GB2093071A - Treatment of cast iron - Google Patents

Abstract

Iron having a low sulphur content is inoculated by introducing into the molten metal a mixture of a source of sulphur and a reactant which forms a sulphide therewith, which sulphide is capable of acting to provide nuclei in the formation of the graphite from the melt.

Description

SPECIFICATION Treatment of cast iron This invention relates to a process for the treatment of cast-iron, more particularly to the inoculation of molten iron.

In the production of flake graphite iron castings, several benefits (such as improved mechanical properties and machineability) are achieved by inoculating the molten iron before it enters the mould.

Inoculation is a process of controlling the solidification behaviour of the austenite -- graphite eutectic and avoiding the formation of the austenite -- carbide eutectic in grey, cast irons. The purpose of the inoculation is to provide potent substrates for the effective heterogeneous nucleation of the graphite phase in the iron-carbon eutectic.

Heterogeneous nucleation is accomplished by inoculating the molten iron with a solid, stable particulate substrate which can provide preferred sites where a nucleus of a suitable size can form.

The inoculants in common use are alloys, or mixtures, of such elements as calcium, silicon, graphite, barium, strontium, aluminium, zirconium, cerium, magnesium, manganese and titanium.

These materials have proved to be effective for the inoculation of molten irons with a sulphur content of above 0.04 per cent. Recent trends in iron melting produce sulphur contents lower than this, and these irons are much more difficult to inoculate. For these low sulphur irons, very few inoculants are effective, such as those containing calcium, magnesium, cerium ans strontium, etc.

Experimental evidence has shown that sulphides nucleate the graphite phase in molten iron, but that the lattice structure of the sulphide must be compatible with that of graphite. Also the lattice disregistry between the inoculating element and graphite should be less than 6 per cent. This encompasses the sulphides of calcium, cerium, lanthanum, silicon and barium, but not the other common inoculating elements. It has also been found that clean surfaces on the substrate are preferable so that atom layers of such elements as oxygen do not interfere with the lattice matching. For this reason, substrates formed by reactions in the melt are generally preferable to those added in powder form.

In order to improve the response of low sulphur irons to inoculation it is sometimes the practice to add iron.sulphide to the melt to increase the sulphur content. But as the lattice disregistry between iron sulphide and graphite is greater than six p.er cent, it cannot act directly as a nuclei, but must rely on secondary reactions with other elements of a suitable lattice compatability. For these reasons it is only partially effective and may induce certain undesirable side effects.

It is an object of the present invention to provide an improved method for the inoculation of molten irons having a low sulphur content.

According to the invention the molten iron is inoculated with a mixture of a source of sulphur and a reactant which forms a sulphide therewith which is capable of acting to provide nuclei in the formation of graphite from the melt.

The sulphide forming mixtures include a source of sulphur and a reactant capable of forming a sulphide with the source of sulphur and being such that the sulphide so formed is capable of acting as a nucleating agent in the formation of graphite from the melt.

As a source of sulphur, granular or powdered sulphur or sulphide minerals, such as chalcocite, bornite, chalo-pyrite, stannite, iron sulphide, covellite, or their industrial equivalents may be used.

As sulphide forming reactants powdered or granular calcium silicide, calcium carbide, cerium or strontium alloys, rare earths, or magnesium may be used.

When these sulphide forming mixtures are blended with conentional inoculants, any of the iatter such as ferro-silicon, graphite, silicon carbide, calcium silicide, barium alloys, zirconium alloys, strontium alloys, cerium alloys, magnesium ferro silicons, manganese alloys, titanium alloys, may be used.

The sulphide-forming materials may be added to the melt in mixed powder or granular condition.

In other form of the invention, the reacting materials are bonded together in the form of shaped blocks. When powdered sulphur is one of the reactions, it is also used as the bonding agent by putting the mixture into a mould and raising it above the melting point of sulphur (1 200C). On cooling the block is quite rigid and may be removed from the mould. Other bonds may also be used which are compatible with the use of the blocks.

These mixed materials may be added to the melt solely for the purpose of increasing the sulphur content and thus render it more responsive to inoculation. They may also be blended with conventional inoculants to increase their effectiveness. The mixtures and inoculants may be added to the molten iron in a conventional way that is either before or after filling of the ladle or to the metal stream in the launder carrying the metal from the furnace to the ladle, no special equipment being necessary.

The following Examples illustrate the invention: EXAMPLE 1 Sulphide forming mixtures A B C D E Calcium Silicide, granular 80 60 50 - 70 Sulphur, powder 20 20 30 30 30 iron Sulphide, FeS granular - 20 20 - - Calcium Carbide, granular - - - 70 EXAMPLE 2 inoculants incorporating sulphide forming mixtures F G H I J Sulphide mixture B - - - 30 Sulphide mixture D Sulphide mixture E 40 40 - - 40 Sulphide mixture C - - 40 - Graphite 20 - 30 - 18 Ferro-Silicon 20 30 30 40 6 Silicon Carbide 20 - - - 30 Calcium Silicide - 30 - - Silicon-Manganese Zirconium - - - 30 Ferro-Silicon Magnesium - - - - 6 EXAMPLE 3 In this Example there are summarised the graphite structures obtained when the base metals of the constitution given were treated in the molten form with CaSi as an inoculant and also with mixtures F and J. The results obtained when no inoculation has been carried out are given by way of comparison.

It will be seen (a) that where the sulphur content of the base metal is greater than about 0.026% then CaSi can be used on its ovvn to give a graphite structure Random A (indicated below as A) which indicates heterogeneous nucleation of the graphite phase in the iron-carbon eutectic, whereas when the sulphur content is equal to or below 0.026% the structure of the iron is described as being interdendritic E (indicated below as E) which means that formation of the austenitic-carbide eutectic has taken place.

In the column where mixture F is used the sulphur value is that obtained from the sum of the original sulphur 0.026% and the sulphur from the added mixture.

In the column where mixture J is used, the sulphur value is that obtained from the sum of the original sulphur 0.04 and the sulphur in the added mixture. The inoculant CaSi and the inoculant mixtures F and J were added to the metal during the filling of the ladle and was completed before the ladle was completely full.

Summary of Results

After Addition of: Metal Type GC Before 0.4% 0.4% 0.4% 0.4% Steel Scrap/Pig Iron Addition CaSi J CaSi J Metal Temperature "C 1400 1370 1355 1360 1370 Wedge Value (3/4" back wedge) 12 4 2 4 2 UaT.S. Tons in 2 19.78 19.90 19.78 19.82 19.85 Brinell Hardness value 212 212 201 212 197 Graphite Structure Eutectic Cell Count 571 2286 1758 2832 2857 Sulphur 0.04 0.03 0.06 0.04 0.06 Carbon Equivalent Value 3.92 3.97 3.91 3.92 3.86 Total Carbon 3.32 3.31 3.28 3.26 3.23 Silicon 1.79 1.97 1.88 1.98 1.89 Manganese 0.67 0.66 0.65 0.66 0.64 Photo-Micrograph and Wedge No. 1 2 3 4 5

After Addition of:- Metal Type GC Before 0.4% 0.4% Steel Scrap/Pig Iron Addition F CaSi Metal Temperature C 1380 1370 1370 Wedge Value (3/4" back wedge) 10 2 4 U.T.S. Tons in 2 Brinell Hardness value Graphite Structure E A E Eutectic Cell Count Sulphur 0.026 0.045 0.026 Carbon Equivalent Value 3.79 3.79 3.79 Total Carbon 3.20 3.20 3.20 Silicon 1.76 1.76 1.76 Manganese 0.48 0.48 0.48 Photo-Micrograph and Wedge No. 6 7 8 In the accompanying drawings the structure of the iron is indicated for each iron treated.

Claims (9)

1. A method for the inoculation of iron having a low sulphur content, wherein there is introduced into the iron in the molten form a mixture of a source of sulphur and a reactant which forms a sulphide therewith shich sulphide is capable of acting to provide nuclei in the formation of the graphite from the melt.

2. A method as claimed in claim 1 in which the source of sulphur is sulphur or sulphide mineral in granular or powdered form.

3. A method as claimed in claim 2 in which the sulphide mineral is chalcocite, bornite, chalcopyrite, stannite, iron sulphide or covellite or an industrial equivalent thereof.

4. A method as claimed in any of claims 1 to 3 in which the sulphide forming reactant is calcium silicide, calcium carbide, a cerium or strontium alloy, a rare earth and/or magnesium.

5. A method as claimed in any of claims 1 to 4 in which the mixture added to the iron is blended with conventional inoculants.

6. A method as claimed in claim 5 in which the conventional inoculant is ferro-silicon, graphite, silicon carbide, calcium silicide, a barium alloy, a zirconium alloy, a strontium alloy, a cerium alloy, a magnesium ferrosilicon, a manganese alloy and/or a titanium alloy.

7. A method as claimed in any of claims 1 to 6 in which the reacting materials are bonded together in the form of shaped blocks.

8. A method as claimed in claim 1 substantially as herein described with reference to the Examples.

9. Cast iron when made by a method as claimed in any of claims 1 to 8.