US2299186A - High speed steel hardening bath - Google Patents

Description

Patented Oct. 20, 1942 HIGH SPEED STEEL HARDENING BATH Haig Solakian and Louis Sarvis, NewHaven, Conn.

No Drawing. Application April 29, 1939,

Serial No. 270,915

1 Claim.

The present invention relates to a fused salt bath especially constituted for the treatment of high speed steels.

This is in part a continuation of our prior application Serial No. 120,933, filed January 16, 1937.

The materials comprising the bath are combined in such manner as to constitute a composition which has a melting point at a relatively low temperature such as 700 to 720 F.

An important feature of the present invention is that in view of the low melting point of the bath and the complete fluidity at temperatures around 900 F., the salts readily drain off the articles treated and the slight residue when they are removed from the bath is readily removed in boiling water due to the combined action of dissolving and mechanical disintegration so that the hardened parts are easily cleaned after the hardening operation.

A further feature of the bath is that the bath may be replenished and renewed by additions of the combined materials in order to replace the material which is removed from the bath mechanically due to the sticking of the materials to the surface of the articles taken from the bath.

The high speed steels for which this bath is especially constituted absorb both carbon and nitrogen from the bath and, therefore, the action of this bath is known in the art as nitriding.

Another advantage of the present composition is that, since it may be used at a relatively low temperature it will produce a balanced type of nitriding and there is little tendency of distortion of work. This is important where work such as tools, dies, etc., must be maintained relatively close to predetermined dimensions.

The preferred constituents of the composition are: barium chloride, or any other alkaline earth metal chloride; sodium cyanide, or any other alkali metal cyanide; sodium carbonate, or any.

other alkali metal carbonate; potassium carbonate, or potassium chloride or any other alkali metal chloride; calcium fluoride, or any other alkaline earth metal fluoride, or sodium fluoride, or any other alkali metal fluorides, or sodium chloride.

The preferred formula comprising the following ingredients in substantially the following tages as are given above.

proportions and wherein the preferred percentages are given by weight:

Per cent Barium chloride 12 Sodium cyanide 33 Sodium carbonate 43 Potassium carbonate 8 Calcium fluoride 4 It is, to be understood that th'ese proportions are only the preferred proportions and that the .bath may be operated by the materials in the proportions having substantially the following limits:

Per cent Barium chloride -1 5 to 25 Sodium cyanide 10 to 45 Sodium carbonate 15 to 45 Potassium carbonate 5 to 20 Calcium fluoride 2 to 10 Sodium fluoride may be substituted for calcium fluoride within the same limits of percen- The barium chloride, sodium cyanide, sodium carbonate and potassium carbonate are thoroughly fused together and then the calcium fluoride or sodium fluoride is added to the top of the bath and the fusing is continued until the materials are thoroughly combined. The mass is then allowed to cool and is broken up into small pieces suitable for packaging and shipping. These broken pieces are adaptable for use by being melted in the nitriding pot, and the bath is maintained at the desired volume by adding the broken pieces thereto from time to time.

.With the materials in substantially the above preferred proportions, the melting'point of the composition is approximately from 700 F. to

720 F. The advantage of this low melting point is that the bath is very liquid at temperatures of 900 F. which makes it particularly suitable for producing secondary hardness on high speed steel tools at temperatures from 900 F. to 1150 F. The work may be heated in this bath for a time period ranging from five minutes to several hours, both temperature and time being dependent upon the size of the piece of Work and its particular use.

This bath as has been previously stated is more especially designed for use with high speed steels Per cent Tungsten 17.5 to 18.5 Chromium 3.75 to 4.25 Vanadium 1.0 to 4.0 Carbon 0.60 to 0.85

Another type of high speed steel known the art as molybdenum type A comprises an alloy of iron with I Per cent Chromium 3.75 to 4.25 Tungsten 2.0 to 3.0 Vanadium 1.0 to 4.0 Molybdenum 6.0 to 8.0 Carbon 0.65 to 0.85

Another type of molybdenum liigh speed steel is still known in the art-as molybdenum type B in which iron is alloyed with Per cent Molybdenum 8.0 to 10.0 Chromium 3.75 to 4.25 Vanadium 1.0 to 4.0 Carbon 3.75 to 4.25

A still further type of high speed steel is known as the cobalt type and comprises iron alloyed with Per cent Tungsten 12.0 to 18.0 Chromium 3.75 to 4.25 Vanadium 1.0 to 2.0 Cobalt 4.0 to12.0 Carbon 0.65 to 0.85

The high speed steels referred to prior to treatment in this salt bath have a hardness which usually is between 63 to 65 on the C-scale, Rockwell. After high speed steel has been treated fifteen to thirty minutes in this salt bath at a temperature of approximately 1050" F., the surface hardness increases to about 75 C-scale, Rockwell as measured by the superficial Rockwell method and converted to the standard Rockwell reading on'the C-scale. This surface hardness is substantial and closely approaches the hardness of a diamond.

A polished surface of high speed steel treated as above specified when viewed under a microscope having one thousand diameter magnification appears as though there had been some segregation of carbides on the surface. Apparently there has also been an absorption of nitrogen and the formation of nitrides as well .as carbides. Tools and dies treated as above specified with this surface hardening bath resist wear and have a greatly increased cutting efficiency. The time and temperature referred to depends upon the use of the tools. The preferred depth of penetration of the high speed steel is extremely shallow in the magnitude of .0001 or .0002 of an inch. However, its effect on the cutting efficiency of the tool is out of proportion to the depth as it increases the efliciency of high speed steel tools from two to ten times.

During the operation of the bath, a black crust forms on the top of the bath which acts as a blanket and aids in reducing the oxidation effect by the atmosphere on the bath. This 'fidllces substantially the rate of sodium cyanide decomposition.

Another important effect of the crust on top of the bath is that it increases the vapor pressure within the bath and this in turn increases the effective operation of the bath and causes a deeper penetration of the case in a lesser time than would be possible if there were no crust on the bath. The main reason for the crust, however, and its most desirable characteristic, is to preserve the sodium cyanide content and its interaction with the other materials. The bath in accordance with the present invention does not form a sludge as do ordinary cyanide baths? The barium chloride in the bath appears to act as a carrier of the cyanogen radical and contributes to the reducing of the melting point of the bath. Preferably, this particular chemical is used in small proportions in combination with the other materials to reduce the amount of the insolubles as its interaction with the sodium carbonate tends to produce barium carbonate and sodium chloride as salts. The barium carbonate is not easily soluble in water but when combined with the other chemicals of the present bath, the residue on the articles treated is easily removed in a water wash.

The sodium carbonate is preferably used in a relatively high percentage of the composition in that its chemical reaction with the other constituents reduces the melting point of the bath and it has been found in actual use that the relation of the sodium carbonate in its proportion to the other materials operates best in accordance with the proportions in the preferred formulae given above.

Potassium carbonate also contributes materially to the fluidity of the bath and thus aids in reducing the melting point as well as in aiding the washing of th materials taken from the bath in that it is readily soluble in hot water. The effect of the potassium carbonate is not clearly understood, but it apparently has an affinity for and an interaction with the calcium or sodium fluoride when the fluoride is introduced into the bath.

The sodium cyanide is the main source of the carbon and nitrogen which is consumed during the treatment of high speed steel articles in the bath. The amounts of sodium cyanide in the formulae are based on the assumption that a consistent, uniform loss from the bath will occur relatively to a given area of high speed steel treated in the bath, and its proportional amount with the other chemicals will produce suificient CO gases to maintain its equilibrium for treatment of high speed steel articles at the desired temperatures.

The calcium or sodium fluoride appears to be effective as a catalyst in the bath and reacts with the other chemicals. thereby acting in generating free carbon whichitends to form the black film or crust on the top surface of the bath as heretofore specified. The stability of the bath and its slow rate of decomposition appear to result directly from the calcium fluoride and in that the factor of the fluoride composition in the bath is held at a low point which prevents accelerated decomposition.

Potassium chloride may be substituted for potassium carbonate; but the rate of decomposition is somewhat increased and the melting point of the bath is raised to a slight degree where this substitution is made.

Calcium chloride may be substituted for calcium or sodium fluoride. While calcium chloride tends to act as a catalyst, it is not as stable a material as'calcium or sodium fluoride and it tends to produce more washing insolubles apparently as a result of its interaction with potassium carbonate and the iron pot in which the bath is melted. The calcium chloride has a tendency to attack the iron 'or steel pot forming an insoluble compound of calcium and iron and thus forms as.

crust which adheres to the side of the pot or forms a sludge which settles to the bottom of the pot. In either case, it-causes local over-heating carbonate. This causes a somewhat greater re- \bspedf steel articles comprising the fusion of the action between the cyanide and the base chemicals than would otherwise occur if the calcium chloride could be added at the surface of the bath after .the other ingredients are fused as is 5 the case with the calcium or sodium fluoride.

The calcium chloride also tends to promote a rust condition which is not desirable, on high steel tools treated in the bath. I

"-13 what wegclaim is:

salt bath for surface hardening high following ingredients in substantially the following percentages by weight: barium chlorid 12%;

sodium ycyanide 33%; sodium carbonate 43%;

15 potassiunrcarbonate' 8%; and calcium fluoride 4%. 1 HAIG SOLAKIAN.

LOUIS SARVIS.