to third shot peening treatments have not been performed.
In Experimental Example 1, the first to third shot peening treatments were performed on the same test piece, a fatigue test piece shown in
A mark H in
It was found in
A bending fatigue curve J in
In Experimental Example 1, from results shown in
Further, from a compressive residual stress distribution shown in
when the first shot peening treatment is omitted, a compressive residual stress is decreased in an area deeper by 25 μm or more from a surface decreases, and when the second shot peening treatment is omitted, a compressive residual stress in an area until 25 μm from a surface is decreases.
In Experimental Example 2, a test material that was obtained by applying said spherical graphite cast iron to an austempering heat treatment to be made a tensile strength to be 1200 MPa or more was used.
With respect to such the test materials, in a manner the same as that of Experimental Example 1, a first shot peening treatment was performed with shots having a hardness of 600 Hv or more and with a shot particle size (φ) of 0.5 to 0.8 mm, to one test material,
a second shot peening treatment was performed with shots having a hardness of 600 Hv or more and with a shot particle size (φ) of 0.1 to 0.3 mm, to the other test material, and
a third shot peening treatment has been performed with shots having a hardness of 600 Hv or more and a shot particle size (φ) of 0.1 mm or less, to the further other test material.
Results of the above-mentioned Experimental Example 2 are the same as that shown in
Further, with respect to the same test material, the first to third shot peening treatments have been performed and a compressive residual stress distribution in said test piece was examined. Results of said examination were the same as the results of
With a test material on which the first to third shot peening treatments have been performed, a fatigue test piece the same as that of Example 1 was prepared, and a rotating bending fatigue test was carried out.
Results of such the fatigue test are shown in
In
As obvious from results of Experimental Example 2, it was found that when an austempering treatment is performed with respect to a spherical graphite cast iron that contains, by mass percentage, C=2.0 to 4.0%, Si=1.5 to 4.5%, Mn=2.0% or less, P=0.08% or less, S=0.03% or less, Mg=0.02 to 0.1%, and Cu=1.8 to 4.0% to impart a tensile strength of 1200 MPa or more, and the first to third shot peening treatments are performed, a bending fatigue strength being the same as that (about 350 MPa) of a carburizing and quenching steel material can be obtained.
When a first shot peening treatment is performed with respect to a test piece used in Experimental Example 1 (the spherical graphite cast iron, which contains 2.0 to 4.0% C, 1.5 to 4.5% Si, 2.0% or less Mn, 0.08% or less P, 0.03% or less S, 0.02 to 0.1% Mg, and 1.8 to 4.0% Cu, by weight ratio, and was applied quenching and tempering heat treatment thereto), a fatigue test of bending fatigue strength was performed to test pieces, which is manufactured in a manner the same as that of Experimental Example 1, except that shots having a particle size larger than 0.8 mm (particle size: 0.9 mm, 1.0 mm, and 1.1 mm) were used.
In
Although in a case that a shot particle size is 0.8 mm, the fatigue strength the same as that (about 600 MPa) of a carburized and hardened steel material was obtained (“◯” in
From
When the shot particle size is larger than 0.8 mm in the first shot peening treatment, it is considered that shots are not conveyed by an air flow when shots are blasted off, and therefore, sufficient impacts can not be imparted to the test piece.
In a manner being similar to that of Experimental Example 1, except that shots having a particle size of 0.5 mm or smaller (particle size: 0.5 mm, 0.4 mm, 0.3 mm) were used in a first shot peening treatment, the fatigue test was performed relating to the bending fatigue strength.
In
As shown in
From
It is considered in a case that a shot particle size is smaller than 0.5 mm in the first shot peening treatment, although the compressive stress on a surface side of a steel material becomes larger, the compressive stress inside the steel material becomes smaller.
In a manner similar to that of Experimental Example 1, except that shots having a particle size of 0.3 mm or larger (particle size: 0.3 mm, 0.4 mm, 0.5 mm) were used in a second shot peening treatment, the fatigue test was performed relating to the bending fatigue strength.
In
As shown in
From results of
Although the second shot peening treatment is a treatment for improving the compressive residual stress of the outermost surface (a region where a distance from a surface is 50 μm) of a cast iron test piece, it is assumed that a peak of the compressive residual stress is not generated on the most surface and the fatigue strength was not improved, in a case that a shot particle size is larger than 0.3 mm.
In a manner similar to that of Experimental Example 1, except that shots having a particle size of 0.1 mm or smaller (particle size: 0.1 mm, 0.07 mm, 0.01 mm) were used in a second shot peening treatment, the fatigue test was performed relating to the bending fatigue strength.
In
As shown in
From
It is assumed that when a particle size of shots used in the second shot peening treatment is small, a surface of a cast iron is smoothened merely, the compressive residual stress of the outermost surface of a steel material was not generated, and the fatigue strength could not be improved.
Gears (gears on which the first to third shot peening treatments were performed) Z being manufactured with a test material of Experimental Example 1 and gears Y being manufactured with a test material to which the third shot peening treatment was not applied, were prepared.
As to gears (gears on which the first to third shot peening treatments were performed) Z being manufactured with a test material of Experimental Example 1, the sliding properties of an engagement surface were good.
By contrast, as to gears Y being manufactured with a test material to which the third shot peening treatment was not applied, the sliding properties of an engagement surface showed abnormality.
In more detail, in
By contrast, the gears Y were not good in touch and sliding properties between engagement gear surfaces, generated fine cracks on a gear surface, and could not clear the predetermined endurance test (shown by “×” in
From
According to the third shot peening treatment, a surface being roughened by the first and second shot peening treatments is smoothened, and an irregularity of a gear surface becomes smaller; accordingly, in fine irregularity, oil stays therein to exert a lubrication operation.
It is assumed that the test material, to which the third shot peening was not applied, could not exert such the lubrication operation and that sliding abnormality was generated on an engagement surface.
Illustrated embodiments are merely examples and do not intend to limit a technical range of the present invention.
For example, illustrated embodiments can be applied to a cum of a valve operating system, con rod, and various kinds of pumps for supplying a gear high pressure oil.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-266054 | Nov 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/075035 | 10/31/2011 | WO | 00 | 5/29/2013 |
