High temperature-resistant wearproof sintered alloys

Abstract

This invention relates to iron base sintered alloys which excel in high temperature resistance and wear resistance and are especially suited for use in the valve seats of an internal combustion engine. These high temperature-resistant, wear resistant sintered alloys are characterized by the fact that they contain Fe as their main component, together with Mo 3-20%, C 0.5-1.5%, Co 3-25% and Pb 1-15% by weight; and that the Fe matrix has very hard particles, which contain intermediate phase .epsilon., and Pb dispersed therein.

Description

BACKGROUND OF THE INVENTION

Conventionally, valve seats have been made mostly of special cast iron or heat-resistant steel. These materials perform well when the gasoline contains an anti-knock agent such as tetraethyl lead, but perform poorly when lead-free gasoline is used. In the first case, various organic leads added to the gasoline for anti-knock purposes become lead oxides when the gasoline burns and are deposited on the working surfaces of valves and valve seats, thereby serving to protect or lubricate the surface of the valve seats or absorb the impact energy of the valves, thus preventing wear of the valve seats. When, however, the gasoline is lead-free, the anti-wear effect of lead is lost and, in consequence, valve seats of conventional material suffer heavy wear, resulting in a poor fit between the valve and valve seat, which leads to a drop in the output of the internal combustion engine and a failure to operate normally. After much research to eliminate the above-mentioned drawback, the present inventors have successfully perfected the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscopic composition photograph illustrating a sintered alloy of the present invention,

FIG. 2 is an optical microscopic composition photograph illustrating a reference alloy,

FIG. 3 is an explanatory diagram illustrating the wear resistance structure of an invented sintered alloy.

SUMMARY OF THE INVENTION

The present invention relates to high temperature-resistant, wear resistant sintered, iron-based alloys containing Mo 3-20%, C 0.5- 1.5%, Co 3-25%, and Pb 1-15% by weight. These alloys are characterized by the fact that the Fe matrix has very hard particles and Pb dispersed therein. If Ni 1-15% or Cr 3-25% or both Ni and Cr totaling 2-30% by weight are added, further improvement of the heat resistance and wearproof resistance will be achieved.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is to provide high temperature-resistant, wear resistant sintered alloys, suitable for use as valve seat material, which assure the good performance of an internal combustion engine which burns non-lead gasoline or light oil or LPG, which is orginally free from organic lead, which are suitable as wear resistant materials for bearings and siding parts to be used under high temperature conditions.

The sintered alloys according to the present invention are high temperature-resistant wear resistant sintered iron-based alloys comprising Mo 3-20%, Co 3-25%, C 0.5-1.5% and Pb 1-15% by weight, characterized by the fact that their relatively soft iron matrix has dispersed evenly therein a phase mainly composed of Fe and Mo with a particle size of several tens of microns and a hardness of Hv 600-1300, and Pb.

If in these sintered alloys the above composition is modified by the further addition of Ni 1-15% or Cr 3-25% or both Ni and Cr totaling 2-30% by weight, their heat resistance and wear resistance will be still further enhanced.

Some examples of the present invention will now be described:

The present invention is characterized by restricting the range of components as well as by suitably controlling the sintering period and temperature. Generally, it is preferable that the production of Fe--base sintered alloys containing a considerable amount of Mo and Co is processed by sintering at a temperature of 1,300.degree.C for comparatively long periods of time to sufficiently diffuse alloy atoms into Fe.

The present invention, however, is characterized in that even in case a considerable quantity of Mo is added into the alloy, sintering can be made at temperatures ranging from 1,130.degree. to 1,180.degree.C for a comparatively short period of time, such as 25 - 45 minutes in order to diffuse Mo or Cr into the Fe--matrix as well as to control diffusion of Fe into Mo powder, and further characterized in that the content of Mo in the Fe matrix is less than 3%, Mo concentration in Mo powder is within 40 - 80% and the hardness of the alloy is Hv (10) 600 - 1,300. The characteristic features of the present invention will be concretely described below with reference to the Figures.

FIG. 1 shows an optical microscopic composition photograph for the specimen manufactured under the conditions given above, which obviously shows the Mo-phases scattered in particles. FIG. 2 shows another optical microscopic photograph for the specimen which contains ingredients identical to those of specimen 4 and sintered for 1 hour at a temperature of 1,300.degree.C, but the Mo--phases as in FIG. 1 can not be seen and .epsilon. phases are significantly precipitated on the grain boundary. With such a structure as shown in FIG. 2, the alloy itself becomes fragile and moreover, improvement in wear-resistance is hopeless due to peeling of the phases precipitated on the grain boundary. As an example, the wear-loss of the alloy presented in FIG. 2 is 5.74mm.

FIG. 3 is an explanatory diagram showing wear-resistance structure at high temperature of the alloys according to the present invention, and numbers 1, 2 and 3 in the FIG. indicate the Fe--base matrix, hard Mo particles and Pb, respectively. At the start of using the alloys according to the present invention, comparatively soft surfaces of the Fe--base matrix are abraded, but hard Mo particles form (.alpha. + .epsilon.) or Mo + .epsilon.) phases which are stable at high temperature and highly wear-resistable thereby lumpy and pitted surfaces are produced. Pb melted at high temperature, permeates into the pitted portions, becomes lead oxide and adheres.

FIG. 3 shows the above state, and the sintered alloys according to the present invention demonstrate superb wear-resistance at high temperature due to wear-resistance of hard Mo particles themselves and also, due to the lubricating ability of lead oxide adhered to the pitted portions.

Below, concrete descriptions are given by referring to the embodiment according to the present invention.

In principle, reduced -100 mesh (less than 147.mu.) Fe--powder, reduced -200 mesh (less than 74.mu.) Co--powder, nickel carbonyl powder in 3.mu. average granularity, pounded -200 mesh Cr--powder, flaky graphite powder and pounded -100 mesh Pb-powder were used. For speciments No. 1,4,5,6,7,8 and 9, the alloys containing 63 percent of pounded -200 mesh Fe--Mo alloy powder and the balance of Mo powder were used, and for specimens 2 and 3, reduced Mo--powder with 3 - 6 granularity were employed and admixed to obtain compositions given in Table 1. For admixing, a V-type power mixer was used by adding 0.5 percent zinc stearate to each specimen as lubricant.

Each of the admixed specimens was compressed and shaped at the density of 6.8g/cm.sup.2 by means of a hydraulic press. For specimens 1 - 5, sintering was made in a decomposed ammonium gas atmosphere, and specimens 6 - 9 were sintered in a vacuum greater than 10.sup.-.sup.3 mmHg in order to prevent oxidization due to the content of Cr.

The sintering temperature was set within a range from 1,130.degree. to 1,180.degree.C for restricting diffusion of Mo into the Fe matrix at less than 3%.

Specimen 3 was made by adding and mixing pounded Pb--powder and other ingredient powders at the same time, but for other specimens, addition was processed by heating at a temperature of 900.degree.C for 30 minutes in the decomposed ammonium gas atmosphere and subjecting the specimens to Pb infiltration after sintering.

In this table, the amount of wear is expressed in terms of a decrease (mm) in the height of a square speciment mounted in cast iron after 100 hours of testing on a so-called "sliding high-cycle impact" wear testing machine which rotates the specimen at 10 rpm under a high temperature of 500.degree.-550.degree.C, and subjects it to 2,500 impacts per minute with a contact pressure of 30 kg/cm.sup.2, using a jig made of heat-resistant steel.

TABLE __________________________________________________________________________ Hardness Tensile Specimen Chemical composition Room 600.degree.C strength Wear No. by weight (%) temperature (600.degree.C) Hv(10) Hv(5) Kg/mm mm __________________________________________________________________________ Example 1 (Fe-10% Mo-10% Co 240 180 29 0.73 1.2%C) -12%Pb Example 2 (Fe-3%Mo-3%Co-0.5%C) 150 120 13 0.61 15%Pb Example 3 Fe-20%Mo-25%Co-1.5%C 250 180 21 0.67 1%Pb Example 4 (Fe-10%Mo-10%Co-1%Ni 290 185 23 0.64 1.2%C) -12%Pb Example 5 (Fe-10%Mo-10%Co-15%Ni 340 220 37 0.51 1.2%C) -12%Pb Example 6 (Fe-10%Mo-10%Co-3%Cr 280 210 30 0.62 1.2%C) -12%Pb Example 7 (Fe-10%Mo-10%Co 320 230 46 0.48 35%Cr-1.2%C) -12%Pb Example 8 (Fe-10%Mo-10%Co-1%Ni 300 205 28 0.60 1%Cr-1.2%C) -12%Pb Example 9 (Fe-10%Mo-10%Co-5%Ni 330 240 54 0.46 25%Cr-1.2%C) -12%Pb Control 1 Fe,-3.5%C,-25%Si,-1%Mn, 280 220 30 7.42 (Cast iron) 0.5%P,-0.5%Cr,-0.5%Mo,-0.1%V Control 2 Fe,-0.4%C,-2%Si,-15%Cr, 300 260 45 6.88 (Heat-resist- 15%Ni,-2%W,-0.5%Mn ant steel) __________________________________________________________________________

As seen from this Table, the amount of wear in the invented sintered alloys is considerably less than that in the conventional valve seat materials, i.e., cast iron and heat-resistant steel; and if necessary, the high temperature strength can be further improved through addition of Ni and Cr.

Next, the effects and the reasons for limiting the amounts of the component elements in the sintered alloys according to the present invention will be described.

In the sintered alloys according to the present invention, carbon permeates into the iron in the form of a solid solution thereby forming a perlite, and has the effect of increasing the hardness and wear resistance of the alloy as well as improving its mechanical properties and, combining with Mo to form carbides of the Mo.sub.2 C or MoC type, it has the effect of increasing the wear resistance of the alloy. When the carbon content is less than 0.5% however, the effect is unsatisfactorily small, and when it is more than 1.5%, the carbides and cementite are precipitated so excessively as to substantially impair the machinability of the alloy to a great extent. For this reason, the carbon content should be limited to 0.5-1.5%.

Both molybdenum and lead are basic elements in the sintered alloys according to the present invention. In these alloys the Mo is not diffused evenly in the Fe matrix, but it is scattered as grains of several tens of microns in size. Meanwhile, by controlling the Fe--diffusion into the Mo, the Mo included is in the .alpha.+ .epsilon. or (Mo + .epsilon.) phase, thereby making the hardness equal to Hv 600-1,300.

The (Mo + .epsilon.) or (.alpha. + .epsilon.) phase thus obtained retains its hardness even at 600.degree.C and exhibits high wear resistance at high temperature. When the Mo--content is less than 3%, namely, the number of particles of (.alpha. + .epsilon.) or (Mo + .epsilon.) are insufficient, the wear resistance is still insufficient and accordingly, an Mo--content of over 3% is desirable. On the contrary, an Mo--content of over 20% is not so effective as expected in improving the wear resistance and it even lowers the mechanical properties of the alloy. Therefore the Mo--content should lie within the 3-20% range. Meanwhile, Mo in partial solid solution in Fe has the effect of increasing its resistance to temper-softening at high temperatures and improving its anti-impact properties. On the other hand, the precipitated or quasi-precipitated Mo, forming Mo oxides at high temperatures, contributes to the improvement of wear resistance through a decrease in the coefficient of friction. Mo may be employed either in the form of Mo--powder or in the form of an Fe--Mo alloy powder. Co is also added to the sintered alloys of the present invention. Co, as a total solid solution in Fe, has the effect of inhibiting the growth of ferrite crystal grains, preventing a drop in hardness at high temperatures progressively as the Co content increases, and thereby improving the mechanical properties. This effect, coupled with the effect of strengthening the bond between the Fe--matrix and Mo grains, results in an improvement of wear resistance at high temperatures. However, the effect attributable to Co for solid-solution hardening the ferrite is small; and since it does not cause a great increase in the hardness of the alloy at room temperature, Mo has the advantage of not deteriorating the machinability so much as it increases the heat resistance. This advantage will not become practically available at a Co--content of less than 5%, but when the content exceeds 25%, the advantage will not be as great as would be expected from the increased content. Moreover, at 36-69%, a super-lattice is partially formed, resulting in an embrittlement of the alloy. Thus, the addition of Co should be limited to 5-25%.

During service of the alloy lead is thinly deposited on the surface of the alloy to form a lead oxide, which acts as a lubricant and has the effect of improving the wear resistance of the alloy. The remarkable increase in wear resistance at high temperatures of the invented sintered alloys is the result of the combined effects of Pb to provide lubrication, Mo to increase the wear resistance and Co to increase the heat resistance. Also, Pb has the effect of substantially improving the machinability, but this effect is low at a Pb content of less than 1%, while at over 15% the high temperature strength of the alloy obtained is inadequate. Therefore, the content of Pb should be 1-15%.

Thus, according to the present invention, through adequate combination of Fe, Mo. Co, C and Pb and effective utilization of the obtained wear resistance, heat resistance and lubrication, an improved high temperature wear resistance can be achieved as an overall effect, and if necessary, the heat resistance and the wear resistance associated with it can be further increased by adding Ni and Cr either singly or in combination, without sacrificing the above-mentioned effect.

The additional element Ni, which, like Co, can enter into a solid solution with Fe in any ration, has the effect of increasing the hardness and toughness of the alloy; and if Ni is added in great quantity, it also has the effect of improving the heat resistance.

Meanwhile, Mo, which can enter into a solid solution with Ni containing up to 20% Mo, even at room temperature, has the effect of strengthening the bond between the Fe-matrix and Mo and of further enhancing the wear resistance of the sintered alloy according to the present invention. This effect, however, is poor when the Mo content is less than 1%. From the stand point of wear resistance 5% will suffice but, depending on the required degree of wear resistance, the content may be increased. At over 15%, however, the effect of increased content in terms of heat resistance will not be so great. Thus, its addition is normally limited to less than 15%.

As for the Cr content, this element, as a solid solution in Fe, makes Fe strong and tough; and in coexistence with Fe.sub.3 C it forms composite carbides such as (Fe.sub.3 C).sub.18, Cr.sub.4 C, (Fe.sub.3 C).sub.9, Cr.sub.4 C and Fe.sub.3 C . CrC, which improve the hardness and wear resistance of the alloy. Also, it has the effect of minimizing the deterioration of material due to temperature rise and enhancing the heat resistance. At less than 3%, its effect is not great, and at over 25% its effect is less than might be expected from the increased content; rather it aggravates embrittlement of the material. Therefore the preferred Cr content is less than 25%.

When Ni and Cr are added at the same time, the desirable total addition is 2-30%. If either Ni or Cr is singly added the effect of simultaneous addition will not fully appear unless each content is more than 0.5%.

According to the present invention grains several tens of microns in size of (Mo + .epsilon. ) phase with Hv 600-1,300, which is stable even at high temperatures, are evenly dispersed in the matrix of a heat resistant sintered alloy of a relatively soft Fe--Co--C system, thereby increasing the wear resistance of the alloy. Pb is added to provide lubrication, and through the overall effect of these additions, an excellent wear resistance at high temperatures is secured in the alloy obtained. Depending on the need therefore, Ni or Cr singly or in combination may be added to further increase the heat resistance and wear resistance. For this reason, the sintered alloys of the present invention are particularly suitable for use as materials for valve seats in an internal combustion engine which burns the fuel which is free from anti-knock agents, such as non-leaded gasoline, LPG (liquefied propane gas) or light oil; and equally suitable for use as materials for bearings to operate under unlubricated and high temperature conditions such as bearings in hot press rolls.

Claims

What is claimed is:

1. High temperature resistant, wear-resistant sintered alloy consisting essentially of:

3-20% by weight of molybdenum,

0.5-1.5% by weight of carbon,

3-25% by weight of cobalt,

1-15% by weight of lead, and

balance mainly iron,

said alloy being in the form of particles consisting essentially of molybdenum and iron, said particles being several tens of microns in size and having a Vickers hardness of 600-1,300, and lead, dispersed in an iron-based matrix softer than said molybdenum-iron particles.

2. Alloy as claimed in claim 1 in which said molybdenum-containing particles comprise an .epsilon. phase.

3. Alloy as claimed in claim 1 in which said matrix contains less than 3% molybdenum.

4. Alloy as claimed in claim 3 containing from 5-15% molybdenum.

5. Alloy as claimed in claim 1 containing from 1-15% nickel.

6. Alloy as claimed in claim 1 containing 3-25% chromium.

7. Alloy as claimed in claim 1 containing nickel and chromium in a total amount lying between 2 and 30%.

8. Alloy as claimed in claim 1 containing molybdenum and chromium in a total amount of at least 18%.

9. Alloy as claimed in claim 1 in which said alloy has a Vickers hardness lying between 150 and 330.

10. Alloy as claimed in claim 1 consisting essentially of the constituents claimed in claim 1, and at least one additional ingredient selected from the group consisting of 1-15% nickel and 3-25% chromium.