RU2184795C2 - Method of producing flat section from zirconium alloys - Google Patents

Abstract

FIELD: metallurgy rolling; applicable in production of flat section used in structural materials in active zones of nuclear reactors, in chemical and oil-and-gas producing industries. SUBSTANCE: method includes melting of ingot; preparation of blank for hot rolling by hot molding and subsequent machining; hot rolling; intermediate heat treatment; cold rolling and final heat treatment. In this case, blanks for hot rolling are subjected to hardening at temperature exceeding point of phase transition from intermediate (alpha + beta) region to beta region by 50-150 C, and tempering at temperature of existence of (alpha + beta) region of zirconium. EFFECT: eliminated hereditary nonuniformity of structure, higher quality of articles and their yield. 5 dwg, 3 tbl, 3 ex

Description

 The invention relates to the field of metallurgy, to rolling production, and is intended, in particular, for the manufacture of a flat profile of zirconium alloys used as structural materials in the active zones of nuclear reactors, in the chemical and oil and gas industry.

 A known method of manufacturing a flat profile of a zirconium alloy or a hafnium alloy by hot rolling after re-heating with infrared radiation and the use of alloys (French Patent 2716897, C 22 F 1/16, 1996). The method is suitable for the manufacture of elements from zirconium or hafnium alloys for nuclear water reactors.

For the manufacture of sheet material or tapes of zirconium or hafnium alloys, a hot workpiece is crimped, then it is subjected to hot rolling in the alpha region. Rolling is carried out in several passes with one or more heating in the furnace to a temperature T _k , which improves the deformability of the workpiece. Next, cold rolling is carried out with one or more rolling and heat treatment cycles. In this case, one pass of hot rolling is carried out, following the last heating in the rolling mill, at least 100 ^° C at a speed of 4 ^° C per second by IR radiation with a wavelength of 0.8-5 μm, followed by hot rolling until the final a thickness of rolled equal to 0.8 of the thickness of the workpiece subjected to hot rolling.

 However, the application of this method requires the creation of special equipment for heating by infrared radiation. The implementation of hot rolling in the temperature region of alpha due to the narrow range of its existence limits the technological capabilities of zirconium alloys to obtain high-quality tack.

 A known method of manufacturing sheets of zirconium alloy with high corrosion resistance to deformation under irradiation (French Patent 2723965, C 22 F 1/16, 1997), in which for the manufacture of sheets of zirconium alloy suitable for making boiler elements, vacuum casting is first obtained from the corresponding alloy, which is subjected to forging and hot rolling, then the workpiece is subjected to hardening after repeated heating in the beta region, after heating, it is subjected to hot rolling, heat treatment in the alpha region, then cold rolling with subsequent heat treatment in the alpha region and final cold rolling with recrystallization annealing in the alpha region. Moreover, hot rolling of the sheet after hardening, starting from the beta region, is carried out in the first direction, then in the direction perpendicular to the first rolling direction.

 The disadvantage of this method is its low productivity due to numerous heatings. In addition, the use of sheet rolling in various directions requires the use of a special mill with elongated rolls, otherwise rolling in the direction perpendicular to the first direction is impossible, especially blanks for long sheets.

A known method of manufacturing sheets used for the manufacture of hull parts of nuclear reactors with a homogeneous structure from alloys of Circular 2 or Circular 4 (French Patent 2683828, C 21 D 8/02, 1993). The method involves heating a sheet of Zirkaloi 2 or Zirkaloi 4 with a thickness of 9 mm to a temperature above 1020 ^o C, quenching in water at a temperature below 15 ^o C. The duration of the transition between the furnace for heating and the quenching medium is less than 15 seconds, then after thermomechanical and / or mechanical processing the sheet is subjected to final heat treatment at 700-750 ^o C for 1-3 minutes.

 The disadvantage of this method is the short exposure time during the final heat treatment of the sheet, this time is not enough for the structural phase changes to occur in the alloy, which determine the required properties and operational reliability of the products. The process of hardening after hot rolling of the sheet up to 9 mm leads to the fact that the workpiece before hot rolling has low processability, although the main deformation is carried out precisely at the stage of hot rolling. In addition, this method is applicable only to alloys of compass 2 or compass 4.

A known method of manufacturing sheets of zirconium alloy - 2.5 wt.% Niobium, including obtaining a workpiece by forging, heating, hot rolling, intermediate heat treatment, cold rolling and final heat treatment, characterized in that the heating and the beginning of hot rolling is carried out in the range below the temperature α + β $\genfrac{}{}{0ex}{}{\text{← \_}}{\text{\_ →}}$ β - transformations at 70-150 ^o С (Patent of the Russian Federation 2021043, В 21 В 3/00, publ. 15.10.94. B.I. 19). This method, as the closest in technical essence to the claimed method, is selected as a prototype.

According to the description of the prototype, upon receipt of a workpiece by forging, deformation of the initial ingot takes place in the areas of beta and (alpha + beta) zirconium, as a result of which hot-rolled workpieces have a different structural state and phase composition in length and cross section. In addition, in this method, after forging there is no heat treatment necessary to relieve stresses arising in the workpiece during deformation of 82.5-83% and subsequent machining. Application of heating a billet for hot rolling in the range below the temperature α + β $\genfrac{}{}{0ex}{}{\text{← \_}}{\text{\_ →}}$ β - transformations at 70-150 ^o C cannot eliminate the initial heterogeneity of the workpieces, which is confirmed by the results of the control of finished sheets. The batch of sheets obtained from such blanks are characterized by instability of mechanical properties even within the same sheet, the metal structure along the length and thickness of the sheet is heterogeneous. In addition, this method is applicable only to an alloy of zirconium with 2.5 wt.% Niobium.

 The invention solves the problem of eliminating the hereditary heterogeneity of the structure of a flat profile of various zirconium alloys in the manufacturing process, improving the quality and yield of products.

The technical result is achieved by the fact that in the known method, including the smelting of an ingot, obtaining from it by hot molding and subsequent mechanical processing of the workpiece for rolling, hot rolling, intermediate heat treatment, cold rolling and final heat treatment, the following operations are introduced:
- hardening of the billet for rolling from a temperature of 50-150 ^o With exceeding the phase transition point from the intermediate (alpha + beta) region to the beta region of zirconium;
- tempering tempering in the intermediate (alpha + beta) -region of zirconium.

 Quenching of the billet for hot rolling leads the alloy to a homogeneous structural state of the martensitic type with maximum dissolution when heated to quenching impurity or intermetallic phases. Subsequent tempering of tempered billets leads to a uniform distribution of impurity and intermetallic formations, in addition, it reduces the hardness of the billets and increases the processability of the alloy for subsequent hot rolling.

 From the analyzed sources of patent and scientific and technical literature analogues that discredit the inventive method for producing a flat profile from zirconium alloys have not been identified.

 Examples of the proposed method.

Example 1. An ingot was made from a binary zirconium alloy Zr-2,5Nb. The ingot was forged. From forgings were made by machining the workpiece. The billets were quenched from a temperature of 960-990 ^o C (temperature polymorphic transformation of the alloy T _PP = 860-870 ^o C) in water. The hardened billets were tempered at a temperature of 740-770 ^° C (the transition temperature from the alpha region to the intermediate (alpha + beta) zirconium region was 610 ^° C). Then, a substandard metal layer was removed mechanically from the workpiece. After that, it was heated and hot rolled. The resulting tackle after annealing was subjected to cold rolling. The final annealing of the sheets was carried out after the last cold rolling. The annealed sheets were subjected to finishing operations to obtain a finished product with a thickness of 1.0 mm

Example 2. An ingot was made from a multicomponent zirconium alloy of Zirkaloy 4. The ingot was forged. From forgings were made by machining the workpiece. The billets were quenched from a temperature of 990-1030 ^o C in water. The hardened billets were tempered at a temperature of 730-770 ^o C. Then, an substandard metal layer was removed from the billet by mechanical means. After that, the processed workpieces were heated and subjected to hot rolling. The resulting tackle after annealing was subjected to cold rolling. The final annealing of the sheets was carried out after the last cold rolling. The annealed sheets were subjected to finishing operations until a finished product with a thickness of 1.5 mm was obtained.

 Table 1 shows the hardness values of tempered tempered billets of Zr-2.5Nb alloy manufactured by the present method, depending on the time of vacation.

 Table 2 shows the operation modes of heat treatment of semi-finished products from zirconium alloys according to the known (prototype) and the claimed methods.

Table 3 shows the mechanical properties of the products during tensile tests (Tisp. = 320 ^o C) obtained by the method described in example 1, and to compare the properties of products obtained by the known method.

 The properties of products made by the prototype and the claimed method are presented in figures 1-5.

 Figure 1 shows the macrostructure of the workpieces made of Zr-2.5Nb alloy after quenching by the present method at various temperatures.

 Figure 2 shows changes in the intensity of reflections of the beta phase of zirconium, impurity phases containing fluorine and silicon, half-widths of reflexes of the alpha zirconium phase in the Zr-2.5Nb alloy at various quenching temperatures.

 Figure 3 presents the microstructure of the workpieces from alloys Zr-2,5Nb (a) and Circular 4 (b) after quenching and tempering by the present method.

 Figure 4 - the macrostructure of the workpieces from alloys Zr-2,5Nb (a) and Compound 4 (b) after hardening and tempering operations according to the claimed method.

 Figure 5 presents the microstructure of sheets of alloy Zr-2,5Nb, manufactured by the present method (b) and its prototype (a).

The criterion for choosing the quenching mode is to ensure complete phase recrystallization of the alloy when heated to a temperature of 50-150 ^o With exceeding the phase transition point from the intermediate (alpha + beta) region to the beta region of zirconium. A further increase in the hardening temperature leads to grain growth (Fig. 1). The decrease in the temperature of quenching from the proposed does not ensure the formation of a homogeneous structural state of the alloy with the maximum dissolution of impurity and intermetallic phases (figure 2).

The tempering of the hardened billets of zirconium alloys in the proposed temperature range leads to the decomposition of the supersaturated solid solution with the release of fine particles of the beta phase, impurity or intermetallic phases uniformly distributed throughout the zirconium matrix (figure 3). A wide range of tempering temperatures is due to a significant difference in phase transition temperatures for binary and multicomponent zirconium alloys. So for the multicomponent Zirkaloi 2 alloy, the phase transition point from the alpha region to the intermediate (alpha + beta) zirconium region corresponds to 810 ^° C (US Pat. No. 2,894,866), and for the Zr-2,5Nb binary alloy, 590-610 ^° C (A . S. Zaimovsky, A. V. Nikulina, N. G. Reshetnikov. Zirconium alloys in nuclear energy. M.: Energoatomizdat, 1994).

 The structure of sheets obtained from hardened and tempered workpieces (table 2) is uniform in sheet thickness, with equiaxed grains (Fig. 4, 5b), in contrast to sheets made by the known method (Fig. 5a).

 The above changes in the structural state of the sheets obtained by the claimed method, lead to stabilization of mechanical properties and reduce the number of defects (table 3).

 From the above examples of obtaining sheets by the present method and an example of obtaining similar products according to the prototype, it can be seen that the inventive method provides products from multicomponent and binary zirconium alloys of high quality.

 At present, OJSC "CHEPETSK MECHANICAL PLANT" is conducting pilot-industrial production of a flat profile from zirconium alloys using the proposed method.

Claims (1)

A method of manufacturing a flat profile from zirconium alloys, including the ingot smelting, hot forming and subsequent machining of a billet for hot rolling, hot rolling, intermediate heat treatment, cold rolling and final heat treatment, characterized in that the hot rolling billets are tempered , 50-150 ^o With exceeding the phase transition point from the intermediate (alpha + beta) region to the beta region of zirconium, and leave at the temperature of existence (alpha + beta) -regions of zirconium.

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