US2390238A - Quenching of aluminum-magnesium alloys - Google Patents

Description

Dec. 4, 1945. w. A. DEAN QUENCHING 0F ALUMINUM-MAGNESIUM ALLOYS Filed Feb. 1, 1943 TIME SECONDS INVENTOR WALTERA.DEAN

ATTORNEY Patented Dec. 4, 1945 2,390,238 I QUENCHING or ALUMINUM-MAGNESIUM ALLOYS Walter A. Dean, Lakewood, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application February 1, 1943, Serial No. 474,323

11 Claims. (Cl. 148-211) This invention relates to a' method of heat treating and quenching articles of aluminum base alloys, and more particularly to a method of cooling articles of aluminum base alloys containing appreciable amounts of magnesium from the temperature at which they receive solution heat treatment.

It has been a common practice heretofore in heat treating articles, especially castings, of aluminum base alloys containing between 6 and 15 per cent magnesium, in order to obtain high tensile and fatigue properties, to heat them from 1 to 40 hours at a temperature between 775 and 850 F., rapidly cool them in boiling water to a temperature of approximately 212 F. and then remove the articles from the water and permit them to cool in air to room temperature However, when high resistance to stress corrosion is required of these articles of aluminum-magnesium alloys, they have heretofore been quenched from the solution heat treating temperature inhot oil to a temperature usually somewhat higher than the temperature of the boiling water quench. When the articleshave cooled to the temperature of the hot oil, they have been removed from the oil and quenched in water or permitted to cool in air to room temperature. Stress corrosion tests carried out in the familiar salt-hydrogen peroxide solutions directly after the quenching treatment reveal little difference between a hot oil and a boiling water'quench. After aging at room temperature for an appreciable time, however, the hot oil quenched mate-.- rial is found to have the higher resistance to corrosion. It has been found that this difference between the two quenches in the accelerated stress corrosion test can be rapidly developedby subjecting the quenched material to an aging treatment, for example, of hours at 212 F.

The alloy articles quenched inthe hot oil possess high tensile properties as well as high resistance to stress corrosion, but thereare a number of objections associated with the hot oil .quench which make it undesirable, particularly from the standpoint of production. A few of the more notable objections to the hot oil quench arise from the extra equipment and space it requires, the fouling of the water tanks, the fire hazard, the extra time required to make .the quench, and the necessity of sand blasting the articles after the quench to remove the charred oil. These objectionable factors increase the fabricating cost of oil quenched aluminum-magnesium alloy articles.

The phenomenon known as stress corrosion occurs in metal alloy articles when stresses are created therein by strains set up between the various sections thereof, whether the strains are the result of externally applied forces or of a system of forces created by previous fabricating or quenching operations. While the rapidity with which an alloy article deteriorates by stress corrosion depends to a great extent upon the severity of the stresses within the article and upon the corrosive environment to which the article is subjected, the rate of corrosion is also influenced by the internal structure of the alloy. It appears that the internal structure having maximum resistance to stress corrosion is char,-

acterized by a type of uniformly distributed. pre} cipitate throughout the alloy of dissolved conistituents from the supersaturated aluminum solid solution, whereas the alloy structure in which stress corrosion proceeds relatively rapidly is characterized by a selective precipitation of.

the alloy constituents at or near the grain boundaries. It has been found that castings of aluminum-magnesium alloys which have been solution heat treated and quenched have often been susceptible to stress corrosion.

It is an object of my invention to provide a method of quenching solution heat treated aluminum-magnesium alloy articles to obtain therein high tensile properties and high resistance to stress corrosion without the disadvantages attendant upon a hot oil quench. Another object is to provide a method of quenching aluminummagnesium alloy articles, which will leave the surface of the articles in a condition satisfactory for use or further fabricating operations without the necessity of first cleaning or otherwise reconditioning them following quenching. Still another object of my invention is to provide'a method of quenching aluminum-magnesium alloy'artieles, which is readily adaptable to the facilities now generally employed in the fabrication of such articles; A particular object is to provide a method of quenching aluminum-magnesium alloy castings to obtain therein high tensile properties and high resistance to stress corrosion.

The term aluminum-magnesium alloy" as employed throughout this specification and appended claims refers to an aluminum base alloy containing at least per cent of aluminum and,

in addition thereto, magnesium as the predominant added alloying element; however, other elements, such as copper, manganese, chromium,- zinc,iron and, silicon, may be present either as intentionally added ingredients or as impurities.

My invention is predicated on the discovery that high resistance to stress corrosion in articles of aluminum-magnesium alloys is obtained while also obtaining high tensile properties by subjecting them to a solution heat treatment. preferably at a temperature between 775 and 850 F. for 15 minutes to 40 hours, to cause solution of a substantial portion of the soluble constituents in the alloys, quenching the articles in boiling water to an intermediate temperature not lower than the knee of their cooling curve, and then removing them from the water and permitting them to cool in air to room temperature. If castings are being treated, it is generally necessary to heat them at the solution temperature for at least 1 hour. By high resistance to stress corrosiop is meant that the alloys have a relatively long life, i. e. freedom from failure due to stress asooma time which has-been found to applyto most sizesv of aluminum-magnesium alloy articles in commercial use is approximately seconds; however, a period as short as 5 seconds for thin or.

.verysmallarticlesandasmuchasaminuteand as compared with the same alloys which have The articles may be arti- The expression knee of the cooling curve," as

herein employed, refers to that portion of the time-temperature cooling curve for the metal body in a boiling aqueous solution where there is a marked change in the downward direction of the curve, which appears to be associated with a collapse of the steam envelope around the body. It will be understood that the cooling curve of a metal body in a quenching medium refers to the pro essive decrease in temperature the body undergoes with an increase in time from the instant the body is introduced into the medium until approximately the lowest temperature is reached which can be attained in the quenching medium employed. The mass of the metal body will, of course, affect the rapidity of the drop in temperature and consequently the slope of the cooling curve, especially during the early part of the cooling. The temperature at which the kneev .below the knee of the cooling curve, the resistance to stress corrosion is considerably decreased. When the alloy articles are being quenched in boiling water from the solution heat treating temperature, rather vigorous boiling immediately results. The intensity of the boiling continues at a relatively even rate until a temperature of a half or longer for very large articles may be required. In terms of a temperature range my preferred practice is to quench the articles to a temperature between the knee of the cooling curve and the temperature representing the maximum solubility of the magnesium content of the alloy in aluminum on the basis of the binary aluminum-magnesium system. The knee in the cooling curve shown in the drawing occurred between v 25 and 30 seconds after the articles, still hot from a solution heat treatment, were introduced into the boiling water. The length of time required to hold the articles in boiling water may be determined in some cases by leaving them in the boiling water until maximum ebullition occurs, then immediately withdrawing them from the water and cooling to room temperature, preferably in still air.

Although the invention contemplates the use of boiling water for the quenching medium, water at a temperature of a few degrees below the boiling point has been found to produce satisfactory Furthermore, good results may be obtained by quenching the articles in water boiling, as a reabout 500! is reached where boiling becomes very violent. This violent boiling soon subsides and the rate at which the articles cool in the water increases and the increased rate of cooling continues until the temperature of the articles approaches the temperature of the boiling water. on a cooling curve for aluminum-mag nesium alloy articles. the maximum ebullition occurs at the knee 'of the curve. 4 l

The temperature to which the alloy are quenched in the boilingwater from the ao lu.

' tion heat treating temperature is above the temperature of the boiling water: eon.-

suit of low atmospheric pressure, at a temperature'of a few degrees less than 212 1''. While oftbecoolingcurveisnotlowerthanioo'l'. The term boiling water as used in this specification is intended to include, in addition to pure boiling aqueous solutions; and

' said-articles and temperature at which the maxias those quenched in the hot oil as mentioned herein. Furthermore, the alloy articles have a more uniform distribution of the precipitated constituents throughout the aluminum base.

As an example, aluminum base alloy test bars nominally containing per cent magnesium were sand cast', given a solution heat treatment at 810 F. for 16 hours, and a third of them, group 1, was quenched in boiling water and held therein until they reached the temperature of the water. Another third, group.2, was quenched in oil at a temperature of about 300 F. in the conventional manner until the bars reached the temperature of the oil, and the remaining third, group 3, was quenched in boiling water to nearly the knee of the'cooling curve and then removed and permitted to cool in still air to room temperature. Specimens for stress corrosion tests were given a thermal treatment at 212F. for 30 hours to simulate the internal structural condition which exists after the alloy has remained at room temperature for a long period of time. then subjected to an accelerated stress corrosion test lasting for a maximum of 14 days which consisted of subjecting the test bars to a stress of approximately 75 per cent of their yield strength and immersing them in a standard 5 per cent sodium chloride and 0.3 per cent hydrogen peroxide aqueous corroding solution. The average results of the corrosion test on bars from each group shown by the number of days before failure occurred, together with the tensile properties of bars from each group, are given in the following table:

1 Did not break during the test, i. e., 14 days.

From this example it is apparent that greater resistance of aluminum-magnesium alloys .to stress corrosion is attained by my method than is attained by the previous commercial methods of quenching. The tensile properties are considered to be high for this alloy and the difierences in these properties between the three groups are not to be regarded as significant with respect to the type of quench employed. It is thus apparent that my quenching procedure has increased the resistance to stress corrosion without substantial reduction in the tensile properties.

I claim:

1. A method of heat treating and quenching articles of aluminum-magnesium alloys to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at a temperature and for a time suilicient to cause substant al solution of soluble constituents, quenching said articles in boiling water to a temperature above the knee of the cooling curve of The three groups were substantial solution of soluble constituents,

mum amount of the magnesium in the alloy is soluble in aluminum and thereafter immediately removing them from said boiling water and cooling to room temperature.

3. A method of heat treating and quenching articles of aluminum-magnesium alloys to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles tensile properties, comprising heating said articles at a temperature between 775 and 850 F. for 15 minutes to 40 hours, quenching said articles in boiling water to a temperature above about 400 F. and thereafter immediately removing the articles from the boiling water.

5. A method of heat treating and quenching articles of aluminum-magnesium alloys to ob tain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at a temperature between 775 and 850 F. for 15 minutes to 40 hours, quenching said articles in boiling water to a, temperature above 400 F., immediately removing said articles from the boiling water and thereafter permitting said articles to cool in still air to room temperature.

6. A method of heat treating and quenching articles of aluminum-magnesium alloys containing 6 to 15 per cent magnesium to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at a temperature between 775 and 850 F. for 15 minutes to 40 hours, quenching said articles in boiling water to a temperature above the knee of the cooling curve of the articles being quenched and thereafter immediately removing the articles from the boiling water.

7. A method of heat treating and quenching articles of aluminum-magnesium alloys containing 6 to 15 per cent magnesium to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at a temperature between 775 and 850 F. for 15 minutes to 40 hours, quenching said articles in. boiling water to a temperature above 400 F., immediately removing said articles from the boiling water and thereafter permitting said articles to cool in still air to room temperature.

8. A method of heat treating and quenching articles of aluminum-magnesium alloys containing 6 to 15 per cent magnesium to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at a temperatur between 775 and 850 F. for 15 minutes to 40 hours, quenching said articles in boiling water for a period of time between 5 and seconds, immediately removing said articles from the boiling water and thereafter permitting said articles to cool in still air to room temperature.

9. A method of heat treating and quenching articles of aluminum base alloys containing 9 to 11 per cent magnesium to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said articles at 775 to 850 F. for 15 minutes to 40 hours, quenching said articles in boiling water for a period of ings at a temperature between 775 and 850 F. q

for one to 40 hours, quenching said castings in boiling water to a temperature above the kneeof the cooling curve of th castings being quenched and thereafter immediately removing the cutings from the boiling water.

' 11. A method of heat treating and quenching castings of aluminum-magnesium alloys to obtain both high resistance to stress corrosion and high tensile properties, comprising heating said castings at a temperature between 775 and 850 F. for one to 40 hours, quenching said castings in boiling .water to a temperature above about 400 F. and thereafter immediately removing the castings from the boiling water.

WALTER A. DEAN.

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