WO1994010354A1

WO1994010354A1 - Gray cast iron

Info

Publication number: WO1994010354A1
Application number: PCT/BR1993/000040
Authority: WO
Inventors: Paulo Sérgio ESPÓSITO
Original assignee: Cofap - Companhia Fabricadora De Peças
Priority date: 1992-10-26
Filing date: 1993-10-26
Publication date: 1994-05-11
Also published as: BR9204244A

Abstract

Gray cast iron, which is used in the manufacture of piston rings and sleeves for internal combustion engines, and which comprises: a bainitic, martensitic, pearlitic or ferritic metallic matrix, with an amount of equivalent carbide from 4 to 5 % and including from 2 to 5 % of steadite and from 1 to 10 % of cementite, which has been modified by the presence from 0.02 to 2 % by weight of boron and from about 0.5 to 5 % by volumne of particles of at least a high hardness MC type carbide, said particles being dispersed in the cast metallic matrix, M being at least one of the elements selected from the group consisting of Ti, Ta, Zr, Hf, Cr, V and Nb, said metallic matrix presenting a first basic surface and a second wear surface, which is spaced from said first surface and which is defined by the projection of the hard particles existing in said matrix.

Description

GRAY CAST IRON

Field of the Invention The present invention refers to a new composition of gray cast iron, which is particularly useful in the manufacture of piston rings and sleeves for internal combustion engines. Background of the Invention It is already known the use of gray cast irons containing the addition of harder materials, such as steadite, which are mainly employed in the manufacture of piston rings and sleeves for internal combustion engines, which require a high wear resistance. Nevertheless, despite the good wear resistance achieved with such materials, in many applications said resistance is not sufficient to endure the high stresses imparted to the piston rings and sleeves in the internal combustion engines. Moreover, some compositions using the known gray cast irons do not present good machinability, thereby preventing said compositions from being used in the manufacture of piston rings and sleeves of the above mentioned internal combustion engines. Disclosure of the Invention

Thus, it is the general object of the present invention to provide a new composition of gray cast iron, which is particularly useful in the manufacture of piston rings and sleeves of internal combustion engines and which presents a wear resistance that is optimized in relation to the pieces made of gray cast iron known up to now.

According to a first aspect of the invention, the gray cast iron in question is formed by a pearlitic, bainitic, martensitic or ferritic basic gray cast iron matrix, containing from 0.5 to 5%, preferably from 1 to 2% by volume of high hardness MC type carbide particles dispersed therein, where M is at least one of the elements selected from the group consisting of Ti, Ta, Zr, Hf, Cr, V and Nb, said matrix further presenting from 0.2 to 5%, preferably from 0.5 to 1% by volume of steadite type hard particles and from 1 to 10%, preferably 4 to 6.5%, of cementite type hard particles, which are modified by the presence of boron, from 0.02 to 2%, preferably from 0.04 to 0.09% by weight. The above mentioned gray cast iron presents hard particles dispersed in the metallic matrix, thus allowing the achievement of a substantial increase in the resistance to abrasive wear, since by using a piece produced with this material, the hard particles become protruding in relation to the basic surface of the surrounding matrix.

In the particular case of piston rings and sleeves for internal combustion engines, the metallic matrix made of gray cast iron tends to be submitted to abrasive wear on the contact surfaces during the engine operation, said wear being reduced in the subject material by the superficial projection of said particles or islands made of harder material, whereby a reduction of the contact pressure in the matrix is also obtained, thereby minimizing the "scuffing", i.e., the wear that occurs due to local icrofusing.

Said islands of hard material are dispersed in the gray cast iron in a discontinous way, so as not to form a path of crack propagation, as it occurs with the conventional gray cast irons. Summarizing, it can be said that during the abrasive wear of a piece produced with the present material, the hard material particles function as blocking elements, stopping the wear scratches responsible for the gradual removal of material from the matrix, thus avoiding the scratching damage.

Brief Description of the Drawings The invention will be described below, with reference to the attached drawings, in which:

Fig. 1 illustrates, in a metallographic representation, a known prior art gray cast iron, containing graphite in the lamellar form and steadite, being involved by a pearlitic matrix;

Fig. 2 illustrates, in a metallographic representation, the islands of niobium carbide, vanadium carbide, steadite and boron modified cementite, said islands being dispersed, as a hard phase, in a gray cast iron matrix represented by pearlite, according to the present invention;

Fig. 3 illustrates, in a metallographic representation, a situation similar to that shown in figure 2, where the hard particles of niobium carbide, vanadium carbide, steadite and boron modified cementite are shown stopping the scratches of an abrasive wear being imparted to the piece; and Fig. 4 illustrates, in a metallographic representation, a situation of contact between a portion of a gray cast iron piece of the present invention and a portion of a piece made of another material, indicating the contact surfaces of the first piece. Best Mode of Carrying Out the Invention As illustrated in figure 1, the presence of steadite

(Fe₃PC) in the conventional gray cast iron containing lamellar graphite from 5 to 15% by weight, preferably from 10 to 13%, leads to the formation of cast pieces presenting the particles of much harder material (steadite) , dispersed in the cast iron matrix in the typically eutectic form.

According to the illustrations of figures 2-4, the gray cast iron of the present invention comprises a pearlitic matrix containing lamellar graphite, steadite (Fe₃PC) , boron modified cementite (Fe₃BC) , as well as dispersed particles of carbide and other alloy elements, such as niobium and vanadium. The boron (B),as it presents a very small atomic size,it modifies the properties of the intermetallic compounds (Fe₃C,VC, NbC, etc),in such a way as to propitiate the wear resistance of the gray cast iron. The addition of boron to the cast iron matrix promotes the dissociation of the steadite, producing the cementite as described above. Among the alloy elements available for application in cast irons, some are of major interest, such as those that form with carbon the composites having high hardness and high melting point, with minimum interference in the reactions of solid state and liquid state of the cast iron, and with little alteration in the manufacture steps when the other alloy elements are maintained inaltered. Elements with such properties are the formers of MC type carbides, where M may comprise at least one of the following elements: Cr,Ti, Ta, Zr, Hf, V and Nb. The present invention preferably uses the niobium carbide (NbC) and/or vanadium carbide (VC) , whose hardness characteristics will be described hereinafter, in a MC Carbide Property Table. The addition of said carbides to the prior art gray cast iron causes a substantial increase in the wear resistance of the material, protecting it against abrasive wear, due to the contact with other working surfaces of friction and abrasion, as shown in the tables herein presented. This protection agaisnt wear is achieved by the formation of a working surface consisting of hard particles, such as from niobium carbide (NbC) , vanadium carbide (VC) , steadite and boron modified cementite, which are projected from the matrix basic surface, thus creating a gap between said surfaces that avoids the direct wearing contact of other working surfaces with the less resistant surface of the basic matrix.

The choice of the niobium and vanadium carbides is due to their availability in the market, as well as to their particularly interesting characteristics, which are very close to the ideal conditions described above. Niobium reacts with carbon at a high temperature, besides having a low solubility in the ferrous matrix when in the carbide form, which fact can be observed by studying the product of the solubility of this element in said matrix, with the additional advantage of presenting a 90% recovery in the additions. Moreover, the niobium carbide presents a high hardness (2400 HV - hard Vickers) , superior to the other carbides. Nevertheless, other combinations between two or more of said different carbides cited above can be used, in order to obtain the above mentioned results, though at least one of said carbides should be the niobium carbide, due to the above cited characteristic. Gray cast iron pieces with the above described characteristics are obtained through a conventional casting process, after which the pieces are submitted to a subsequent machining of the piston ring and sleeves for internal combustion engines. Table 1 below shows typical hardness values for some carbides, as well as for metallic matrices.

TABLE 1

Modified

Cementite Fe3PC 800 a 1000

Steadite Fe3PC 600 a 800

Ferrite 200

Matrix Pearlite 350

Tempered

Martensite 450 Table 2 below shows the variation in the dimension of the piston ring and sleeves, for internal combustion engines, obtained with the prior art gray cast iron and with the gray cast iron of the present invention, besides the increase in the wear resistance when niobium carbide, vanadium carbide, steadite and boron modified cementite are added, in the respective percentile ranges by weight of gray cast iron. The wear was measured in a second groove ring, which was tested for 200 hours in a MWM engine, with three rings being made of each of said materials . It has been observed that with this new material the measurement of the wear of the tested pieces presents a value about 1/3 the value of the wear presented by the common gray iron.

TABLE 2

Table 3 below shows the basic composition of the tested alloys.

TABLE 3 Analyzed Chemical Composition Common Gray Gray Cast Iron Alloyed

Cast Iron with B/V/Nb

C 3.78 C 3.81

Si 2.42 Si 2.45

The niobium, boron and vanadium contents will vary within the above range, according to the severity of the application, the variation of said contents towards the maximum limits of the above range corresponding to an increase in the severity of the application. Nevertheless, it should be understood that the above description is valid for gray cast irons presenting other matrices, such as: bainitic, martensitic or even high hardness ferritic. The usefulness of this concept of improving the wear resistance is applied for dimensional variations in the radial direction for compression rings and oil rings and for sleeves used in internal combustion engines, as well as for minimizing the wear in the height of the compression rings of the first and second grooves.

Claims

1. Gray cast iron, which is used in the manufacture of piston rings and sleeves for internal combustion engines or the like, characterized in that it comprises: a bainitic, martensitic, pearlitic or ferritic metallic matrix, with an amount of equivalent carbide from 4 to 5% (by weight) and including from 2 to 5% (by volume) of steadite and from 1 to 10% (by volume) of cementite, which has been modified by the presence from 0.02 to 2% by weight of boron and about 0.5 to 5% by volume of particles of at least a high hardness MC type carbide, said particles being dispersed in the cast metallic matrix, M being at least one of the elements selected from the group consisting of Ti, Ta, Zr. Hf, Cr, V and Nb, said metallic matrix presenting a first basic surface and a second wear surface, which is spaced from said first surface and which is defined by the projection of the hard particles existing in said matrix.

2. Gray cast iron, according to claim 1, characterized in that in the metallic matrix at least one of said MC type carbides is of the niobium carbide type (NbC) .

3. Gray cast iron, according to claim 2, characterized in that the metallic matrix contains, in dispersion, vanadium carbide particles (VC) .

4. Gray cast iron, according to claim 3, characterized in that the metallic matrix preferably presents from 4.3 to 4.8 % of equivalent carbide; from 0.5 to 1% of steadite; from 4 to 6.5% of cementite modified by the presence from 0.07 to 0.09% by weight of boron; from 1 to 2% of niobium carbide and from 1 to 2% of vanadium carbide.