Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1994-03-15
1996-11-12
Lin, Kuang Y.
Metal founding
Process
Shaping liquid metal against a forming surface
164127, B22D 1500, B22D 2720
Patent
active
055730574
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for the manufacture of chromium containing, chill cast, cast iron camshafts.
Camshafts for use in, for example, internal combustion engines have been made in cast iron. There are two production methods which have been used most extensively, these are; either to cast the camshaft in a hardenable iron followed by, for example, induction hardening of the cam lobes, or to incorporate cold metal chills in the casting mould to produce a white iron chill cast structure in at least the cam lobes during the casting process. It is the latter production process with which the present invention is principally concerned.
Camshafts generally comprise an elongate shaft on which the valve operating cams are disposed in varying orientations together with camshaft bearing journals and also sometimes other features, such as ancillary equipment drive gears or various projections, for example, which require post-casting machining. Indeed, the shaft itself often requires a bore to be produced along the shaft centre, the bore usually being produced by the technique known as "gun-drilling". The white iron structure of cast iron is ideally confined to the cam lobe regions where it is desirable for its wear resistant properties which stem from the high hardness of this type of structure. White iron comprises iron carbides in a pearlite matrix; the iron carbides rendering the metal so hard that the cam lobes are normally finished by grinding. Where metal cutting operations need to be performed on portions of the cast camshaft it is desirable that such portions solidify as grey iron which has a structure comprising graphite flakes in a pearlite matrix and which is readily machinable by normal metal cutting techniques.
Which form of cast iron is produced on solidification will depend, principally, upon several factors which include the chemical composition of the iron being cast, the cooling rate of the metal during solidification and the degree of nucleation applied to the molten metal.
Co-pending British Patent application number 9106752.0 of common ownership herewith describes the manufacture of chill-cast camshafts from substantially unalloyed cast-iron. In some applications, it is necessary that the camshaft be made of an inherently stronger material than basic unalloyed cast-iron. For this reason chromium is frequently used as an alloying addition to cast-iron. Chromium increases the mechanical properties such as, for example, fatigue resistance, tensile strength, shear strength, torsional strength and hardness of grey iron. It is the grey iron constituent which gives the camshaft its strength and rigidity. Some engines, particularly diesel engines have auxiliary services driven from the camshaft. Such services might include a fuel injection pump and a hydraulic pump in the case of agricultural vehicles for example. Where drives for these services are taken from the camshaft, it is necessary that the material strength is significally greater than with unalloyed cast iron to limit twisting of the shaft in operation. The effect of chromium in increasing the hardness of grey iron is also important for the shaft bearing journals.
Vanadium has a similar effect to chromium, but it is very much more costly as a raw material and, therefore, its use tends to be sparing.
With conventional chill cast camshafts, white iron is produced at the cam lobe surface by the use of metal chills placed in the casting mould, which is generally composed of sand. The metal chills produce a sufficiently high cooling rate such as to ensure solidification of the molten cast iron as white iron adjacent the chills. A problem arises in some designs of camshafts where a particular feature, which requires subsequent machining, has a relatively low metal volume compared to the area of the adjacent sand mould material. In this instance the cooling rate produced by the sand mould itself may be sufficiently high to produce white iron in such features, thus causing machinability problems.
The form
REFERENCES:
patent: 3977867 (1976-08-01), Cochran et al.
patent: 4153017 (1979-05-01), Behnke
"Development of Precision Inoculation to Control Microstructures of Cast Irons" by J. R. Nieman, Reprinted from 1976 AFS Transactions, pp. 175-182.
Lin Kuang Y.
Lydmet Limited
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