US2655457A - Method of heat-treating tungstennickel alloys - Google Patents

Description

C. VERDI Oct. 13, 1953 METHOD OF HEAT-TREATING TUNGSTEN-NICKEL. ALLOYS Filed July 26 1951 2 Sheets-Sheet l INTERIM TEMPBQATURE 4002: 50cc a Ian: M:

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u O m 1 m 7 0 u A mmmm: M22 mwmumw lf uu w x Q A E 6 N w Patented Oct. 13, 1953 METHOD OF HEAT-TREATING TUNGSTEN- NICKEL ALLOYS Columbus Verdi, Waukegan, Ill., asslgnor to Fansteel Metallurgical Corporation, North Chicago, III., a corporation of New York Application July 26, 1951, Serial No. 238,598

Claims.

1 This invention relates to high density tungsten alloys and more specifically to a method of improving the physical and mechanical properties oi certain high density alloys of tungsten and nickel by a heat treatment of the alloys.

. nickel containing from about 5% to about 45% nickel, when subjected to the heat treatment described hereinafter, exhibit an unexpected substantial increase in the mechanical properties of the alloys.

The alloys or compositions are produced by conventional powder metallurgy methods. A thorough and intimate mixture of the tungsten and nickel powder is prepared by any desired method. For example, the metal powders may be intimately mixed by a prolonged ball milling. or, in those instances where the nickel content is low. the nickel may be added to the tungsten powder in the form of a reducible nickel compound such as nickel chloride which is subsequently reduced in hydrogen to form nickel, or the mixture may be prepared by forming an intimate mixture of tungstic acid and a reducible nickel salt such as nickel chloride and reducing the tungstic acid and nickel salt simultaneously in an atmosphere 01' hydrogen. The metal powder is then pressed into the desired shape and sintered in a hydrogen atmosphere or in a dissociated ammonia gas atmosphere, in accordance with conventional practices. Sintering temperatures for these tungsten-nickel alloys lie within the range of from about 1250 C. to about 1450" C. The period of sintering will vary inversely with the specific sintering temperature employed.

In general, it has been discovered that sintering temperatures in the range from about 1300 C. to about 1400 C. produce alloys having maximum hardness and mechanical characteristics in the as sintered condition. Higher sintering temperatures promote a higher degree of grain growth and the resulting products exhibit, in most instances, a slightly lower hardness and mechanical strength.

I have discovered that these alloys when heated to a temperature above about 1000 C., preferably to about 1200 C., and when rapidly cooled,

as by quenching in a liquid or in a gas stream. from this temperature exhibit an appreciable increase in the mechanical strength without an appreciable effect on the hardness. In general, the transverse rupture increases about 20% to 25% while the averagedecrease in hardness is from about 1 to 2 points on the Rockwell C scale.

I have also discovered that the mechanical strength of the alloys may be further appreciably increased by subsequently subjecting the alloys to a heat treatment consisting of heating to a temperature between about 500 C. and 700 C. for from about 1 hour to about 2 hours. This treatment results in a further increase of from 20% to 25% in the transverse rupture strength and also increases the hardness to about that of the alloys before heat treatment.

I have further discovered that the hardness may be increased by subjecting the alloys to a heat treatment which consists of heating the alloys to a temperature between about 600 C. and 900 C. for from about 1 hour to 2 hours. Where the temperature does not exceed about 800 C., the increase in hardness is obtained without decreasing the strength below that of the sintered alloy. Where the strength is not controlling and a harder material is desired, the heat treatment maybe extended to about 900 C.

In order to illustrate the improvements in the physical and mechanical characteristics of the tungsten-nickel alloys when subjected to the heat treatment of this invention a series of such alloys were prepared. The nickel content of this series of alloys varied from 5.18% nickel to 43.60% nickel. Bars were prepared from the various powder mixtures by pressing the powder at about tons, per square inch. Groups of pressed bars of the various powder mixtures were subsequently sintered at temperatures between 1300" C. and 1325 0., between 1400 C. and 1425 C.. and between 1440 C. and l450 C. Sintering was carried out in hydrogen and in dissociated ammonia gas. The bars were approximately 5" in length and varied from about 0.2" to about 0.3" in width and thickness.

The hardness of the bars in the as sintered condition was determined on a standard hardness testing apparatus employing the Rockwell 0 scale. The series of bars of the various compositions were mounted and subjected to predetermined loading so as to determine the transverse rupture strength. Another series of the bars were heated in hydrogen to a temperature of about 1200 C. and maintained at this tem-' perature for about 15 minutes to insure uniform heating throughout the bars. This series of bars such temperature for about one hour and finally V removing the bars from the furnace and allowing them to cool in the atmosphere. Groups of bars were subjected to such heat treatment at temperatures of 100 C. to 900 C. in intervals of 100 C. Each group of bars after being sub- Jected to its heat treatment was tested for hardness and transverse rupture.

The data so obtained was employed in plotting the graphs of-Figs. 1 and 2.

In Fig. 1 the transverse rupture strength is plotted against the heat treatment. It will be noted that the quenching treatment increases the transverse rupture strength an average of from 25% to 30% for each series of sintering temperatures.

In Fig. 2 the hardness is plotted against the heat treatment. It will be noted that quenching for all practical purposes, effects no alteration of the hardness.

Treatment of the alloys by heating them to temperatures of from about 100 C. to about 500 C. generally decreases the transverse rupture strength to a value which lies between the strength of the alloys in their as sintered condition and in their quenched condition. Similarly. in most instances, the hardness of the alloys subjected to the subsequent heat treatment within this temperature range varies slightly and lies between the hardness of the alloys in their as crease in the hardness of the alloys. Heat treatment in the range or from 800 C. to 900 C. etiects a further increase in the hardness accompanied by a decrease in the mechanical strength.

The explanation for the unusual increase in strength without an accompanying increase in hardness eflfected by the quenching and heat treatment is not apparent. The phase diagram of these tungsten-nickel alloys does not indicate that the alloys rich in tungsten are precipitation hardenable.

Iclaim:

1. The method or increasing substantially the mechanical strength without increasing appreciably the hardness of sintered tungsten-nickel alloys containing from about 5% to about 45% nickel and the balance tungsten, which includes heating the sintered alloy to a temperature between about 1000 C. and 1200 C., quenching the heated alloy and subjecting the quenched alloy to heat treatment at a temperature of from about 500 C. to about 700 C.

2. The method of increasing substantially the mechanical strengthwithout increasing appreciably the hardness of sintered tungsten-nickel alloys containing from about 5% to about nickel and the balance tungsten, which includes heating the sintered alloy to a temperature between about 1000 C. and 1200 C., quenching the heated alloy and subjecting the quenched alloy to heat treatment at a temperature of the order of 600 C.

3. An an article of manufacture, a sintered tungsten-nickel alloy consisting essentially of from about 5% to about 45% nickel and the balance tungsten, formed by heating the sintered alloy to a temperature between about 1000 C.

I and 1200 C., quenching the alloy from such temsintered condition and in their quenched condition.

Higher heat treatment effects a further increase in the strength as compared to the strength in the quenched condition and the increase varies from about 20% to about 40% depending upon the sintering temperature.

From the graphs it is apparent that the heat treatment at a temperature of about 600 C. produces the optimum increase in the strength. Heat treatment at this temperature efiects an increase in strength as compared to the strength in the sintered condition of from about to about 80% depending upon the sintering temperature.

In general, the hardness of the alloys begins to increase with a heat treating temperature of about 500 C. and the hardness of the alloy when subjected to heat treatment at 600 C. is the same as the hardness of the alloys in their as sintered condition. It is to be understood that this variation in hardness is within a range of about 2 points on the Rockwell C scale.- As the temperature is increased above 600 C., th transverse rupture strength rapidly decreases and the hardness rapidly increases. ,The optimum heat treating temperature range for the production of maximum transverse rupture strength without an appreciable alteration of the hardness lies in the temperature range of from about 500 C. to about 700 C. In most instances heat treatment at 800 C. results in a strength about that of the alloys in their as sintered condition; however, such a temperature produces a substantial inperature and heat treating the quenched alloy at a temperature of from about 500 C. to about 700 C. and being characterized by having a substantially greater mechanical strength than, but

by having approximately the same hardness as, a corresponding sintered alloy in its as sintered condition.

4. As an article of manufacture, a sintered tungsten-nickel alloy consisting essentially of from about 5% to about 45% nickel and the balance tungsten, formed by heating the sintered alloy to a temperature between about1000 C. and 1200 C., quenching the alloy from such temperature and heat treating the quenched alloy at a temperature of the order of 600 C. and being characterized by having a substantially greater mechanical strength than, but by having approximately the same hardness as, a corresponding sintered alloy in its as sintered condition. 5. The method of increasing substantially the mechanical strength without increasing appreciably the hardness of a sintered tungsten-nickel alloy containing from about 5% tov about 45% nickel and the balance tungsten, which has been sintered at a temperature between about 1300 C. to 1325 C., said method comprising heating the sintered alloy-to a temperature between about 1000 C. and 1200 C., quenching the heated alloy and subjecting the quenched alloy'to heat treatment at a temperature of the order of 600 C.

COLUMBUS VERDI.

' References Cited in the file of this patent I Transactions of The American Societyior Metals, volume 28, pages 619-643, especially p es 636-642 (1940). I

Claims (1)

1. THE METHOD OF INCREASING SUBSTANTIALLY THE MECHANICAL STRENGTH WITHOUT INCREASING APPRECIABLY THE HARDNESS OF SINTERED TUNGSTEN-NICKEL ALLOYS CONTAINING FROM ABOUT 5% TO ABOUT 45% NICKEL AND THE BALANCE TUNGSTEN, WHICH INCLUDES HEATING THE SINTERED ALLOY TO A TEMPERATURE BETWEEN ABOUT 1000* C. AND 1200* C., QUENCHING THE HEATED ALLOY AND SUBJECTING THE QUENCHING ALLOY TO HEAT TREATMENT AT A TEMPERATURE OF FROM ABOUT 500* C. TO ABOUT 700* C.

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