GB2146409A

GB2146409A - Cylinder liners

Info

Publication number: GB2146409A
Application number: GB08421732A
Authority: GB
Inventors: Norman Tommis
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1983-09-06
Filing date: 1984-08-28
Publication date: 1985-04-17
Also published as: FR2551499B1; ZA846990B; MX161251A; GB8323844D0; ES535631A0; KR850002111A; IT8422552A0; DE3431971A1; BR8404455A; KR910000560B1; IN162274B; DD236773A5; AU563425B2; FR2551499A1; AU3277084A; IT1176665B; GB8421732D0; TR23244A; CA1225311A; JPS60155666A

Abstract

A cylinder liner is produced from a mild steel by pressing, drawing or extruding to a tubular shape and then finish machining. The finally shaped liner is then nitrocarburised with a mixture of carburising and nitrogenous gases to form a hard "epsilon" layer on the liner surfaces. The liner so produced requires no subsequent finishing processes, can be pressed into an engine block without brittle fracture and is resistant to both exterior wear from water, in a wet liner, or from the engine block, in a dry liner, and interior bore wear from an associated piston and piston rings.

Description

SPECIFICATION Cylinder liners The invention relates to cylinder liners for internal combustion engines.

In many internal combustion engines, the or each piston reciprocates in a cylinderformed buy a generally cylindrical dry liner commonly press-fitted, shrunk or otherwise inserted into the engine block.

The inner surface (or "bore") of the cylinder liner contacts the associated piston and so is subjected to wear and scuffing. In addition dry liners can fret against the associated engine block and so a satisfactory dry cylinder liner should be capable of resisting such wear at the same time being capable of being readily press-fitted or slipped into and disassembled from the associated engine block.

One material commonly used for such liners is low carbon mild steel but this does not have satisfactory wear characteristics. For this reason, various techniques have been used to improve wear characteristics. One such technique is to use a material which is harder and more wear resistant than the mild steel.

For example, cast irons or steels which are high in nickel, chromium and molybdenum may be used, particularly when hardened or tempered. However, although these are highly wear resistant, they have the disadvantage that their ductility is much lower than mild steels so they are correspondingly difficult to machine to a required finished shape. In addition, a finished cylinder liner from such a material can be brittle and this can lead to fractures when the liner is press-fitted into a cylinder block.

A second technique is to provide a hard surface layer on a cylinder liner formed from a mild steel our a cast iron. One such layer is a hard chromium plate on the bore of the liner. Chromium plating has the disadvantages, however, that it is an expensive process, so increasing the cost of the liners, and that such plating does not readily retain oil and hence is prone to scuffing in service. In addition, chromium plating can soften at temperatures above 300"C thus reducing its wear resistance, and requires finishing which adds to the expense. Alternative hard surface treatments are correspondingly expensive to apply.

According to the invention, there is provided a method of manufacturing a mild steel cylinder liner for an internal combustion engine and comprising forming a cylinder liner to a final shape from a mild steel, placing the shaped cylinder liner in a chamber from which air is excluded, and then supplying to the chamber a gaseous mixture of a carburising gas and a nitrogenous gas in the ratio of from 25:75 to 75:25 (% by volume) at a temperature of from 500"C to 650"C to nitrocarburise the cylinder liner.

The following is a more detailed description of some embodiments of the invention, by way of example only.

Example 1 A dry cylinder liner is formed of a low carbon steel.

For example, the material may have the following composition: Mild Steel (% by weight) Carbon: 0.05 to 0.30 Silicon: 0.10 to 0.35 Manganese: 0.40 to 1.4 Sulphur: 0.050 max.

Phosphorous: 0.050 max.

Nickel) May be present either as trace elements Chromium) or as significant low alloying Molybdenum) additions Balance: iron.

A billet of such a material is first punched, drawn or extruded through a suitably shaped die to form a generally cylindrical blank. This blank is then machined to the final shape of the cylinder liner by the machining of an end flange, if required, and the shaping to required dimensions of cylindrical inner and outer surfaces.

The shaped cylinder liner is then placed in a chamber from which air is excluded. Next a nitrogenous gas, such as ammonia, and a carburising gas, such as an exothermic hydrocarbon gas, are fed into the chamber at a temperature of between 500"C and 650 C. The proportion of the two gases, nitrogenous to carburising, may be between 25:75 (% by volume) and 75:25 (% by volume) although tests with ammonia and exothermic hydrocarbon gas have shown that ratios of 50:50 (% by volume) or 60:40 (% by volume) give improved results.

The gases contact the surfaces of the cylinder liner and carbon and nitrogen from the gases diffuse from these surfaces into the mild steel of the liner forming a thin layer (so called 'epsilon' layer) between 25 and 20 micrometers thick from which diffusion takes place into the body of the liner. For a particular material, the total depth of penetration depends on the time for which the gases are supplied and this may be regulated to give, for example, an 'epsilon' layer 10 micrometers thick and a total penetration of 0.1 mm to 0.3mm. For example, the time may be 2 to 4 hours. A surface hardness of 800 HV or more is achievable decreasing progressively but nonuniformly to the hardness of the basic material. This hardness is maintained on subsequent exposure of the liner to operating temperatures of up to 550"C.

The cylinder liner is then removed from the chamber and is immediately ready for use without any further finishing steps. The cylinder liner has a hard wear resistant outer surface and a ductile core.

Additionally, the treatment increases significantly the fatigue strength of the liner. This can be of particular importance where a flange is provided on the liner and projects significantly outwardly of the liner because the high fatigue strength increases the fatigue strength of the flange and so reduces the incidence of flange breakage and the incidence of cracks in the flange region.

Example 2 A dry cylinder liner was formed from a mild steel having a carbon content of 0.18%, in any of the ways described above with reference to Example 1. The shaped liner was then placed in a chamber from which air was excluded and was nitrocarburised as described above with reference to Example 1; the temperature being 570"C and the time of treatment 3 hours.

After treatment, the liner was fast gas cooled and then removed from the chamber and examined. The bore and the exterior surface of the liner were found to have equal nitrocarburised layers. A white surface "epsilon" layer 0.04mm thick overlaid a complex structure of iron nitride needles in ferrite grains to a depth of over 0.2mm with a total observed penetration of 0.3mm. In the bulk structure, the hardness decreased from 540 HV just below the surface layer to 380 HV at 0.15mm depth. The core hardness of the material was 160/178 HV at 0.35mm below the surface.

Cylinder liners treated as described were then used in a turbo-charged diesei engine. After 300 hours running, the wear on the surface of the liner was negligible. The oil consumption was acceptable at 0.5% of the fuel consumption and the power about 1% greater than with untreated cylinder liners, due to the low friction liner surface which, after running, had a glass-like appearance. After 550 hours, the oil consumption had slowly risen to 0.7% of the fuel consumption due to bedding-in of the piston rings, but was still acceptable. The power had increased by 1.3% without any increased fuelling.

The methods of manufacture described above and the cylinder liners so manufactured have a number of important advantages. Since the liner is made from a ductile mild steel and since all machining is performed before the nitrocarburising is effected, the shaping of the cylinder liner can be readily and rapidly achieved. Suitable mild steels and cast irons are inexpensive so reducing the cost of the cylinder liners. The nitrocarburising step provides the cylinder liner with a surface finish which is highly wear resistant, which remains effective at elevated temperatures (up to 550 ) and which penetrates the surface to a substantial depth (0.01 mm to 0.3mm).

Such a liner can be pressed into an engine block without fear of brittle fracture because there remains a ductile central core. The outer surface is resistant to wear by fretting with the engine block. The inner piston receiving surface is resistant to the wear and scuffing of the associated piston and piston rings because the surface is well wetted by an oil film.

There are particular benefits where the piston rings are also nitrocarburised for example as described in British Patent Publication No. 2 025. The hard surface of the piston rings would tend to wear the cylinder liner in the absence of a correspondingly hard surface on the liner. Where the liner is a wet liner, the outer surface has been found to resist well cavitation erosion.

It should also be noted that cylinder liners made as described above have increased resistance to atmospheric oxydation, thus reducing the cost of packaging and transportion.

Claims

1. A method of manufacturing a mild steel cylinder liner for an internal combustion engine and comprising forming a cylinder liner to a final shape from a mild steel, placing the shaped cylinder liner in a chamber from which air is excluded, and then supplying to the chamber a gaseous mixture of a carburising gas and a nitrogenous gas in the ratiio of from 25:75 to 75:25 (% by volume) at a temperature of from 500"C to 650"C to nitrocarburise the cylinder liner.

2. A method according to claim 1, wherein the nitrogenous gas is ammonia and the carburising gas is an exothermic hydrocarbon gas.

3. A method according to claim 1 or claim 2, wherein the proportions of the gases are from 50:50 to 60:40 (% by volume).

4. A method according to any one of claims 1 to 3, wherein the temperature is 550"C.

5. A method according to any one of claims 1 to 4, wherein the cylinder liner is treated for a time such that the total depth of penetration of the nitrocarburised layer into the surface of the cylinder liner is from 0.1 to 0.3mm, with an epsilon surface layer of from 0.005 to 0.020mm in thickness.

6. A method according to claim 5, wherein the treatment time is from 2 to 4 hours.

7. A method according to any one of claims 1 to 6 and further comprising forming a generally tubular cylinder blank of mild steel and then machining the cylinder blank to form a finally shaped cylinder liner before the nitrocarburising.

8. A method according to claim 7, wherein the tubular cylinder blank is formed by an extrusion or pressing process or by deep drawing.

9. A method of manufacturing a mild steel cylinder liner for an internal combustion engine substantially as hereinbefore described with reference to any of the foregoing Examples.

10. A cylinder liner when made by the method of any one of claims 1 to 9.

11. A cylinder liner substantialiy as hereinbefore described with reference to the Examples.