WO1992002654A1

WO1992002654A1 - Fabrication of zircaloy mill products for improved microstructure and properties

Info

Publication number: WO1992002654A1
Application number: PCT/US1991/005143
Authority: WO
Inventors: Chun T. Wang; Buford A. Mcclanahan; Craig M. Eucken
Original assignee: Teledyne Industries, Inc.
Priority date: 1990-08-03
Filing date: 1991-07-25
Publication date: 1992-02-20
Also published as: JPH05500539A; EP0495978A1; EP0495978A4

Abstract

A novel process for making zircaloy mill products with improved fabricability and free of large carbide (silicide or phosphide) particles by a high β phase heat treatment and/or a low β phase quenching process followed by rapid cooling and/or air annealing after extrusion or primary fabrication. This also provides final product with uniform equiaxed α grain structure and improved aqueous steam corrosion resistance.

Description

FABRICATION OF ZIRCALOY MILL PRODUCTS FOR IMPROVED MICROSTRUCTURE AMD PROPERTIES

This is a continuation-in-part application of co-pending application Serial No. 07/726,964, filed July 8, 1991, which is a continuation-in-part application of application Serial No. 07/562,576, filed August 3, 1990, now abandoned.

BACKGROUND OF THE INVENTION

Field of the invention This invention relates to novel processes for improving the fabricability of zirconium alloy tubes, fuel rod spacers or boiling water reactor channels by providing for homogenous smaller size metal grains. More specifically, it relates to extrusions of annealed zircaloy-containing objects produced by adding a carbide-phosphide-silicide β solution heat treatment, and by cooling the extrusion more rapidly to achieve excellent microstructure and nodular corrosion resistance.

Zircaloy undergoes an allotropic transformation upon heating. The room temperature hexagonal close-packed alpha phase exists up to about 815°C. At temperatures between 980"C and the melting point of the alloy, the body- center-cubic β phase exists. The alloying elements, iron, chromium and nickel are in solution in β phase, but precipitate as intermetallic compounds, essentially Zr(Fe, Cr)₂ and Zr₂(Fe, Ni) , in a phase. However, the particles of this kind are generally coarse, and their effect on the microstructure of the product is insignificant. In the conventional processes for zircaloys, a solution treatment in the lower β phase region, 980-l040^eC, and/or quenching at an early stage of fabrication is essentially to cause the alloying elements to be in supersaturation to promote corrosion resistance of the final product. On the other hand, interstitial elements such as carbon, phosphorus and/or silicon, are dissolved only in the upper region of β phase, and these elements precipitate as zirconium compounds in the lower region of the β phase (Ref: Use of Anodization Techniques for Optical Microscopy Determination of Solubility Limits for Carbon in Zircaloy, C.T. Wang and P.E. Danielson. Materials Characterization 24:87-92, 1990). If the alloy is heated in the upper β phase region, and cooled rapidly through the lower β phase region, these precipitates are numerous and small, sub-micron in size. On the contrary, in conventional processes for zirconium alloys a primary forging operation is generally carried out in the lower β region, 980-1040°C, for a time of, say, 1/2 to 8 hours. Under these conditions, the dissolution of small precipitates and the growth of large ones occur due to Ostwald Ripening (Ref: Kahlweit, M. , et al., Ostwald Ripening of Precipitates, in Decomposition of Alloys, Proceedings of the Second Acta/Scripta Metallurgical Conference, ed. P. Haasen, Pergamon, 1983, pp. 61-69). The precipitates serve as the sites where the phase nucleates from the parent β phase during cooling. When the carbide, suicide and/or phosphide particles are small and numerous, the α platelets nucleate in a wide diversity of orientations and the growing platelets rapidly impinge on each other. This forms small colonies of α platelets with diverse orientations known as a "basketweave" structure which is most desirable. If insufficient carbides, suicides and/or phosphides are present the platelets will nucleate on the β grain boundaries. Certain orientations of the grain boundaries are conductive to rapid lengthwise growth of the α platelets, and the platelets lengthen until they impinge on the opposite β grain boundary, at which time they have consumed the entire β grain. Because there are relatively few β grain boundaries, the diversity of orientations possible for a platelet growth is highly restricted compared to when platelets nucleate at numerous carbide silicide and/or phosphide sites. The limited number of orientations formed when α platelets heterogeneously nucleate on β grain boundaries coupled with the rapid lengthwise growth of certain orientations of the platelets forms an undesirable microstructure designated as a "parallel platelet" structure. Parallel platelet structures are undesirable because of their limited diversity of orientations and large size. Experiments have shown that the elastic compliance coefficients (a measure of stiffness of the material) can vary as much as 300% depending on the orientation of the stress with respect to the σrystallographic structure. Thus, certain orientations resist deformation during subsequent fabrication operations.

If a parallel platelet colony is oriented in a direction such that when an external stress is applied it is in a direction of high elastic compliance, it will "shed" that stress to surrounding grains which are more favorably oriented for deformation. The result is that the uniquely oriented parallel platelet colonies survive the deformation process intact.

A dominant principle in refining metal microstructures is to uniformly deform the individual grains of the metal matrix. During subsequent heating or annealing, the deformed grains form new, finer grains in a process known as recrystallization. However, the undeformed parallel platelets will not recrystallize and thus will survive the annealing treatment substantially the same as their original size. When this occurs, a duplex microstructure consisting of large parallel platelets surrounded by relatively fine equiaxed α grains is formed.

Such duplex microstructures are undesirable because they have a propensity to crack during subsequent deformation. Duplex microstructures can be prevented by appropriate control of silicide carbide particles early in the process.

Description of the Prior Art

At the present time zirconium alloy such as Zircaloy-2 and Zircaloy-4 extruded from β-quenched billets suffer from large grain size variations, typically ASTM G.S. 7 to 10. In addition, the process is very lengthy. For example, the current practice for tubeshell annealing is performed in a vacuum furnace held at 600-750^eC for 1-4 nominal hours. Since the full load in the furnace usually consists of 7000 - 8000 lbs., an annealing cycle lasts about 24 hours. For such a run, the material located at the center of the load may be heated above 600"C for more than 12 hours (Fig. 1) . Furthermore, the slow heat up in the temperature range of 500-600^βC causes the material to be stress relieved, and when it reaches a higher temperature, the driving force for recrystallization is so little that the material cannot be fully recrystallized. The step wise process of heating ingots, hot forging and extruding billets to be machined to desired length, then drilled to hollows is known. Rapid heating of the billet to β-phase (1015-1200°C) followed by rapid aqueous quenching produces a billet which is then milled and extruded. Induction heating at less than 650° softens the zircaloy billet before extrusion to the tubeshells. Vacuum annealing at 620-700°C for one to 3 hours produces a zirconium alloy containing structure with the large grain size variation but good nodular corrosion resistance. More uniform smaller grain size can be produced by rolling the ingot to a larger diameter before machining and heating into the β-phase. However, quenching from a larger size, i.e. slower cooling, has been shown to be detrimental to nodular corrosion resistance. So also has additional heating operations after the quench proven to be detrimental to nodular corrosion resistance.

The prior art includes various processes for step- wise hot working, annealing, cold working, annealing and such, before a final product is achieved. In U.S. Patent 4,775,428 (Cezus) , intermediate anneals are followed by cold deformation processes and a final anneal in order to obtain only partially recrystallized material of 20-40%, and elongated structure. Similarly, U.S. Patent 4,238,251 (Williams) claims only a partial transformation from the α-phase to the β-phase followed by quenching and rejects any particular advantage of working above the α-β phases including the cost to provide so much more energy. MaσEwen et al. (U.S. Patent 4,000,013) indicates a solution treatment is a heat treatment to dissolve alloying elements, which include Nb, Mo, Ni or Cr. In the case of zircaloys, the lower limit for the β phase is 980"C, while for Zr-2.5Nb, the lower limit for the β phase is 850°C. No mention is made of solution treatment in the upper β phase region for carbides, phosphides and/or suicides which are present as impurities, not as alloying elements. Dissolution and redistribution of alloying elements can be accomplished at lower temperatures within the β phase region, and after shorter holding times, than dissolution of these impurity elements precipitate.

The present invention, in contrast thereto includes the heat treatment in the upper region of β phase, from 1075'C to 1300*C, which is important for dissolving interstitial impurities of carbon, silicon and phosphorus and having them precipitating as numerous and fine particles, which serve the purpose of producing basketweave microstructure and uniform equiaxed α grain structure for the final product. This high β phase heat treatment also prevents the formation of large (greater than 1 μm) inclusions of carbides, suicides and/or phosphides, which is detrimental to the fabricability in the subsequent processing. The grain structure of the intermediate product is not considered. The present invention, not only achieves improvement in steam corrosion resistance but it particularly improves the fabricability of the intermediate product by the formation of finely dispersed carbide, suicide and/or phosphide particles and by preventing the formation of detrimental large particles of them.

European patent (0071193, Hitachi) also mentions a solution treatment, but the preferred temperature range is in the plus β range, 860-930° and low β range, IOOO-1100'C. Its purpose is to obtain alloying elements in supersaturation in order to improve steam corrosion resistance for the final product. It is the multiple cold plastic working steps ("at least twice") which produce the desirable properties of high corrosion resistance, high strength, high toughness, and small grain size in the final product.

It is an objective of the present invention to provide a process to produce zirconium alloy products having substantially uniform microstructure of fine grain size which imparts good fabricability to the zircaloy tube blanks made therefrom.

It is still another objective of this invention to provide a process to improve the orientation of the metallic crystals in zirconium alloy products such as tubing.

It is yet another objective of this invention to provide improved processes for producing zircaloy tubeshells whose metallic microstructure is substantially uniformly recrystallized by the rapid cooling step producing retained stress.

It is still another objective of this invention to avoid the α+β heating range process of the zirconium alloys processing into tubeshells and other mill products.

These and other advantages of the present invention will become more readily apparent to those skilled in the art from the following specification and examples.

BRIEF SUMMARY OF THE INVENTION

In accordance with the above objectives, it has been unexpectedly found, as will be further described hereinafter, that superior zircaloy tubeshells having metallic microstructure of uniform grain structure and imparting superior fabricability and nodular corrosion resistance are obtained by processes for heat treating a zircaloy billet, having a cross section between approximate 100 to 160 in² in a temperature range of upper region of β phase, from about 1075°C to about 1300°C for about 0.5 to about 8.0 hours sufficient to dissolve carbides, phosphides and/or suicides or mixtures thereof. This step is then followed by rapid cooling through the temperature range from 1075°C to 980"C either by water quenching or by air cooling at a rate preferably greater than 3°C per minute. At this cooling rate carbides, suicides and/or phosphides will precipitate into small and numerous particles which will act as nucleation sites for α platelets. This step is essential to the process because they are important for the formation of basketweave structure and subsequently for uniform equiaxed grain structure for the product. Furthermore, large (greater than 1 μm size) carbide, suicide and/or phosphide particles will form stress raisers and will be detrimental to the material in the subsequent fabrication processes. Large particles will be formed if the cooling rate in the said temperature range is substantially slower than 3'C per minute. From 980*C to ambient temperature, the material may be cooled in air or water.

The next step is reducing the cross sectional area of the billet to about 30 in², followed by preheating to the lower region of β phase, say, between 980^eC to 1040°C, or above and quenching into water, effecting a cooling rate of more than 1*C per second and preferably at 10 to 60°C per second in order to keep iron, chromium and nickel in super- saturation in the alloy. Special caution should be paid to the second β heating, that prolonged heating in the lower region of β phase more than a few minutes should be avoided, because this will reduce the number of particles and coarsen the precipitates of carbides, phosphides and/or suicides.

When the product line is tubing, the billet is usually made hollow by machining or other means prior to the β quench. After quench, the hollow billet is then preheated at about 500^eC to about 800"C for less than 10 minutes and extruded into a tubeshell.

The extruded tubeshells may be next annealed by the rapid heating to a temperature in the range of about 550°C to about 790"C for about 5 to about 60 minutes at temperature. As will be seen, the heating process is done only in the α phase and annealing conducted in air or inert atmosphere such as argon, helium, or nitrogen. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the fast and slow heating/cooling curves for a conventional vacuum anneal run for 621"C/2 hrs.

Figure 2 is a photomicrograph showing a duplex microstructure of zircaloy tubeshell which was vacuum annealed at 643°C/2 hrs.

Figure 3 is a photomicrograph showing uniform microstructure of zircaloy tubeshell which was air annealed at 670°C/50 min.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that by using solution heating to treat zirconium alloys only in the high β phase region followed by rapid cooling to α phase region, superior properties of fabricability and corrosion resistance are achieved through the formation of a uniform fine grain structure and a fine dispersion of precipitates through the metal matrix.

This solution heat treatment is performed at an intermediate stage of the process. It is conducted above the solvus lines for carbide and silicide precipitation to dissolve coarsened carbide silicide particles back into the β-phase matrix. Controlled cooling below the solvus promotes nucleation of fine and well distributed carbide silicide particles. As the zircaloy is further cooled the upper β transus (980°C) is reached and the α platelets have many finely distributed, carbide and silicide particles on which to nucleate. This results in an α phase structure comprised of numerous colonies of α platelets in a diversity of orientations, e.g., the desirable basketweave transformation structure now exists.

As will be seen from the following examples, it is important that the control of ZrC and Zr₃(Si,P) particles and their size and number density be achieved for the overall microstructural development of uniform, equiaxed α grains in zircaloy containing products. EXAMPLE I

A forged log of Zircaloy-2, measuring 14" diameter x 30" length is heated above the carbide solvus at about 1100"C for about 2 hours sufficient to promote dissolution of any pre-existing coarse carbide and silicide particles. This is followed by air cooling to room temperature. The temperature drop measured 3.6^βC in about 1 minute. All subsequent processing including additional forging and hot work is done at temperature well below the carbide solvus 1040^βC to avoid inadvertent coarsening of the finely distributed carbide silicide particles as a result of "Ostwald ripening".

A final β phase quench is conducted to dissolve intermetallic Zr-Fe, Zr-Cr, Zr-Ni particles to provide good corrosion resistance in aqueous steam. The β quench is conducted at a temperature range of from about 980°C to 1040*C. Zircaloy is reheated into this range at a smaller cross sectional dimension (about 6-8" in diameter) and is plunged into water. The low temperature β preheat, and limiting the holding time to a few minutes, preserves the nucleation sites formed from the first high temperature β solution heat treatment/cooling operation. It ensures that the Zr-Fe, Zr-Cr, Zr-Ni intermetallic compounds are totally dissolved, and it ensures that they too will be finely and uniformly distributed because these intermetallic compounds delineate the boundaries of the finely separated α platelets that nucleate on the preexisting carbide/silicide particles.

EXAMPLE II

Following the process mentioned in Example I, the Zircaloy-2 material was machined into a billet, 6" OD x 1.650" ID x length, and extruded, at 650"C to a tubeshell, 2.5" OD x .430" W x length. It was then air annealed at 670"C for 50 minutes. The material was fully recrystallized with a typical grain size of ASTM No. 10-1/2, which is shown in Fig. 3. A photomicrograph of Zircaloy-2 tubeshell which was conventionally vacuum annealed at 643"C for 2 hours is shown in Fig. 2 for comparison. Note that although the grain size in this case is similar, but there are areas of nonrecrystallized structure stretched diagonally across the photo in Fig. 2.

EXAMPLE III Samples of Zircaloy-2 tubeshells were vacuum annealed at between 620°C to 643°C and air annealed at temperature ranges from 643°C to 732°C. These samples were examined under a microscope and the results are shown in Table 1. Samples which were vacuum annealed at 620°C for up to 2 hours and at 643"C for up to 1 hour showed some bands of non-recrystallized areas or partially wrought structures (as marked with stars in the table) , while samples air annealed at 643°C for 1 hour or at higher temperature for shorter periods of time were essentially fully recrystallized. The drawback of the vacuum anneal is that, for a nominal 2-hour run, the material located at the center of the load may be heated above 600"C for more than 12 hours (see Fig. 1) . Furthermore, the slow heat up in the temperature range of 500-600°C causes the material to be stress relieved, and when it reaches a higher temperature, the driving force for recrystallization is so little that the material cannot be fully recrystallized even with longer nominal periods of time.

TABLE 1

Annealing Parameters vs. Grain Size and Microstructure for Zr2 Tubeshells

2.5" OD x .430" W x L Condition Zr2

VA 620'C/l hr. 11*

VA 620"C/2 hrs. 11*

VA 643^βC/l hr. 11 *

AA 643'C/l hr. 10 VA 643^βC/l hrs. 10.

AA 643^βC/l_ hrs. 11

VA 643°C/2 hrs. 11

AA 670^βC/10 min. 11%

AA 670^βC/15 min. 11. AA 670^βC/25 min. 10%

AA 670-C/50 min. 10

AA 700^βC/15 min. 11

AA 732^βC/5 min. 11

AA 732^βC/10 min. 11 VA 732^βC/15 min. 10%

VA = Vacuum Annealed AA - Air Annealed

* With bands of non-recrystallized area or partially wrought structure.

The affinity of zirconium for oxygen is very high.

The conventional practice of using vacuum annealing in industry is to avoid the possibility of picking up oxygen during air annealing. Since oxygen embrittles and hardens zirconium, oxygen pickup would have detrimental effects on the product properties. Table 2 shows results of oxygen analysis of samples of Zircaloy-2 barrier tubeshells (a barrier tubeshell is one with an inner liner of zirconium which is metallurgically bonded to the alloy by extruding) which were air annealed at temperatures ranging from 643°C to 750^*C for a few minutes to a few hours. No appreciable oxygen pickup for the outer shell or the liner is noticed when comparing with those of. the as-extruded samples or ingot.

TABLE 2

Oxygen Analysis of Annealed Liner Tubeshell

2.5" Outer Shell x .430" W x L

(Part Per Million)

Condition Outershell fZr2. Liner (Zr) As Extruded 1260 490 643°C/10 min. 1240 410 643^βC/20 min. 1270 440 643°C/20 min. 1250 460 643°C/25 min. 1260 450 643^βC/l% hrs. 480 643°C/2 hrs 470 643°C/3 hrs. 450 670-C/10 min. 1230 460 670"C/15 min. 1250 440 670°C/20 min. 1250 440 670°C/25 min. 1270 440 700-C/10 min. 1320 510 700°C/12 min. 1260 440 700°C/15 min. 1230 450 700°C/17 min. 1280 450 700°C/20 min. 1250 470 700^βC/l hr. 480 700°C/1% hrs. 510 700"C/2 hrs. 490 750°C/30 min. 620 750°C/1 hr. 450 750°C/2 hrs. 450

In addition to oxygen, another concern for air annealing is the possible picking up of hydrogen which will also have detrimental effects on the product properties. Even very low concentration of hydrogen will result in precipitation of zirconium hydrides which act to embrittle the product. ^' Table 3 shows hydrogen analysis for outer shell Zircaloy-2 and zirconium liner of the air annealed samples along with the as-extruded samples. No appreciable hydrogen increase for the air annealed samples is apparent.

TABLE 3 Hydrogen Analysis of Air Annealed Zircaloy-2 Liner Tubeshells

2.5" OD X .430" W X L (Parts Per Million)

Condition Outershell .Zr2. kjner (Zy) As Extruded 12 <5 620"C Anneal 15 min. 10 <5 30 mi. 9 6 60 min. 7 <5 643°C Anneal 5 min. 10 6 15 min. 8 <5 30 min. 9 <5 60 min. 7 <5 180 min. 10, 6, 6* 700'C Anneal 5 min. 10 <5 15 min. 12 <5 30 min. 7 <5 90 min. 20, 6, 6* 120 min. 7 732°C Anneal 5 min. 8 <5 15 min. 8 9 30 min. 9 <5 750^JC Anneal 5 min. 7 <5 15 min. 7 <5 60 min. 9, 10, 5* 120 min. 7

* These samples have been rerun; indicated are 3 numbers for the same sample. TABLE 4

Effect on Air annealing Parameters on the Nodular Corrosion Behavior of Zircaloy-2 2.5" Outershell x .430" W x L Tubeshells Test: 500^βC, 1500 psi, 24 Hrs. Number of Nodules Observed Lead End Sample Tail End Sample

Outside Dia. Inside Dia. Outside Dia. Inside Dia.

0 0

0 0 0 0 0

0 0 0

0 0 0

(As a result of the autoclave test results of the air annealed samples above shown, no nodules were observed on the outside or the inside diameter of the alloy tubeshell) As this invention can be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is, therefore, merely illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them and all changes that fall within the metes and bounds of the claims or that from their functional as well as conjointly cooperative equivalent are, therefore intended to be embraced by claims.

Claims

CLAIMSWhat is claimed is:

1. A process for producing zirconium-based alloy mill products characterized by a fine uniform grain structure free of particles greater than 1 μm in size of a member selected from the group of carbide, silicide, phosphide or mixtures thereof having finely dispersed intermetallic particles and improved nodular corrosion resistance including the steps of: a) heat treating a zirconium-based alloy billet or hollow billet with cross sectional area about 100- 160 in² in the upper region of the β phase, from 1075 to 1300"C, for about 0.5 to about 8 hours, then rapidly cooling in the temperature range from 1075°C to 980"C at a rate of greater than 3°C per minute by air cooling or water quench, followed by reducing the cross sectional area of the billet to about 30-90 in², then reheating to about 980°C to about 1050°C for a few minutes and quenching in water with a cooling rate of 10-60°C per second; b) extruding the billet or hollow billet into a rod, sheet or tubeshell at a temperature of about 500- 800°C; c) annealing the extruded product by rapid heating to about 550-790°C for about 5 minutes to about 60 minutes at temperature.

2. The process of Claim 1 in which said zirconium-based alloy mill product is a tubeshell.

3. The process of Claim 1 in which annealing is performed in air or inert atmosphere.

4. A zirconium-based alloy mill product characterized by a fine uniform grain structure free of particles greater than 1 μm in size of members selected from the group consisting of carbides, suicides, phosphides or mixtures thereof, said product having nodular corrosion resistance and a fine dispersion of precipitates through the metal matrix, made by the process consisting essentially of the steps of: a) heat treating a zirconium-based alloy solid billet or hollow billet with cross sectional area of about 100-160 in², by heating to a temperature in the upper region of the β phase, from about 1075 to 1300°C, for about 0.5 to about 8 hours, then rapidly cooling in the temperature range from about 1075°C to 980*C at a rate of greater than 3"C per minute by air cooling or water quench, followed by reducing the cross sectional area of the billet to about 30- 90 in², then reheating to about 980"C to about 1050"C for a few minutes and quenching in water with a cooling rate of about 10-60"C per second; b) extruding the billet or hollow billet into a rod, sheet or tubeshell at a temperature of about 500- 800"C; c) annealing the extruded product by rapid heating to about 550-790'C for about 5 minutes to about 60 minutes at temperature.

5. The product of Claim 1 , wherein the mill product is a tubeshell.