GB2265154A - Nodular cast iron and method for making it - Google Patents

Description

2265154 1 CAST NODULAR IRON AND PROCESS FOR OBTAINING CAST NODULAR IRON

The present invention refers to a new cast nodular iron composition, particularly useful in the manufacture of piston rings for internal combustion engines, and to a process for obtaining said new cast nodular iron.

It is already known the use of cast nodular iron having high mechanical characteristics, similar to those of steel in the manufacture of piston rings for internal combustion engines. Nevertheless, even with such high mechanical characteristics, these cast nodular iron piston rings of the prior art do not always fulfill the requirements of wear resistance in the internal combustion engines. Thus, it is an object of the present invention to provide a new particularly useful in the manufacture of piston rings for internal combustion engines, which presents a wear resistance that is optimized in relation to the cast nodular iron parts known up to cast nodular iron composition, now.

A further object of the present invention is to provide a process for obtaining said cast nodular iron.

A still further object of the present invention is to provide a piston ring made of cast nodular iron, which is more resistant to wear.

According to the first aspect of the present invention, the cast nodular iron in question is formed by a bainitic, martensitic, pearlitic or ferritic matrix, having a purity degree of about 99.6% and containing from 10 to 12% by volume of spheroidal graphite particles dispersed therein, and about 0.2 to 1.2% by volume of high hardness 2 carbide particles of the MC type, where M is at least one of the elements selected from the group consisting of Ti, Ta, Zr Hf, V e Nb.

The cast nodular iron mentioned above presents hard particles dispersed in the metallic matrix, 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 wear surface of the surrounding matrix.

In the particular case of piston rings for internal combustion engines, the metallic matrix made of cast nodular iron tends to be submitted to abrasive wear, in the region of the most external edge of the ring, during engine operation. Said wear, however, is reduced in the subject material, by the superficial projection of said particles or islands of much harder material. A reduction of the contact pressure in the matrix is also obtained, minimizing the adhesive wear.

Summarizing, it can be said that, during the abrasive wear of a piece made with the subject 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.

Though not discussed in more details in this specification, it should be noted that the hard material particles also tend to reduce the abrasion caused by the corrosive wear of the pieces made with the new cast nodular iron.

According to a second aspect of the present invention, the process for obtaining the cast nodular iron in question comprises the steps of:

3 a)- Adding to a bath of gray cast iron containing less than 0.015% of sulfur and less than 0.04/lo of phosphorus, a load of about 0.2 to 1.2% by bath weight of at least one ferroalloy having the formula Fe-FeM or FeMC, where M is at least one of the elements selected from the group consisting of Ti, Ta, Zr, Hf, V and Nb; b)- Effecting at least one nodulization of the bath, by introducing ferrosilicon magnesium therein.

By using the above mentioned process, possible to obtain the cast present invention, with the that the nodulization of the it is nodular iron of the further observation gray cast iron bath containing, still in only one phase, the element or elements which form the hard particles, causes a drastic alteration, after the cast of the piece, in the morphology of the MC carbides, particularly when M is niobium, from a Chinese script to the formation of idiomorphic polygonal islands, having a hard phase upon the solidification of the bath in a cast piece.

Said hard material idiomor];nic polygonal islands, which are dispersed in the cast nodular iron matrix, are discontinuous, forming no paths of crack propagation, as it occurs with the hard phases of Chinese script type in the gray cast irons. Furthermore, these idiomorphic polygonal islands of hard material present a mean width that is slightly larger than that of the 8arathes of abrasive wear made onto the pieces formed with the cast nodular iron of the present invention.

The invention will be described now, with reference to the attached drawings, in which:

Fig.1 illustrates in a metallographic 4 representation, niobium carbide (Nbc) islands dispersed, as a hard phase, in a matrix of gray cast nodular iron of the prior art, said islands presenting a morphology of the Chinese script type;

Fig.2 representation, illustrates in a niobium carbide metallographic (Nbc) islands dispersed, as a hard phase, in a matrix of cast nodular iron according to the present invention, said islands presenting an idiomorphic polygonal morphology; Fig.3 illustrates, in an enlarged metallographic representation, a matrix of cast nodular iron of tempered martensite, containing idiomorphic polygonal islands of niobium carbide, which stop the scratches of abrasive wear to which the piece has been submitted; and Fig.4 illustrates in a metallographic representation, a situation similar to that shown in figure 3, indicating the mechanism for stopping the wear scratches in the piece, said mechanism being defined by the niobium carbide hard particles in the matrix of cast nodular iron of tempered martensite.

As illustrated in figure 1, the addition of MC 25 carbides to the gray cast irons (lamellar graphite) leads to the formation of cast pieces presenting the particles of a much harder material, such as niobium carbide, dispersed in the cast iron matrix, in the form of elongated lines of the Chinese script type. These elongated lines of much harder material define natural paths for the propagation of cracks in the piece and have reduced width, thus limiting its capacity to stop the scratches of abrasive wear in the piece formed by said material.

Figure 1 illustrates the lamellar graphite islands and the elongated lines of niobium carbide (NbC) dispersed in the matrix of gray cast iron (FoFo).

In figure 2, the much harder material particles formed of MC carbides, are dispersed in the matrix of cast nodular iron (nodular FoFo), in the form of islands having a discontinuous morphology and presenting a mean area whose cross section is larger than the width of the wear scratches, as shown in figures 3 and 4.

The example illustrated in figures 2, 3 and 4 refers to a matrix of tempered martensite containing, in dispersion, niobium carbide (NbC) particles. However, it should be understood that the same effect can be expected for other matrices, such as: bainitic, pearlitic and even high hardness ferrite. The ferrous matrix should present a purity degree, such that sulfur is less than 0.015% and that phosphorus is less than 0.04%_ Among the alloy elements available for aplication 20 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: Ti, Ta, Zr, Hf, V and Nb, with a broad mutual solubility.

Among the above cited elements, niobium was chosen as the Me former, due to its availability in the market, as well as its particularly interesting characteristics, with little interference in the nodulization reactions, thus being very close to 6 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 can be observed by studying the product of the solubility of this element in said matrix, with the additional advantage of presenting a additions.

The process for obtaining the cast nodular iron of the present invention comprises the initial step of providing a bath of gray cast iron having a purity degree such as to present amounts of sulfur and phosphorus lower than 0.015% and 0.04,'o', respectively, this bath consisting of a solid metallic load supplied to a furnace which is adequate to the casting thereof, such as a threephase electric are furnace. This metallic load is calculated as a function of the final product to be obtained and comprises a basic load consisting of pig iron, steel and returns (scrap, chips and sprinkles).

After the loading in the furnace, the basic load is melted, then it receives the addition of about 0.2 to 1.2% of ferroalloys of Si, Ni, Mo and Nb base, the alloy components being chemically analysed, in order to achieve the final desired composition. Elements C and Si may, eventually, suffer some corrections.

In the previous paragraph, there was mentioned the addition of a niobium-base ferroalloy, besides the ferroalloys already known for additions of that kind, such as ferrosilicon, ferronickel and ferromolybdenum. However, as previously mentioned, the new element of ferroalloy may consist of at least one of the elements selected from the group 90% recovery in the 7 comprised of Ti, Ta, Zr, Hf, V and Nb.

The correction of the final bath is made in the electric furnace (blast furnace), into which niobium (in ferroalloy) is added, as it needs high temperature for dissolution. Only slight final corrections of niobium (or other elements of the addition alloy) may be effected afterwards, out of the furnace and before the discharge into the molds.

In the case of niobium addition, it has been found more adequate to work with a ferroalloy containing about 60 to 85/% of niobium.

After the complete melting of the addition alloys with the basic metallic load,. the' bath is transferred, through a melting pot, to another furnace, such as a Detroit furnace, where some slight final adjustments of the new ferroalloy may still be made- The nodulization of the melting iron bath is achieved, at least in afirst step, in a reaction pan and by inoculating a certain quantity of iron, silicon and magnesium (Fe, Si, Mg) in the bath.

The proportion of the nodulizing alloy is previously determined, by known methods and as a function of the quantity of cast units to be produced in the reaction.

Just after the first phase of nodulization, the bath is lead to the final discharge in less than three minutes, by controlling the metal temperature in the nodulization reaction, the chill mold temperature, the quantity of protecting paint applied to the chill mold and the chemical analysis of the metal being melted.

During the metal discharge into the molds, the 35 melting load is submitted to a second phase of a inoculation, by way of a second step of Fe-Oi inoculation.

After removed from the molds, the cast nodular iron pieces are normally subjected to Jet blast and identified for each batch. At each batch, there is made a metallographic analysis of the last molded piece, this type of analysis allowing a 100% evaluation of the reactions.

The pieces thus produced may still have their 10 ferritic properties increased for further machining of the piston ring, which is also quenched and tempered.

Table 1 shows typical hardness values for some carbides, as well as metallic matrices.

TABLE 1 PHYSICAL PROPERTIES OF SOME CARBIDES AND "PHASES" CONDITION CARBIDE VICKERS3 HARDN.

TaC 1800 NbC 2400 TIC 2500 Carbonetos VC 2800 puros W2C 3000 2400 1300 1300 WC Cr3C2 Cr23C6 MELTING(OC) 3067 3420 3928 2648 3600 3983 2760 Compiexo, M6e 1100 to 1650 Carbonetos MC 1800to 2200 Matrix Ferrite 200 Pearlite 350 Tempered Martensite 450 Table 2 shows the Increase of wear resistance when 9 niobium is added in the range of 0.2 to 1.2% by weight of cast nodular iron, in relation to other nodular with the other variables being inaltered. Wear was measured in an oil ring (077 h 2mm), which has been tested for 94 hours in an Otto cycle engine having 4 cylinders (1.6 liters 60 KW), with two rings in each material.

TABLE 2 RESULTS OF THE WEAR IN RINGS WITH OR WITHOUT NbC VARIATIONS IN THE DIMENSIONS OF THE RINGS (mm) Position Plain Nodular Nodular with Nb Radial 0,038 0,032 Gap 0,23 0,19 Average in a pair. 5 measurements per ring in the radial direction.

Table 3 shows the base composition of the tested alloys.

TABLE 3

ANALYSED CHEMICAL COMPOSITION (weight ALLOY 1 c....

Si.... Mn.... P..... S.... Ni... 30 Mo..

... 3.62 -2.14 ... 0.21 ... 0.038 ........ 0.016 .... 0.54 ..... 0.18 Mg 0.053 Nb.........

ALLOY 2 C 3.63 Si 2.17 Mn 0.21 p 0.038 S 0.022" Ni 0.54 M0 0.18 Mg 0.53 Nb 0.2-1.2% The niobium contents will vary within the above range, according to the requirements of application. More severe requirements will correspond to the displacement of the niobium contents to said range upper limit.

The usefulness of this concept of increase in wear resistance is valid for dimensional variations in the radial direction for compression rings and oil rings, as well as for minimizing the height loss of compression rings in the first groove.

J1

Claims (9)

1. Cast nodular iron, charactQrized in that it comprises: a bainitic, martensitic, pearlitic or ferritic metallic matrix, having a purity degree of about 99.6%, in which the sulfur and phosphorus contents are maintained lower than 0.015% and 0.04%, respectively; from 10 to 12% by volume of graphite particles and about 0.2 to 1.2% by volume of high hardness carbide particles of the MC type, said particles being dispersed in the melted metallic matrix, where M is at least one of the elements selected from the group consisting of Ti, Ta, Zr Hf, V e Nb.

2. Cast nodular iron, as in claim 1, charactt:,rized 15 in that said metallic matrix of tempered martensite contains, in dispersion, niobium carbide particles (NbC).

3. Process for obtaining cast nodular iron, charactpri7ed for comprising the steps of: adding to a bath of gray cast iron containing less than 0.015% of sulfur and less than 0.04% of phosphorus, a load of about 0.2 to 1.2% by bath weight of at least one ferroalloy having the formula Fe-FeM or Fe-MC, where M is at least one of the elements selected from the group consisting of Ti, Ta, Zr, Hf, V and Nb; effecting at least one nodulization of the bath, by inoculating low ferroBilicon magnesium therein; and discharging the nodulized metallic bath into the molds.

4. Process, as in claim 3, nhnrantnrized in that said nodulization is achieved in at least two steps of inoculating a low ferrosilicon magnesium into the metallic bath, the second inoculation being effected during the discharge of the bath.

5. Process, as in claim 3, charaQterized in that ) 2 said bath of gray cast iron is achieved by adding, to a basic bath of pig iron, steel and returns, ferroalloys of Si, Ni and Mo base and at least one of the elements selected from the group consisting of Ti, Ta, Zr, Hf, V and Nb.

6. Process, as in claim 5, characterized in that said ferroalloys include a ferroalloy presenting about 60 to 85% of niobium.

7. Cast nodular iron substantially as hereinbefore described with particular reference to the Examples.

8. Process for obtaining cast nodular iron substantially as hereinbefore described with particular reference to the Examples.

9. Cast nodular iron when produced by a process as claimed in any one of claims 3 to 6 and 8.