US2562065A - Carburizing salt bath - Google Patents

Description

Patented July 24, 1951 Edna L. Sawyer, Stamford, and Northrup W. Marr, Springdale, Conn., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing.

Thepresent invention relates to salt mixtures which, when in a molten condition at elevated temperatures, are adapted for carburizing ferrous metal, a bath made from such salt mixtures, and a process of using the bath.

,It s an object of this invention to provide a case carburizing bath that will give efiective case depths of at least 16 mils in one hour at 1600 F. consistently, but which, at the same time, will not leave an insoluble sludge on the article after it is removed from the bath, oil quenched, and shaken in hot water. It is a further object of the present invention to provide an easy washing carburizing bath operable over a temperature range of 1300 to 1750 F. It is a still further object to provide an inexpensive, novel, and efiective activator for such a bath, namely, an alkali ii'ietalsilicate. Still other objects will be obviou from the description of the invention given below.

A cyanidin bath containing unactivated sodium cyanide does not coat the work with insoluble sludges, but is unsuitable as a carburizing bath because it carburizes slowly and gives effective casedepths of only'about 12 mils at the highest temperature at which it can be operated, namely, around 1550 F., and is further unsatisfactory as a carburizing means since it nitrides the Workas well as carburizes it, the case at the surface analyzing about 0.6% C and about 0.5%N.

1 The usual liquid carburizing bath provides sat Application November 29, 1950, Serial No. 198,226

I soiai s'l (Cl. 148-27) f and unacceptable level. For example, while th isfactory case depths through the activation of calcium and barium salts. Such a bath may give an effective case depth of from 16 mils inan hour at 1600 F. to 125 mils in 16 hours at 1750 F. A disadvantage of a bath activated with calcium and barium salts is that these salts sludge, forming oxides and carbonates that adhere to the article when it is withdrawn from the bath and that cannot be dissolved in hot water, but must be removed from the article by chemical and/or mechanical means. v V 'A'n'easy washing carburizing bath has now been found that incorporates the easy washing characteristics of a cyaniding bath with the high carburizing activity'of a calcium salt-activated bath.

highly alkaline bath might give case depths of 16 mils inthe beginning of its operation, penetration would soon drop to the order of 12-14 mils or less. It was believed that this reduced inactivity was caused by stabilization of the bath by the alkaline materials present. A standard method oiv correcting excess alkalinity has been to add acid materials to the bath. Examples of such acid mate rials are free silica and the sodium acid silicates. Thus corrected, the bath retained its reactivity and gave good carburizing penetration overa long period of time.

The present invention demonstrates that a highly alkaline bath can be prepared containing substantial quantities of alkali metal containing no acidic groups, and that the bath gives consistent high penetration over a long period of time. This combination of high alkalinity and alkali metal silicatescontaining no acidic elements is unobvious and, so far asthe inventors are aware, there is no suggestion of the combination in the prior art. In the following description, wherever alkali metal silicates are mentioned, it is to be understood they are the normal salts, containing no acidic-hydrogen. They may be added as such or formed in situ in the bath. The bathcontemplated in the present invention gives effective case depths on fracture up to about 32 mils during a four-hour treatment at l600 1?. in the bath, and the. film of adherent salts after an oil quench is so soluble that it may be removed simply by shaking the work in hot water a few times. Adherent salts from holes A; inch indiameter and /2 inch deep are easily removed by i such means. i

The bath of the present invention, unlike a cyf aniding bath, is operable up to 1750 F. and gives satisfactory case depths even though it contains no alkaline earth activator. The bath of the present invention is further characterized by its high activity in commercial-size pots, its ease of maintaining a good cover, low sodium cyanide loss, and low nitriding.

The present. invention involves a completely new concept in carburizing baths. In the past,

When making up the bath, a-salt mixture with the following range of ingredients may be use (part by weight) Sodium chloride 20 -50 Sodium carbonate 25 45., Sodium cyanide 15 35 Anhydrous sodium silicate, sufficient to provide an SiOz content of 0.5- 5.5 Graphite 0.3- 1.0

Various equivalents may be used for the above substances. For example, various other water: soluble cyanogen-forming compounds maybe used instead of sodium cyanide, namely, other alkali metal cyanides, cyanates, cyanamides, and the like. Organic cyanogen compounds are also suitable as, for example, cyanamide, dicyandiamide, melamine, melam, melem, melon, am: meline, ammelidahydrogen cyanide polymers,

and the like. i The sodium carbonate of the bath may be substituted for, in total or in part, by other alkali metal carbonates, and the sodium chloride maybe similarly substituted by other alkali metal chlorides. The sodium silicate used,

may vary widely in its NazO:Siz ratio and may be replaced by potassium silicate. However, a sodium silicate containing about 50% NazO and 50% Si02, conforming to the approximate molecular formula NazSiOs, is preferred.

Although the quantity ranges listed above are operable and are contemplated by the invention, the following composition represents the preferred materials and their preferred ranges (parts by weight):

NaCN, 20-25. NaaSiOa, 3-10, equivalent to 1.5- parts of contained silica. NazCOs, 30-35. Graphite, O.3- 1;0. NaCl, remainder to ,give 100 parts total.

"The 1.5-5.0 parts of silica in the sodium silicate isf'airly-critical, as -a content of less than 05 part fails to provide the necessary carbon penetra-tion, while a content of more than 5.5 parts results-in excessive sludging in the bath. When using the bath in the silica range of 154 parts 'oflcontained silica (3-8 parts alkali metal silidate) the bath should be replenished daily to hiaintain a satisfactory carburizing activity. A- significant advantage of the bath of this invention is its ability to give good case depths ve'r'f aj widerange of temperature without developing detrimental working characteristics. In'- 'the past it has been customary to use one bath to cover the range l5'001650 F. and a second bath to cover the range 1600-1'750 F. It was found that the first bath could not be heated substantially above 1-650 without foaming or sludging or both, and that the second bath, while having excellent cover and sludge characteristics theneighborhood of 1700 would not give good case depths below 1650" F. In contrast, the bath of the present invention gives excellent case depths with good cover performance and little or no sludging over a range-of l300-'1'7 50 F. I Itjhas =furtherbeen found that parts carburized infthe bath of the present invention have a good clean metallic appearance when Washed after an 'c'il'quench. It is common knowledge that parts similarly washed after treatment in a barium activated bath are frequently discolored. Another advantage of the present bath over previous --carburizing baths is that it maybe operated at a temperature as low as 1400 F., at which temperature it gives a case depth (microscopic) of 8 mils in 4 hours. This possibility permits carburization of intricate work without distortion...

lhe following example illustrates but doesnot limit the use of the bath.

Example 1 j iA-rniixture of saltsconsisting of 20% sodium cyanide, 30% sodium carbonate, 40% sodium chloride, and 10% sodium silicate (in the form of .a commercial alkaline cleaner) with 0.3% graphite was melted and heated to the operating temperature.

As the bath melted, the graphite formed a soft blanket which reduced the loss of cyanide and prevented evaporation of the bath while in use.

To maintain the energizing effect, and to compensate for cyanide loss and drag out from the bath, it is desirable but not absolutely necessary to add to the bath, in the amount of 78% of the total bath weight, every working day, a mixture consisting of 10% sodium silicate, 40% sodium cyanide, 50% sodium chloride and 0.3% graphite.

Case Depths Obtained on y." Dia. SAE 1020 Steel Rods on Fracture, in MilS Treatment No.

Time, Hours Temp.,

Essentially, case penetration is equal to that produced using a barium carburizing salt bath.

This is a continuation-in-part of applicants"? co-pending Serial No. 89,576, filed April 25, 1949, now abandoned.

The sodium silicate used in the herein examples contained about 50% silica.

Example 2 Carburizing comparable to that of Example is obtained with a bathmade upfrom a mixture consisting of 29% sodium cyanide, 39% sodium carbonate, 29% sodium chloride, 3% sodium silicats, and 0.2% graphite replenished daily with a mixture consisting of 40% sodium cyanide,.28%- sodium carbonate, 29% sodium chloride, 3% sodium silicate, and 0.2% graphite. The bath so used avoids a slight .sludging occasionally emcountered in the higher silicate ranges ,of the bath .of Example 1. 7

While the invention 5113.5 been described with particular reference to specific embodiment, it is to :be understood that it is not to .be limited thereto but is to be construed broadly and re. stricted solely by the scope of the appended claims. 1 We claim:

1. A mixture adapted upon fusion for the car; burizing of ferrous metal articles which consists essentially of alkali metal chloride about .2.050 parts, cyanogen compound about 15-35 parts, anhydrous alkali metal carbonate about 25-45 parts, alkali metal silicate equivalent to about 0.5-5.5 parts of silica, and graphite about 0.-1-1'.0 part, the parts being by weight. i

2. A mixture adapted upon fusion for the carburizing of ferrous metal articles which consists essentially of sodium chloride about 40 parts, sodium cyanide about 20 parts, sodium carbonate about 30 parts, anhydrous sodium silicate equi aa lent to about 5 parts of silica and graphite about 0.3 part, the parts being by weight.

3. A mixture adapted upon fusion for the carburizing of ferrous metal articles which 'con-' sists essentially of alkali metal carbonate about 39 parts, anhydrous alkali metal silicate equivalent to about 1.5 parts of silica, alkali metal-cyanideabout .29 parts, graphite about 0.3 partand alkali met-a1 chloride to make parts, the parts being by weight.

EDNA L. .SAWYER. NORTI-IRIJP W. ,MARR.

REFERENCES CITED The following references are of record :in file of this patent:

Number "$5 385

Claims (1)

1. A MIXTURE ADAPTED UPON FUSION FOR THE CARBURIZING OF FERROUS METAL ARTICLES WHICH CONSISTS ESSENTIALLY OF ALKALI METAL CHLORIDE ABOUT 20-50 PARTS, CYANOGEN COMPOUND ABOUT 15-35 PARTS, ANHYDROUS ALKALI METAL CARBONATE ABOUT 25-45 PARTS, ALKALI METAL SILICATE EQUIVALENT TO ABOUT 0.5-5.5 PARTS OF SILICA, AND GRAPHITE ABOUT 0.1-1.0 PART, THE PARTS BEING BY WEIGHT.