阅读说明：本技术 一种铁素体/马氏体钢六角管控制变形的制造方法 (Manufacturing method for controlling deformation of ferrite/martensite steel hexagonal pipe ) 是由 李峻宏 苏喜平 杜爱兵 冯伟 任媛媛 王明政 黄晨 杨勇 刘兴民 杨孔雳 张东辉 于 2020-11-13 设计创作，主要内容包括：本发明属于冶金材料技术领域,涉及一种铁素体/马氏体钢六角管控制变形的制造方法。所述的制造方法依次包括如下步骤：(1)热挤压；(2)中间冷轧和退火；(3)圆管轧六角形管；(4)中间管正火和回火；(5)成品冷轧；(6)最终热处理。利用本发明的铁素体/马氏体钢六角管控制变形的制造方法,能够使得到的六角管满足快堆堆芯组件对铁素体/马氏体钢六角管直线度、壁厚、内对边距和扭曲度等严格的要求。(The invention belongs to the technical field of metallurgical materials, and relates to a manufacturing method for controlling deformation of a ferrite/martensite steel hexagonal pipe. The manufacturing method sequentially comprises the following steps: (1) hot extrusion; (2) intermediate cold rolling and annealing; (3) rolling a hexagonal pipe with a round pipe; (4) normalizing and tempering the intermediate pipe; (5) cold rolling of finished products; (6) and (4) final heat treatment. By using the manufacturing method for controlling deformation of the ferrite/martensite steel hexagonal pipe, the obtained hexagonal pipe can meet the strict requirements of the fast reactor core assembly on the straightness, the wall thickness, the internal alignment edge distance, the torsion degree and the like of the ferrite/martensite steel hexagonal pipe.)

1. The manufacturing method for controlling deformation of the hexagonal tube of ferrite/martensite steel is characterized by sequentially comprising the following steps of:

(1) hot extrusion: after a hole is drilled in a forged bar, preheating and induction heating are carried out in an annular furnace, heat preservation and hot extrusion are carried out, and after water cooling, pierced billet annealing, roller straightening, pipe cutting and surface polishing are carried out to obtain an extruded pipe;

(2) intermediate cold rolling and annealing: carrying out degreasing cleaning on the extruded circular tube after intermediate cold rolling, straightening the extruded circular tube after black annealing heat treatment, and then inspecting and polishing the extruded circular tube;

(3) rolling a hexagonal pipe with a round pipe: rolling a forming tube with hexagonal outer contour and hexagonal inner contour of the radial section by adopting a cold rolling process, and then degreasing and cleaning to obtain an intermediate tube;

(4) normalizing and tempering the intermediate pipe: carrying out normalizing and tempering heat treatment on the cleaned intermediate pipe by using a vertical vacuum furnace, and then inspecting, straightening and coping the intermediate pipe;

(5) cold rolling of finished products: carrying out final cold rolling on the intermediate pipe, and then carrying out degreasing cleaning;

(6) final heat treatment: and (4) carrying out tempering heat treatment on the pipe obtained in the step (5).

2. The manufacturing method according to claim 1, characterized in that: in the step (1), induction heating is carried out to 1120-.

3. The manufacturing method according to claim 1, characterized in that: in the step (2), the feeding amount of the intermediate cold rolling is controlled to be 3-6 mm/time, and the cold rolling speed is controlled to be 20-200 times/min.

4. The manufacturing method according to claim 1, characterized in that: in the step (2), the temperature of the black annealing heat treatment is controlled at 820-880 ℃, and the heat preservation time is 60-90 min; the straightened straightness is 0.01-1 mm/m.

5. The manufacturing method according to claim 1, characterized in that: and (3) rolling the forming pipe with the hexagonal outer contour and the hexagonal inner contour of the radial section by adopting a cold rolling process combined by a plurality of processing rollers.

6. The manufacturing method according to claim 1, characterized in that: in the step (3), the inner corner radius of the cold rolling process is controlled to be R2.5-R5.0mm, the cold rolling feed amount is 1-3 mm/time, and the rolling speed is 30-100 times/min.

7. The manufacturing method according to claim 1, characterized in that: in the step (4), the furnace temperature uniformity of the vertical vacuum furnace is within the range of +/-10 ℃; controlling the normalizing temperature at 1020-; the tempering temperature is controlled at 720-780 ℃, the vacuum heat preservation is carried out for 60-90min, and the argon is filled for rapid cooling after the furnace is cooled to a certain temperature.

8. The manufacturing method according to claim 1, characterized in that: in the step (5), the final cold variable is controlled to be 20-25%, and the final cold rolling speed is 10-100 times/min.

9. The manufacturing method according to claim 1, characterized in that: in the step (6), a vertical vacuum furnace is used for tempering heat treatment, the heat treatment temperature is controlled at 720-780 ℃, and the heat preservation time is 60-90 min.

10. The method according to claim 1, characterized in that the ferritic/martensitic steel has the composition, in weight percent:

si: 0.04 to 0.30 percent; n: 0.0040% -0.0700%; c: 0.17% -0.22%; mn: 0.40% -0.70%; p: less than or equal to 0.015 percent; s: less than or equal to 0.010 percent; ni: 0.40% -0.70%; cr: 11.00% -12.50%; mo: 0.80% -1.05%; v: 0.25% -0.35%; w: 0.40% -0.60%; nb: less than or equal to 0.05 percent; al: less than or equal to 0.020%; ti: less than or equal to 0.010 percent; zr: less than or equal to 0.010 percent; cu: less than or equal to 0.10 percent; sb: less than or equal to 0.0030 percent; sn: less than or equal to 0.0055 percent; as: less than or equal to 0.0050 percent; pb: less than or equal to 0.0010 percent; co: less than or equal to 0.015 percent; o: less than or equal to 0.0040 percent; h: less than or equal to 0.0005 percent; the balance being Fe.

Technical Field

The invention belongs to the technical field of metallurgical materials, and relates to a manufacturing method for controlling deformation of a ferrite/martensite steel hexagonal pipe.

Background

As a fourth generation nuclear energy system, a sodium-cooled fast neutron reactor (hereinafter referred to as a fast reactor) is required to achieve higher burnup at a higher temperature than a heavy water reactor or a pressurized water reactor.

The hexagonal tube in the fast reactor fuel assembly is one of the most important parts in the reactor core, and is corroded by a coolant sodium and damaged by high-dose irradiation of more than 100dpa when working in a temperature range of 360-600 ℃ for a long time. The hexagonal tubes are structural supports for the core assembly fuel bundles, achieve radial restraint for the bundles, and provide flow channels for the sodium coolant. By designing the proper hexagonal tube box spacing, the contact of the hexagonal tubes of the adjacent assemblies on the plane in the core active area can be prevented. The hexagonal tubes also provide a physical barrier for the fuel bundle to prevent accidents from propagating between assemblies when the fuel rods are broken. The outer sleeve of the fast reactor assembly in the hexagonal shape works in the environment of high temperature and high irradiation damage, neutron irradiation dose and temperature on edges and surfaces close to a reactor core and far away from the reactor core are different, and deformation of the surfaces caused by thermal expansion, irradiation swelling and irradiation creep is inconsistent, so that the hexagonal pipe is affected by bending moment, and long-time safe operation of a fuel assembly is influenced in severe cases. Therefore, the hexagonal tube is required to have good high-temperature strength and radiation swelling resistance after long-term irradiation, and good geometric stability. Meanwhile, after irradiation, the hexagonal tube still has certain plasticity so as to facilitate operations such as component extraction and the like.

The fast reactor core assembly has very strict requirements on the dimensional accuracy (wall thickness, parallelism of opposite sides, straightness, uniformity of opposite sides in three groups, torsion degree and the like), surface quality and deformation control of ferrite/martensite steel hexagonal pipe finished products.

The patent (patent publication No. CN 105112811A) applied by the institute of science and technology of combined fertilizer of Chinese academy of sciences describes in detail an austenitic stainless steel cladding tube for lead bismuth fast reactor and a preparation method thereof, but the application object of the patent is a cladding tube for lead bismuth fast reactor, and the cladding tube described in the patent is not a hexagonal tube, and the manufacturing shape and the type of the used material are different, wherein the manufacturing process is very different, the deformation form is different, and the heat treatment system is completely different. Therefore, the method and the process for preparing the austenitic stainless steel cladding tube cannot be used for manufacturing the ferrite/martensite steel hexagonal tube for the fast reactor core assembly.

Disclosure of Invention

The invention aims to provide a manufacturing method for controlling deformation of a ferrite/martensite steel hexagonal pipe, so that the obtained hexagonal pipe can meet strict requirements of a fast reactor core assembly on straightness, wall thickness, internal alignment margin, torsion and the like of the ferrite/martensite steel hexagonal pipe.

To achieve this object, in a basic embodiment, the present invention provides a method for manufacturing a hexagonal tube of ferritic/martensitic steel with controlled deformation, said method comprising the following steps in sequence:

(1) hot extrusion: after a hole is drilled in a forged bar, preheating and induction heating are carried out in an annular furnace, heat preservation and hot extrusion are carried out, and after water cooling, pierced billet annealing, roller straightening, pipe cutting and surface polishing are carried out to obtain an extruded pipe;

(2) intermediate cold rolling and annealing: carrying out degreasing cleaning on the extruded circular tube after intermediate cold rolling, straightening the extruded circular tube after black annealing heat treatment, and then inspecting and polishing the extruded circular tube;

(3) rolling a hexagonal pipe with a round pipe: rolling a forming tube with hexagonal outer contour and hexagonal inner contour of the radial section by adopting a cold rolling process, and then degreasing and cleaning to obtain an intermediate tube;

(4) normalizing and tempering the intermediate pipe: carrying out normalizing and tempering heat treatment on the cleaned intermediate pipe by using a vertical vacuum furnace, and then inspecting, straightening and coping the intermediate pipe;

(5) cold rolling of finished products: carrying out final cold rolling on the intermediate pipe, and then carrying out degreasing cleaning;

(6) final heat treatment: and (4) carrying out tempering heat treatment on the pipe obtained in the step (5).

In a preferred embodiment, the invention provides a method for manufacturing hexagonal tube of ferrite/martensite steel for controlling deformation, wherein in the step (1), the induction heating is carried out to 1120-.

In a preferred embodiment, the present invention provides a method for manufacturing hexagonal tubes of ferritic/martensitic steel with controlled deformation, wherein in step (2), the feed amount of the intermediate cold rolling is controlled to 3 to 6 mm/time and the cold rolling speed is controlled to 20 to 200 times/min.

In a preferred embodiment, the invention provides a manufacturing method for controlling deformation of a hexagonal tube of ferrite/martensite steel, wherein in the step (2), the temperature of the black annealing heat treatment is controlled at 820-880 ℃, and the holding time is 60-90 min; the straightened straightness is 0.01-1 mm/m.

In a preferred embodiment, the present invention provides a method for manufacturing a hexagonal tube of ferritic/martensitic steel with controlled deformation, wherein in step (3), a forming tube having a hexagonal outer profile and a hexagonal inner profile in a radial section is rolled by a cold rolling process using a plurality of work rolls.

In a preferred embodiment, the present invention provides a method for manufacturing hexagonal tubes of ferritic/martensitic steel with controlled deformation, wherein in step (3), the inside radius of the cold rolling process is controlled to be R2.5-R5.0mm, the cold rolling feed is 1-3 mm/time, and the rolling speed is 30-100 times/min.

In a preferred embodiment, the present invention provides a method for manufacturing hexagonal tubes of ferritic/martensitic steel with controlled deformation, wherein in step (4), the furnace temperature uniformity of the vertical vacuum furnace is within ± 10 ℃; controlling the normalizing temperature at 1020-; the tempering temperature is controlled at 720-780 ℃, the vacuum heat preservation is carried out for 60-90min, and the argon is filled for rapid cooling after the furnace is cooled to a certain temperature.

In a preferred embodiment, the present invention provides a manufacturing method of hexagonal tube controlled deformation of ferritic/martensitic steel, wherein in the step (5), the final cold variation is controlled to be 20-25%, and the final cold rolling speed is 10-100 times/min.

In a preferred embodiment, the invention provides a method for manufacturing hexagonal tube of ferrite/martensite steel for controlling deformation, wherein in the step (6), a vertical vacuum furnace is used for carrying out tempering heat treatment, the heat treatment temperature is controlled at 720-780 ℃, and the holding time is 60-90 min.

In a preferred embodiment, the present invention provides a method for manufacturing hexagonal tubes of ferritic/martensitic steel with controlled deformation, wherein the composition in weight percent of the ferritic/martensitic steel is:

si: 0.04 to 0.30 percent; n: 0.0040% -0.0700%; c: 0.17% -0.22%; mn: 0.40% -0.70%; p: less than or equal to 0.015 percent; s: less than or equal to 0.010 percent; ni: 0.40% -0.70%; cr: 11.00% -12.50%; mo: 0.80% -1.05%; v: 0.25% -0.35%; w: 0.40% -0.60%; nb: less than or equal to 0.05 percent; al: less than or equal to 0.020%; ti: less than or equal to 0.010 percent; zr: less than or equal to 0.010 percent; cu: less than or equal to 0.10 percent; sb: less than or equal to 0.0030 percent; sn: less than or equal to 0.0055 percent; as: less than or equal to 0.0050 percent; pb: less than or equal to 0.0010 percent; co: less than or equal to 0.015 percent; o: less than or equal to 0.0040 percent; h: less than or equal to 0.0005 percent; the balance being Fe.

The manufacturing method has the beneficial effects that by using the manufacturing method for controlling deformation of the ferrite/martensite steel hexagonal pipe, the obtained hexagonal pipe can meet the strict requirements of the fast reactor core assembly on the straightness, the wall thickness, the internal alignment edge distance, the torsion degree and the like of the ferrite/martensite steel hexagonal pipe.

The invention provides a novel manufacturing method for controlling deformation of a ferrite/martensite steel hexagonal pipe, which aims at solving the problem that the existing manufacturing process and heat treatment mode of austenitic stainless steel hexagonal pipes can not meet the strict requirements of a fast reactor core assembly on the dimensional accuracy (wall thickness, parallelism of opposite sides, straightness, uniformity of distances between three groups of inner sides, torsion resistance and the like), surface quality, deformation control, chemical components, structural uniformity, mechanical properties and the like of the ferrite/martensite steel hexagonal pipe.

Detailed Description

The manufacturing method of the hexagonal tube deformation control of the ferrite/martensite steel comprises the following steps (the designed ferrite/martensite steel comprises, by weight, 0.04-0.30% of Si, 0.0040-0.0700% of N, 0.17-0.22% of C, 0.40-0.70% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.40-0.70% of Ni, 11.00-12.50% of Cr, 0.80-1.05% of Mo, 0.25-0.35% of V, 0.40-0.60% of W, less than or equal to 0.05% of Nb, 0.020% of Al, less than or equal to 0.010% of Ti, less than or equal to 0.010% of Zr, less than or equal to 0.10% of Cu, less than or equal to 0.0030% of Sb, less than or equal to 0.0055% of Sn, less than or equal to 0.0050.0050.0010% of Pb, less than or equal to 0.015% of Co, and the balance of H, less than or equal to 0.0005% of Fe.

1. Hot extrusion

After the hole of the forging bar is drilled, preheating and induction heating are carried out in an annular furnace, the bar is heated to 1120-plus-1200 ℃, heat preservation is carried out for 1-3min, then hot extrusion is carried out, the extrusion ratio is 5-15%, and after water cooling, pierced billet annealing, roll straightening, pipe cutting and surface polishing are carried out, thus obtaining the extruded pipe.

2. Intermediate cold rolling and annealing

And (3) carrying out cold rolling on the extruded round pipe for 1 pass, and controlling the feeding amount of the intermediate cold rolling for 3-6 mm/time in order to ensure the dimensional precision and the structural uniformity of the cold-rolled pipe, wherein the cold rolling speed does not exceed 200 times/min. And degreasing and cleaning after cold rolling. Adopting black annealing heat treatment, controlling the temperature at 820-880 ℃, and keeping the temperature for 60-90 min. And straightening after annealing heat treatment, wherein the straightness is not more than 1 mm/m. Then, inspection and coping are performed.

3. Rolling hexagonal pipe with round pipe

And rolling a forming pipe with hexagonal outer contour and hexagonal inner contour of the radial section by adopting a cold rolling process combined by a plurality of processing rollers, wherein the inner corner radius is controlled to be R2.5-R5.0 mm. The cold rolling feeding amount is 1-3 mm/time, and the rolling speed is 30-100 times/min. And degreasing and cleaning after cold rolling.

4. Intermediate pipe normalizing and tempering

In order to ensure that the dimensional accuracy (uniform wall thickness, parallelism of opposite sides, uniformity of distances between three groups of inner opposite sides, straightness and torsion resistance) and the surface quality of the finished hexagonal tube meet the requirements, the intermediate tube cleaned cleanly needs to be subjected to normalizing and tempering heat treatment by a vertical vacuum furnace. The furnace temperature uniformity of the vertical vacuum furnace is within a range of +/-10 ℃, the normalizing temperature is controlled to be 1020-1080 ℃, the vacuum heat preservation is carried out for 15-25min, and the furnace is cooled to a certain temperature and then argon is filled for rapid cooling. Controlling the tempering temperature at 720-780 ℃, carrying out vacuum heat preservation for 60-90min, cooling to a certain temperature along with the furnace, and then filling argon for cooling. And after normalizing and tempering heat treatment, carrying out inspection, straightening and coping.

5. Cold rolling of finished products

The final cold variable is controlled at 20-25%. In order to ensure the strict size precision, surface quality and tissue uniformity of the finished product, the final cold rolling speed is not more than 100 times/min, and degreasing and cleaning are carried out after the final cold rolling.

6. Final heat treatment

And (3) tempering heat treatment is carried out by using a vertical vacuum furnace, in order to meet the requirement of 8-10 grade of grain size of a finished product, the heat treatment temperature is controlled to be 720-780 ℃, and the heat preservation time is 60-90 min.

7. Inspection and packaging

And (3) carrying out tests on the finished hexagonal tube such as dimensional accuracy, surface quality, chemical components, mechanical properties, metallographic structure, ultrasonic flaw detection and the like, wherein the standard flaw of the ultrasonic flaw detection is 0.15mm multiplied by 0.3mm multiplied by 3.0mm (depth multiplied by width multiplied by length). And finally, code spraying identification is carried out on the hexagonal pipes one by one. In order to prevent the ferrite/martensite steel from being oxidized and rusted, each hexagonal tube is independently packaged by a plastic bag, two ends of each hexagonal tube are sealed by heat sealing, and nitrogen is filled in each hexagonal tube to be used as protective gas. The two ends and the middle part of the hexagonal tubes put into the packing box are separated by a specially-made cushion block, so that mutual collision or friction in the transferring process is prevented, and the hexagonal tubes cannot deform due to external force.

The product obtained by this deformation control and manufacturing process has the following characteristics:

1) dimensional tolerance

(1) Wall thickness tolerance: plus or minus 0.1 mm;

(2) inner pair of pitch tolerances: +0.5mm or less;

(3) straightness accuracy: less than or equal to 0.7mm/m, and the total length is less than or equal to 1.0 mm;

(4) torsion resistance: the total length is less than or equal to 40';

(5) and the parallelism of opposite sides: less than or equal to 0.1 mm;

(6) three groups of inner-to-outer-side distance uniformity: less than or equal to 0.2 mm;

2) mechanical properties

TABLE 1 mechanical Properties

3) Metallographic structure

The original austenite grain size is measured according to the method specified in GB/T6394, and the original austenite grain size is 8-10 grades. Detecting the content of delta-ferrite under a microscope according to a method specified by YB/T4402, wherein the content of delta-ferrite in the worst view field is less than or equal to 8 percent.

4) Non-metallic inclusions

TABLE 2 non-metallic inclusions

5) Surface quality

The inner surface and the outer surface of the hexagonal tube are not allowed to have macroscopic defects such as oxide scale, debris, cracks, folding, delamination, holes, scabbing and the like; the finish Ra of the inner surface and the outer surface of the finished pipe is less than or equal to 1.6 mu m.

6) Ultrasonic flaw detection

The hexagonal tube was subjected to ultrasonic flaw detection at 100% with standard flaw sizes of 0.15mm by 0.3mm by 3.0mm (depth by width by length).

The manufacturing method of the ferrite/martensite steel hexagonal pipe controlled deformation of the present invention exemplified above is exemplified as follows.

Example 1: manufacture of hexagonal tube of ferrite/martensite steel for controlling deformation

The dimensions of the ferrite/martensite steel hexagonal tube prepared by the embodiment are as follows: 115mm (outside-to-outside margin) x 3mm (wall thickness), comprising the following steps.

1) Hot extrusion

Heating the forged bar, carrying out hot extrusion, controlling the extrusion ratio to be 5-15%, carrying out roller straightening, pipe cutting and surface polishing after water cooling, and obtaining the extruded pipe.

2) Intermediate cold rolling and annealing

And (3) carrying out 1-pass intermediate cold rolling on the extruded tube, controlling the rolling speed not to exceed 200mm/min, and then carrying out degreasing cleaning. The intermediate heat treatment adopts black annealing, the annealing temperature is controlled at 820-880 ℃, and the heat preservation time is 60-90 min. And after intermediate heat treatment, internal and external surfaces are polished and straightened.

3) Hexagonal forming cold rolling and normalizing and tempering heat treatment

When the hexagon is formed, the inner angle radius is controlled to be 4.5. The cold rolling feed amount is 2 mm/time, and the rolling speed is 80 times/min. And degreasing and cleaning after cold rolling. Normalizing and tempering by using a vertical vacuum furnace, controlling the normalizing temperature at 1040 ℃, and carrying out vacuum heat preservation for 25 min. Controlling the tempering temperature at 760 ℃, and carrying out vacuum heat preservation for 90 min. Then straightening, checking and coping are carried out.

4) Cold rolling of finished products

The final cold variation was 20%. In order to ensure the strict size precision, surface quality and tissue uniformity of the finished pipe, the final cold rolling speed is not more than 100 times/min, and degreasing and cleaning are carried out after cold rolling.

5) Final heat treatment

And (3) carrying out final tempering heat treatment by using a vertical vacuum furnace, wherein the heat treatment temperature is controlled at 760 ℃ and the heat preservation time is 90min in order to meet the requirement of 8-10 grade of grain size of a finished product.

6) Examination of

And (5) carrying out detection on the finished pipe on size, surface quality, chemical components, mechanical properties, metallographic structure, ultrasonic flaw detection and the like. Wherein the ultrasonic flaw detection is 100%, and the standard flaw is 0.15mm multiplied by 0.3mm multiplied by 3.0mm (depth multiplied by width multiplied by length).

The total length 3300 + -0.5 mm, the inner pair margin 109-109.5mm, the straightness less than 0.7mm/m, the wall thickness (3 + -0.1) mm, the twist degree less than 40'/total length, the parallel degree of the opposite side less than 0.1mm, the uniformity of the three groups of inner pair margin less than 0.2mm, the surface finish Ra of the finished product tube is less than 1.6 um. The grain size is 9 grade, the mechanical property meets the requirements of table 1, and the metallographic structure, the nonmetallic inclusion and the ultrasonic flaw detection all meet the technical requirements of the fast reactor core assembly on the hexagonal pipe.

Example 2: manufacture of hexagonal tube of ferrite/martensite steel for controlling deformation

The dimensions of the ferrite/martensite steel hexagonal tube prepared by the embodiment are as follows: 59mm (outside-to-outside margin). times.3 mm (wall thickness). The preparation procedure is substantially the same as in example 1, except that the intermediate cold rolling pass is 2 passes, and the standard flaw of ultrasonic flaw detection is 0.15mm × 0.3mm × 3.0mm (depth × width × length).

The total length of the prepared hexagonal tube is 1449mm +/-0.5 mm, the wall thickness is within the range of (3 +/-0.1 mm), and the inner opposite edge distance is 53_-0.1 ^+0.3Within the range of mm, the inner angle radius R is 3.0mm, the straightness of the ridge line is less than 0.7mm/m, the straightness of the full length is less than or equal to 1.0mm/m, the torsion resistance is less than or equal to 40'/full length, and the inside and outside surface smoothness Ra of the finished tube is less than or equal to 1.6 um. The grain size is 9 grade, the mechanical property meets the requirement of table 1, and the metallographic structure and the nonmetallic inclusions meet the technical requirement of the fast reactor core assembly on the hexagonal pipe.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is intended to include such modifications and variations. The foregoing examples or embodiments are merely illustrative of the present invention, which may be embodied in other specific forms or in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention should be indicated by the appended claims, and any changes that are equivalent to the intent and scope of the claims should be construed to be included therein.