GB2112811A

GB2112811A - A method for the manufacture of hardened cast camshafts

Info

Publication number: GB2112811A
Application number: GB08225661A
Authority: GB
Inventors: James Bryce Mcintyre
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-09-12
Filing date: 1982-09-08
Publication date: 1983-07-27
Also published as: GB2112811B

Abstract

Hardened surface camshafts are cast from an alloyed flake graphite iron and subjected to an austenitising treatment followed by an interrupted quench and austempering. The alloy content of the iron is low and the composition range given is:- Carbon 3.0% to 3.6% Silicon 1.75% to 2.6% Manganese 0.5% to 0.9% Chromium 0.1% to 0.4% Tin 0.1% maximum Sulphur 0.1% maximum Phosphorus 0.2% maximum Iron & the balance Impurities Other alloying elements of Copper, Titanium and Molybdenum may be included. The camshafts are austenitised at 850 DEG -920 DEG C, followed by austempering at 320 DEG - 400 DEG C and then air cooling to produce a structure of bainite with a significant proportion (e.g. 30%) of retained austenite.

Description

SPECIFICATION A method for the manufacture of hardened cast camshafts This invention concerns a method for the manufacture of hardened cast iron camshafts, and has particular application for the mass manufacture of camshafts for internal combustion engines.

It is already known to manufacture camshafts in cast iron, indeed the majority of camshafts for internal combustion engines are of grey cast iron and these have superseded forged steel camshafts for reasons of economy in mass manufacture for the automotive industry.

In order to provide the required hard, wear and scuff resistant characteristics for the surface of the cam lobes whilst retaining the strength, toughness and machinability in the shaft, special manufacturing processes and iron alloys have been developed to employ cast iron.

One known process requires hardening only the cam lobe surface after machining the casting.

Such hardening comprises localised heating of the lobe section by induction or flame with subsequent quenching. Special alloyed cast irons are required, these often being called "hardenable iron", and in the eventual microstructure the hardened zone of the lobe section is martensitic with the shaft zone being fine grained pearlitic.

Problems arise with this process because of the risk of distortion of the shaft due to the localised heating and subsequent quenching, and special steps are required to attempt to obviate this.

Additional complications arise because special jigs and fixtures are required for each design of camshaft, and usually the actual hardening treatment must be varied to suit each camshaft design because of differences in sectional thicknesses, surface area and axial position of one or more cam lobes.

Another known process called "chill-casting" comprises casting the iron into special moulds wherein heat sinks (called chills) are located at specific positions in the moulding sand adjacent to the void defining the cam lobe(s). On casting, the molten iron at these localised positions adjacent to the chills is rapidly cooled to give a hard white iron microstructure at the cam lobes whilst the microstructure of the shaft is that of close grained grey iron, e.g. flake graphite in a pearlitic matrix. Special alloyed cast irons for this process have been developed. However, this process has the disadvantage of requiring labour intensive preparation of the moulds for accurate placement of the chills as well as the receovery, sorting and assembly of the chills on knock-out.

Even with sophisticated automated systems for making the moulds, it is difficult to ensure reliability in accurate location and retention of the chills in place during production and casting.

It is an object of this invention to provide a method of manufacturing camshafts particularly suitable for mass manufacture using simple founding procedures and avoiding separate hardening of localised areas of the camshaft after machining.

According to the broadest aspect of this invention, I provide a method of manufacturing a hardened surface camshaft comprising the steps of: (a) casting the camshaft from an alloyed flake graphite iron having the following composition by weight: Carbon 3.0% to 3.6% Silicon 1.75% to 2.6% Manganese 0.5% to 0.9% Chromium 0. 1% to 0.4% Tin 0.1% maximum Sulphur 0.1% maximum Phosphorus 0.2% maximum Iron s Impurities the balance (b) subjecting the casting to an austenitising treatment for a period of from 30 minutes to 2 hours at a temperature selected within the range of 850or to 9200C; (c) quenching the austenitised casting to a selected austempering temperature;; (d) subjecting the quenched casting to an isothermal treatment at the selected austempering temperature within the range from 3200C to 4000C for a period selected to convert most of the austenitic phase of the microstructure in the zone at and adjacent to the surface of the casting to bainite; and (e) air cooling the austempered casting to ambient temperature to provide a significant proportion of retained austenite in the microstructure of the surface zone of the casting.

Preferably, the significant proportion of retained austenite is at least 30%.

By my invention, the hardened surface of the camshaft is an essentially bainitic structure containing some retained austenite, and this hardened surface provides the desired good wear resistant characteristics to the surface of the cam profile(s). This surface zone of the cam profile(s) is capable of work hardening by conversion of retained austenite to martensite so that a wear resistant surface is continuously renewed in use.

The central core of the casting remains mechanically strong but soft enough to permit machining of oilways or the like.

The isothermal treatment and subsequent cooling (sometimes called "interrupted austempering") provides the retained austenite which is stable up to 3000C. This microstructure thus provides superior wear resistance in camshafts particularly under the commonly arising operating condition of a dry start with poor lubrication causing overheating. With camshafts hardened by the aforementioned traditional methods, the overheating and wear under load causes deterioration of the surface condition of the cam surface.

The chemical composition of the alloyed flake graphite iron is selected to ensure that this essentially bainitic microstructure is produced by the austempering treatment.

It is notable that the alloy content of the iron is relatively low, and the hardenability is restricted.

This is distinctly different to another known method of manufacturing camshafts in which special high alloy irons are used with the castings being through hardened.

The composition of the alloyed flake graphite iron may also include: Copper 0.15% maximum Titanium 0.2% to 0.1% These optional amounts of Copper and Titanium have been found to be useful in promoting the desired microstructure following the austenitising and austempering treatments as given above.

Furthermore, the composition of the alloyed flake graphite iron may also include: Molybdenum 0.2% to 0.5% The alloying elements and the ranges thereof are selected to ensure that the desired microstructure can be obtained in the surface zone, and the presence of titanium, copper and molybdenum in the prescribed optional ranges enhance the formation of the surface layer.

A preferred composition of the alloyed flake graphite iron comprises: Carbon 3.2% to 3.6% Silicon 2.0% to 2.6% Manganese 0.5% to 0.9% Chromium 0.1% to 0.4% Copper 0.15 maximum Tin 0.1% maximum Titanium 0.05% to 0.1% Sulphur 0.1% maximum Phosphorus 0.2% maximum Iron 8 Impurities the balance As will be appreciated, the casting of the selected alloyed flake graphite iron can be carried out in the conventional manner without any special "chills" or the like. Thus, by my method automated founding processes can be employed to produce the camshafts.

Additionally, the subsequent treatments of austenitising and interrupted austempering can be carried out on a mass production basis because the complete casting are subjected to the same treatment in the selected temperature and cooling environments.

The hardening of the camshaft achieved by the austenitising and austempering treatments is not throughout the entire section of the casting, and the hardening presented by the bainitic microstructure is provided at the surface zone where the hardness and wear resistance is required in service.

In an exemplary embodiment of this invention, test bars of 2 inches (5.08 cms) diameter were sand cast from an alloyed flake graphite iron having the following composition: Carbon 3.46% Silicon 2.32% Manganese 0.85% Chromium 0.22% Copper 0.10% Tin 0.02% Titanium 0.07% Sulphur 0.099/0 Phosphorus 0.18% Iron Ei Impurities Balance The cooled test bars were subjected to an austenitising treatment at 9000C for 2 hours and were then fast quenched in a fused salt bath at a selected austempering temperature of 3600 C.

The bars were then subjected to the isothermal austempering treatment at 3600C for 75 minutes and allowed to cool in air to ambient temperature.

A section of the test bar showed a mixed bainitic-austenitic structure having the significant proportion of bainite with some retained austenite in the areas in and adjacent to the surface.

The hardness of the cross-section was taken and a gradient of from 340 Brinell in the surface zone to 280 Brinell in the centrai core was recorded.

Furthermore, wear test have been carried out on the work-hardened surface of the test bars, and these have shown good wear resistance. The optimum resistance to sliding and rolling wear is secured when the essentially bainitic microstructure of the surface zone includes a significant proportion, preferably at least 30% of high carbon retained austenite.

In general terms, the invented method employing the special alloyed flake graphite iron includes subjecting the castings to a heat treatment comprising an austenitising treatment, followed by the isothermal treatment of interrupted austempering. Following the austenitising treatment, the fast quench and austempering causes the austenite to be mainly converted to the harder constituent bainite. The conversion is dependent mainly on time, and by restricting the time of the austempering treatment prior to the slow air cooling, the preferred proportion of the austenite can be retained in the microstructure of the hardened surface zone.

The austenitising and austempering treatments can be carried out in any suitable medium in known manner.

Claims

1. A method of manufacturing a hardened surface camshaft comprising the steps of: (a) casting the camshaft from an alloyed flake graphite iron having the following composition by weight: Carbon 3.0% to 3.6% Silicon 1.75% to 2.6% Manganese 0.5% to 0.9% Chromium 0. 1% to 0.4% Tin 0.1% maximum Sulphur 0.1% maximum Phosphorus 0.2% maximum Iron Ei impurities the balance (b) subjecting the casting to an austenitising treatment for a period of from 30 minutes to 2 hours at a temperature selected within the range of 8500C to 9200C; (c) quenching the austenitised casting to a selected austempering temperature;; (d) subjecting the quenched casting to an isothermal treatment at the selected austempering temperature within the range of 3200C to 4000C for a period selected to convert most of the austenitic phase of the microstructure in the zone at and adjacent to the surface of the casting bainite; and (e) air cooling the austempered casting to ambient temperature to provide a significant proportion of retained austenite in the microstructure of the surface zone of the casting.

2. The method according to Claim 1 wherein the significant proportion of retained austenite is 30%.

3. The method according to Claim 1 or Claim 2 wherein the composition of the alloyed flake graphite iron further includes: Copper 0.15% maximum Titanium 0.2% to 0.1%

4. The method according to any one of Claims 1, 2 or 3 wherein the composition of the alloyed flake graphite iron further includes: Molybdenum 0.2% to 0.5%

5. The method according to Claim 1 or 2 wherein the alloyed flake graphite iron has the following composition by weight:- carbon 3.20% to 3.6% Silicon 2.0% to 2.6% Manganese 0.5% to 0.9% Chromium 0. 1% to 0.4% Copper 0.15% maximum Tin 0.1% maximum Titanium 0.5% to 0.1% Sulphur 0. 1% maximum Phosphorus 0.2% maximum Iron 8 Impurities the balance

6. The method of manufacturing a hardened surface camshaft substantially as hereinbefore described with reference to the exemplary embodiment given.

7. A hardened camshaft made by the method according to any one of the preceding Claims.