EP0277931A2 - Wear-resistant cast iron with a high phosphorus content - Google Patents

Abstract

A cast iron of high phosphorus content, intended especially for the manufacture of brake shoes, offering good wear resistance, is produced by directly incorporating approximately 10% by weight of ferrophosphorus in a liquid casting melt collected in a casting ladle or in a mixer associated with this casting ladle.

Description

The present invention relates to a cast iron with a high phosphorus content intended in particular for the manufacture of friction braking devices, offering good resistance to wear and composed of the following constituents:
85 to 95% by weight of iron
2 to 10% by weight of phosphorus
less than 6% of the total weight of carbon and silicon
less than 5% of the total weight of the following metals: manganese, cobalt, nickel, vanadium, tungsten and molybdenum,
and having a Brinell hardness of less than 270 HB. The cast iron according to the invention can be used in devices involving cast iron parts subjected to friction wear, in particular friction brake shoes intended to equip railway equipment.

Belgian patent No. 717,428 discloses iron / phosphorus alloys which, thanks to their resistance to wear, can be used in friction braking devices. The addition of at least 2% by weight of phosphorus reduces friction wear compared to that of conventional gray cast iron as well as reducing the emission of sparks during braking.

This phosphorus is generally added to cast iron in the form of ferro-phosphorus. The incorporation is carried out during a second melting in a cupola in which alternating charges of coke and of a mixture of iron and ferro-phosphorus are previously put in place. According to the aforementioned Belgian patent, the cast iron thus obtained has a Brinell hardness of between 207 and 255 HB, while the ferrite content of the alloy is not specified.

Measurements of the hardness carried out by the applicant according to DIN standard 50 35l demonstrate however that the Brinell hardness of the iron / phosphorus alloys having the composition described in Belgian patent n ° 717.428 have a hardness much higher than that mentioned above, of the less if no special precautions are taken. Brinell hardness values between 250 and 350 HB have been recorded when the cast iron is prepared in the conventional way by refining in cupola. Obtaining a high Brinell hardness in phosphorus cast iron is easily explained by the influence that phosphorus plays on secondary cast iron, which generally has a carbon content of more than 2% by weight.

Phosphorus promotes the segregation of graphite and gives rise to the formation of a ternary eutectic, steadite, which has a Brinell hardness of approximately 440 HB. Its melting point is around 950 ° C. Steadite has a much higher hardness than that of ferrite or even perlite.

Thus, ferrite has a Brinell hardness of approximately 125 HB and perlite a Brinell hardness of approximately 220 HB.

It is easy to understand that a reduction in Brinell hardness can only be obtained if it promotes the formation of a ferritic structure.

Knowing that the addition to a gray pig iron of secondary fusion containing approximately 2% by weight of carbon, approximately 3% by weight of phosphorus, involves the formation of approximately 30% by weight of steadite, it can therefore be affirmed that l he resulting ferrous alloy should contain at least 20% by weight of ferrite to have a Brinell hardness of less than 225 HB, as mentioned in Belgian Patent No. 717,428.

However, most of the railway networks in European states establish extremely strict specifications for the production of brake shoes. The most significant requirements are that the Brinell hardness of the cast iron cannot exceed 270 HB for a ferrite content of less than 8% by volume.

The requirements for maximum hardness and for ferrite content below a given level seem to be incompatible and impossible at the same time.

• 1. Forçage de la teneur en carbone de la fonte de seconde fusion par ajout au cubilot de carbure de si­licium, de manière à atteindre des valeurs de 3 à 3,3 % en poids de carbone. Le silicium, en formant du siliciure de fer plus stable que la cémentite, provo­ que la ségrégation du carbone. Or, les lamelles de graphite se comportent pratiquement comme des zones de dureté nulle, qui permettent d'abaisser la dureté Brinell de la fonte traitée:
• 2. Incorporation de manganèse à la fonte de seconde fusion, en vue de favoriser une structure perlitique.

For a secondary melting, only two solutions are likely to be adopted to date to reduce the hardness. These solutions are as follows:

• 1. Forcing the carbon content of secondary smelting by adding silicon carbide to the cupola, so as to reach values of 3 to 3.3% by weight of carbon. Silicon, by forming iron silicide more stable than cementite, provo than carbon segregation. However, the graphite lamellas behave practically like zones of zero hardness, which make it possible to lower the Brinell hardness of the treated cast iron:
• 2. Incorporation of manganese in secondary smelting, with a view to promoting a pearlitic structure.

The value of the maximum hardness of cast iron has been fixed to take account of the usual hardness of the wheel treads of rolling stock.

Document FR-A-2,145,607 describes alloys of cast iron with a characteristic microstructure comprising phosphorus present in the form of a substantially continuous network of a degenerate phosphorus eutectic between voluminous primary dendrite crystals. The degenerate eutectic comprises a substantially continuous matrix of iron predominantly in the form of ferrite or perlite. These cast iron alloys are obtained by melting together iron and ferro-phosphorus waste in an induction furnace and then annealing in an electrically heated mold.

This process provides a relatively low hardness ferritic iron. To make the parts harder, it is therefore necessary to carry out an annealing which is obtained by prolonged heating for 1 to 6 hours of the parts cast at a temperature between 850 and 1,500 ° C., preferably 910 to 920 °. vs.

The limitation of the ferrite content is linked to a phenomenon of micro-welding between the ferritic structure of the brake shoe and that of the wheel tread, this which has the effect of causing premature wear of said wheel tread.

Finally, it is known from document US-A-4,352,416 a brake shoe made from a cast iron alloy containing sulfur. This alloy has the following composition:
carbon: 2.5 to 3.5% by weight;
silicon: 1.6 to 2.2% by weight:
phosphorus: more than 2 and at most 10% by weight;
sulfide and manganese, both present in a weight ratio S: (Mn / 1.8)> 1.

The presence of sulfur in the cast iron alloy is disadvantageous in the sense that it makes the pieces fragile and porous. It is observed in fact that the sulfur, reacting with traces of slag, causes blowing in the alloy. The sulfur is generally eliminated by the addition of manganese, in very precise proportions known to those skilled in the art.

In order to meet the requirements of maximum hardness and of ferrite content higher than a determined level dictated by the specifications for the manufacture of brake shoes the invention proposes a cast iron with a high phosphorus content intended in particular for the manufacture of devices friction brake, offering good resistance to wear, and composed of the following constituents:
85 to 95% by weight of iron
2 to 10% by weight of phosphorus
less than 6% of the total weight of carbon and silicon
less than 5% of the total weight of the following metals: manganese, cobalt, nickel, chromium, tungsten and molybdenum,
and having a Brinell hardness of less than 270 HB.

This pig iron is essentially characterized in that it comprises a pearlitic structure containing at least 3% by weight of carbon, 2% by weight of phosphorus and less than 5% by volume of ferrite, and a manganese content at least equal to that found by the formula Mn = 1.72 S + 0.30 where S is the sulfur content.

qui régit l'équilibre entre le carbone, le phosphore et le silicium de la fonte dans une installation de fusion, montre qu'une teneur croissante en phosphore abaisse d'autant la teneur en carbone, pour une même teneur en silicium.According to the invention, the production of a pig iron corresponding to the preceding paragraph is carried out by adding ferro-phosphorus to the liquid pig iron collected in a ladle or a mixer and not in the melting installation. The direct incorporation of ferro-phosphorus into the ladle also makes it possible to obtain better accuracy of analysis of the phosphorus content.
Lévi's experimental formula:

which governs the balance between carbon, phosphorus and silicon in pig iron in a smelter, shows that an increasing content of phosphorus lowers the carbon content by the same amount, for the same silicon content.

The cast irons according to the invention, with a phosphorus content of between 2 and 10% by weight and a carbon content of at least 3% by weight, meet the hardness and ferrite content requirements.

They also have the advantage of dampening noise during braking. This property is due to the fact that the graphite lamellae, present in greater numbers in a larger size, are excellent vibration absorbers.

According to a feature of the invention, the cast iron contains at least 2.3% by weight of carbon in the form of lamellar graphite.

Other features and details of the invention will appear during the detailed description of a particular form of implementation of the method according to the invention.

Cast iron generally has the following typical composition:
carbon: 3 to 4% by weight
silicon: 1 to 3% by weight
manganese: 0.4 to 1.0% by weight
phosphorus: 0.1 to 1.0% by weight
sulfur: 0.08 to 0.15% by weight.

According to known methods, cupola fusion is accompanied by the incorporation of ferro-phosphorus when it comes to obtaining a wear-resistant ferrous alloy. The present invention suggests subjecting the cast iron to a different treatment.

She suggests bringing the cast iron into a ladle or a mixer to add about 10% by weight of ferro-phosphorus.

It was found that when ferro-phosphorus was added to the ladle, all of the phosphorus resulting from the decomposition of ferro-phosphorus was found in liquid pig iron, while a significant proportion of the iron present in ferro-phosphorus floated in the pocket, in the form of slag.

Undissolved iron therefore does not participate in the dilution of carbon, so that the carbon content decreases to a lesser extent than what could have been theoretically predicted and that it remains greater than 3%.

The addition of ferro-phosphorus is adjusted so as to bring the phosphorus content to a value between 2.25 and 4.00% by weight.

The alloy has a composition typically corresponding to gray cast iron, except as regards the high phosphorus content, greater than 2% by weight and a ferrite content less than 5% by volume.

The cast iron according to the invention is perfectly suitable for the manufacture of brake shoes for railway equipment. It has excellent wear resistance, as described in Belgian Patent No. 717,428 and considerably reduces the formation of sparks. It also reduces noise emission, since the graphite slats help to dampen the propagation of vibrations in the brake shoes.

The cast iron according to the invention has good flowability, due to the presence of phosphorus. It therefore allows relatively complicated parts to be produced with precision.

Claims (4)

1. High phosphorus content iron intended in particular for the manufacture of friction braking devices, offering good resistance to wear, and composed of the following constituents:
85 to 95% by weight of iron
2 to 10% by weight of phosphorus
less than 6% of the total weight of carbon and silicon
less than 5% of the total weight of the following metals:
manganese, cobalt, nickel, vanadium, chromium, tungsten and molybdenum,
and having a Brinell hardness of less than 270 HB, characterized in that it comprises a pearlitic structure containing at least 3% by weight of carbon, 2% by weight of phosphorus, less than 5% by volume of ferrite and a manganese content at least equal to that found by the formula Mn = 1.72 S. + 0.30 where S is the sulfur content due to the sulfides bound by manganese. 2. Cast iron according to claim 1, characterized in that it contains at least 2.3% by weight of carbon in the form of lamellar graphite. 3. Method for producing a pig iron as described in claim 1 or 2, characterized in that the formation of a pearlitic structure is favored by adding ferro-phosphorus to the liquid pig iron collected in a ladle or a mixer. 4. Method according to claim 3, characterized in that the amount of ferro-phosphorus added amounts to about 10% by weight of the liquid iron.