GB2176206A - Wear-resistant sintered alloys - Google Patents

Abstract

A sintered alloy having improved shrinkage consistency consists of, in percentage by weight:- Carbon 1.25-2.5 Chromium 2-8% Manganese 0 - 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% The alloy is of particular use in the manufacture of fabricated camshafts because of reduced concavity in cam side surfaces upon sintering.

Description

SPECIFICATION Wear-resistant sintered alloys The present invention relates to sintered alloys having good wear-resistance and has particular, but not exclusive, application to the manufacture of fabricated camshafts.

Conventionally, camshafts are case from white case irons, i.e. cast irons which do not have free graphite in their microstructures. Such cast irons are hard and have the high abrasion resistance required of cams and like parts. However, the casting process is relatively inefficient both in terms of materials utilisation and energy usage. In particular, excess metal is required to fill the mould and substantial further processing by way of machining and/or grinding is required to finish the cast parts to the required dimensional accuracy.

It has recently been proposed to fabricate camshafts using powder metallurgy techniques and such fabricated camshafts are likely to replace cast camshafts for many applications, especially in internal combustion engines. Powder metallurgy is more efficient than the casting process in that materials utilisation and energy usage are both lower and less finishing is required for the same dimensional accuracy.

Further, fabricated camshafts are lighter than cast camshafts and hence particularly attractive for automobile applications.

Camshafts are fabricated by locating green compact or pre-sintered cams in position on low carbon steel shafts and subsequent sintering to simultaneously sinter-the cams and diffusion bond them to the shaft. To the best of our knowledge, the fabricated camshafts proposed to date have all been manufactured using copper-containing alloys for the cams. A disadvantage of such alloys is that the extent of shrinkage during sintering is inconsistent, usually exceeds 5% (linear shrinkage) and a concavity of about 0.015 inch (0.04 cm) forms in the nose wearing (ie. side edge) surfaces of the cams. As a result, the sintered cams are not formed with close tolerances and require substantial grinding to the required dimensional accuracy.

It is an object of the present invention to provide a power mixture which can be compressed and sintered into cams with close tolerances and little or no concavity in the nose wearing surfaces.

Surprisingly, it has been found that replacement of copper by nickel in the previously known alloys increase the consistency of shrinkage during sintering and reduces the concavity in the nose wearing surfaces.

According to one aspect of the present invention, there is provided a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.5 - 0.8% Silicon 0 - 2% Tungsten 0- 5% the balance being iron and less than 2% by weight impurities.

According to a second aspect of the present invention, there is provided a powder mixture containing graphite, chromium, nickel, phosphorus and iron and, optionally, manganese, molybdenum, silicon andl or tungsten which can be compressed and sintered to a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

According to a third aspect of the present invention, there is provided a process of manufacturing a sintered alloy article, which process comprises mixing powdered graphite and a lubricant and, optionally, tungstic oxide and/or ferro-alloy with pre-alloyed powder, compressing the mixture into a shaped article; and sintering said article, wherein the said components are present in such relative proportions as to produce a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0-5% the balance being iron and less than 2% by weight impurities.

According to a fourth aspect of the present invention, there is provided a method of manufacturing a fabricated camshaft, which method comprises mixing powdered graphite and a lubricant and, optionally, tungstic oxide and/or ferro-alloy with a pre-alloyed powder, compressing the mixture into a pre-sintered cam; mounting the cam on a metal shaft; and sintering the cam in situ on the shaft, wherein the said components are present in such relative proportions as to produce a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

It is preferred that the sintered alloy has a density greater than 7.0 g/cc and preferably greater than 7.4 g/cc. For most applications density in the range 7.4 g/cc to 7.8 g/cc is advantageous. It also is preferred that the sintered alloy has a hardness of at least 70 Rockwell A.

The sintered alloy suitably is formed by compressing and sintering a powder mixture of graphite, lubricant and a pre-alloy of the other components. The pre-alloy advantageously is obtained by atomizing a molten mixture of the iron, chromium, nickel and phosphorus and, optional manganese, molybdenum, silicon and/or tungsten. In order to ensure compressibility of the powder and avoid hard phases, the carbon content of the pre-alloy usually is maintained below 0.1%. Whilst the oxygen content preferably is maintained as low as possible, it has not been found necessary to take exceptional steps to limit the oxygen content and levels of up to about 0.5% by weight usually are tolerated.

The pre-alloy powder is sieved and suitably the fraction passing 100 mesh (BSS) is used. The sieved fraction is mixed with a lubricant, eg 1% by weight zinc stearate, and the requisite quantity of graphite powder. The amount of graphite powder is dependant upon, inter alia, the composition of the pre-alloyed powder and the required properties of the sintered alloy and allowance, usually about 0.2% by weight, is made for graphite loss during sintering. The loss is predominantly due to oxidation of the constituents of the sintering gas and/or reaction with metal oxides in the pre-alloyed powder.

The mixture is compacted in a die at, suitably, a pressure of 30 to 45 tsi (4.5 x 105 to 7 x 105 kPa) to give a green compact having a density typically in the range 5.9 to 6.3 g/cc.

Optionally, and often preferably, the green compact is pre-sintered at, for example, 925 to 1000"C for a period of, for example, 15 to 90 minutes in a sintering atmosphere as aforementioned. The higher the pre-sintering temperature and the longer the pre-sintering time, the greater will be the strength of the pre-sintered component. Such a component is easier to handle and less subject to damage if any assembly operations, for example, mounting a cam on a shaft, are to be performed before the final sintering operation.

The green compact or, where pre-sintering has been carried out, the pre-sintered component is then sintered in conventional manner, usually in an atmosphere of nitrogen, nitrogen/hydrogen mixture, hydrogen, or dissociated ammonia and having a dew point of at least -10 C. The sintering temperature depends upon the composition of the pre-alloyed powder and the quantity of graphite but usually is in the range 1050 to 1180 C. Sintering times are typically 15 to 90 minutes and the sintering operation conveniently is carried out in a conventional mesh belt furnace with a water cooled section at its exit end.

The required sintering temperature is best explained by referring to the alloying constituents in the pre-alloyed powder, and giving the reasons for their inclusion.

Chromium and, if present, molybdenum combine with carbon to produce some of the hard phases in the final component. Consequently sufficient graphite must be present to form these carbides or otherwise the final hardness of the sintered alloy will be too low and the wear resistance will be impaired.

Tungsten, if present, improves the general strength of the matrix especially at elevated temperatures.

Although more than 5% may be added, the alloys become increasingly expensive with increase in tungsten content.

Iron, carbon, phosphorus, chromium and, if present, silicon together produce a liquid phase during the sintering process. Iron, carbon and phosphorus exert the strongest influence on the production of this phase and hence the quantities of carbon and phosphorus need to be selected to ensure that the correct amount of liquid phase is present at the sintering temperature. Generally the greater the quantity of liquid phase, the lower the melting point of the liquid phase. The presence of the liquid phase is essential for correct sintering to take place.

Sintering, that is the densification of the compacted powders, takes place by atomic diffusion of the elements to reduce the number of internal surfaces, i.e. holes, due to thermodynamic considerations.

Diffusion through a liquid phase is substantially greater than that through a solid phase, consequently, the presence and amount of the liquid phase are very important for the attainment of high density components. If too much liquid phase is present, the compacted powder looses its shape as the liquid phase is formed due to the overall loss of strength of the compacted powder at the sintering temperature.

The sintering temperature is important because the amount of liquid phase formed depends upon the sintering temperature, consequently a particular compacted powder may not have sufficient liquid phase present at one temperature, but by increasing the sintering temperature by, say, 20"C, sufficient liquid phase can be formed to ensure correct sintering and high density compact.

As is clear from the above, the amount of carbon and phosphorus and also the sintering temperature must be adjusted to produce the correct amount of liquid phase and consequently, alloys of high density.

Nickel and, if present manganese and molybdenum also improve the hardenability of the material in that they ensure that the matrix phase after sintering is present as a hard constituent (martensite). As indicated above, molybdenum has a double effect in that it also produces hard carbide phases. Nickel is almost completely dissolved in the matrix and provides an entirely effective and independant means of controlling hardenability. Too great a quantity however, results in retention of the softer high temperature phase (austenite) and this is to be avoided if a high assintered hardness is required.

Phosphorus in addition to assisting with the production of a liquid phase, also produces hard phosphide phases. Too great a phosphorus content results in too much of this hard phase, especially at grain boundaries, and this results in a brittle material.

Examples of the inter-relationship of carbon, phosphorus and the sintering temperature are given in Table 1 of Example 1.

A variation of the process described above is to produce a pre-alloyed metal powder containing all of the required elements, except carbon and tungsten. This powder is then mixed with lubricant, graphite and tungstic oxide and processed as before. In this variation, the graphite addition has to be increased by an amount dependant upon the quantity of tungstic oxide added. This is because graphite is used during the sintering process to remove some of the oxygen from the tungstic oxide. The resultant tungsten then diffuses rapidly into the pre-alloyed powder when the liquid phase present is formed at the sintering temperature. If the liquid phase were not present, tunsten would diffuse very slowly at the sintering temperature into the pre-alloyed powder and a heterogeneous material would result.

In a similar manner, chromium and, if present, molybdenum could be added as ferro-alloys and diffused into the pre-alloyed powder at the sintering temperature using the liquid phase present at the sintering temperature to speed the diffusion process.

After sintering, the hardness of the components will usually be above 60 Rockwell A, and preferably above 70 Rockwell A. The metallurgical structure of the components will also consist of hard carbide and phosphide phases in a martensitic, or bainitic matrix.

The invention is illustrated in the following non-limiting Examples.

Example 1 An atomized pre-alloyed powder consisting of, in percentages by weight, 0.014% carbon, 4.44% chromium, 0.99% manganese, 0.89% molybdenum, 0.64% nickel, 0.56% phosphorus, 1.27% silicon and the balance iron, and passing through a 100 mesh (BSS) sieve was mixed with 1% by weight zinc stearate and varying amounts of graphite as set forth in Table 1 below. The mixtures were compressed at 40 tsi (6 x 105 kPa) into 1 inch (2.5 cm) diameter samples of 0.76 inch (1.9cm) thickness and sintered in a hydrogen atmosphere with a dew point of -20 C using a tube furnace. The sintering temperature for each sample is set forth in Table I below.

The density of the sintered discs and their hardness (in Rockwell A) were measured and the results are set forth in Table I below. Further, the out of roundess of the sintered samples was measured and the results also are set forth in Table I below.

TABLE 1 Effect of amount of liquid phase on suitability % added Sintering Sintered Out of Hardness graphite Temperature g/cc roundness "C RA inch (cm) 1.25 1056 6.28 .001 (.003) 58 1.25 1162 7.12 .001 (.003) 78 1.50 1049 6.28 .001 (.003) 57 1.50 1132 7.26 .001 (.003) 74 1.50 1168 7.48 .009 (.025) 76 1.75 1050 6.27 .000 (.00) 59 1.75 1114 7.45 .005 (.015) 74 1.75 1150 7.51 .022 (.055) 79 2.00 1056 6.55 .001 (.003) 66 2.00 1084 7.46 .001 (.003) 79 2.00 1162 7.54 .097 (.245) 81 2.25 1050 6.53 .001 (.003) 63 2.25 1103 7.55 .005 (.015) 72 2.25 1160 7.52 .133 (.340) 73 2.50 1078 6.37 .000 (.00) 60 2.50 1162 7.16 .005 (.140) 70 The results set forth in Table 1 show that:: (a) for the same addition of graphite, density increases with sintering temperature because there is more liquid phase present and hence diffusion is more rapid; (b) for the same sintering temperature, density increases with increasing addition of graphite, which increases the liquid phase and hence diffusion; and (c) if there is too much liquid phase, the product has high density but the out-of-roundness is pronounced because the sample has low strength at the sintering temperature.

Example 2 The procedure of Example 1 was repeated except that the pre-alloy was mixed with 6.25% tungstic oxide and varying amounts of graphite. Samples were of a similar size to those of Example 1 and were sintered to a hydrogen atmosphere with a dew point of -10"C. The graphite addition and sintering temperatures are set forth in Table 2 below together with the density and hardness (in Rockwell A) of the sintered alloys.

TABLE 2 Sintering Density %added graphite temp. "C gicc Hardness 2.25 1080 6.45 64 2.25 1120 6.82 60 2.75 1080 7.43 70 It can be seen from the results of Table 2 that, compared with Example 1, additional graphite is required to reduce the tungstic oxide.

Example 3 A pre-alloy powder/zinc stearate/graphite mixture as described in Example 1 was compressed at 40 tsi (6 x 105 kPa) into green cams of 6.1 g/cc density. The cams were pre-sintered at 950"C for 30 minutes and then assembled in a steel shaft and located in position thereon. The assembly was sintered at 1100"C for 30 minutes in a conventional mesh belt furnace with a water cooled exit chamber, in an atmosphere of dissociated ammonia with a dew point of at least -10 C.

The cams had a nominal length of 1.760 inches (4.47 cms), a nominal width perpendicular thereto of 1.360 inches (3.455 cms), and nominal thickness of 10mm with an internal shaft-receiving hole of 1.024 inches (2.60 cms) diameter. After sintering, the side edge surfaces of the cams were substantially flat having at most 0.002 inch (0.005 cm) concavity. Further, the length and width were both very consistent, having variations of 0.002 to 0.003 inch (0.005-0.007 cm) in length and 0.001 to 0.002 inch (0.002 to 0.005 cm) in width. This consistency and lack of concavity was significantly less than that of similar cams sintered from prior art copper-containing alloys.

Claims (18)

1. A sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5 Chromium 2 - 8% Manganese 0-1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

2. An alloy as claimed in Claim 1, which contains manganese in an amount not exceeding 1.5% by weight.

3. An alloy as claimed in Claim 1 or Claim 2, which contains molybdenum in an amount not exceeding 3% by weight.

4. An alloy as claimed in any one of the preceding claims, which contains silicon in an amount not exceeding 2% by weight.

5. An alloy as claimed in any one of the preceding claims, which contains tungsten in an amount not exceeding 5% by weight.

6. An alloy as claimed in any one of the preceding claims, wherein the alloy has a density of at least 7.0 g/cc.

7. An alloy as claimed in Claim 6, wherein the alloy has a density of at least 7.4 g/cc.

8. An alloy as claimed in any one of the preceding claims, wherein the alloy has a hardness of at least 70 Rockwell A.

9. An alloy as claimed in Claim 1 and substantially as hereinbefore described.

10. A powder mixture containing graphite, chromium, nickel, phosphorus and iron and, optionally, manganese, molybdenum, silicon and/or tungsten which can be compressed and sintered to a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0 - 1.5% Molybdenum 0 - 3% Nickel 0.5 - 0.8% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

11. A powder mixture as claimed in Claim 10, wherein the said sintered alloy is as claimed in any one of Claims 2 to 9.

12. A powder mixture as claimed in Claim 10 and substantially as hereinbefore described.

13. A process of manufacturing a sintered alloy article, which process comprises mixing powdered graphite and a lubricant and, optionally, tungstic oxide and/or ferro-alloy with pre-alloyed powder, compressing the mixture into a shaped article; and sintering said article, wherein the said components are present in such relative proportions as to produce a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

14. A process as claimed in Claim 13, wherein the said sintered alloy is as claimed in any one of Claims 1 to 9.

15. An article when manufactured by a process as claimed in Claim 13 or Claim 14.

16. A method of manufacturing a fabricated camshaft, which method comprises mixing powdered graphite and a lubricant and, optionally, tungstic oxide and/or ferro-alloy with a pre-alloyed powder, compressing the mixture into a pre-sintered cam; mounting the cam on a metal shaft; and sintering the cam in situ on the shaft, wherein the said components are present in such relative proportions as to produce a sintered alloy consisting of, in percentages by weight: Carbon 1.25 - 2.5% Chromium 2 - 8% Manganese 0- 1.5% Molybdenum 0 - 3% Nickel 0.5 - 3% Phosphorus 0.3 - 0.8% Silicon 0 - 2% Tungsten 0 - 5% the balance being iron and less than 2% by weight impurities.

17. A method as claimed in Claim 16, wherein the said sintered alloy is as claimed in any one of Claims 1 to 9.

18. A fabricated camshaft when manufactured by a process as claimed in Claim 16 or Claim 17.