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Title:
LOW-ALLOY CAST IRON TAPPET, APPARATUS, AND PROCESS FOR ITS MANUFACTURING
Document Type and Number:
WIPO Patent Application WO/1998/047648
Kind Code:
A2
Abstract:
Low-alloy cast iron tappet, apparatus, and process for it solve the problem of obtaining tappet face surface microstructure. The microstructure consists of fine complex carbide in the martensite form, which is highly resistant to wear, features high fatigue (rolling) resistance, and is at the same time tough enough not to allow pitting during the engine operation. When pouring the molten mass into moulds (Fig. 3), the mass cooling is done under equal conditions, as each casting is provided with the same type of the cooling element. Application of cooling metal insert elements allows uniform cooling of castings, and consequently more uniform formation of the malleable white iron layer on the tappet face surface. Prior to the mechanical processing (turning and drilling), the castings are softened and annealed just lower critical point by a combined process of thermal treatment. After the machining process is completed, the castings are hardened to the prescribed hardness. They can be hardened in salt baths either by induction hardening or by flame hardening, depending on the equipment of the production plant where the procedure is to be performed.

Inventors:
JAKIR MILE (SI)
Application Number:
PCT/SI1998/000010
Publication Date:
October 29, 1998
Filing Date:
April 20, 1998
Export Citation:
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Assignee:
JAKIR MILE (SI)
International Classes:
B22D15/00; C21D1/56; C21D1/84; C21D5/00; C21D9/00; C22C37/00; F01L1/00; F01L35/00; (IPC1-7): B22D15/00; C21D5/00
Foreign References:
US1727565A1929-09-10
US1560832A1925-11-10
DE416783C1925-07-25
US1999790A1935-04-30
US3122822A1964-03-03
GB356795A1931-09-07
JPH0230360A1990-01-31
Other References:
P. PEPPLER : "Schalenhartguss ....." KONSTRUIEREN + GIESSEN,1979, pages 12-18, XP002080213 D}sseldorf, DE
KLAUS R\HRIG: "Jahres}bersicht Legiertes Gusseisen" GIESSEREI, vol. 71, no. 16/17, 6 August 1984, pages 654-655, XP002080214 D}sseldorf, DE
RICHARD MEYER: "DER HARTGUSS" 1954 , VEB WILHELM KNAPP VERLAG , HALLE (SAALE), DE XP002080283 * page 103 - 113 * see page 104 and 110, table 34 a
Attorney, Agent or Firm:
Marn, Jure (2000 Maribor, SI)
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Claims:
CLAIMS
1. A lowalloy cast iron tappet characterized by said tappet face surface (5) comprising at least 5 mm deep essentially flat layer comprised essentially of malleable white iron (8).
2. Apparatus for manufacturing of a lowalloy cast iron tappet comprising a sand mould and at least one cooling means (11) connected thereto for obtaining a tappet surface comprised essentially of malleable white iron layer by connecting said tappet surface to at least one cooling means (11) during casting characterized in that said cooling means (11) comprises a bottom sand shell (3) into which at least one metal cooling element (4) is inserted.
3. A process for manufacturing of a lowalloy cast iron tappet by casting in a sand mould where tappet face surface comprised of essentially malleable white iron layer is obtained by connecting said tappet surface to at least one cooling means (11) characterized in that essentially flat malleable white iron layer (1) of varied depth is obtained within said tappet surface (8) by pouring molten material into said mould and transferring heat from bottom surface of said molten material to at least one metal cooling element (4) inserted into a bottom sand shell (3).
4. A process for manufacturing of a lowalloy iron tappet cast in a three part sand mould (1,2,3) whereby following annealing said tappet is further mechanically processed and/or said tappet surface is heat treated characterized in that said annealing is comprised of heating said tappet to temperature between 800"C and 900"C, stagnating at said temperature range for period not to exceed 2 hours, cooling on air until temperature reaches not less than 650"C and not more than 710°C, stagnating at said temperature range for period not less than 2.5 and not more than 4 hours, and cooling in furnace for period not less than 7 and not more than 9 hours at a rate essentially 50"C per hour.
5. An invention according to any of previous claims characterized in that said heat treatment comprises of a hardening in salt bath, said hardening comprising of heating of said tappet by heating means to temperature not less than 420"C and not more than 500"C at a rate less than 500"C per hour, heating to temperature not less than 820"C and not more than 850"C at a rate more than 50"C per minute, stagnating at said temperature range for period not to exceed 20 minutes, cooling in an oil to a temperature less than 1 50°C, and cooling in an air to essentially room temperature; an annealing, said annealing comprising of heating of said tappet by heating means to temperature not less than 1800C and not more than 380"C, stagnating at said temperature range for a period not less than 2,5 hours and not more than 4 hours, and cooling to essentially room temperature.
6. An invention according to any of previous claims characterized in that following said hardening in salt bath said tappet undergoes measurement of its surface face hardness within period not to exceed 24 hours and is classified accordingly, and said annealing temperature is chosen according to said tappet surface face hardness whereby said tappet with said hardness of 64 HRC is annealed at essentially 380"C whereas said tappet with said hardness of 56 HRC is annealed at essentially 1800C.
7. An invention according to any of previous claims characterized in that said heat treatment of said tappet surface face comprises of induction and/or flame hardening said hardening comprising of heating of said tappet by induction heating means to essentially 850"C within period not less than 10 and not more than 20 seconds, stagnating at said temperature for period not less than 10 and not more than 13 seconds, and cooling with a water jet to essentially room temperature, annealing said annealing comprising of heating of said tappet by heating means to temperature not less than 1600C and not more than 220"C, stagnating at said temperature range for a period not less than 2,5 hours and not more than 4 hours, and cooling to essentially room temperature.
8. An invention according to any of previous claims characterized by hardness of said tappet surface face between 55 and 58 HRC.
Description:
LOW-ALLOY CAST IRON TAPPET, APPARATUS, AND PROCESS FOR ITS MANUFACTURING The subject of this invention is a technological procedure used for manufacturing of various types of tappets, which assure stability of valve clearances during the operation of internal combustion engines.

This invention gives a technical solution by establishing a technological procedure which will allow mass production of a tappet that will ensure better stability of adjusted valve clearances, and consequently make the contact surface between the tappet front surface and the camshaft resistant to unexpected wear. There are several production procedures known world-wide which deal with making of tappets. The oldest known procedure applied is forging of round sections made of suitable types of steel which, however, does not give a quality solution due to high manufacturing costs and very poor damping force of impact loads extending from the camshaft.

Another known solution is casting of tappets made of low-alloy chilled cast iron. This standard solution known for years proves quite inefficient in terms of the valve clearance issue due to pitting of tappet face surface during the engine operation.

Characteristical features of tappets made of low-alloy chilled cast iron are their structural properties because of the malleable white iron and grey iron combination. Low-alloy malleable white iron is highly resistant to wear. A tappet stem made of grey iron combined with lamellar graphite, on the other hand, has considerable damping force of impact loads.

Mechanical processing of tappets causes a noticeable decrease in resistance to wear of malleable white iron layer between the tappet face surface and the stem. Very damaging to tappet castings is a partial penetration of malleable white iron into the areas where turning and drilling are performed. All these drawbacks increase the costs of machining and produce too many castings, too hard to be able to be mechanically processed.

Further scope of applicability of the present invention will become aparent from the detailed description given hereinafter and the accompanying drawings.

However, it should be understood that the detailed descriptions and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention yet not specifically described hereinafter will become apparent to those skilled in the art from the detailed description.

When using this standard casting procedure, the malleable white iron layer on the tappet face surface is obtained by positioning the sand mould on the metal cooling plate (see Fig. 1). Fig. 2 shows structural pattern of the casting.

Fig. 1 Cross section of a part of a standard type sand mould depicting an upper sand shell (1), a bottom sand shell (3), and a metal cooling plate (11).

Fig. 2 Cross section of a casting with an essentially parabolic malleable white iron layer of the tappet face surface (5), a transitional layer (6), and a pearlite part of the tappet stem (7).

This type of cooling the chilled cast iron makes the malleable white iron layer solidify to a shape of a parabola, which is shown in Figure 2. However, the malleable white iron layer of the tappet face surface has not been found favourable due to damaging effect on the machining tools applied and due to too long production time. From the very beginning of tappet making, wear caused by the camshaft was ascribed to insufficient hardness of the tappet face surface. The tappet face pitting has resulted from embrittlement of the basic malleable white iron layer of the face surface. Such structure, obtained by casting, is brittle and incapable of receiving constant dynamic impacts generated by the camshaft in a sufficient degree. Sooner or later this leads to pitting of the tappet face surface, and consequently to changing of valve clearances.

A common characteristic of hitherto known solutions is valve quality issue of internal combustion engines, which so far has not been solved yet.

This invention solves this problem with a new technological procedure of tappet making. The invention is additionally described in two versions (embodiments) on a model, and further illustrated in the figures: Fig.3 Partial cross section of the mould according to this invention comprising of upper sand shell (1), a middle sand shell (2), a bottom sand shell (3), a metal cooling insert element (4).

Fig. 4 Cross section of the casting cast to shape of the mould according to Fig. 3 whereby the tappet face surface is comprised essentially of malleable white iron layer (8), transitional part (9), and pearlite stem part (10).

Fig. 5 Heat treatment chart of the tappet prior to mechanical processing of drilling and turning.

Fig. 6 Heat treatment chart of salt bath treatment following mechanical processing aimed at achieving proper hardness of the tappet's face surface (Version 1).

Fig. 7 Annealing chart of hardened casting according to this invention (Version 1).

Fig. 8 Induction and/or flame hardening according to this invention of mechanically processed tappet aimed at achieving proper hardness of the tappet's face surface.

Fig. 9 Annealing chart of hardened casting according to this invention (Version 2).

The technological procedure dealt with in this invention and concerning manufacture of tappets made of low-alloy chilled cast iron consists of several consecutive manufacturing operations, which are to be performed in production plants.

Fig. 3 shows a three-section mould, the partial cross section of which is illustrated on the model. Function of the upper shell (1) is to form pouring channels and feeding system. The middle shell (2), Fig. 3, forms cavities according to the shape required for the tappet castings. The bottom shell (3), Fig. 3, features cavities into which prior to the mould assembly metal cooling insert elements are mounted (4), Fig. 3, which are adapted to the shape of the casting they cool. Dimensions of these cooling elements can be obtained according to the following factors: diameter P = (0.7 - 1.0) D and height = (1.1 - 1.5) D, where D stands for bottom casting diameter. Material used for the cooling procedure is either the Beryllium-bronze or Chromium-bronze alloy.

The cooling insert elements can also be made of fire-resistant steels or grey iron, but have poorer cooling effect. The best cooling effect is achieved with the elements made of electrolyte copper, which on the other hand get easily damaged due to their softness. The size of the cooling elements within the given parameters depend mainly on chemical structure required for castings, as well as on the shape of the casting face surface and the size of the machining allowance. Higher the percentage of carbide-building elements in the structure, smaller the diameter of the cooling element. The cooling element diameter of the tappet with the form shown in Fig. 4 is bigger than that of the tappet with a mushroom-type face surface (the stem diameter is smaller than the face diameter).

When the prepared molten mass is poured into moulds (Fig.3), castings with malleable white iron layer are obtained, as illustrated in Fig. 4. Thus solidified tappet face layer has a thickness from 5 to 7 mm, and the minimum permitted hardness of 48 HRC. There is no limit on maximum hardnesses of castings.

Cleaned castings are submitted to thermal treatment according to the chart in Fig. 5. In a very short time they are heated up to glowing heat which causes partial melting of the tappet face malleable white layer down to 50 % of the whole microstructure. In this part of the process, which takes about an hour, the remaining 50 % of the layer are made spheroidised. The result of the next stage, i.e. air cooling up to approximately 650(C, is fine-grained lamellar perlite. This is followed with soft annealing of castings (in compliance to the same chart) and, if necessary, with heating from 650(C up to 710(C. The heating process at a certain temperature takes approximately three hours. After the soft annealing process is finished, softened castings with spheroidised perlite and the remaining malleable white iron are obtained. The hardnesses on the tappet face surface of thus softened castings range between 40 to 46 HRC.

With each new quantity of castings, depending on the amount of the malleable white cast iron layer on the tappet face surface, new working temperatures have to be defined for the first and the second stage of the process (Fig. 5).

The mentioned temperatures range from 800(C to 900(C for the first stage and 650(C to 710(C for the second stage of the process.

In such a way prepared castings are further mechanically processed, i.e.

turned and drilied. Characteristic for this stage of manufacturing operations is that it is possible to organise the processes to run quite fast and efficient on high capacity multi-spindle machine tools. This makes production time much shorter, and consequently the product much cheaper.

The castings mechanically processed by turning and drilling are then put into salt bath with a view to achieving prescribed face surface hardnesses.

Hardening is performed according to the chart on Fig. 6. Within 24 hours at the latest after completion of the hardening process, tappet face surface hardnesses have to be measured so as to arrange the tappets by groups with the difference in hardness up to 2 HRC. The measured tappets are further submitted to stressfree annealing, which is to be performed according to the chart in Fig. 7. In order to achieve that all processed tappets have hardnesses within 55 to 58 HRC, it is necessary to anneal the classified tappets by groups.

The hardest tappets are annealed at higher temperatures so as to obtain final hardness within the given limits. Sometimes the level of hardness may decrease even by 4 to 6 HRC per piece. For the tappets the hardness of which increases after hardening only by 1 to 3 HRC, when compared to the prescribed hardnesses, the stress-free annealing process is carried out according to the chart at lower temperatures. Selective stress-free annealing ensures that hardnesses of finally processed tappets are within the prescribed limits. The result of all above mentioned procedures are finally processed tappets with the difference in hardness between the highest and the lowest point being up to 3 HRC, and the microstructure consisting of fine complex carbides in the martensite form (Version 1).

According to Version 2, the mechanically processed castings are further surface hardened. During induction hardening the tappet put into a rotating device is positioned into the glow wire, which is powered with a medium frequency A.C. Heating is performed in compliance with the chart in Fig. 8 so as to finally obtain a hard tappet face surface. The inspection of tappet hardnesses is followed by stress-free annealing according to the chart in Fig.

9. The hardnesses and microstructure are similar to those of Version 1.