US2340725A - Quenching of metals - Google Patents

Description

Patented Feb. 1, 194-4 unuuu nu LHLHUL,

UNITED STATES PATENT OFFICE QUENCHING OF METALS Delaware No Drawing. Original application January 12,

1942, Serial No. 426,536. Divided and this application July 13, 1943, Serial No. 494,531

3 Claims.

This invention relates to the quenching of metals; and it is particularly concerned with improved quenching oil compositions comprising a light mineral quenching oil and dried rubber latex in a controlled amount adequate to produce a quenching oil composition having an initial five second quenching speed of at least about 22.0 per cent without substantially modifying the stress-reducing characteristics of the oil; and it further comprises an improved method of quenching metals wherein the preheated metal is quenched by immersion in a quenching oil bath consisting essentially of a mineral quenching oil and a controlled amount of dried rubber latex adequate to impart to said bath an initial flve second quenching speed of at least about 22.0 per cent without substantially modifying the stress-reducing characteristics of the oil; all as more fully hereinafter set forth and as claimed.

While our invention is useful in the quenching of any metal which is advantageously quenched in an oil bath having a high initial quenching speed, it is particularly useful in the quenching of iron base alloys and it will be described hereinafter in detail in connection with the quenching of steel.

Many metal alloys, particularly iron base alloys, such as carbon steels and alloy steels, require heat treatment for the development of maximum properties of hardness and strength. These properties are dependent upon the establishment of certain physical structures in the metallic components of the alloy. In steel, hardenability is determined by the extent to which a martensitic structure is established in the alloy. The production of this structure in steel is usually accomplished by arresting at the desired point, the changes in the internal structure of the alloy which take place during the cooling of the steel from high temperatures. The fact that these physical changes require time for their completion makes it possible to arrest them at the desired point by suitable quick cooling.

Quenching of the steel in aqueous or oil quenching baths is generally used to arrest these physical changes. It is advantageously carried out in such manner that the physical changes in the steel are arrested at or near the point at which maximum hardness is obtained, and is then followed by a tempering treatment involving heating at relatively low temperatures to impart the desired ductility or toughness to the metal at some sacrifice in hardness.

Quenching in aqueous quenching media develops the properties of strength and hardness to the maximum obtainable for a given section of metal. However, quenching in aqueous quenching media is undesirable in many cases because these media tend to set up excessive amounts of internal stress in the steel resulting in distortion and warping and, in the extreme case, cracking of the quenched piece. As a result, aqueous quenching media have been supplanted by mineral oil quenching media where such mineral oil quenching media have sufficiently high quenching speeds to produce the desired properties in the quenched metal because such mineral oil quenching media are particularly adapted to minimize internal stresses and distortion in the quenched product. This results from the fact that the quenching speeds of mineral oil quenching media are substantially slower than those of aqueous quenching media, due to considerable extent to the formation of a more or less persistent vapor envelop about the quenched piece at the beginning of the quenching cycle, and the fact that in the later stages of the quenching cycle the quenching speeds of mineral oil quench ing media are sufliciently slow so that the internal stresses developed in the metal in the early stages of the cycle tend to be relieved. However, since the overall cooling efficiency of the mineral oil quenching media heretofore known is not as great as that of aqueous quenching media, it has been difllcult or impossible to effect in mineral oil quenching baths quenching of pieces formed of steels having high critical cooling rates, sufiiciently rapidly to develop the properties of strength and hardness to their maximum values, or the quenching of pieces having high mass to surface ratios, sufliciently rapidly to develop the properties of hardness and strength to the maximum depth.

It is desirable, therefore, to improve the quenching of metals in mineral oil quenching media so that the cooling efiiciency in the early stages of the quenching cycle is increased to more nearly approach the cooling efiiciency of aqueous quenching media without increasing the quench- -ing speed in the later stages of the cycle suf- OUHI'UH HUD"! ficiently to deleteriously affect the normal stressrelieving properties of the mineral oil.

It is an object achieved by the present invention to provide a method of quenching metals, particularly iron base alloys, in mineral oil quenching media having quenching speeds greater than the mineral oil quenching media heretofore known. It is a further object achieved by the present invention to provide mineral quenching oil compositions having cooling efllciencies more nearly approaching the cooling eificiency of water while retaining the stress-preventing characteristics of oil quenches.

The critical cooling rate of a metal is defined as the lowest cooling rate at which maximum hardness is developed. For steel the critical cooling rate is the lowest cooling rate which will produce a full martensitic structure in the steel. Thus it is generally most desirable to effect initial quenching at a speed sufiicient to produce a cooling rate at least equal to the critical cooling rate for as great a depth in the metal as possible and to eifect the quenching with a minimum of distortion.

The cooling eiiiciency of a quenching medium is customarily defined in terms of quenching speed and the initial five second quenching speed is a convenient measure of this property of quenching oil compositions, and is the property with which this invention is most concerned. The initial five second quenching speed of a quenching oil composition as referred to throughout this specification and in the appended claims is determined according to the following formula:

Temperature rise in 5 sec. test Temperature rise in cold quench test Per cent available heat removed m ilrst five seconds furnace equipped with an automatic temperature control. Two quarts of the quenching oil composition to be tested are placed in a calorimeter which has a wire screen positioned in the center of the oil bath to insure constant depth of immersion of the samples. position is heated to an initial temperature of 100 F.

One of the preheated test pieces is immersed in the quenching bath by means of light tongs for a period of five seconds without agitation. The test piece is then removed and the bath agitated and the maximum temperature reached is measured. A second preheated test piece is immersed in the bath at an initial temperature of 100 F. and the bath is agitated until the maximum rise in temperature has been produced. The maximum temperature rise measured in this cold quench represents the available heat of the test piece. By substituting the values thus determined for the five second quench and for the cold quench in the above formula a measure of the heat removed in the first five seconds as compared with the total available heat is obtained and is referred to herein as the initial five second quenching speed.

We have discovered that dried rubber latex has the property of increasing the initial five second quenching speed of mineral oils without lessening their stress-reducing characteristics, and that by incorporating it in suitable amounts in light The quenching commineral oils of the type heretofore used in quenching metals, we can produce quenching media having initial five second quenching speeds of at least 20.0 per cent, while retaining the distortion-minimizing characteristics of mineral oil quenching media. We have found further that by quenching pieces formed of metals having high critical cooling rates or pieces difiicult to quench because of a high ratio of mass to surface, by immersion in quenching oil compositions containing dried rubber latex, we can eilect cooling of the metal at a rate more nearly approaching or equal to the critical cooling rate of the metal without creating excessive stresses in the metal. In this way we have been able to develop the hardness of metals having high critical cooling rates to a degree heretofore unattainable with oil quenching media and have been able to effect hardening to a greater depth in pieces having high ratios of mass to surface, and have achieved both results with a minimum of distortion.

Dried rubber latex incorporated in a mineral quenching oil in very small amounts produces quenching compositions having reasonably good quenching speeds. Amounts between about 0.25 and 2.0 per cent by weight produce quenching media having initial five second quenching speeds between about 22.0 and 28.0 per cent, the highest values being obtained at about 1.0 per cent by weight. The effect of this material in increasing the initial quenching speed of the oil increases directly with the amount added up to a maximum and then decreases.

In compounding the quenching oil compositions of our invention we select for the quenching bath a mineral oil of the type customarily used for quenching steel and other metals. The oils used for this purpose may be either naphthenic or parafiinic oils and are usually acid treated neutral oils having a viscosity of about 70 to 200 S. U. V. at F., relatively high flash and fire points, and substantial heat stability and resistance to sludging. For optimum results we have found that paiaffinic oils which usually have an initial five second quenching speed between about 17.0 and about 19.0 per cent are most advantageous.

The best results are obtained according to our invention by incorporating in the oil an amount of dried rubber latex sufiicient to produce a quenching oil composition having an initial five second quenching speed which can eifect cooling of the metal to be quenched at a rate approaching the rate obtainable with aqueous quenching media. In the case of many steels, initial five second quenching speeds of at least about 22.0 per cent are required for this purpose, and particularly for the quenching of many pieces having high mass to surface ratios these high initial five second quenching speeds are particularly advantageous.

The actual quenching operation using the quenching compositions of our invention is usually carried out by immersion of the preheated metal in the quenching oil composition until sufficient heat has been removed to reduce its temperature the desired amount. The quenching oil may be used repeatedly with only slight deterioration.

In the following example there is illustrated specifically the results obtainable with several concentrations of dried rubber latex used in accordance with the process of our invention.

A series of quenching oil compositions was made by using a neutral parafllnic mineral oil cause litl LlitllUt having a viscosity of 100 S. U. V. at 100 F. and varying amounts of dried rubber latex between 0.5 and 1.5 per cent. The initial five second quenching speeds of the respective compositions were then determined as above described and the following results were obtained:

Quenching speed Quenching oil No. 2 16.98 Quenching oil No. 2+0.5% latex 26.54 Quenching oil No. 2+1.0% latex 27.78 Quenching oil No. 2+1.5 latex 25.16

It will be observed from the results of this test that the optimum concentration of dried rubber latex is about 1.0 per cent and that all concentrations between 0.5 and 1.5 per cent produce quenching oil compositions having initial to such specific embodiments except as hereinafter defined in the appended claims.

What we claim is:

1. An improved quenching oil composition comprising a mineral quenching oil and dried rubber latex .in controlled amount adequate to impart to said quenching oil composition an initial five second quenching speed of at least about 22.0 per cent without substantially modifying the stress-reducing characteristics of the oil.

2. An improved quenching oil composition comprising a light mineral quenching oil and 0.25 to 2.0 per cent by weight of dried rubber latex.

3. A method of quenching metals comprising heating the metal to a temperature above its critical temperature and thereafter cooling the metal by immersion in a quenching oil bath comprising a light mineral oil and a controlled amount of dried rubber latex adequate to impart to the quenching oil bath an initial five second quenching speed of at least about 22.0 per cent.

CLIFFORD G. ZUR HORST. BLAINE B. WESCOTT. LESLIE W. VOLLMER.

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