Case hardening is a heat treating practice by which a thin, hardened "skin" is formed on a steel article. Case hardening generally designates any of three types of hardening processes: carburizing wherein carbon is added to low carbon steel, nitriding wherein nitrogen is added, and nitrocarburizing wherein both carbon and nitrogen are added to the steel. In carburizing, the article is exposed to a gaseous carbon containing atmosphere (gas carburization) or packed in a carbonaceous powder (pack carburization) or immersed in a cyanide-salt bath (liquid carburizing) and heated so that the carbon diffuses into the surface of the steel to form the skin. In nitriding, the steel article is placed in a heated nitrogen containing atmosphere (gas nitriding), or the article is placed in a heated cyanide bath (liquid nitriding) so that the nitrogen from the gas or bath diffuses into the surface to form the skin. (In both liquid carburizing and liquid nitriding, the medium is a molten cyanide salt bath; the temperature at which the case hardening is carried out determines the result). Ferritic nitro-carburizing (carbonitriding) is a combination of gas carburization and nitriding wherein a nitrogen gas, usually ammonia, and an endothermic gas are added to the carburizing gas.
These processes are carried out at temperatures sufficiently high so that nitrogen from the disassociated ammonia or carbon from the carbonaceous powder can diffuse into the steel to form a hardened skin by making iron nitrides and alloys of nitrides and/or carbides through reactions with various trace elements at and just below the surface. Typical gas nitriding temperatures are 925.degree. to 1050.degree. F. whereas typical gas carburization temperatures are above 1600.degree. F. and most often at 1700.degree. F. to 1900.degree. F. Carbonitriding is carried out at temperatures below those required for carburizing, that is at 1400.degree. F. to 1650.degree. F.
During case hardening, anti-nitriding and anti-carburizing compositions are sometimes employed to shield selected metal surfaces from the nitriding or the carburizing medium, whether gas, liquid or powder. These compositions are called "stop-offs" or "resists."
Stop-offs are typically employed to prevent a predetermined portion of the steel from forming a hardened skin so as to facilitate later machining operations on the unhardened areas.
Prevention of case hardening is a matter of preventing nitrogen or carbon from reaching the surface of the steel by providing a physical shield between the steel and the case-hardening atmosphere or environment. The high temperatures at which case hardening processes are carried out present special problems in formulating stop-offs. To be commercially acceptable, a stop-off should exhibit certain desirable performance characteristics: the stop-off must be capable of effective application to the articles at room temperatures (preferably by an easily practiced technique, such as by "painting" the article with the stop-off); the stop-off must provide a gas impervious shield which not only resists gaseous penetration at high temperatures but also withstands the stresses and strains caused by thermal expansion and contraction of the underlying article; and the stop-off must be easily removable from the article after the case hardening process is complete.
Carburizing takes place at substantially higher temperatures than nitriding or ferritic nitrocarburizing so that carburizing stop-off formulations have been different from those used in nitrogen processes.
One early stop-off technique was to coat metal with pulverized boric acid. The boric acid melted at the elevated operating temperatures to provide a fluent, adherent glaze-like shield. Such a technique is disclosed in U.S. Pat. No. 1,190,937, which teaches a boric acid coating to prevent the decarbonization of steel during heat tempering. Other carburizing stop-off materials have been proposed which employ various alternative formulations, such as the refractory clay and borax or boric acid formulations in U.S. Pat. Nos. 1,567,632, and 3,151,002. The later patent teaches the use of a synthetic resin lacquer as a binder to hold the boric acid crystals in place until an operating temperature sufficiently elevated to melt the boric acid and to coke the resin is reached.
Nitriding and ferritic nitro-carburizing (nitrogen processes) are carried out at temperatures lower than those for carburizing. Nitrogen process frequently employ a layer of tin on the surfaces of the article to be shielded. Electroplating was one technique of providing a tin layer, and where electroplating was not feasible, various tin-bearing coatings have been employed to shield parts of the article. When the article was brought to an elevated operating temperature, the tin in the coating melted to form a coherent, fluent shield. A tin technique, albeit for prevention of carburization in a cyanide bath, is disclosed in U.S. Pat. No. 2,485,176, which teaches a copper/tin (bronze) composition in an organic, vehicle.
In order to be easily applied by spraying or painting, some prior art stop-offs were formulated with combustible or flammable resins, as taught in U.S. Pat. No. 2,485,176, wherein the finely powdered metal particles are dispersed in an organic vehicle so as to be sprayable or paintable on surfaces not required to be case-hardened. If further thinning or dilution of these stop-offs was needed, for example after a period of storage or standing open so as to cause the resin to evaporate, additional flammable resin or solvent therefor had to be handled. Thus, use of many prior stop-offs involved the risk of fire or explosion. It can be appreciated that such stop-offs required special ventilation of the work place as well as special handling and storage facilities. Lastly, flammable stop-offs present transportation problems in that they require special packaging, labeling and handling.
In order to ameliorate the dangers and difficulties attendant the use of flammable stop-offs, various proposals have been made for providing non-flammable formulations. U.S. Pat. No. 3,178,321 teaches a wide temperature range (i.e., from about 800.degree. F. to about 1900.degree. F.) protective coating comprising refractory clays in a water-dispersible resin. U.S. Pat. No. 3,454,433 discloses low temperature (i.e., below about 1600.degree. F.) protective ceramic coatings comprising frits and refractory materials in both organic solvent type binder carrier and water-base binder carrier. U.S. Pat. No. 3,661,820 teaches an anti-carburization compound containing boric acid or borax in "water reducible" resins. Although the greater water content of these stop-offs may have improved safety, there was a trade-off in terms of drying techniques and times. The '321 patent reports oven drying at 180.degree. F. In the '433 patent, air drying is followed by baking the water-soluble resinous varnish-containing formulation at about 180.degree. C. to thermoset the binder; drying times are reported in the '820 patent to be around eight hours at room or factory temperatures. Satisfactory use of such stop-offs required either the provision of special elevated temperature drying rooms to hasten drying or a great deal of advance planning coupled with storage space so that the coated articles awaiting treatment could be stored away during drying so as to avoid damage to the wet stop-off surfaces.
Another disadvantage encountered with prior formulations for water-based stop-offs included short-shelf life in contrast to organic solvent based stop-offs; problems with prior formulations included such settling during storage that the particulates could not be stirred up again, as well as rapid hardening after opening the container.
It would be advantageous, therefore, to have a truly water-based stop-off useful in nitrogen processes and characterized by ease of application and quickness of drying before heat treatment, ease of removal afterward, and enhanced safety in the work place.