EP0099067B1

EP0099067B1 - Wear-resistant sintered ferrous alloy and method of producing same

Info

Publication number: EP0099067B1
Application number: EP19830106624
Authority: EP
Inventors: Yoshihiro Maki; Takaaki Oaku; Yasuzi Hokazono
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1982-07-09
Filing date: 1983-07-06
Publication date: 1987-06-16
Also published as: JPS599151A; DE3372110D1; AU1644783A; AU536739B2; EP0099067A3; EP0099067A2

Description

Background of the invention

This invention relates to a wear-resistant sintered ferrous alloy for parts subjected to rubbing friction and a method of producing the same.
A typical example of metal parts that are forced to make continuous rubbing contact with another metal part is the rocker arm of an internal combustion engine. The body of the rocker arm is formed by casting or by forging, but the tip part where the rocker arm makes rubbing contact with a cam must be afforded with high wear resistance. Therefore, it is usual to harden the tip portion of the rocker arm by a surface treatment such as carbrizing, nitriding, chromium plating or plasma-spraying of a hard coating material, or alternatively to form the tip part separately from the main part of the rocker arm by chilled casting or by a powder metallurgy method and attach the tip part to the rocker arm body by soldering or by insert-casting.
As the performance requirements to the recent internal combustion engines for automotive uses have become more and more severe, there is the tendency to force the rocker arms to make rubbing contact with the cams under severer conditions. Then there arises a problem that the rocker arm tip parts produced by conventional materials and techniques and/or the cam surfaces undergo intolerably significant wear.

Summary of the invention

It is an object of the present invention to provide a sintered alloy which has such high wear resistance as is sufficient for parts subjected to severe rubbing friction such as the rocker arm tips in recent automotive internal combustion engines but is relatively weak in the tendency to abrade another metal material brought into rubbing contact with the sintered alloy parts.
It is another object of the invention to provide a sintered alloy which is very high in wear resistance and can be produced at relatively low costs.
It is still another object of the invention to provide a method of producing a sintered alloy according to the invention.
The present invention provides a method of producing a wear-resistant sintered ferrous alloy, the method comprising the steps of preparing 100 parts by weight of a powder mixture by mixing 16 to 50 parts by weight of a powder of a Fe-Cr-B alloy which contains 10 to 35% by weight of Cr and 1.0 to 2.5% by weight of B, 1.0 to 3.5 parts by weight of a graphite powder and as balance a phosphorus-containing powder which comprises a Fe-P alloy powder such that the prepared powder mixture contains 0.2 to 1.0% by weight of P, compacting the powder mixture into a body of a desired shape, and sintering the compacted body in a nonoxidizing atmosphere.
Either a Fe-P alloy relatively low in the content of P alone or a mixture of a Fe-P alloy relatively high in the content of P and an iron powder, which may be a low-alloy iron powder, can be used as the phosphorus-containing powder in the initial step.
Preferably the amount of the Fe-Cr-B alloy powder in the initial step is in the range from 20 to 30 parts by weight.
As the product of the above stated method, a wear-resistant sintered ferrous alloy according to the invention consists of 1.6 to 17.5% of Cr, 0.16 to 1.25% of B, 1.0 to 3.5% of C, 0.2 to 1.0% of P, by weight, and the balance Fe and incidental impurities.
Preferably the content of Cr in this sintered alloy is in the range from 2.0 to 10.5% by weight and the content of B is in the range from 0.20 to 0.75% by weight. Also it is preferred that the porosity of the sintered alloy is not greater than 20%.
The matrix of a sintered alloy according to the invention is principally of iron, and adequate amounts of hard phases of Fe-Cr-B-C compounds and Fe-C-P compounds are dispersed in the iron matrix. This sintered alloy is very high in wear resistance but is relatively weak in the tendency to abrade another metal material with which the sintered alloy makes rubbing contact. Accordingly, when this sintered alloy is used for rocker arm tips in the recent automotive internal combustion engines both the rocker arm tips and the cam faces become very small in the amounts of wear. This sintered alloy does not use very costly metals such as Mo and W, and can easily be produced by using conventional powder metallurgy techniques. In principle this sintered alloy can be used in the state as sintered without need of any post-sintering heat treatment or surface treatment. Accordingly various parts of this excellent sintered alloy can be produced at very low costs.
An important feature of the invention is the use of a Fe-P alloy powder as an essential component of the starting powder mixture so that the sintered alloy contains an adequate amount of P.
Before completion of the present invention, we recognized that a sintered ferrous alloy relatively high in wear resistance can be obtained by using a powder material composed of 16-50% by weight of the aforementioned Fe-Cr-B alloy, 0.5-2.5% by weight of graphite powder and the balance of Fe powder. However, there are some problems in practical application of this sintered alloy to the rocker arm tips. When the powder material in compacted form is sintered at a temperature above the melting point of the Fe-Cr-B alloy the sintered alloy contains relatively coarsely grown hard phases of borides and/or carbides of Fe and/or Cr at the grain boundaries of the matrix, and therefore the cams making rubbing contact with the rocker arm tips exhibit increased amounts of wear. When the sintering temperature is below the melting point of the Fe-Cr-B alloy, the wear resistance of the rocker arm tips remain insufficient though the wear of the cams decreases. We tried to compensate for the disadvantageous property resulting from the sintering at a relatively high temperature by replacing a portion of the Fe powder in the powder material (2-15% of the total weight of the powder material) by Cu powder, Pb powder and/or Sn powder, but this resulted in lowering of the overall hardness of the sintered rocker arm tips and significant increase in the amounts of wear of the rocker arm tips though the wear of the cams decreased.
In the present invention, P added in the form of a Fe-P alloy to the starting powder mixture has the effect of producing a liquid phase of steadite, which is an eutectic crystal of Fe-Fe₃P-Fe₃C, during the sintering process and hence promoting the sintering. Accordingly it is possible to achieve sintering at a temperature below the melting point of the Fe-Cr-B alloy to thereby prevent growth of relatively coarse particles of the aforementioned hard phases. In the sintered alloy, the hard steadite is finely dispersed in the matrix. Therefore, the rocker arm tips of the sintered alloy is sufficiently hard and resistant to wear but is relatively weak in the tendency to abrade the cam faces.

Description of the preferred embodiments

Throughout the following description, the amounts of the elements in the respective alloys are given in percentages by weight.
In the present invention a Fe-Cr-B alloy powder is used as the source of Cr and B on which the wear resistance of the sintered alloy primarily depends. In the process of sintering of the compacted powder material, the Fe-Cr-B alloy powder diffuses into the iron base matrix by solid phase diffusion and/or by liquid phase sintering that takes place because of the presence of a Fe-P-C liquid phase as the effect of P contained in the powder material or a Fe-Cr-B-C liquid phase resulting from combining of the Fe-Cr-B alloy powder with the graphite powder. The content of Cr in the Fe-Cr-B alloy is specified to be from 10 to 35% and the content of B to be from 1.0 to 2.5% for the following reasons.
Cr combines with B contained in the same alloy and also with C of the graphite powder to form borides and carbides which are distributed over the iron base matrix of the sintered alloy. Accordingly it is important that the amount of Cr be balanced with the amounts of B and C. When the content of Cr in the Fe-Cr-B alloy is less than 10% it is difficult to produce a sintered alloy in which the content of Cr is sufficient to afford desirably high wear resistance to the sintered alloy. However, a Fe-Cr-B alloy containing more than 35% of Cr is too high in the hardness of the alloy powder particles so that the alloy powder is inferior in formability.
As mentioned above, B combines with Cr to form hard borides. When the content of B in the Fe-Cr-B alloy is less than 1.0% the precipitation of borides remains insufficient. When the content of B exceeds 2.5% the precipitation of borides becomes more than sufficient, and the particles of the precipitated borides become undesirably coarse and, besides, the powder becomes inferior in formability.
Usually Fe-Cr-B alloy powders are produced by an atomizing method. A small amount of Si may be added to a Fe-Cr-B alloy for use in the present invention with a view to improving the fluidity of the molten alloy and suppressing the oxidation of the molten alloy in the production of the alloy powder by an atomizing method. In that case the amount of Si should be limited so as not to degrade the important properties of the Fe-Cr-B alloy. When the amount of added Si is less than 0.5% of the alloy the expected effects of Si are hardly appreciable, but it is undesirable to add more than 3.0% of Si because it causes lowering of the hardness and wear resistance of the sintered alloy.
A powder mixture to be compacted and sintered is prepared so as to contain 16 to 50% by weight of a Fe-Cr-B alloy powder. When the amount of the Fe-Cr-B alloy powder is less than 16%, the amounts of hard phases of borides and carbides in the sintered alloy remain insufficient to afford desirably high wear resistance to the sintered alloy. However, an increase in the amount of the Fe-Cr-B alloy powder in the powder mixture beyond 50% no longer produces a corresponding effect on the wear resistance of the sintered alloy and renders the formability of the powder mixture inferior. In the sintered alloy produced by using a powder mixture containing 16-50% of a Fe-Cr-B alloy which contains 10-35% of Cr and 1.0-2.5% of B, the content of Cr is 1.6-17.5% and the content of B is 0.16-1.25% by calculation. It is preferred to limit the amount of the Fe-Cr-B alloy in the starting powder mixture within the range from 20 to 30% by weight. Then the content of Cr in the sintered alloy becomes 2.0-10.5% and the content of B becomes 0.20-0.75%.
The powder mixture must contain 1.0 to 3.5% by weight of a graphite powder. Graphite or C diffuses into the iron base matrix of the sintered alloy with the effect of enhancing hardness and physical strength of the matrix and, besides, diffuses into the Fe-Cr-B alloy powder to form carbides. When the amount of graphite in the powder mixture is less than 1.0% the overall hardness and wear resistance of the sintered alloy remain insufficient. However, when the powder mixture contains more than 3.5% of graphite the sintered alloy becomes brittle and stronger in the tendency to abrade another metal material brought into rubbing contact with the sintered alloy by reason of the precipitation of excessively large amounts of carbides.
In the present invention, P is introduced into the starting powder mixture by using a Fe-P alloy powder with a view to minimizing evaporation loss of P during the sintering process. It is suitable to use either a relatively low-phosphorus Fe-P alloy containing 0.2 to 1.0% of P or a relatively high-phosphorus Fe-P alloy containing 20 to 40% of P. These two types of Fe-P alloys in powder form are both available as commercial materials. When using a Fe-P alloy powder relatively high in the content of P, the Fe-P alloy powder is mixed with either a substantially pure iron powder or a low-alloy iron powder to thereby prepare a powder mixture containing a sufficient amount of Fe and an optimum amount of P. As a substantially pure iron powder, use can be made of atomized iron powder, reduced iron powder or carbonyl iron powder for example. A suitable low-alloy iron powder, which may contain very small amounts of Mh, Cr, Mo and/or V for example, can be selected from low-alloy iron powders used in the current powder metallurgy.
The amount of the Fe-P alloy powder is controlled such that the prepared powder mixture contains 0.2 to 1.0% by weight of P. When the content of P in the powder mixture is less than 0.2% the expected effects of P remain insufficient. However, it is undesirable to increase the content of P in the powder mixture beyond 1.0% firstly because an excessively large amount of liquid phase is produced in the process of sintering the powder material to result in that the sintered alloy has coarse surfaces and is unsatisfactory in the dimensional precision, and secondly because there occurs extraordinary growth of steadite phase with the result that the sintered alloy becomes inferior in the smoothness of its rubbing or sliding contact with another metal material.
A powder composition prepared in the above described manner is compacted into a desired shape by a conventional compacting method. Preferably the compaction is performed by application of a compacting pressure of about 5000-8000 kg/cm², When the compacting pressure is too low the sintered alloy will suffer from insufficient mechanical strength, but the employment of an unnecessarily high compacting pressure shortens the life of the metal dies for compacting.
The compacted material is subjected to sintering. It is preferred to perform the sintering in vacuum, but it is permissible to perform the sintering in either a reducing atmosphere or an unreactive gas atmosphere on condition that the sintering atmosphere is practically free of oxygen and moisture.
The sintering temperature should be determined carefully. When the sintering temperature is too low it is difficult to realize sufficient dispersion of the Fe-Cr-B alloy in the iron matrix, and therefore the sintered products will possibly suffer from pitting resulting from separation of the borides and carbides of chromium when subjected to rubbing friction. When the sintering temperature is so high as to exceed the melting point of the Fe-Cr-B alloy, the sintered alloy tends to have a very hard phase consisting of relatively coarse particles of borides and/or carbides of iron and/or chromium at the grain boundaries and, therefore, seriously abrades the opposite material when subjected to rubbing friction. Accordingly it is necessary to employ a sufficiently high sintering temperature which does not exceed the melting point of the Fe-Cr-B alloy used as a raw material.
It is difficult to strictly specify an optimum sintering temperature firstly because the melting point of a Fe-Cr-B alloy varies depending on its composition and secondly because the temperatures at which are produced liquid phases of Fe-Cr-B-C, Fe-Cr-B-P and/or Fe-Cr-B-C_-P and the amounts of such liquid phases very depending on the proportions of C and P added to the Fe-Cr-B alloy. Usually, however, a suitable range of the sintering temperature is from about 1050°C to about 1140°C. When employing a sintering temperature within this range, it is suitable to perform sintering for about 30 to 60 min. If the duration of sintering is made shorter there arises a possibility of insufficient sintering, but an extension of the sintering time beyond about 60 min is of little effect and sometimes results in softening of the once formed hard borides and carbides.
As to the porosity of the sintered alloy products, the existence of some pores raises no problem and is rather favorable for wear resistance because of the possibility of affording the products with a self-lubricating property by impregnation with oil. However, an unduly high porosity becomes a cause of buckling of the alloy matrix subjected to high load and resultant denting of the alloy surface in rubbing contact with another material. Therefore, it is preferred not to make the porosity of the sintered alloy above 20%.
A sintered alloy according to the invention is excellent in wear resistance. In principle the rocker arm tips or other parts formed of this sintered alloy can be used without need of any post-sintering treatment such as heat treatment or surface treatment. However, it is possible and optional to further enhance the wear resistance of the sintered parts by making a heat treatment such as quenching and tempering or a surface treatment such as nitriding insofar as the enhanced hardness of the sintered parts is not seriously unfavorable to the materials subjected to rubbing contact with the sintered alloy parts.
The invention will further be illustrated by the following nonlimitative examples.

Example 1

A powder mixture was prepared by mixing 65 parts by weight of Fe powder which was a powder of reduced iron, 30 parts by weight of a Fe-Cr-B alloy powder containing 20% of Cr and 1.5% of B, 2.5 parts by weight of graphite powder and 2.5 parts by weight of a Fe-P alloy powder containing 27% of P. Each of the powders used as the raw materials consisted of particles that passed through a 100-mesh sieve. With the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture, thorough mixing was performed for 15 min in a V-shaped blender. In the obtained ferrous powder mixture, the contents of essential alloying elements were as shown in Table 1.
The powder mixture was compacted into the shape of a rocker arm tip for an automotive internal combustion engine by application of a pressure of 7000 kg/cm², and the compacted body was sintered in vacuum (8x 10-4 Torr) at 1100°C for 60 min. The sintered alloy in the form of the rocker arm tip had a porosity of 4%.

Example 2

A powder mixture was prepared by mixing 66 parts by weight of a low-alloy Fe powder containing 1.0% of Cr, 0.8% of Mn and 0.3% of Mo, 30 parts by weight of a Fe-Cr-B alloy powder containing 15% of Cr and 2.0% of B, 2.5 parts by weight of graphite powder and 1.5 parts by weight of a Fe-P alloy powder containing 27% of P. The low-alloy Fe powder consisted of particles that passed through an 80-mesh sieve, and the other powders each consisted of particles passed through a 100-mesh sieve. Thorough mixing was carried out with the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture. In the obtained powder mixture, the contents of essential alloying elements were as shown in Table 1.
The powder mixture was compacted into the shape of the aforementioned rocker arm tip by application of a pressure of 8000 kg/cm², and the compacted body was sintered in vacuum at 1120°C for 45 min. The sintered alloy in the form of the rocker arm tip had a porosity of 8%.

Example 3

A powder mixture was prepared by mixing 78 parts by weight of a low-alloy Fe powder containing 3.5% of Cr, 0.3% of Mo and 0.3% of V, 16 parts by weight of a Fe-Cr-B alloy powder containing 25% of Cr and 1.2% of B, 3.0 parts by weight of graphite powder and 3.0 parts by weight of a Fe-P alloy powder containing 21.7% of P. The low-alloy Fe powder consisted of particles that passed through an 80-mesh sieve, and the other powders each consisted of particles passed through a 100-mesh sieve. Thorough mixing was carried out with the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture. In the obtained powder mixture, the contents of essential alloying elements were as shown in Table 1.
The powder composition was compacted into the shape of the rocker arm tip by application of a pressure of 8000 kg/cm², and the compacted body was sintered in vacuum at 1100°C for 60 min. The sintered alloy in the form of the rocker arm tip had a porosity of_.5%.

Example 4

A powder mixture was prepared by mixing 77.5 parts by weight of a Fe-P alloy powder containing 0.6% of P, 20 parts by weight of a Fe-Cr-B alloy powder containing 18% of Cr and 1.8% of B and 2.5 parts by weight of graphite powder. The Fe-P alloy powder consisted of particles that passed through an 80-mesh sieve, and the other powders each consisted of particles passed through a 100-mesh sieve. Thorough mixing was carried out with the addition of zinc stearate amounting to 0.75% of the above powder mixture. In the obtained powder mixture, the contents of essential alloying elements were as shown in Table 1.
The powder mixture was compacted into the shape of the rocker arm tip by application of a pressure of 7000 kg/cm², and the compacted body was sintered in vacuum at 1140°C for 60 min. The sintered alloy in the form of the rocker arm tip had a porosity of 6%.

Endurance test

The sintered rocker arm tips produced in Examples 1 to 4 were individually attached to rocker arms, which were used in a 2-liter automotive engine. The cams with which the rocker arm tips.made rubbing contact were produced by chilled casting of a cast iron consisting of about 3% of C, 2.2% of Si, 0.7% of Mn, 0.2% of P, 0.5% of Cu and the balance of Fe. The hardness of the cam surface was above H_RC 55. The engine was operated over a period of 100 hr to examine the wear resistance and durability of the respective rocker arm tips. To accelerate the wear, the engine was operated with augmented force of the valve spring and with addition of water to the lubricating oil. The details of the test conditions were as follows.
The results of the test are presented in the following Table 2 together with the corresponding data obtained by testing the comparative rocker arm tips produced in the reference experiments described below.

Reference 1

A powder mixture was prepared by mixing 78 parts by weight of the Fe powder used in Example 1, 20 parts by weight of the Fe-Cr-B alloy powder used in Example 1 and 2 parts by weight of graphite powder. With the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture, thorough mixing was carried out for 15 min in a V-shaped blender.
The powder mixture was compacted into the shape of the rocker arm tip by application of a pressure of 8000 kg/cm², and the compacted body was sintered in hydrogen gas, which was passed through a dehydrating agent in advance, at 1175°C for 30 min. The sintered alloy in the form of the rocker arm tip had a porosity of 15%.

Reference 2

A powder mixture was prepared by mixing 68.5 parts by weight of the low-alloy Fe powder used in Example 2, 30 parts by weight of the Fe-Cr-B alloy powder used in Example 2 and 1.5 parts by weight of graphite powder, with the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture.
The powder mixture was compacted into the shape of the rocker arm tip by application of a pressure of 8000 kg/cm², and the compacted body was sintered in vacuum at 1190°C for 45 min. The sintered alloy in the form of the rocker arm tip had a porosity of 5%.

Reference 3

A powder mixture was prepared by mixing 71 parts by weight of Fe powder, which was an atomized iron powder consisting of particles passed through a 100-mesh sieve, 20 parts by weight of a Fe-Cr-B alloy powder which contained 30% of Cr and 1.5% of B and consisted of particles passed through a 100-mesh sieve, 1.0 part by weight of graphite powder, 5 parts by weight of electrolytic Cu powder smaller than 105 um in mean particle size, 2 parts by weight of atomized Pb powder consisting of particles passed through a 200-mesh sieve and 1 part by weight of atomized Sn powder consisting of particles passed through a 200-mesh sieve, with the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture.
The powder mixture was compacted into the shape of the rocker arm tip by application of a pressure of 6000 kg/cm², and the compacted body was sintered in purified hydrogen gas at 1165°C for 60 min.
The sintered alloy in the form of the rocker arm tip had a porosity of 20%.

Reference 4

A powder mixture was prepared by mixing 78 parts by weight of the low-alloy Fe powder used in Example 3, 16 parts by weight of the Fe-Cr-B alloy powder used in Example 1, 1 part by weight of graphite powder and 5 parfs by weight of a leaded bronze powder which contained 10% of Pb and 10% of Sn and consisted of particles passed through a 100-mesh sieve, with the addition of zinc stearate amounting to 0.75% by weight of the above powder mixture.
The powder mixture was compacted into the shape of the rocker arm tip by application of a pressure of 8000 kg/cm², and the compacted body was sintered in purified hydrogen gas at 1170°C for 30 min. The sintered alloy in the form of the rocker arm tip had a porosity of 13%.
In the sintered alloys prepared in References 1 to 4, the contents of Cr, B and C were always within the ranges specified in the present invention, but these sintered alloys were all prepared without using P as an alloying element.
As can be seen in Table 2, the sintered alloy rocker arm tips produced in Examples 1 to 4 were all very high in wear resistance and very low in the tendency to abrade the cams and can be judged to be superior to the sintered alloy rocker arm tips of References 1 to 4.

Claims

1. A method of producing a wear-resistant sintered ferrous alloy, the method comprising the steps of: preparing 100 parts by weight of a powder mixture by mixing 16 to 50 parts by weight of a powder of a Fe-Cr-B alloy which contains 10 to 35% by weight of Cr and 1.0 to 2.5% by weight of B, 1.0 to 3.5 parts by weight of a graphite powder and as balance a phosphorus-containing iron powder which comprises a Fe-P alloy powder such that the prepared powder mixture contains 0.2 to 1.0% by weight of P;

compacting said powder mixture into a body of a desired shape; and

sintering said body in a nonoxidizing atmosphere.

2. A method according to Claim 1, wherein said phosphorus-containing powder is substantially entirely said Fe-P alloy powder.

3. A method according to Claim 2, wherein said Fe-P alloy contains 0.2 to 1.0% by weight of P.

4. A method according to Claim 1, wherein said phosphorus-containing powder is a mixture of a Fe-P alloy powder and an iron powder.

5. A method according to Claim 4, wherein said Fe-P alloy contains 20 to 40% by weight of P.

6. A method according to Claim 1, wherein said phosphorus-containing powder is a mixture of a Fe-P alloy powder and a low-alloy iron powder.

7. A method according to Claim 6, wherein said Fe-P alloy contains 20 to 40% by weight of P.

8. A method according to Claim 1, wherein the amount of the Fe-Cr-B alloy in the first step is in the range from 20 to 30 parts by weight.

9. A method according to Claim 1, wherein said Fe-Cr-B alloy contains up to 3.0% by weight of Si.

10. A method according to Claim 1, wherein the sintering is performed at a temperature in the range from about 1050°C to about 1140°C.

11. A wear-resistant sintered ferrous alloy consisting of 1.6 to 17.5% of Cr, 0.16 to 1.25% of B, 1.0 to 3.5% of C, 0.2 to 1.0% of P, by weight, and the balance Fe and incidental impurities.

12. A sintered ferrous alloy according to Claim 11, wherein the content of Cr in the alloy is in the range from 2.0 to 10.5% by weight, and the content of B is in the range from 0.20 to 0.75% by weight.

13. A sintered ferrous alloy according to Claim 11, wherein the porosity of the sintered alloy'is not greater than 20%.