GB2054660A - Fused salt baths containing lithium ions - Google Patents

Abstract

A fused salt bath useful for heat treating metal parts and containing silicon carbide in a melt which includes lithium ions and carbonate ions. The lithium ions prevent encrustation of the bath surface by silcate slag. The bath charge may be prepared from silicon carbide and a melt of an inorganic lithium salt, e.g. the chloride or carbonate, sodium or potassium carbonate as the source of carbonate ions, and sodium or potassium chloride as a melt depressant. The invention has particular application to the carburisation of steel.

Description

SPECIFICATION Improvements in or relating to fused salt baths The present invention relates to the heat treatment of metal in fused salt baths.

Heat treatment of metals in fused salt baths is a well established method of obtaining the desired properties for the metal to suit its particular end use. One example of such a heat treatment is the case carburisation of steel.

It is known to case carburise mild steel components in molten cyanide baths obtained by fusing sodium (or potassium) cyanide and sodium lor potassium) carbonate (optionally together with a melting point depressant such as sodium or potassium chloride). The process is effected simply by immersing the surface-of the component to be treated in the bath for a predetermined period of time, the carburisation being effected by a carburising species generated within the melt.

The use of cyanide baths does however have a number of severe drawbacks. One such drawback is the relatively high cost of the sodium or potassium cyanide which must continually be added to the bath. Furthermore, the process leads to the production of noxious cyanide containing waste products, such as a sludge formed at the bottom of the bath, waste wash water with which the carburised components have been washed and exhausted salts. These waste products pose special handling problems and require comparatively expensive treatment before they can be safely disposed.

An alternative to the fused cyanide bath is a fused salt bath containing solid silicon carbide in a melt of sodium or potassium carbonate (or alternatively an alkaline earth metal carbonate) which optionally additionally includes a melting point depressant such as sodium chloride or potassium chloride. In this case the carburising species is believed to be generated by reaction between the silicon carbide and the carbonate ions.

However it is found that such baths rapidly become unworkable due to the formation of a thick silicate crust on the melt surface. This causes the advantages obtained by selection of silicon carbide, namely lack of toxicity and cheapness compared to cyanide to be lost.

It is an object of the invention to provide a silicon carbide containing fused salt bath, and a method of heat treating metal therein, which bath can be used without encrustation of the surface thereof.

According to a first aspect of the invention, we provide a fused salt bath containing silicon carbide, in a melt including carbonate ions and lithium ions.

In accordance with a second aspect of the invention metal is heat treated in the fused salt bath defined in the preceding paragraph.

The basis of the invention lies in the use in the melt of lithium ions which we have found prevent encrustation of the melt surface. The lithium ions may be provided by fusion of any suitable lithium compound, which will for preference be an inorganic salt, most preferably the carbonate or chloride.

The use of lithium ions is particularly effective in a fused salt bath containing silicon carbide in a melt of sodium or potassium carbonate and, optionally, a melting point depressant such as sodium or potassium chloride. The amount of lithium compound, e.g. the carbonate, may vary within a wide range with respect to the total weight of the bath composition whilst still giving satisfactory results. There will of course be a minimum amount above which the lithium compound must be used to prevent encrustation, this minimum amount being dependent on the remaining components, and their amounts, in the bath. Amounts of lithium compound significantly above this minimum may not lead to any advantages, and may in fact give rise to an unnecessary increase in the cost of the process.

Generally the amount of lithium compound used will be in the range of 1 to 20% by weight of the total weight of the bath.

The use of a melting point depressant allows the working temperature of the bath to be adjusted to within a preferred range of 8500--9500C. Due to its high melting point, silicon carbide is of course solid at these temperatures. If the melt is produced from sodium carbonate then the melting point depressant will preferably be potassium chloride, since it is found that a mixture of potassium and sodium ions is required for the most effective operation of a silicon carbide salt bath. Preferably the potassium chloride and sodium carbonate are used in equimolar amounts. Alternatively, of course, the co-fusion could be prepared with potassium carbonate and sodium chloride.

As indicated above, sodium or potassium carbonate is the preferred compound for providing carbonate ions in the melt. It is however possible to use a variety of compounds which decompose or oxidise to carbonate at the temperature of the melt, e.g. bicarbonate, formate acetate and oxaiate.

It is possible to use any grade of silicon carbide in the process of the invention. The amount of silicon carbide used will depend on a number of factors which are discussed below.

The fused salt bath may be used for case carburising metal, particularly steel components, or may be operated as a "neutral" bath. A "neutral" bath is one which gives no case carburisation and merely thermaliy treats the metal surface. In this case, the amount of silicon carbide used in the bath will be such that there is no increase in the carbon content of the surface layer of the metal part.

If the bath is to be operated as a carburising bath then the amount of silicon carbide must be above a certain minimum amount otherwise the bath will either function as a neutral bath, or may even be decarburising with respect to steel to be treated. It will generally be found that the carburising bath may comprise 2-40% by weight silicon carbide, although values outside this range may still give acceptable results. The optimum weight percentage of silicon carbide will however vary with its particie size, particle size range, and also on the treatment temperature.As an example, it has been found that about 5% by weight of 20--60 grit Firesand seems optimal at 9200 C, although 20-40 grit in the metallurgical grade and 12, 24, 60 and 80 grits in the first quality grade have been found satisfactory when used in amounts of 240% at temperatures of 900--9500C.

The time for which the metal is carburised in the bath will depend on the temperature thereof as well as the required depth of carburisation. It is most preferred that the bath be operated at a temperature of 850-9500C since lower temperatures, e.g. 8000 C, may give slow carburisation and higher temperatures, e.g.

11 000C, may give carburisation which is impractically fast. A typical treatment time at 920 C would be about 2 hours, but times of 24 hours have been used.

In use, the bath composition will become gradually depleted in silicon carbide and for most effective operation the bath would be topped up daily with 16% of silicon carbide and a small quantity of sodium carbonate. The bath can be operated continuously over a period of several weeks, but it is generaily necessary to dredge every 12 hours the sludge formed at the bottom of the bath so that this does not hinder the working of the process.

It will be appreciated from the foregoing description that the composition with which the bath is charged may vary within wide limits. The following composition is one specific, but nonlimiting example of charge which may be used for carburising mild steel components at a temperature of 900--9500C.

Na2CO3 50% by weight) equimolar KCI 36% by weight) Li2CO3 4% by weight SiC 10% by weight The invention has been described with specific reference to neutral baths and to carburising processes. It is envisaged however that the process of the invention could be operated as a nitriding or carbonitriding process by providing a source of nitrogen in the bath.

Claims (14)

1. A fused salt bath containing silicon carbide in a melt including lithium ions and carbonate ions.

2. A bath as claimed in claim 1 wherein the lithium ions have been provided by fusion of a lithium compound in an amount of 1 to 20% by weight based on the total weight of the bath charge.

3. A bath as claimed in claim 1 or 2 wherein the lithium ions have been provided by fusion of an inorganic lithium salt.

4. A bath as claimed in claim 3 wherein the lithium salt is lithium chloride or lithium carbonate.

5. A bath as claimed in any one of claims 1 to 4 wherein said carbonate ions are provided at least partially by fusion of sodium carbonate or potassium carbonate.

6. A bath as claimed in any one of claims 1 to 5 wherein a melting point depressant is used in the production of the melt.

7. A bath as claimed in claim 6 when dependent from claim 5 wherein the melting point depressant is potassium chloride and the carbonate ions are at least partially provided by sodium carbonate.

8. A bath as claimed in claim 7 wherein the potassium chloride and sodium carbonate are used in equimolar proportions.

9. A bath as claimed in claim 6 when dependent on claim 5 wherein the melting point depressant is sodium chloride and the carbonate ions are at least partially provided by potassium carbonate.

10. A bath as claimed in any one of claims 1 to 9 wherein the amount of silicon carbide present in the bath is from 240% by weight based on the total weight of the bath charge.

11. A method for the heat treatment of metal comprising treating the metal in a fused salt bath according to any one of the preceding claims.

12. A method as claimed in claim 11 wherein the working temperature of the bath is 850 C to 950"C.

13. A method as claimed in claim 11 or 12 wherein the metal is steel.

14. A method as claimed in claim 13 wherein the bath is carburising to the steel.

1 5. A fused salt bath substantialiy as hereinbefore described.

1 6. A method for the heat treatment of metal substantially as hereinbefore described.