GB2043699A - Improvements in cyanide-free carburizing process and composition - Google Patents

Abstract

A cyanide-free molten salt carburizing process and composition comprising an alkali metal salt bath 10 having a cover 26 of finely divided graphite particles in a size range of about 100 U.S. mesh to about 300 U.S. mesh, is operated in the temperature range of about 1550 DEG F. to 1900 DEG F. and is provided with mechanical agitation 12 to produce entrainment of air through the surface cover 26 and sufficient mobility of carbon monoxide coated graphite particles to produce carbon diffusion by contact into the surface of the parts immersed in the bath. Total graphite content is between about 11 DIVIDED 2% to about 7% of the active portion of the bath. The graphite is preferably of the synthetic type so as to produce an ash content below about 1%. <IMAGE>

Description

SPECIFICATION Improvements in cyanide-free carburizing process and composition This invention relates to improvements in carburizing processes of the type utilizing a cyanidefree molten salt bath into which ferrous metal parts are immersed for carbon diffusion purposes.

Case hardening of ferrous metal parts has been accomplished for many years by way of carbon diffusion processes which utilize and/or produce highly toxic cyanide compounds. A cyanide process is disadvantageous for several reasons; for example, the cost of cyanide disposal treatment is very high due to the toxicity of the material; in addition, the cyanide bath is incompatible with commonly used low temperature quench salts of the nitrate/nitrite type.

Work has been done to provide a commercially acceptable salt bath carburizing process of the noncyanide type thereby to reduce or eliminate the difficulties attending the use and handling of cyanide compounds; see, for example, Canadian Patent No.

944,665 "Cyanide Free Carburizing Composition, Apparatus and Process" granted April 2, 1974 to the Park Chemical Company, assignee of the present invention. The aforesaid Canadian patent discloses a carburizing process in which carbon is diffused into ferrous metal parts by immersion in a bath consisting of a molten chloride/carbonate salt and having a cover of graphite particles.

While the cyanide-free carburizing process of the above-identified prior art was frequently capable of producing satisfactory results, we have determined that the actual carburizing mechanism was not phenomenologically understood. Hence, processes which appeared to be carried out in accordance with the prior art disclosures often produced unacceptable results, and the diagnosis of functional problems was difficult or impossible due to a lack of understanding of the basic process.

We have discovered that the achievement of a stable cyanide-free molten salt carburizirig bath which consistently produces commercially acceptable results lies in a critical balance of certain factors including: graphite character and composition in the active bath, the physical parameters of the cover, agitation and the resulting character and composition mobility of the graphite particles throughout the bath, and operating temperature. Inthe course of determining the criticality and effectiveness of this balance, we have discovered that the phenomenon which gives rise to carbon diffusion involves the spontaneous generation of nescent carbon monoxide as a gaseous coating around mobile graphite particles in the bath as a result of drawing air through the cover.Hence, the graphite particles serve not only as carbon sources but also as carriers of carbon monoxide to parts immersed in the bath.

According to the present invention, there is provided a non-cyanide carbon diffusion process for case-hardening ferrous metal parts comprising the steps of: producing a molten bath consisting essentially of an alkali metal chloride and an alkali metal carbonate in the temperature range between about 1 550 F and 1 900 F; producing a cover on the bath consisting essentially of uniformly distributed graphite particles of low ash content and having particle sizes in the range of about 100 U.S. mesh to about 300 U.S.

mesh; and mechanically agitating the bath sufficiently to produce a substantially continuous flow of graphite particles from the cover into and throughout the bath as well as the entrainment of air through the cover to generate nascent carbon monoxide bubbles around graphite particle carriers which, by flowing through the bath, produce carbon diffusion into the surface of low carbon ferrous metal parts immersed therein.

Thus, our process, as more fully set forth hereinafter, involves steps which promote and enhance the graphite-carbon monoxide carrier system. By this mechanism, nascent carbon monoxide is brought into contact with the surface of low carbon ferrous metal parts to produce a quench hardenable Cementite case.

The quantity of graphite in the cover is preferably such as to produce, by suitable agitation, a bath comprising between about 1a% by weight and 7% by weight graphite evenly distributed and moving throughout the bath. This is accomplished by agitation using a propeller-type device which entrains air into the bath and gently but positively produces mobilization of the suspended particles.

In addition, the graphite is selected so as to exhibit an ash content of less than about 1%. This is best achieved by using synthetic rather than natural graphite. In addition, it is believed advantageous, although not essential, that the graphite particles be present in a mixture of particle sizes within the preferred range, the smaller graphite particle sizes contributing to the generation of carbon monoxide while the larger graphite particle sizes make up the necessary mechanical carriers. The concept of particle size mixture is not to be viewed as an essential component of the invention but rather as a theoretical adjunct to the basic phenomenological mechanism of cyanide-free carburizing set forth herein.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing in which the single figure is a diagram of a hardening apparatus for low carbon ferrous metal parts, and representing the nature and composition of the molten salt bath which forms a principal part of the apparatus essential to said process.

Referring to the drawing, the apparatus comprises an open container 10, preferably but not necessarily ceramic, of such size and shape as to provide a molten chloride/carbonate salt bath suitable for receiving commercial quantities of low carbon ferrous metal parts for the initial phase of a case-hardening process. Disposed over and adjacent the container 10 is an air motor 12 having an output shaft 14 which extends into the container and has connected thereto at spaced locations a setofthree-blade propeller type impellers 16 and 20 disposed within cylindrical rings 18 and 22 respectively. Container 10 is substantially filled with a chloride/carbonate salt, the composition of which is more fully set forth hereinafter. In addition, a cover 26 of finely divided graphite particles is disposed on top of the salt mixture 24 in a fairly uniform fashion.The dimensions of the apparatus are such that the cover 26 lies five to eight inches above the impeller 16; impeller 20 is preferably spaced five to eight inches about the floor of the container 10.

Adjacent container 10 is a second container 28 containing a low temperature nitrate-nitrite quench bath 30. Both containers 10 and 28 include provisions for heating the contents thereof; for example, immersible electric heaters or exterior gas heaters.

In practice, the low carbon ferrous metal parts are immersed in the bath 24 of container 10 for a sufficient period of time to produce a carbon diffusion into the surface of the part or parts, after which the part or parts are removed from bath 24 and directly quenched in bath 30 to produce a hardened case. An intermediate quench may also be employed, depending upon the desired end result. Suitable quenchants include nitrate/nitrite salts, oil, and water.

In accordance with the invention, the alkali metal salt/carbonate mixture may comprise the combination of sodium or potassium chloride, present in sufficient quantity to make up between about 100% to 0% of the bath by weight, and sodium or potassium carbonate present in sufficient quantities to make up between about 0% and 100% of the bath by weight Other alkali metal salts and other alkali metal carbonates may be employed; for example, barium and lithium.

Graphite cover 26 is preferably finely divided, high purity, synthetic graphite having an ash content of less than about 1%. The graphite cover comprises particles of a mesh size within the range of about 100 U.S. mesh to 300 U.S. mesh and preferably exhibiting a distribution of mesh sizes within the range. The cover should be reasonably uniform and maintained in a thickness of about " to 1", covering more than 90% of the salt bath surface and being easily broken by agitation provided by the impellers 16 and 20 so as to permit the entrainment of air through the cover 26 from the surface of the bath. When the impellers 16 and 20 are operating, the active portion of the bath 24 should be made up of about 1%to to about 7% graphite by weight, i.e., a sample taken below the cover would show such a percentage of suspended graphite particles.We have found that the lower limit of 19% is that point at which carburizing activity appears to fall off rapidly, it being understood that some variation around this point should be expected. The high end of the range shows a less definite falling off of carburization and considerable latitude should be given here. The 7% figure has been found to represent that point at which the balance between good carburization activity and excessive cover begins to break down; i.e., greater percentages can be achieved only by creating more cover and inhibiting the air-entrainment function.

The operation and nature of the agitator means and its effect on the cover and mobilized graphite is extremely important to the proper and satisfactory attainment of carburizing in accordance with the invention. Agitation is considered proper when it can pull some of the graphite from the cover 26 into the bath and provide substantial mobility of the graphite particles, with the carbon monoxide coating thereon, uniformly throughout the bath. Agitation is excessive when a vortex is created. Over-agitation pulls too much air into the bath giving rise to an excess oxygen condition which produces carbon dioxide rather than carbon monoxide. In addition, it has been found that over-agitation tends to separate the carbon monoxide gas coatings from the graphite carriers, after which the carbon monoxide merely rises to the surface and burns off. Since the invention is dependent upon the mobility of carbon monoxide carrying graphite particles throughout the bath, over-agitation is to be avoided.

On the other hand, agitation is insufficient when it fails to pull graphite from the cover and entrain sufficient air forth generation of carbon monoxide around the graphite particles in the bath. The following table is illustrative of the proper agitation combinations for various bath configurations and sizes.

Three-bladed propellers with peripheral rings around the blades have been found satisfactory. It is to be understood, however, that other agitation means may be used in accordance with the principles of the present invention.

SALT BATH DATA AGITATOR DATA Area Length Molten Diameter No. of Position having to Width Salt of 3 Propellers No. of of Pro- Speed, cover, ft! Ratio Depth, bladed on Each Agita- peller r.p.m. ft. propeller Shaft level Up to 5 1 to 2.5 < 1.5 3 1 1 4" from 600 top 800 UptoS > 2.5 " " 1 2 2 6-11 1to2.5 " 4 1 1 " " 1.5-3 4 2 1 4" from 800 top and 1000 botom " > 2.5 < 1.5 4 1 2 4" from 600 top 800 1.5-3 4 2 2 4" from 800 top and 1000 bottom 12-25 1to3 < 1.5 4 1 2 4"from 600 top 800 > 1.5 4 2 2 4" from 800 top and 1000 bottom

TABLE An air motor is preferred to an electric motor in view of the intense heat which is present near the surface of the bath. The motor should be situated at least one foot or more away from the top of the salt bath and a heat deflecting circular shield near the upper end of the shaft 14 may be advisable.

Example I The bath 24 is started with a base salt consisting of 60% sodium chloride and 40% sodium carbonate, no graphite cover being present initially. After melting, the temperature of the bath is raised to 15000F. and air motor 12 is actuated to begin agitation. Finely divided synthetic graphite particles having a U.S.

mesh size according to the following distribution are slowly added to sparsely cover the bath, agitation continuing during the application of the cover: 24% +200 mesh 34% 200-325 mesh 42% -325 mesh All graphite has an ash content of about 10/o. The graphite cover is maintained in a thickness of about 'to 1", should be easily broken by agitation and cover about 900/c of the bath surface. Graphite is stir red into the bath by agitation; a sample shows graphite content about 4% of the active bath by weight.

The bath is further heated to a normal operating temperature of 1700"F. Approximately five hours are then permitted to lapse before low carbon parts are added for carburizing. This break-in period is necessary only on the initial start-up. When the bath is fully active the cover continuously emits small pops of flame.

When carburizing small parts, nesting or stacking should be avoided because of the necessity for a surface contact of the carbon monoxide carrying graphite particles with the low carbon metal parts. If flat surfaces are in close contact, the carburizing media may not be able to penetrate thoroughly thus producing an uneven case. Parts and baskets should be packed randomly and repositioned occassionally by brief shaking.

Parts immersed in the above bath for 1.5 hours at 1700"F. exhibited a total case depth of .030" and sur face hardness after brine quench of 62 Rc.

Example II Basic operating conditions the same as Example I except the bath is 10% potassium chloride and 90% sodium carbonate. The bath is operated at a car burizing temperature of 1900"F. with graphite parti cles of about 200 U.S. mesh present in quantity so as to make up 3.16% of the bath by weight. Low carbon ferrous metal parts are carburizedfortwo hours producing a total case depth of .066" and surface hardness, after brine quench, of 63 Rc.

Example III Basic operating conditions the same as Example I except the bath is stabilized at approximately 1600"F.

Low carbon ferrous parts are immersed for one hour and produce an effective case of 0.008", 62 Rc (equivalent).

Example IV Basic operating conditions the same as Example I except mobilized graphite content in the active bath is about 19% by weight. Surface hardness resulting: about 61 Rc. Below 1D/o, surface hardness falls off; example, 0.85% graphite produces surface hardness of only about 58.2 Rc.

It is to be understood that the foregoing examples are illustrative in nature and are not to be construed in a limiting sense.

Claims (10)

1. A non-cyanide carbon diffusion process for case-hardening ferrous metal parts comprising the steps of: producing a molten bath consisting essentially of an alkali metal chloride and an alkali metal carbonate in the temperature range between about 1550"F. and 1900 F.; producing a cover on the bath consisting essentially of uniformly distributed graphite particles of low ash content and having particle sizes in the range of about 100 U.S. mesh to about 300 U.S.

mesh; and mechanically agitating the bath sufficiently to produce a substantially continuous flow of graphite particles from the cover into and throughout the bath as well as the entrainment of air through the cover to generate nascent carbon monoxide bubbles around graphite particle carriers which, by flowing through the bath, produce carbon diffusion into the surface of low carbon ferrous metal parts immersed therein.

2. A non-cyanide carbon diffusion process as claimed in claim 1 wherein the agitation is sufficient to produce a graphite content in the active portion of the bath of between about 10/o by weight and 7% by weight.

3. A non-cyanide carbon diffusion process as claimed in claim 1 or 2, wherein the ash content of the graphite is less than 1%.

4. A non-cyanide carbon diffusion process as claimed in any of claims 1 to 3, wherein the agitation is provided by immersed impeller means.

5. A non-cyanide carbon diffusion process as claimed in claim 1 substantially as hereinbefore described.

6. A non-cyanide carbon diffusion process substantially as hereinbefore described with reference to any of the foregoing Examples I to IV.

7. Aferrous metal part when case-hardened by the process as claimed in any preceding claim.

8. A molten salt composition for use in a noncyanide carbon diffusion process for case hardening ferrous metal parts consisting of: a molten alkali metal chloride in the temperature range between about 1550 F. and 1900 F.; and graphite particles of low ash content having a mesh size in the range of about 100 U.S. mesh to about 300 U.S. mesh forming a cover on the bath and being uniformly dispersed throughout the bath.

9. A molten salt composition as claimed in claim 8, wherein the graphite content in the bath is bet ween 10/o and 7% by weight.

10. A molten salt composition substantially as hereinbefore described with reference to any of the foregoing Examples I to IV.

10. A molten salt composition as claimed in claim 8 or 9, wherein the ash content of the graphite is less than 1%.

11. A molten salt composition as claimed in claim 8, substantially as hereinbefore described.

12. A molten salt composition substantially as hereinbefore described with reference to any of the foregoing Examples I to IV.

New claims or amendments to claims filed on 15 Oct 1979.

Superseded claims 1-12.

New or amended claims: 1. A non-cyanide carbon diffusion process for case-hardening ferrous metal parts comprising the steps of: producing a molten bath consisting essentially of an alkali metal chloride and an alkali metal carbonate in the temperature range between about 1550 F. and 1900"F.; producing a cover on the bath consisting essentially of uniformly distributed graphite particles of low ash content and having particle sizes in the range of about 100 mesh to about 300 mesh; and mechanically agitating the bath sufficiently to produce a substantially continuous flow of graphite particles from the cover into and throughout the bath to produce a graphite content in the active portion of the bath of between about by weight and about 7% by weight; and to produce the entrainment of air through the cover, to produce carbon diffusion into the surface of low carbon ferrous metal parts immersed in the bath.

2. The process as defined in claim 1 wherein the ash content of the graphite is less than about 1%.

3. The process as defined in claim 1 or2 wherein the agitation is provided by immersed impeller means.

4. A non-cyanide carbon diffusion process as claimed in claim 1 substantially as hereinbefore described.

5. A non-cyanide carbon diffusion process substantially as hereinbefore described with reference to any of the foregoing Examples I to IV.

6. A ferrous metal part when case-hardened by the process as claimed in any preceding claim.

7. A molten salt composition for use in a noncyanide carbon diffusion process for case-hardening ferrous metal parts consisting of: from about 0.001% to about 99.99% of a molten alkali metal chloride and from about 0.001% to about 99.99 /O of a molten alkali metal carbonate in the temperature range between about 15500F. and 1900 F.; and graphite particles having a mesh size in the range of about 100 mesh to about 300 mesh forming a cover on the bath and being uniformly dispersed throughout the bath and present in the amount of about 10/o by weight to about 7% by weight of the bath and having an ash content of less than about 1%.

8. A molten salt composition as claimed in claim 7 in which the ash content of the graphite is less than 1%.

9. A molten salt composition as claimed in claim 7 substantially as hereinbefore described.