EP0946775A1

EP0946775A1 - Iron based metal powder mixture and component made therefrom

Info

Publication number: EP0946775A1
Application number: EP97939050A
Authority: EP
Inventors: Paritosh Maulik
Original assignee: Brico Engineering Ltd
Current assignee: Federal Mogul Coventry Ltd
Priority date: 1996-10-11
Filing date: 1997-09-08
Publication date: 1999-10-06
Anticipated expiration: 2017-09-08
Also published as: KR20000048952A; GB2318126B; GB2318126A; DE69703589D1; GB9621232D0; GB9718854D0; DE69703589T2; WO1998016666A1; ES2152106T3; EP0946775B1

Abstract

A powder mixture suitable for use in a compaction and sintering process comprises three constituents. The first constituent is substantially carbon-free and is formed by one or more alloyed powders and has a composition lying in the range expressed in % by weight: 8 to 12 chromium, 0.5 to 3 molybdenum, optionally up to a toal of 10 of at least one metal selected from vanadium, cobalt and tungsten, a maximum of 2 of other materials including impurities, and the balance iron. The second constituent is formed by sufficient carbon powder to bring the total carbon content of the mixture to 1.5 to 3.0 % by weight. The third constituent is formed by sufficient ferrophosphorus powder to bring the total phosphorus content of the mixture to 0.1 to 0.5 % by weight.

Description

IRON BASED METAL POWDER MIXTURE AND COMPONENT MADE THEREFROM

This invention is concerned with iron-based powders which can be compacted and sintered to form components. In particular, the invention is concerned with powders for forming components in a cost effective way, the components having high wear resistance and high resistance to Hertzian stress, ie the contact stress which acts when elastic contact is made between two bodies whose geometries are defined by circular arcs. Such stresses are experienced, eg, when components make rolling contact with one another. An example of such a component is a roller follower in the valve train of an internal combustion engine in which the roller follower, instead of the more common sliding tappet, engages a cam lobe. The invention is also concerned with components made from such powders.

It is well known to manufacture components by a powder metallurgy (PM) route in which an iron-based powder is compacted (to form a "green" body) and is then sintered. A one step cold compaction and sintering process can produce components with a density of the order of 85-90% of theoretical density. The achievable density is limited by the stresses on the compaction tools and by the characteristics of the powder. In some cases, the powder is pre-alloyed to ensure a uniform distribution of the alloying elements but, where carbon is added, this is usually in the form of graphite powder. However, hitherto, components made by a one step cold compaction and sintering route have proved to have a low endurance limit under rolling contact conditions.

Since an increase in density normally increases the strength of the component and often increases its ability to cope with high Hertzian stress, more complex processing routes such as warm compaction, sinter forging to full density, and double press/sinter have been used to raise the sintered density and the endurance limit. These routes are, however, expensive. It is also possible to apply surface treatments, eg nitriding, or heat treatments, eg hardening and tempering, to the sintered component but this is also expensive.

It is known to alter the formulation of the powder to enable a one step compaction route to achieve increased density of the sintered component. WO 91/18123 describes an iron-based powder which contains 3 to 15% by weight of molybdenum and/or 3 to 20% by weight of tungsten. The powder also comprises 0.2 to 1% by weight of phosphorus and 0.5 to 1.5% by weight of carbon. This powder is stated to enable high density and hardness to be achieved. The phosphorus and carbon are added, as a mixture of Fe₃P and graphite powder. WO 95/32827 describes an iron-based powder which contains 0.6 to 2.0% of molybdenum, 0.2 to 0.8% of phosphorus, and 0.2 to 0.8% by weight of carbon. The powder is stated to be useful for manufacturing components of high tensile ductility by sintering the mixture of Fe₃P, graphite powder and pre-alloyed low alloy steel powder. The carbon content is kept below 0.8% by weight in order to prevent the component from being brittle. EP 0 480 495 A describes a sintered component made from powder mixture of iron powder, manganese sulphide, graphite, and a pre-alloyed powder comprising 8 to 12% by weight chromium, 0.5 to 3% by weight molybdenum, 1.5% by weight maximum vanadium, 0.2% by weight maximum carbon, 2% by weight maximum impurities and the balance iron, the carbon level in the mixture being up to 1.5% by weight.

It is an object of the present invention to provide an iron-based powder which can be made into components which will tolerate high Hertzian stress by a single step compaction and sintering process. The invention provides a powder mixture suitable for use in a compaction and sintering process, characterised in that the mixture comprises a first constituent which is substantially carbon-free and is formed by one or more alloyed powders, the first constituent having a composition lying in the range expressed in % by weight: 8 to 12 chromium, 0.5 to 3 molybdenum, optionally up to a total of 10 of at least one metal selected from vanadium, cobalt and tungsten, a maximum of 2 of other materials including impurities, and the balance iron, the mixture also comprising a second constituent formed by sufficient carbon powder to bring the total carbon content of the mixture to 1.5 to 3.0% by weight, and a third constituent formed by sufficient ferrophosphorus powder to bring the total phosphorus content of the mixture to 0.1 to 0.5% by weight.

It is found that a powder according to the invention can be cold compacted in a single step to form a green body, eg at compaction pressures of less than 800 MPa, and can be sintered in a dissociated ammonia or other protective atmosphere in excess of 1130°C to a density of about 7.1 to 7.4 Mgm^"3. The components so produced have a high hardness (above 60 HRA) . Rolling contact fatigue tests in which the components are subjected to Hertzian stress enable a calculation of the endurance limit to be made, the endurance limit being the safe stress level which the component can endure rolling contact for 200 million cycles. Components according to the invention were found to be capable of enduring stresses in the vicinity of 2,000 MPa. Conventional materials made by a one step cold or warm compaction and sintering route have an endurance limit below 1800 MPa (or in most cases below 1500 MPa) .

In relation to said first constituent, the term "substantially carbon-free" is intended to mean that the constituent contain less than 0.1% by weight of carbon, ie it is a stainless iron. In a powder according to the invention, said first constituent may be formed from a single alloy powder, having the composition stated, or may be formed by two or more alloy powders which in combination have the composition stated. Such alloy powders are well- known and are produced by water atomisation. Where one or more of vanadium, cobalt and tungsten is included, this may form 5 to 10% by weight of the first constituent.

In a powder mixture according to the invention, the chromium range (which is preferably 9 to 11% by weight) is selected to give strength without greatly reducing compressibility. The molybdenum range is selected to be high enough to give sufficient strength but not so high as to cause brittleness (the molybdenum range is preferably 1.5 to 2.5% by weight). The phosphorus range is selected to be high enough for sufficient densification but not so high as effect shape or size control or cause brittleness. The carbon range is selected to be high enough for good hardness but not so high that control of the sintered structure is lost, resulting in brittleness, or loss of size or shape control during sintering.

The invention gives a low cost route for achieving the object of the invention. Where additional wear resistance is required, the option of including one or more of vanadium, cobalt and tungsten in the first constituent is selected.

A powder mixture according to the invention may also optionally have a solid lubricant and/or a machining aid added thereto. Examples include mica, hexagonal boron nitride, manganese sulphide, molybdenum disulphide, calcium fluoride, and barium fluoride. It is, of course, necessary that such additions, either due to coarse particle size or excessive volume fraction, do not act as stress raisers locally. The invention also provides a sintered component made by compacting a powder mixture according to the invention and sintering it. Such a component may, for example, be a cam lobe, a tappet shim, or a roller. Preferably, said component has an endurance limit in excess of 1800 MPa. The component can be hydrogen annealed, if desired.

The invention also provides a method of manufacturing a component comprising cold compacting a powder mixture according to the invention in a one step operation, and sintering. The cold compacting may be at a pressure of less than 800 MPa. The sintering may be one step carried out in a continuous sintering furnace.

There now follow detailed descriptions, to be read with reference to the accompanying drawing, of two examples which are illustrative of the invention.

The drawing is a graphical representation of the results of tests carried out on components made according to Examples 1 and 2 and on components made from comparative powder mixtures.

Example 1

In example 1, a powder mixture having three constituents was prepared. The first constituent was a substantially carbon-free base alloy powder having a nominal composition of 10% by weight chromium, 2% by weight molybdenum, and a balance of iron except for inevitable impurities. The second constituent was graphite powder and the third constituent ferrophosphorus powder, ie an alloy of iron and phosphorus, comprising 26% by weight of phosphorus. The constituents were mixed in the proportions by weight 96.5:2:1.5 giving a powder mixture having the approximate composition by weight: 9.7% chromium, 1.9% molybdenum, 2% carbon, 0.4% phosphorus, balance iron and inevitable impurities. A standard fugitive compaction lubricant was added to the powder mixture. The powder was compacted at a pressure of 620 MPa and sintered in a conventional continuous sintering furnace at a temperature in excess of 1130°C in a dissociated NH₃ atmosphere to make components in the form of 23mm diameter cylinders 10mm thick. The density of the green body was 6.3 Mgrn^"3 and that of the final component was 7.4 Mgm³. The components had a hardness of 74 HRA. No subsequent heat treatments or surface treatments were applied.

The components were ground to a 20mm outside diameter and a surface roughness of 0.4 microns Ra. Four of the components were subjected to a rolling contact fatigue test to establish their endurance. The samples were tested by being loaded between two tungsten carbide rollers and lubricated with oil. The stress applied varied between 2000 and 2200 MPa and the rotation was at 7000 rpm. The test machine was operated in such a way that it ran for 10⁸ cycles or until a critical level of pitting or surface damage in the component caused vibration beyond a certain threshold whereupon a trip switches off the machine.

In the drawing, the results obtained with the components made in Example 1 are represented by triangles, with the X-axis being a log scale of number of cycles to failure and the Y- axis being a linear scale of Hertzian stress in MPa. Specifically, one component was stressed at 2223 MPa and failed at 15.62 million cycles, the second was stressed at 2150 MPa and failed at 17.39 million cycles, the third was stressed at 2071 MPa and failed at 32.97 million cycles, and the fourth was stressed at 2015 MPa and failed at 28.64 million cycles. The plot obtained for stress against number of cycles enables the endurance limit to be calculated and this was found to be in the vicinity of 2000 MPa.

Example 2 Example 2 was the same as example 1 except that the constituents were mixed in the proportions by weight 96:2.5:1.5 giving a powder mixture having the approximate composition by weight: 9.6% chromium, 1.9% molybdenum, 2.5% carbon, 0.4% phosphorus, balance iron and inevitable impurities. The density of the green body was 6.4 Mgm^"3 and that of the final component was 7.4 Mg ³. The components had a hardness of 75 HRA. No subsequent heat treatments or surface treatments were applied.

The drawing shows the test results for the components made according to Example 2 as octagons. Specifically, one component was stressed at 2214 MPa and failed at 8.4 million cycles, the second was stressed at 2132 MPa and failed at 8.82 million cycles, the third was stressed at 2074 MPa and failed at 17.22 million cycles, and the fourth was stressed at 2013 MPa and reached 100 million cycles without failure (this point is plotted on the 100 million line) . The plot obtained for stress against number of cycles enables the endurance limit to be calculated and this was found to be in the vicinity of 2000 MPa.

Comparative Examples

The drawing also shows the results of identical tests for components made according to four comparative examples. In comparative example 1 (represented by stars in the drawing) , components were made by warm compaction and sintering from a powder containing 1.8% by weight nickel, 0.5% by weight molybdenum, and 1.5% by weight copper as an alloy with iron, and 0.5% by weight carbon added as graphite. Specifically, the results were: stress 1361 MPa, failures at 7.14 and 7.06 million cycles (shown as one point on the drawing as the points are substantially at the same location); stress 1241 MPa, failure at 13.02 million cycles; and stress 1463 MPa, failure at 4.54 million cycles. In comparative example 2 (represented by the squares in the drawing) , components were made by warm compaction and sintering from a powder containing 1.5% by weight molybdenum as an alloy with iron, and 0.5% by weight carbon added as graphite. Specifically, the results were: stress 1430 MPa, failures at 5.54, 4.03, 3.78 and 3.53 million cycles (three of these points merge in the drawing) .

In comparative example 3 (represented by the crosses in the drawing) , components were made by warm compaction and sintering from a powder containing 1.8% by weight nickel, 0.5% by weight molybdenum, and 1.5% by weight copper as an alloy with iron, and 0.5% by weight carbon added as graphite. After sintering, the components were given an additional carburising surface treatment (thus, giving them an advantage relative to the components of Examples 1 and 2) . Specifically, the results were: stress 1900 MPa, failure at 3.53 million cycles; stress 1772 MPa, failure at 73.16 million cycles; stress 1670 MPa, failure at 56.95 million cycles; and stress 1593 MPa, reached 100 million cycles without failure (this point is plotted on the 100 million cycle line) .

In comparative example 4 (represented by the diamonds in the drawing) , components were made by warm compaction and sintering from a powder containing 1.5% by weight molybdenum as an alloy with iron, and 0.5% by weight carbon added as graphite. After sintering, the components were given an additional carburising surface treatment (thus, giving them an advantage relative to the components of Examples 1 and 2) . Specifically, the results were: stress 2075 MPa, failure at 26.21 million cycles; stress 2019 MPa, failure at 32.51 million cycles; stress 1862 MPa, failure at 92.48 million cycles; and stress 1674 MPa, reached 100 million cycles without failure (this point is plotted on the 100 million cycle line) . As is readily apparent from the drawing, the components made according to Examples 1 and 2 performed considerably better in the tests than components made according to comparative examples 1 to 4 , despite the advantage given to the comparative example components by the warm compaction, and, in two cases, by an additional surface treatment.

Claims

A powder mixture suitable for use in a compaction and sintering process, characterised in that the mixture comprises a first constituent which is substantially carbon-free and is formed by one or more alloyed powders, the first constituent having a composition lying in the range expressed in % by weight: 8 to 12 chromium, 0.5 to 3 molybdenum, optionally up to a total of 10 of at least one metal selected from vanadium, cobalt and tungsten, a maximum of 2 of other materials including impurities, and the balance iron, the mixture also comprising a second constituent formed by sufficient carbon powder to bring the total carbon content of the mixture to 1.5 to 3.0% by weight, and a third constituent formed by sufficient ferrophosphorus powder to bring the total phosphorus content of the mixture to 0.1 to 0.5% by weight.

A powder mixture according to claim 1, characterised in that said first constituent is formed from a single alloy powder, having the composition stated.

A powder mixture according to claim 1, characterised in that said first constituent is formed by two or more alloy powders which in combination have the composition stated.

A powder mixture according to any one of claims 1 to

3 , characterised in that the chromium range in said first constituent is 9 to 11% by weight.

A powder mixture according to any one of claims 1 to

4 , characterised in that the molybdenum range in said first constituent is 1.5 to 2.5% by weight. A sintered component manufactured by compacting a powder according to any one of claims 1 to 5, and sintering it.

A method of manufacturing a component comprising cold compacting a powder according to any one of claims 1 to 5 in a one step operation, and sintering.

A method according to claim 7, characterised in that the cold compacting is at a pressure less than 800 MPa.

A method according to either one of claims 7 and 8, characterised in that the sintering is in a continuous sintering furnace.