US3526551A

US3526551A - Process for heat treating metals

Info

Publication number: US3526551A
Application number: US698395A
Authority: US
Inventors: Herbert Sargent
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-01-17
Filing date: 1968-01-17
Publication date: 1970-09-01
Anticipated expiration: 1987-09-01

Description

United States Patent 3,526,551 PROCESS FOR HEAT TREATING METALS Herbert Sargent, 3416 Via Palomino, Palos Verde Estates, Calif. 90274 No Drawing. Filed Jan. 17, 1968, Ser. No. 698,395 Int. Cl. C21d 1/68; C22f 1/04, 1/08 U.S. Cl. 148-131 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF INVENTION In the heat treatment of sheets or bars of some metals and alloys, it has long been noted that there is a bonding or sticking of the surfaces of the metals to each other after cooling. Normally this does not create a problem with sheets or bars since the surface characteristics of these articles are not usually critical. When the article being heat treated, however, must have a highly polished surface or surfaces or when the article is a thin section which is easily deformed, this bonding of the surfaces creates a serious problem.

One area in which this problem is particularly acute is in the heat treatment of thin aluminum alloy memory discs for use in commuters. The sensitivities of the computer operation require that the disc surfaces have an extremely high polish. Any irregularity created on the disc surfaces due to sticking during heat treatment renders the discs unusable so that they must be discarded as scrap. These discs are normally thermally stress relieved between the initial machining where the disces are machined to about 14 to 36 inches in diameter and to 4 inches in thickness and the finishing or polishing. With the conventional heat treating processes, these discs cannot be stacked without a high percentage of scrap being formed. Often if the discs are stacked, all of them must be discarded because the discs stick to each other and become deformed during separation.

Thus, a process is needed for heat treating metal articles, such as the computer memory discs, at elevated temperatures so that they can be stacked or placed contiguously to other articles in an oven without danger of sticking or bonding to other articles in the stack.

SUMMARY OF INVENTION This invention is directed to a process for heat treating metallic articles such as metallic sheets, bars and memory discs for use in computers. The process basically oomprises precoating each article with a layer of an organic compound having a hydroxyl functional group. After the articles are coated, they are stacked in contiguous relation with each other and heat treated at temperatures above 800 F. to relive machining stresses set-up in the articles during their formation. After the heat treating of the articles with the coating of organic compound on the surfaces thereof, they can easily be separated by hand without the need for prying the articles apart or scraping or gouging their surfaces.

The coating material which is applied to the surfaces of the articles may be any organic compound having a ice hydroxyl functional group such as the alcohols including the monohydric alcohols; the dihydric alcohols or the diols or glycols; and the polyhydric alcohols or polyols or polyglycols. The glycols and polyglycols are preferred because they are less volatile than the monohydric alcohols and they have a greater number of hydroxyl functional groups. The compound which has been found to give the most staisfactory results is propylene glycol (1,2-propanediol).

The metals to which this process is applicable are aluminum and aluminum alloys, magnesium and magnesium alloys, carbon steels and any other metals which are bonded or stick to the surfaces of similar metals upon stress relieving or other heat treatment at elevated temperatures. The temperature employed for such heat treatment is usually as high as practicable, but not high enough to impair the structure produced by any previous heat treatment. For example for steel the thermal stress relieving is normally performed at temperatures just below the transformation range. For aluminum alloys thermal stress relieving is generally performed at from about 400 F. to about 850 F.

A feature of the heat treating process of this invention is that metal articles can be stacked one on top of the other or moved into contiguous relationship with the surfaces of similar metal articles in the heat treating oven when heated to elevated temperatures such as stress relieving temperatures without bonding or sticking of the articles to each other so that force must be applied for their separation. For example, when aluminum memory discs are heat treated according to this process in stacks in the oven, they can be easily hand separated from each other with no marks, scratches or other blemishes being formed on the surfaces of the discs. Thus the percentage of scrapped or discarded discs after the heat treating process is dramatically reduced.

DESCRIPTION OF PREFERRED EMBODIMENTS AND DISCUSSION The preferred heat treating process of this invention is to brush coat aluminum alloy discs with propylene glycol to form a thin layer of the propylene glycol on all surfaces of the discs. The discs are then placed in stacks of at least twenty-five at a time in a heat treating oven and heated to a temperature of above 400 F. The discs are maintained within the heat treating oven for a sufficient time to stress relieve the aluminum alloy. This treatment should be at least 15 minutes and preferably 30 minutes. The discs are then slowly cooled by turning the heat treating oven off and allowing them to remain in the oven until it has cooled. This may require leaving the discs in the oven 24 hours. It has been found that when the discs are so precoated there is no sticking or bonding between the discs and they can be easily separated after the heat treatment.

It has been determined that any organic compound having a hydroxyl functional group will produce a reduction in the sticking or bonding of the adjacent metal surfaces during the thermal stress relieving treatment. The preferred compounds, however, because they are readily available in liquid form and easy to work with are alcohols including the monohydric alcohols, the glycols and the polyglycols such as methyl alcohol, ethyl alcohol, butyl alcohol, isobutyl alcohol, secondary butyl alcohol, t-butyl alcohol, lauryl alcohol, vinyl alcohol, allyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, glycerin and mixtures thereof. The best results have been obtained with propylene glycol as the protective layer although it has been found that all of the monohydric and polyhydric alcohols tested have produced the desired effect.

Some of these compounds such as propylene and dipropylene glycol, have the added advantage that they can be used as a cutting oil during the initial machining of the discs or other articles so that it is not necessary to thoroughly wash the articles prior to heat treatment thereof. Some of the more highly volatile alcohols such as methyl and ethyl alcohol are not as satisfactory since they evaporate too quickly and thus must be applied immediately prior to heat treating in order to produce the beneficial results.

The monohydric alcohols, in general, have been found to function satisfactorily in preventing sticking of the adjacent contiguous articles in a stack during the heat treating thereof but the polyhydric alcohols and particularly the glycols due to their higher viscosity and lower volatility have been found to be even more satisfactory. The hydroxyl functional group in some manner acts to prevent bonding of the contiguous metal surfaces during the thermal treatment.

The mechanism by which organic compounds with hydroxyl functional groups prevent sticking of contiguous metal surfaces during the heat treating process has not yet been determined. It has been conjectured however that the hydroxyl radical may combine with the metal to form a compound such as a metal-organic salt which prevents migration of metallic atoms from one surface to another and therefore prevents bonding of the metallic surfaces to each other.

Most of the development work for this process has been done on the copper-aluminum, magnesium-aluminum, and zinc-aluminum precipitation hardenable alloys designated 7075, 6061, 5083, 5086 and 2024 since these alloys have applicability for use as computer memory discs. The process in its broader aspect is applicable to any metals which are bonded or stuck together during heat treatment at stress relieving temperatures such as magnesium alloys, zinc alloys, carbon steels etc.

This invention will be more clearly understood from a consideration of the foregoing discussion when taken in conjunction with the following specific examples.

EXAMPLE 1 A solution of 80% by volume lauryl alcohol and by volume myristyl alcohol was prepared. This solution was added to diisobutyl alcohol to form a treating solution of by volume diisobutyl alcohol and 75% by volume of the lauryl and myristyl alcohol mixture. A set of twenty-five 7075 aluminum alloy discs having a nominal composition of .5% Si, .7% Fe, 1.22.0% Cu, .3% Mn, 2.1-2.9% Mg, .18.40% Cr, 5.16.1% Zn, .2% Ti and less than .15 others were machined to a thickness of inch and a 14 inch diameter and then brush coated with the alcohol solution and stacked one on another in an annealing oven. The temperature of the annealing oven was brought to 500 C. and the discs were held at that temperature in the oven for minutes in an air atmosphere. These discs were then slowly cooled to room temperature, removed from the oven, and hand separated. No appreciable sticking or bonding of the discs was noted.

EXAMPLE 2 A set of twenty-five 14 inch diameter and inch thick 7075 aluminum alloy discs which had been machined to a fairly high surface polish were brush coated with a solution of butyl Cellosolve (ethylene glycol monobutyl ether). These discs were then stacked in a single stack in an annealing oven and annealed at a temperature of 700 F. The discs were then slowly cooled to room temperature, removed from the oven, and hand separated. No appreciable sticking or bonding of the discs to one another was noted.

EXAMPLE 3 A set of twenty-five 2024 aluminum alloy discs having a nominal composition of 5% S .5% Fe, 3.84.9% Cu,

.30.90% Mn, 1.21.8% Mg, .1% Cr, 15% Zn and a total of less than .15% others were machined to a thickness of about inch, a 14 inch diameter and a fairly high surface polish. The discs were brush coated with a layer of triethylene glycol. The discs were then stacked in a single stack in an annealing oven and heat treated at a temperature of 850 C. for 30 minutes. The discs were then slowly cooled by allowing them to remain in the oven for 24 hours after the heat had been turned off, removed from the oven, and hand separated. No appreciable sticking or bonding of the discs was noted.

EXAMPLE 4 A set of twenty-five 2024 aluminum alloy discs were machined to a thickness of about inch and a 14 inch diameter with a fairly high surface finish. These discs were brush painted by hand with a layer of propylene EXAMPLE 5 A set of twenty-five 2024 aluminum alloy discs were machined to a thickness of about A inch and a 14 inch diameter with a fairly high surface polish. These discs were then painted with a brush with a polyethylene glycol 200 solution to form a thin coating on the surfaces of the discs. The discs were then inserted into a heat treating furnace in a single stack and heated to a temperature of 800 F. for at least 30 minutes. The discs were then cooled in the oven, removed from the oven and hand separated. No appreciable sticking or bonding of the discs was noted.

EXAMPLE 6 A set of twenty-five 5086 aluminum alloy discs having a nominal composition of .4% si, .5 Fe, .1% Cu, .2-.7% Mn, 35-45% Mg, .05-.25% Cr, 25% Zn, .15% Ti and less than .15 total others were machined to a 14 inch diameter and a inch thickness with a fairly high surface polish. A thin coating of glycerin was applied by means of a brush to all surfaces of the discs and they were placed in a heat treating oven in a single stack and heated to a temperature of 750 F. for 30 minutes to stress relieve the discs. After this stress relief treatment, the discs were then cooled in the oven for about 24 hours, removed from the oven and hand separated. No appreciable sticking or bonding of the discs was noted.

The discs coated with propylene glycol exhibited a much cleaner surface after the heat treatment than the discs coated with other treating materials. All of the discs, however, exhibited much cleaner surfaces after the treatment with an organic compound having the hydroxyl functional group than discs which were heat treated without a precoating with a compound having hydroxyl functional groups. For example, when kerosene was used to precoat the discs, all of the discs had to be discarded as a result of sticking during the heat treating process. When no coating at all was used on the discs, it was customary to scrap as high as of those discs heat treated if the surfaces of the discs were in contact with each other.

While the invention has been described with partiular reference to the thermal stress relief of aluminum alloys it is more broadly applicable to the treatment of any metals which become bonded to similar metal surfaces during the heat treatment thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A process for heat treating metal articles such as sheets, rods and discs comprising the steps of:

coating said articles with an organic compound having a hydroxyl functional group;

placing said articles in a heat treating oven so that surfaces of said articles are in contact with each other;

heating said articles to a temperature sufiiciently high to stress relieve said articles;

cooling said articles to room temperature in a stress relieved condition; and

separating said articles from each other.

2. A process as defined in claim 1 wherein said compound having a hydroxyl functional group is an alcohol.

3. A process as defined in claim 1 wherein said compound having a hydroxyl functional group is a polyhydric alcohol.

4.. A process as defined in claim 1 wherein said compound having a hydroxyl functional group is a glycol.

5. A process as defined in claim 1 wherein said compound having a hydroxyl functional group is propylene glycol.

6. A process as defined in claim 1 wherein said coating step comprises applying said compound to the surfaces of said articles with a brush.

7. A process as defined in claim 1 wherein said articles are heated to a temperature of from 400 F. to just below the melting point of the metal being treated and maintained at said temperature for at least 15 minutes to relieve internal stresses developed during forming of said articles.

8. A process as defined in claim 1 wherein said articles comprise aluminum alloy memory discs for use in computers.

9. A process as defined in claim 1 wherein said compound is an alcohol having a sufficiently high viscosity to be brush coated on the surfaces of said articles and a sufficiently low volatility so that said alcohol remains on the surfaces of said articles while said articles are being stacked in said heat treating furnace.

10. A process as defined in claim 1 wherein said articles comprise aluminum alloy discs; said discs are heated to a temperature of from 400 F. to 850 F. and said discs are slowly cooled to room temperature by turning off said oven and allowing said discs to remain in said oven for about 24 hours while said oven cools.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R.