EP0425461A1

EP0425461A1 - Continuous solution heat treatment of precipitation hardenable alloys

Info

Publication number: EP0425461A1
Application number: EP90850342A
Authority: EP
Inventors: John H. Schemel
Original assignee: Sandvik Special Metals LLC
Current assignee: Alleima Special Metals LLC
Priority date: 1989-10-27
Filing date: 1990-10-17
Publication date: 1991-05-02
Also published as: JPH03207842A

Abstract

A continuous solution heat treating process for precipitation hardenable alloys. The alloys can be in the form of tubing, strip, or other long products and are heat treated by a continuous process by passing the product along a feed path continuously through a heating zone in a vacuum or inert gas atmosphere followed by rapid cooling through an inert gas quench or water quench at a rate to prevent precipitation of secondary phases in the alloy. The inert gas quench can be performed in combination with passing the product in close proximity to or contact with a cooled surface, such as a water-cooled copper cooling device. The process results in very little surface oxide which can be easily removed. The final product can be resolution treated prior to aging or aged after final cold working of the product.

Description

Background of the Invention

The present invention relates to a method of continuously solution heat treating precipitation hardenable alloy products in an air-free atmosphere followed by cooling in an air-free atmosphere at a rate which prevents precipitation of secondary phases.
U.S.Patent No. 4,802,930 describes a process for the formation of seamless beta phase titanium alloy tubing of commercial tube lengths. This patent teaches that full solution treatment of most beta alloys, which results in optimum properties after aging, requires that the product be cooled from the solution ambient temperature (1350 to 1550° F.) to 500° F. in less than approximately five (5) minutes, depending on the composition. The '930 patent teaches a method by which this can be accomplished for the 8 to 20 foot tube lengths required by hydraulic tubing users.
As explained in the '930 patent, elemental titanium exists in two geometric forms. At temperatures under 1625° F. (885° C.), titanium has a close packed hexagonal structure, which is the alpha phase. At higher temperatures it converts to the beta phase, a body-centered cubic geometry. Alloying elements, or stabilizers, change the temperature at which the beta state beccmes stable. In a beta alloy, exposure to selected elevated temperatures will decompose the beta structure to precipitate a fine dispersion of alpha phase, which increases strength.
The '930 patent also reports that during the tube manufacturing process, before and after hot or cold working, the metal undergoes several types of heat treatments, which require heating at specified temperatures for specific times, followed by cooling. Such heat treatments include stress relief annealing, solution treatment (sometimes called solution annealing), and aging. For instance, cooling in the case of solution treatment, must occur within a specific time to confer the desired properties to the metal. Additionally, contaminants and oxidation products must be removed after heat treatment.
The '930 patent teaches that solution annealing serves to increase fracture toughness and ductility at room temperature and that intermediate solution annealing steps are performed before each successive pilger, or cold deformation, of the product. Solution treatment or solution treatment plus cold working (pilgering) and subsequent aging are used to increase the strength level of the metal. By heating to the solution treatment temperature, 1350 to 1550° F., and fast cooling, beta phase is stabilized to room temperature, and when subsequently aged at lower temperature, 850 to 1250° F., the beta phase decomposes into a stronger structure, due to a fine dispersion of alpha phase which increases the strength of the alloy.
The '930 patent teaches that after solution annealing, either water, air or furnace quenching can be utilized, but each would result in different tensile properties after aging and that the rate of cooling from solution annealing temperatures is critical. According to the '930 patent, if the process is too slow, then partial decomposition of the beta phase occurs during cooling, and the subsequent aging of the beta phase will not result in the desired strengthening effect, optimum ductility for subsequent pilgering is not achieved and aged properties of the final product are unpredictable and result in subnormal combinations of strength and ductility. Furthermore, the '930 patent teaches that full solution treatment of the alloy requires that cooling take place within approximately five minutes, depending on the composition of the alloy.
To avoid the formation of an oxide layer on the surface of the metal and a perceived detrimental effect on the final properties of the metal, it is known in the art to carry out the cooling step in a vacuum furnace. According to the '930 patent, no vacuum furnaces are available which will accommodate tubes over eight feet in length which are required by the aircraft industry. The '930 patent points out, however, that if the formation of an oxide layer is of no consequence, effective quenching can be achieved using available air heat treatment furnaces with air, water, brine or caustic soda solutions being used to achieve the desired cooling rate, which is dependent on the cross sectional thickness and size of the tube.
The '930 patent also teaches that the final steps in the manufacturing process are aging and stress relief. Stress relief treatments decrease undesirable residual stresses from cold forming and straightening to maintain shape stability without loss of yield strength. Aging consists of reheating to intermediate temperatures, causing partial decomposition of the beta phase to increase strength.
According to the '930 patent, all intermediate annealing operations are performed in an air atmosphere. The '930 process begins when the initial material is steam cleaned and pilgered; the product is then degreased from the pilgering process and steam cleaned again; the first of the annealing steps is then performed in an air atmosphere; quenching takes place, utilizing water or room temperature air as needed to cool within five minutes; after annealing, the metal is descaled in a hot salt bath and pickled in a nitric-hydrofluoric acid solution to remove the oxygen contaminated surface layer; the product is then straightened, cleaned and pilgered again; the process continues repeatedly until the desired diameter and thickness of tubing has been achieved, after which the tubing is cleaned and finally aged in a vacuum environment. The '930 process provides stress relief, and the aging required to decompose the beta phase to achieve desired properties.
The '930 patent teaches away from a final treatment consisting of solution treatment and then aging. Instead, the '930 process uses direct aging in a vacuum furnace after pilgering to avoid the surface contamination that would occur if the final solution treatment were performed in air. The vacuum aging also removes hydrogen picked up during previous annealing and pickling operations. The '930 patent also teaches that such a process produces a finer grained product, which is more susceptible to defect detection by ultrasonic testing, and which shows more uniform response to aging from lot to lot, heat to heat and between various tube sizes.
The prior art has encountered problems with respect to non-continuous heat treatment of precipitation hardenable alloys. Such problems include precipitation of secondary phases during cooling from solution heat treatment temperatures. Precipitation of such secondary phases commonly occurs in furnace cooling precipitation hardenable alloys. Another problem encountered in heat treating metals is that of surface contamination of alloys which oxidize at high temperatures.

Summary of the Invention

The present invention provides a process for heat treating a precipitation hardenable metal alloy product comprising passing the product along a feed path while heating the product to a solution heat treatment temperature during a continuous annealing step in an air-free atmosphere followed by cooling the product in the air-free atmosphere at a cooling rate which avoids precipitation of a secondary phase.
The present invention provides a continuous solution heat treating process for precipitation hardenable alloys such as metastable beta titanium alloys, copper alloys, aluminum alloys and zirconium alloys. Such a continuous heat treatment is more convenient than a batch heat treatment process and allows cooling to be performed at faster rates than are obtainable in batch heat treatment furnaces. The alloys can be in the form of tubing, strip, or other long products which are heat treated in a continuous process by passing through a heating zone in a vacuum or inert gas atmosphere followed by rapid cooling through an inert gas quench or water quench. The inert gas quench can be performed in combination with passing the products in close proximity to or contact with a cooled surface, such as a water-cooled copper cooling device. The process results in very little surface oxide which can be easily removed. The final product can be re-solution treated prior to aging or aged after the last cold reduction.

Brief Description of the Drawings

Figure 1 shows an example of apparatus which could be used to carry out the method according to the invention; and
Figure 2 shows a possible sequence of steps according to the method of the invention.

Detailed Description of the Preferred Embodiments

The present invention provides an improved process for heat treating precipitation hardenable alloy products. In particular, such products can be formed by a series of at least one intermediate cold forming step followed by a solution annealing step with rapid cooling of the alloy after the annealing step to achieve optimum physical properties. The present invention provides a heat treatment wherein the alloy is continuously solution annealed such as between a series of cold forming steps or subsequent to a final working step, the continuous annealing step being performed in an air-free environment such as a vacuum or inert gas atmosphere.
The invention is applicable to alloys containing one or more elements in contents which exceed the solid-solubility limit. Such alloys can be heat treated to produce "second-phase" microstructural constituents that may consist of either the pure alloying ingredient or an intermetallic-compound phase.
In the case of aluminum, such pure alloying elements include Si, Sn and Be but in ternary or higher-order alloys, the Si or Sn may form intermetallic-compound phases. Other alloying elements form such intermetallic compounds with Al in binary alloys and more complex phases in ternary or higher-order alloys depending on ratios and total amounts of alloying elements. Metastable conditions may produce phases that are not shown on equilibrium diagrams, for instance, Al₃Fe forms from metastable Al₆Fe on solid-state heating. Examples of precipitation hardening aluminum alloys which can be heat treated in accordance with the present invention can include Mg, Si, Zn, Cr and/or Mn. Some typical precipitation hardenable aluminum based alloys include Al-Cu-Mg, Al-Cu-Si and Al-Cu-Mg-Si of the 2000 series, Al-Mg-Si of the 6000 series, Al-Si-Mg, Al-Si-Cu and Al-Si-Mg-Cu of the 3000 series and Al-Zn-Mg and Al-Zn-Mg-Cu of the 7000 series. The solubility of the solute elements for each of these groups of alloys decreases with decreasing temperature.
Another group of age-hardenable alloys which can be heat treated in accordance with the present invention includes copper alloys such as Cu-Be alloys of the C17000 series, Cu-Zr alloys of the C15000 series, Cu-Cr alloys of the C18000 series, Cu-Ni-P alloys of the C19000 series and Cu-Ni-Si alloys of the C64700 series. Of course, other age hardenable copper alloys can be processed according to the treatment of the present invention.
The process of the present invention is also applicable to reactive metals including Ti alloys such as alpha-beta Ti alloys and beta or metastable beta Ti alloys which can be aged after solution treating followed by cooling at a sufficient rate to prevent precipitation of alpha phase. Such alpha-beta alloys include Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-8Mn, Ti-7Al-4Mo, Ti-6Al-2Sn-4Zr-6Mo, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-6Al-2Sn-2Zr-2Mo-2Zr, Ti-10V-2Fe-3Al and Ti-3Al-2.5V. The beta alloys include Ti-13V-11Cr-3Al, Ti-8Ni-8V-2Fe-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-11.5Mo-6Zr-4.5Sn. Another reactive metal which could be treated according to the present invention is zirconium-based alloys which are age hardenable, for instance, Zr-0.5Cu-0.5Mo.
The present invention is applicable to long length products such as tubing, bars, strip or plate. For thinner cross-sections, the quenching step can be performed by blowing an inert gas on the product. Alternatively, the product can be passed in close proximity to a cooled surface in addition to quenching the alloy with an inert gas or the product can be passed in contact with a cooled surface with or without additional gas quenching. The cooled surface can be a water-cooled copper cooling device. The products can also be water quenched, for instance, in cases where the products have thicker cross-sections. In a preferred embodiment, the product comprises a tube hollow which is cooled after the continuous solution anneal by blowing helium gas on the product simultaneously with passing the product in close proximity to a water-cooled surface. The cooling step is performed in less than 5 minutes.
In a preferred embodiment, the metal product P is continuously heat treated in an enclosure 1 which extends along a feed path A along which the product travels while it is being heat treated, as shown in Fig. 1. The enclosure can have a cross-section taken in a plane perpendicular to the feed path which is larger than a similar cross-section of the product at least with respect to its maximum dimension in the plane through which the cross-section is taken. For instance, in the case where the product is a 3/4 inch diameter tube, the enclosure can have a diameter of around 1 foot.
The enclosure 1 can be provided without a seal at each end provided inert gas supplied to the enclosure flows out each end of the enclosure during the heat treating process.
Alternatively, a relatively tight fit could be provided between the product and the interior of the enclosure, or by the provision of a series of chambers at each end of the enclosure having inert gas supplied thereto to prevent entry of air into the enclosure, whereby a seal at each end of the enclosure can be obviated.
For example, the enclosure 1 can include at least one chamber 6, 11 at each end or a series of chambers 6, 7, 8; 9, 10, 11 at each end, respectively. Inert gas can be fed from a supply S to each of the chambers 6-11 at opposite ends of the enclosure via feed lines 12, 13, 14, 16, 17 and 18, respectively. Also, inert gas can be fed to a central part of the enclosure in which the heating means and cooling means are located by means of at least one feed line 15.
For purposes of heating the product P, at least one heating device 2 can be provided. For example, a series of heating devices 2, 3, 4 can be provided along the feed path A for heating the product P to the solution temperature. For purposes of cooling the product, at least one cooling device 5 can be provided or a series of cooling devices (not shown) could be provided for cooling the product to a desired temperature such as the ambient at a rate which avoids precipitation of a secondary phase of the age hardenable alloy comprising the product P.
Any suitable apparatus can be used for performing the method of the invention provided it accomplishes the objectives of (1) heating the precipitation hardenable alloy product to a solution annealing temperature in an air-free atmosphere and (2) cooling the alloy product in an air-free atmosphere at a rate which avoids precipitation of a secondary phase of the alloy product.
In cases where the product is a tube or a product having another type of curved cross-section, if a water-cooled cooling device is used, the cooling device can be formed in one or more curved sections which confront such curved surfaces of the product. For instance, in the case of tubing, the cooling device can comprise one or more curved sections which confront or contact the outer surface of the tubing. In the case where only gas is used to cool the product, such cooling is achieved by radiation and convection whereas when the product contacts a cooling device the cooling is achieved by conduction.
A purpose of the continuous solution annealing process of the present invention is to avoid aging of age hardenable alloys, such as metastable beta titanium alloys, which normally occurs during the slow cooling inherent in most vacuum furnaces. The present invention avoids this problem by passing the material through a heating zone which can be an induction coil, a radiant heat source or some other method of heating the material. The heating is accomplished in a vacuum or more preferably, in an inert gas atmosphere. Subsequently, the material is rapidly cooled by means of an inert gas quench conduction cooling step, for instance, by passing the material in contact with a water-cooled copper device while blowing an inert gas on the material or by direct water quenching (depending on the metal being treated).
In some cases, water quenching can be used for the cooling step. For instance, during the cooling step, the time available for oxidation would be very short and even if water is used as the quench medium, there would be practically no contamination for some alloys below the surface oxide of the material and this surface oxide could be easily removed. In the case of Ti and Zr alloys, however, a hard tenacious oxide would be formed which is difficult to remove. Accordingly, water quenching would not be satisfactory in all cases.
The final product according to the present invention can be re-solution treated by the method of the present invention prior to aging or the material can be directly aged after the last cold reduction. The selection of using a solution anneal and aging or direct aging after the final cold working step depends on the desired properties of the final product.
Fig. 2 shows a possible sequence of steps according to the method of the invention. In particular, the first step comprises cold working a precipitation hardenable alloy product; the second step comprises passing the product along a feed path in an air-free atmosphere; the third step comprises continuously heating the product to a solution annealing temperature in the air-free atmosphere while moving the product along the feed path; the fourth step comprises cooling the product in an air-free atmosphere at a rate which avoids precipitation of a secondary phase of the alloy product; the fifth step comprises cold working the product; and the sixth step comprises either repeating step three followed by aging or directly aging by heating the product to a temperature at which the secondary phase precipitates in the alloy product.
While the present invention has been described with reference to the foregoing embodiments, many changes and variations may be made thereto which fall within the scope of the appended claims.

Claims

1. A process for heat treating a precipitation hardenable metal alloy product comprising passing the product along a feed path while heating the product to a solution heat treatment temperature during a continuous annealing step in an air-free atmosphere followed by cooling the product in the air-free atmosphere at a cooling rate which avoids precipitation of a secondary phase.

2. The process of claim 1, further comprising a step of aging the product after a final cold working step without a solution annealing step between the final cold working step and the aging step.

3. The process of claim 1, wherein the air-free atmosphere comprises an inert gas atmosphere.

4. The process of claim 1, wherein the air-free atmosphere comprises a vacuum.

5. The process of claim 1, further comprising a step of final continuous solution annealing the product subsequent to a step of final cold forming the product and a step of aging the product subsequent to the final continuous solution annealing step.

6. The process of claim 1, wherein the product comprises tubing.

7. The process of claim 1, wherein the product comprises a precipitation hardenable reactive metal alloy.

8. The process of claim 1, wherein the product comprises a precipitation hardenable titanium alloy.

9. The process of claim 1, wherein the cooling step comprises quenching the product in an inert gas.

10. The process of claim 9, wherein the cooling step further comprises passing the alloy in close proximity to a cooled surface in addition to quenching the product with an inert gas.

11. The process of claim 1, wherein the cooling step is performed by passing the product in contact with a cooled surface.

12. The process of claim 10, wherein the cooled surface comprises a water-cooled copper cooling device.

13. The process of claim 11, wherein the cooled surface comprises a water-cooled copper cooling device.

14. The process of claim 9, wherein the inert gas comprises helium.

15. The process of claim 1, wherein the annealing step is performed by heating the product with an induction coil.

16. The process of claim 1, wherein the annealing step is performed by heating the product with a radiant heat source.

17. The process of claim 1, wherein the cooling step is performed by quenching the product with water.

18. The process of claim 1, wherein the product comprises a precipitation hardenable aluminum alloy.

19. The process of claim 1, wherein the product comprises a precipitation hardenable copper alloy.

20. The process of claim 1, wherein the product comprises a bar.

21. The process of claim 1, wherein the product comprises a strip.

22. The process of claim 1, wherein the product comprises a plate.