FUSED SALT BATH COMPOSITION

Abstract: A composition of a fused salt bath useful for heat treating metal parts contains silicon carbide in a melt which includes lithium ions an carbonate ions. The lithium ions prevent encrustation of the bath surface by silicate slag. Chloride ions may be included to depress the melting point. A bath, used for 2 hours at 920 DEG C to carburise steel, has the composition in parts by weight: 55 1/2 sodium carbonate, 40 potassium chloride, 4 1/2 lithium carbonate, 10 silicon carbide.

Description: - i - FUSED SALT BATH COMPOSITION The present invention relates to a fused salt bath composition. Heat treatment of metals in fused salt baths is a well established method of obtaining desired properties for a metal to suit its particular end use. One example of such a heat treat- ment 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 (or potassium) carbonate (optionally together with a melting point depressant such as sodium or potassium chloride). The surface of the component to be treated is simply immersed in the bath, the carburisation being effected by a carburisinc species generated within the melt. Cyanide baths however have severe drawbacks, for example the high cost of the sodium or potassium cyanide which must continually be added to the bath, and the production of noxious cyanide-containing waste products, including a sludge which forms at the bottom of the bath, water with which the carburised components have been-waghed. and exhausted- salts. 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 πselt surface. This vitiates the advantages of silicon carbide, namely lack of toxicity and cheapness. According to the invention, a fused salt bath composition containing silicon carbide, and carbonate ions is characterised by containing lithium ions. Al « _o in accordance with the invention, metal is heat treated in this fused salt bath. We have found that.lithium ions can 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. an inorganic salt such as 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 or delay it for a sufficient time, 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 lixhium compound used will be in the range of 1 to 2096 by weight of the total weight (excluding the silicon carbide) of the bath, prefer¬ ably at least 2.6, more preferably at least 3%, and preferably up to 10%, more preferably up to 6%. The use of a melting point depressant allows the working temperature of the bath to be adjusted to within a preferred range of 850 - 950C. 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 depressant will preferably be potassium chloride, since it is found thax a mixture of potassium and sodium ions is required for the most effective operation of a silicon carbide salt bath. Thus the melτ. may be produced of sodium carbonate, of which up to 8θ mole?ό (prefer-? ably up to 65 mole..) of the sodium may be substituted by potassium and of which up to 8θ mole% (preferably up to 60 mole.. " > of the carbonate may be substituted by chloride. The potassium chloride and sodium carbonate may be used in equimolar amounts, in which case the co-fusion could in practice be alternatively prepared with equimolar amounts of potassium carbonate and sodium chloride. While sodium or potassium carbonate is the preferred compound for providing carbonate ions in the melt, the melt may be produced of these ions by using a compound which decomposes or oxidises to carbonate at the temperature of the melt, e.g. bicarbonate, formate, acetate and oxalate. 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 thermally 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 1-40%,preferably up to 10%, by weight silicon carbide, (note thax this percentage is additional to the fused salts, which are taken as 100%) although value≤ outside this range may still give acceptable results. The optimum weight percentage of silicon carbide will however vary with its particle size, particle size range, and also the treatment temperature. As an example, it has been found that about 5% by weight (same basis as above) of 20-60 grit in the metallurgical grade seems optimal at 920C, although 4-4^% 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 respective amounts of 30%, 20%, 10% and 10% (generally 2-40%) at temperatures of 850-950C. Also, _i% by weight seems optimal both for 60-120 grit arid 120 - finer (i.e. smaller particles than 120 grit) at 920C, with somewhat less (around 1%) at 950C, and somewhat more (i - 2%) at 850C. The grit sizes are to FEPA standards (Federation of European Producers of Abrasive Products). 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 85O-950C since lower temperatures, e.g. 800C, may give slow carburisation and higher temperatures, e.g. 1100C ; may give carburisation which is impracτically fast. A typical treatment time at 920C 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 1-6% of silicon carbide and a small quantity of sodium carbonate. The bath can be operated contin¬ uously over a period of several weeks, but it is generally 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. The invention is now described by way of example. A salt bath had the following composition. Na 2 C0 3 55τ% by " weight-) .equimolar KC1 4θ% by weightJ i 2 C03 ki% by weight SiC (80 grit 10% by weight first quality) - 5 - A steel component (EN32B steel) was fully immersed in this bath, which was held at 920C, for 2 hours. It was found that carburisation to a discernible degree had penetrated 0.66mm from the steel surface, the upper 0.33mm being fully pearlitic. IREA

Claims: CLAIMS 1. A fused salt bath composition containing silicon carbide and carbonate ions, characterised by containing lithium ions. 2. A bath composition as claimed in Claim 1 wherein the lithium ions are provided by a lithium compound in an amount of 1 to 20% by weight of the total weight of the bath excluding the silicon carbide. 3. A both composition as claimed in Claim 2, wherein said amount is up to 10% by weight. 4. A bath composition as claimed in Claim 3. wherein said amount is up to 6% by weight. 5. A bath composition as claimed in Claime.2, 3 or 4 wherein said amount is at least 2% by weight, 6. A bath composition as claimed in Claim 5. wherein said amount is at least 3% by weight. 7. A bath composition as claimed in any preceding claim wherein the lithium compound is an inorganic lithium salt. 8. A bath composition as claimed in Claim 7. wherein the lithium salt is lithium chloride or lithium carbonate. 9. A bath composition as claimed in any preceding claim wherein said carbonate ions are provided by sodium carbonate of which up to 8θ mole%, preferably up to 65 mole%, of the sodium is substituted by potassium. 10. A bath composition as claimed in any preceding claim further comprising a melting point depressant. 11. A bath composition as claimed in Claim 10 when dependent on Claim 9. wherein the melting point depressant is sodiuα and/or potassium chloride,which is provided in that up to 80 oole%, preferably up to 60 mole%, of the carbonate is substituted by chloride. 12. A bath composition as claimed in Claim 11, wherein poτassium chloride and sodium carbonate are used in substantially equimolar proportions. 13. A bath composition as claimed in any preceding claim, wherein the amount of silicon carbide is from 1 - 4θ% by weight, taking the total weight of the fused salts as 100%. 14. A bath composition as claimed in Claim 13. wherein the amount of silicon carbide is up to 10% by weight. 15. A method of heat treating metal, comprising treating the metal in a fused salt bath having the composition defined in any one of the preceding claims. 16. A method as claimed in Claim 15 1 wherein the temperature of the bath is 85OC to 950C. 17. A method as claimed in Claim 15 or l6, wherein the metal is steel. 18. A method as claimed in Claim 17, wherein the bath is carburising to the steel. 19. A method as claimed in any of Claims 15 to 18, wherein the treatment time is to 24 hours.

Inventors: KERRIDGE DAVID HENRY
Applicants: KERRIDGE DAVID HENRY, NAT RES DEV, NATIONAL RESEARCH DEV CORP
Application year: 1981
Main classification: C23C9
IPC codes: C21D1, C23C8