Thermal And Abrasion Resistant Sintered Alloy

Abstract

An alloy prepared by molding a powdery composition comprising 0.6 to 2% of carbon, 0.5 to 4% of nickel, 0.5 to 5% of molybdenum and 6 to 11% of cobalt, by weight, and the balance being iron, and then sintering the molded composition, has large thermal resistance and abrasion resistance.

Description

BACKGROUND OF THE INVENTION

A publicly known metal such as chromium, cobalt, tungsten, etc. has not only a large abrasion resistance but also is prominent in its characteristics at elevated temperatures and is applied in various fields. However, such a metal has many problems to be solved when it is used as sintered parts for a machine. That is, such a metal has a high melting point so that the sintering temperature is, of necessity, required to be elevated, and the sintering time has to be extended, and, therefore, it is naturally disadvantageous in cost.

SUMMARY OF THE INVENTION

The present invention provides a sintered alloy having large thermal resistance and abrasion resistance suitable for use as a sliding element such as, for example, a valve sheet in which high thermal resistance and high abrasion resistance are required. That is, the present invention comprises a sintered thermal and abrasion resistant alloy comprising a molded and sintered powdery composition consisting of by weight 0.6 to 2% of carbon, 0.4 to 4% of nickel, 0.5 to 5% of molybdenum, 6 to 11% of cobalt and the balance iron.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the abrasion resistance of sintered alloys of the Examples and of a conventional cast iron and a sintered iron alloy when evaluated in a valve sheet abrasion test machine; and

FIG. 2 is a graph showing the hardness at elevated temperatures of sintered alloys of the Examples and of a conventional cast iron and a sintered iron alloy.

DETAILED DESCRIPTION OF THE INVENTION

In the sintered alloy of the present invention, when the carbon content is less than 0.6%, by weight, the alloy becomes a ferrite-excessive structure so that a high hardness cannot be expected while, when the carbon content is more than 2%, the alloy changes to a cementite-excessive structure which is high in britteleness.

Nickel strengthens the base structure of the alloy and improves the thermal resistance and abrasion resistance, however, the effect is small with a nickel content of less than 1%, while, when it becomes more than 4%, the base structure locally changes to martensite so that the hardness increases unnecessarily.

Molybdenum increases the tenacity of alloy as well as the impact strength and endurance limit, and, on the other hand, improves the heat treatment property and stabilizes the structure after sintering, however, there is little effect with less than 0.5% of molybdenum and even if more than 5% is present, no effect corresponding to the increase is obtained.

Cobalt is selected for substantially improving the thermal resistance and the abrasion resistance at elevated temperatures and has been established to be 6 to 11% on the basis of a synergistic effect with the other elements.

In the sintered alloy of the present invention, from a viewpoint of providing the material with a high density and improving the lubricating property, it is very advantageous to impregnate molten lead into the alloy after the alloy is molded and sintered.

In this case, the amount of lead impregnated has been experimentally confirmed to be preferably within the range of 0.05 to 5%. That is, with less than 0.05% the effect of impregnation is not remarkable and the impregnation of more than 5% of lead involves a problem in strength from the relation with the density of material before impregnation.

The present invention will be further illustrated by the following Examples by which the present invention is not intended to be limited. All percents are by weight.

EXAMPLE 1

1.2% of graphite powder (-325 mesh), 2% of carbonyl nickel powder (-250 mesh), 2% (as molybdenum) of ferromolyb-denum powder (-150 mesh), 10% of cobalt powder (-150 mesh) and 1.0% of zinc stearate as a lubricant were added to reduced iron powder (-100 mesh) as iron powder. The mixture was molded under a pressure of 4.5 ton/cm.sup.2 and sintered at 1,120.degree. to 1,170.degree. C for 30 to 60 minutes in an atmosphere of decomposed ammonium gas. The sintered material so obtained had a density of 6.6 g/cm.sup.3 and a Rockwell B scale hardness of 92.

The results of the abrasion test on this sintered material using a valve sheet abrasion testing machine (rotation number 3,000 rpm, spring pressure 35 Kg, valve velocity at the time of valve closing 0.5 m/sec., width of valve 1 mm, test repeating number 8 .times. 10.sup.5, material SUH 31B) are shown in FIG. 1, and the results of the measurement of hardness at elevated temperatures are shown in FIG. 2.

EXAMPLE 2

A sintered material comprising 0.68% of carbon, 0.71% of nickel, 0.66% of molybdenum, 6.92% of cobalt and the balance iron was made under the same conditions as described in Example 1, and impregnated with molten lead. The sintered material so obtained had a density of 6.4 g/cm.sup.3 and a Rockwell B scale hardness of 90. The lead content was 0.07%.

EXAMPLE 3

A sintered material comprising 1.83% of carbon, 3.88% of nickel, 4.79% of molybdenum, 10.62% of cobalt and the balance iron was made under the same conditions as described in Example 1 and impregnated with lead. The sintered material so obtained had a density of 6.7 g/cm.sup.3 and a hardness on the Rockwell B scale of 94. The lead content was 4.7%.

Next, the abrasion test results using a valve sheet test machine on Examples 1, 2 and 3 are shown in FIG. 1 and the test results of hardness at elevated temperatures are shown in FIG. 2. For comparison these tests were run on a conventionally known cast iron and conventional sintered ferro alloy. In this case, the compositions of the cast iron and ferro alloy are as follows:

Ferro alloy: Carbon 1%, chromium 3%, the balance iron.

Cast iron: Carbon 3.02%, silicon 2.01%, manganese 0.48%, chromium 0.81%, the balance iron.

Claims

What is claimed is:

1. A thermal and abrasion resistant sintered alloy consisting essentially of a molded and sintered composition comprising from 0.6 to 2% of carbon, from 0.5 to 4% of nickel, from 0.5 to 5% of molybdenum and from 6 to 11% of cobalt, by weight, the balance being iron.

2. The sintered alloy of claim 1 consisting of the recited components.

3. The sintered alloy of claim 1 exhibiting a hardness (HV) on the order of 500 over the temperature range of from normal temperature to about 500.degree. C.