EP0277931B2 - 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

85 à 95 % en poids de fer

2 à 10 % en poids de phosphore

moins de 6 % du poids total de carbone et silicium

moins de 5 % du poids total des métaux suivants : manganèse, cobalt, nickel, vanadium, tungstène et molybdène,
et présentant une dureté Brinell inférieure à 270 HB. La fonte conforme à l'invention est utilisable dans les dispositifs faisant intervenir des pièces en fonte coulée soumises à une usure par frottement, notamment des sabots de freins à friction destinés à équiper le matériel de chemins de fer.

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 wear resistance 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 wear by friction, in particular friction brake shoes intended to equip railway equipment.

We know from document FR-A-1584012 a cast iron high phosphorus content which, thanks to its resistance to wear is usable in devices of friction braking. Having planned in this cast iron a content of at least 2% by weight of phosphorus reduces friction wear by compared to that of classic gray cast iron which contains only 1% phosphorus. It also allows reduce the emission of sparks when braking.

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

Hardness measurements made by the depositor according to DIN 50 351 demonstrate however that the Brinell hardness of iron / phosphorus alloys having the composition described in Belgian patent n ° 717.428 have a hardness much higher than that mentioned above, at least if no special precautions is not taken. Brinell hardness values included between 250 and 350 HB were noted when the cast iron is prepared in a conventional manner by refining with cupola. Obtaining in phosphorus fonts high Brinell hardness is easily explained by the influence that phosphorus plays on the second melt fusion, which usually has a carbon content greater than 2% by weight.

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

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

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

Knowing that adding to a second gray font fusion containing approximately 2% by weight of carbon, about 3% by weight of phosphorus, causes the formation about 30% by weight of steadite, we can therefore assert that the resulting ferrous alloy, should contain at least 20% by weight of ferrite to present a Brinell hardness lower than 225 HB, as mentioned in Belgian Patent n ° 717.428.

However, most rail networks in the European states establish extremely demanding specifications severe for the realization of clogs of brake. The most prominent requirements are in the fact that the Brinell hardness of cast iron cannot not exceed 270 HB for a lower ferrite content at 8% by volume.

Maximum hardness and content requirements ferrite below a given level seem incompatible and simultaneously impracticable.

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 retained 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, provokes that carbon segregation. However, the graphite lamellae 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, in order to promote a pearlitic structure.

The maximum hardness value of the cast iron was fixed to take into account the usual hardness of the tires rolling stock wheels.

By document FR-A-2.145.607 are described cast iron alloys with characteristic microstructure comprising phosphorus present in the form of a network substantially continuous from a degenerate phosphorus eutectic between voluminous primary dendrite crystals. Degenerate eutectic includes a matrix substantially continuous iron predominantly in the form ferrite or perlite. These cast iron alloys obtained by melting together waste of iron and ferro-phosphorus in an induction furnace and then annealing in a mold electrically heated.

This process provides a hardness ferritic cast iron relatively small. To make the pieces harder, it is therefore necessary to perform an annealing that this is obtained by prolonged heating for 1 to 6 hours of castings at a temperature included between 850 and 1,500 ° C, preferably 910 to 920 ° C.

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

US-A-3767386 describes a high-grade cast iron phosphorus content for brake shoes, containing graphite having a mixed structure very fine nodular and vermicular. Cast iron has a sparse steadite structure, one of which only part is pearlitic so as to leave free field at any percentage of ferrite.

It has a Brinell hardness greater than that of a conventional cast iron but less than 270 HB. It provides the brakes with a longevity greater than that provided by ordinary gray iron, but the low lengthening of longevity still seems insufficient to be profitable.

carbone : 2,5 à 3,5 % en poids;

silicium : 1,6 à 2,2 % en poids;

phosphore : plus de 2 et au maximum 10 % en poids;

sulfure et manganèse, tous deux présents dans un rapport pondéral S : (Mn/1,8) > 1.

Finally, it is known from document US-A-4,352,416 a brake shoe produced 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. We observe that the sulfur by reacting with traces of slag, generates blows in the alloy. Sulfur is generally eliminated by the addition of manganese, in very precise proportions known to man from art.

To meet maximum hardness requirements and ferrite content greater than one level determined dictated by the specifications for the manufacture of brake shoes, the invention proposes a method according to claim. Direct incorporation of ferro-phosphorus in the ladle, not in the fusion plant, also allows to obtain a better analysis accuracy of phosphorus content.

Lévi's experimental formula: VS melting = 2.4 + vs charge 2 - Si + P cast 4 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 invention also relates to cast irons according to claim 3.

They also have the advantage of absorbing the noises when braking. This property is due to fact that the graphite lamellae, present in number higher under a larger size, are excellent vibration dampers.

According to 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.

carbone

: 3 à 4 % en poids

silicium

: 1 à 3 % en poids

manganèse

: 0,4 à 1,0 % en poids

phosphore

: 0.1 à 1,0 % en poids

soufre

: 0,08 à 0,15 % en poids.

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 resistant ferrous alloy to wear. The present invention suggests subjecting cast iron to a different treatment.

She suggests bringing the cast iron into a pouring or blending to add approximately 10% by weight of ferro-phosphorus.

It has been found that when adding ferro-phosphorus in the pocket, all of the phosphorus from of the decomposition of ferro-phosphorus was found in liquid iron, while a significant proportion iron in the ferro-phosphorus floated in the pocket, in the form of slag.

Undissolved iron therefore does not participate in the dilution carbon, so the carbon content goes down to a lesser extent than we could have predict theoretically and that it remains greater than 3%.

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

The alloy has a composition corresponding typically gray cast iron, except for the content high in phosphorus, more than 2% by weight and a ferrite content of less than 5% by volume.

The iron according to the invention is perfectly suitable the manufacture of brake shoes for equipment railroads. It has excellent resistance to wear, as described in Belgian patent no. 717.428 and considerably reduces the formation of sparks. It also reduces the emission noise, since the graphite slats contribute to dampen the propagation of vibrations in hooves brakes.

The cast iron according to the invention has good flowability, due to the presence of phosphorus. She permits therefore to produce parts relatively accurately complicated.

Claims (3)

1. Method for producing a cast iron with a high phosphorus content offering a good resistance to wear and a Brinell hardness of less than 270 HB, intended in particular for the manufacture of friction braking devices and comprising 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,

   characterized in that the cast iron has a manganese content at least equal to that given by the formula Mn = 1.72 S + 0.30 where S is the sulphur content due to sulphides bonded by the manganese, and in that ferro-phosphorus is added to a liquid grey cast iron collected in a pouring ladle or a mixer, in order to adjust the phosphorus content thereof such as to obtain a cast iron with a content of at least 2.3% of carbon in the form of flake graphite and the formation of a perlitic structure, the cast iron comprising less than 5% by volume of ferrite.
2. Method according to claim 1, characterized in that the amount of ferro-phosphorus added amounts to approximately 10% by weight of the liquid grey cast iron.
3. Cast iron with a high phosphorus content intended for the manufacture of friction braking devices, offering a good resistance to wear, and comprising 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 has a manganese content at least equal to that given by the formula Mn = 1.72 S + 0.30 where S is the sulphur content due to sulphides bonded by the manganese, a carbon content of more than 3%, at least 2.3% by weight of carbon in the form of flake graphite and a perlitic structure, the cast iron comprising less than 5% by volume of ferrite.