Claims:We Claim:

1. A method (100) of heat treatment for precipitation strengthened steel using a low alloy steel of grade 23, the method (100) comprising:
? hot bending of a pre-normalized and pre-tempered low alloy steel of grade 23 boiler tube panel (101);
? subjecting panels to a normalizing treatment at austenitizing temperature range (102); and
? a tempering treatment below a low critical temperature for obtaining mechanical strength and toughness (103).

2. The method (100) as claimed in claim 1, wherein the low alloy steel of grade 23 includes 0.04-0.10% carbon, 0.5% (max) Silicon, 0.1-0.6% Manganese, 1.9-2.6% chromium, 0.2-0.3% Vanadium, 0.02-0.08 Columbium, 0.0005-0.006% Boron, 0.03% (max) Nitrogen, 0.03%(max) Aluminium, 0.05-0.30% Molybdenum and 1.45-1.75% Tungsten.

3. The method (100) as claimed in claim 1, wherein the normalising and the tempering treatment are simulated in a thermo-mechanical simulator to study its effect.

4. The method (100) as claimed in claim 1, wherein the method conducts a metallurgical analysis at different cooling rate conditions to observe the phase changes.

5. The method (100) as claimed in claim 1, wherein the method requires further normalising and tempering treatment if bending strain exceeds 20%.

6. The method (100) as claimed in claim 1, wherein the austenitizing temperature for the normalising treatment ranges from 1050°C to 1070°C.

7. The method (100) as claimed in claim 1, wherein the critical temperature for the tempering treatment varies from 750°C to 780°C.

8. The method (100) as claimed in claim 1, wherein the hot bending operation of the low alloy steel of grade 23 is conducted at temperature above 1050°C.

Dated this 25th day of March, 2022

SOMA RANI MISHRA
PATENT AGENT
IN/PA – 1159
OF L. S. DAVAR & CO.,
APPLICANT’S AGENT
, Description:A METHOD OF HEAT TREATMENT FOR PRECIPITATION STRENGTHENED STEEL

FIELD OF THE INVENTION
[001] The present invention relates to a method of heat treatment for normalising and tempering of a low alloy steel of grade 23 boiler tube panels to achieve a required toughness and for ensuring compliance to a boiler manufacturing. More, particularly the present invention relates to a modified heat treatment simulation in which different heat treatment cycle using thermo-mechanical simulator is performed.

BACKGROUND OF THE INVENTION
[002] A low alloy steel of grade T23 is one of the important engineering material used widely in boiler applications. In one such applications, the grade 23 panels are formed in a required shape and are then subjected to normalising, followed by tempering heat treatment to obtain required mechanical properties particularly toughness.

[003] Generally, the steel of grade 23 used undergo certain manufacturing and heat treatment process for achieving required mechanical strength and toughness. The toughness property is evaluated using impact test as a part of qualification requirement. Sometimes, poor toughness is observed due to the changes in the process parameters such as cooling rate, soaking temperature and time.

[004] Moreover, the conventional slow cooling rate tends to form the ferrite phase and reaches poor toughness. The accelerated cooling rate of 1°C/s and above minimises the ferrite formation and good toughness can be achieved. Based on this analysis, accelerated cooling with the help of external mist forming air-cooler is proposed.

[005] The present invention is subjected to normalise heat treatment above 900°C for a sufficient soaking period and is followed by a cooling cycle. The present invention method discloses a modified heat treatment simulation with different heat treatment cycle using thermo-mechanical simulator. Based on the simulation, an optimised cooling rate cycle can be achieved, with required toughness values.

PRIOR ART OF THE INVENTION
[006] There are some systems/methods known in the art heat treatment for a low alloy steel. These are discussed herein below:

[007] CN105714060A titled “Heat treatment process for low-alloy-steel casting” relates to a heat treatment process for castings. In particular, a heat treatment process for a low-alloy-steel casting. The process includes the following steps of normalizing, water quenching, high-temperature tempering and air-cooling. By means of the heat treatment process for the low-alloy-steel casting, the tempering temperature of the low-alloy-steel casting is increased. The mode that the casting is directly discharged from a furnace and air-cooled is adopted, the processed low-alloy-steel casting meets strength and hardness requirements, and meanwhile, toughness of the casting is improved. A water-cooling mode is adopted in the quenching step of the heat treatment process for the low-alloy-steel casting, so that production cost is lowered, environment pollution is reduced, and potential safety hazards are reduced.

[008] US6676777B2 titled “Post-weld heat treatment process of carbon steel and low alloy steel” relates to the creep strength of a welded joint portion. The creep strength is improved, and damage of welded joint portion based on the generation or development of cracks is suppressed. A post-weld heat treatment process, wherein a welded joint made of carbon steel and low alloy steel is held within austenite single-phase temperature range for a given time and subsequently the joint is cooled by air-cooling or by slow cooling at a cooling rate lower than that of the air-cooling.

[009] CN103805753, titled “A Low-carbon alloy steel heat treatment process” relates to a low-carbon alloy steel heat treatment process which comprises the following steps; performing carbo-nitriding treatment, quenching, performing copious cooling treatment and tempering. The step of performing carbo-nitriding treatment comprises the sub-steps; dripping absolute methanol at the temperature of 850-870 DEG C, continuing for 60-90 minutes, dripping kerosene, introducing ammonia gas at a speed of 160-200ml per minute for 150-180 minutes, reducing the temperature to be 650-750 DEG C, dripping the absolute methanol, and introducing the ammonia gas at a speed of 80-120ml per minute for 90-120 minutes to reach room temperature. The quenching step comprises the sub-steps; performing oil quenching on the alloy subjected to the carbo-nitriding treatment at the temperature of 830-850 DEG C, preserving the temperature for 40-60 minutes, and cooling. The step of performing copious cooling treatment comprises the sub-steps; performing copious cooling treatment on a medium liquid nitrogen at the temperature of 150-160 DEG C below zero, and continuing for 24 hours. The tempering step comprises the following sub-steps; preserving the temperature of the alloy subjected to the copious cooling treatment to be 200-220 DEG C for 100 minutes, and performing air cooling. By adoption of the copious cooling treatment, austenite is fully converted into marten site, the strength of the low-carbon alloy steel is further improved, the wear resistance is improved, and the service life is prolonged.

[010] US8858741B2, titled “Methods for treating high-strength, low-alloy steel” relates to a method for treating high-strength, low-alloy steel includes controlling material responses, such as the crystal structure of the steel, through various processing steps. More specifically, the method includes cold treating the steel to achieve predictable increase in a minimum ultimate tensile strength or desired changes in the crystal structure of the steel. In one embodiment, cold treating the steel operates to controllably increase the minimum ultimate tensile strength of the steel within increasing a specified maximum ultimate tensile strength of the steel. Stated otherwise, cold treating the steel may reduce or narrow a minimum-to-maximum ultimate tensile strength range such that the minimum ultimate tensile strength is closer to the specified maximum ultimate tensile strength.

[011] It is evident from the prior arts that there is no such method of efficient heat treatment to meet the requirement of toughness property. In pursuit of this, the present disclosure focuses on achieving desired toughness and other mechanical property.

OBJECTIVES OF THE INVENTION
[012] It is an object of the present subject matter to overcome the aforementioned and other drawbacks existing in the prior art systems and methods.

[013] It a principal object of the present subject matter to propose a method to provide heat treatment on a hot formed grade 23 boiler tube panel to achieve the required toughness.

[014] It is another object of the present subject matter to achieve an optimised cooling cycle rate by simulating different cooling rate conditions using a thermomechanical simulator.

[015] These and other objects and advantages of the present subject matter will be apparent to a person skilled in the art after consideration of the following detailed description taking into consideration with accompanied drawings in which preferred embodiments of the present subject matter are illustrated.

SUMMARY OF THE INVENTION
[016] The present invention relates to a heat treatment method for obtaining code compliant toughness property on a hot formed precipitation strengthened steel like grade 23 material.

[017] According to an embodiment of the present invention, there is provided a method of heat treatment for precipitation strengthened steel using the low alloy steel of grade 23. The method comprises of, hot bending of a pre-normalized and pre-tempered low alloy steel of grade 23 boiler tube panel at high temperature (above 1050 degree centigrade) for obtaining required shape, a normalizing treatment at austenitizing temperature range by subjecting panels, a tempering treatment below low critical temperature for obtaining required mechanical strength and toughness.

[018] In an aspect, in the method the low alloy steel of grade 23 includes 0.04-0.10% carbon, 0.5% (max) Silicon, 0.1-0.6% Manganese, 1.9-2.6% chromium, 0.2-0.3% Vanadium, 0.02-0.08 Columbium, 0.0005-0.006% Boron, 0.03% (max) Nitrogen, 0.03%(max) Aluminium, 0.05-0.30% Molybdenum and 1.45-1.75% Tungsten.

[019] In an aspect, in the method the normalising and tempering treatment are simulated in a thermo-mechanical simulator to study its effect.

[020] In an aspect, in the method the metallurgical analysis at different cooling rate conditions to observe the phase change is conducted.

[021] In an aspect, in the method there is further requirement of the normalising and the tempering treatment if bending strain exceeds 20%.

[022] In an aspect, in the method the austenitizing temperature for the normalising treatment ranges from1050°C to 1070°C.

[023] In an aspect, in the method the lower critical temperature for the tempering treatment is from 750°C to 780°C.

[024] To further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit the scope of the present subject matter.

[025] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING(S)

[026] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which

[027] Figure 1 illustrates a method of heat treatment of a low alloy steel of grade 23 boiler tube panel in accordance with the present embodiment.

[028] Figure 2 (a, b and c) illustrates a schematic graph depicting value of hardness of material in normal and tempered condition and toughness values at different cooling condition in accordance with the present embodiment.

[029] Figure 3 (a, b) illustrates an optical micrograph of material at cooling rate of 0.25°C/s. It depicts image of different proportion 200x and 500x.

[030] Figure 4 (a, b) illustrates an optical micrograph of material at cooling rate of 0.5°C/s. It depicts image of different proportion 200x and 500x.

[031] Figure 5 (a, b) illustrates an optical micrograph of material at cooling rate of 1°C/s. It also depicts different proportion a) 200x b) 500x.

[032] Figure 6 (a, b) illustrates an optical micrograph of material at cooling rate of 2°C/s. It depicts image of different proportion 200x and 500x.

[033] Figure 7 (a, b) illustrates an optical micrograph of material at cooling rate of 3°C/s. It depicts image of different proportion 200x and 500x.

[034] Figure 8 (a, b) illustrates an optical micrograph of material at cooling rate of 5°C/s. It depicts image of different proportion 200x and 500x.

[035] Figure 9 (a, b) illustrates a scanning electron microscopes (SEM) fractography of material at cooling rate of 0.25°C/s and 0.5°C/s.

[036] Figure 10 (a, b) illustrates SEM fractography of material at cooling rate of 1°C/s and 2°C/s.

[037] Figure 11 (a, b) illustrates SEM fractography of material at cooling rate of 3°C/s and 5°C/s.

DETAILED DESCRIPTION OF INVENTION WITH REFERENCE TO THE DRAWINGS OF THE PREFERRED EMBODIMENTS

[038] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[039] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

[040] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such system, or assembly, or device. In other words, one or more elements in a system or device proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or device.

[041] The present disclosure relates to an invention pertaining to a method of normalising and tempering heat treatment of a low alloy steel containing 0.04-0.10% carbon, 0.5% (max) Silicon, 0.1-0.6% Manganese, 1.9-2.6% chromium, 0.2-0.3% Vanadium, 0.02-0.08 Columbium, 0.0005-0.006% Boron, 0.03% (max) Nitrogen, 0.03%(max) Aluminium, 0.05-0.30% Molybdenum and 1.45-1.75% Tungsten. The normalizing heat treatment helps to remove impurities and improve ductility and toughness. During the normalizing process, material is heated. The exact heat applied for treatment varies and is determined based on the amount of carbon content in the metal. Moreover, the process of tempering introduces toughness. These low alloy steels are usually tempered after normalizing, to increase the toughness and relieve internal stress. This can make the metal more suitable for its intended use and hence, is easier to machine.

[042] Figure 1 illustrates an exemplary method (100) using a low alloy steel of grade 23 boiler tube panel proposed in the present disclosure. The order in which the method (100) is described is not intended to be construed as a limitation.

[043] At block (101), the method includes hot bending a low alloy steel of grade 23 boiler tube panel (100) at high temperature of above 1050 degree centigrade to obtain the required shape.

[044] In an aspect the low alloy steel of grade 23 are pre-normalised and pre-tempered.

[045] At block (102), the method includes subjecting the panels to a normalising treatment at an austenitizing temperature range of 1050°C to 1070°C.

[046] At block (103), the method includes a tempering treatment below lower critical temperature from 750°C to 780°C to achieve required mechanical strength and toughness.

[047] In an aspect, after the hot bending operation, if bending strain exceeds 20% the panel of grade 23 material requires further normalising and tempering treatment.

[048] Figure 3 illustrates a predominant bainite and isolated polygonal ferrite which are observed in the specimens with cooling rate of 0.25°C/s. Figure 3(a, b) illustrates observed specimen in proportionate dimension of 200x and 500x respectively.

[049] Figure 4 illustrates a predominant bainite and isolated polygonal ferrite which are observed in the specimens with cooling rate of 0.5°C/s. Figure 4(a, b) illustrates observed specimen in proportionate dimension of 200x and 500x respectively.
[050] Figure 5 illustrates the absence of formation of ferrite phase and presence of only bainitic phase in the specimen at 1°C/s cooling rate. Figure 5(a, b) illustrates observed specimen in proportionate dimension of 200x and 500x respectively.

[051] Figure 6 illustrates the absence of formation of ferrite phase and presence of only bainitic phase in the specimen at 2°C/s cooling rate. Figure 6(a, b) illustrates observed specimen in proportionate dimension of 200x and 500x respectively.

[052] Figure 7 and 8 illustrates the formation of lathe structure (in the bainitic phase) which seems to be more continuous nature within the prior austenite grain boundary compared to the lathe formation in the slower cooling rate specimens. Figure 7 (a, b) and 8 (a, b) illustrates observed specimen in proportionate dimension of 200x and 500x respectively.

[053] In an aspect, the cooling rate reaches to a high value in the range of 3°C/s -5°C/s.

[054] Figure 9 (a, b) illustrates the fracture surface (fractography) analysed in a scanning electron microscope (SEM) shows predominant flat bifurcated type in the specimen treated at the cooling rate of 0.25°C/s -0.5°C/s.

[055] In an aspect, the scanning electron microscope (SEM) which is a critical tool in fractographic study, is used. The SEM has much better resolution and depth of field than optical microscopes, which aids in revealing topographical features of fractured surfaces.

[056] Figure 10 (a, b) illustrates the fractography analysed in the SEM which shows mixed features of flat bifurcation and imprints in the intermediate cooling rate of 1°C/s -2°C/s.

[057] Figure 11 (a, b) illustrates the predominant imprint features that are observed at the higher cooling rate condition of 3-5°C/s.

Test results
[058] This invention aims to simulate different cooling rate conditions in the test specimen. The method includes analysing through a thermal mechanical simulator, after the soaking period of a normalising treatment. Here, the cooling rate is varied to 0.25°C/s, 0.5°C/s, 1°C/s, 2°C/s, 3°C/s and 5°C/s to observe its effect on the microstructure phases and toughness. Now, the sample reaches room temperature. All the specimens are subjected to tempering treatment at 760°C for 1 hour in the furnace environment. After the tempering treatment, microstructure and mechanical properties are evaluated. Also, accelerated cooling with the help of an external mist forming air-coolers is preferred. In some cases, water quenching is conducted to achieve the required cooling rate.

[059] In an aspect, if the toughness is reported to be deteriorating after the normalising and tempering treatment, the material advances are facilitated through the use of thermal-mechanical simulations. This simulator allows the characterization of properties including hot ductility, flow stress and weldability.

[060] In an aspect, the normalising and tempering treatment are simulated in the thermo-mechanical simulator to study its effect. Here, the thermomechanical analysis is used to calculate the structural and thermal behaviour of one or multiple bodies at once. The thermal and structural field results are determined sequentially, in an iterative process, where the results of a thermal step serve as an input for the next structural step.

[061] In an aspect, the heat treatment method on the hot formed precipitation strengthened steel like grade 23 material is performed for obtaining code compliant toughness property. The said method conducts the metallurgical analysis at different cooling rate conditions to observe the phase changes.

[062] Now, the specimens (11x11x86 mm) are heated to normalising treatment at 1050°C and are soaked for 5 minutes. The specimens are cooled to different temperature at the cooling rate of 0.25, 0.5, 1, 2, 3 and 5°C/s. After this, all the specimens are tempered in the furnace at 750°C for one-hour duration. The simulated specimens are hence machined to a required size for impact test (10x10x55 mm) of toughness.

[063] From the test results, it is observed that the hardness of the specimen treated at the cooling rate of 0.25°C/s reaches 250 HV in the normalised condition. On increasing the cooling rate to 5°C/s, the hardness value raises to 310 HV as illustrated in Figure 2(a).

[064] The hardness values are observed to be reducing considerably in the tempering process. The hardness of the sample in normalised and tempered (N&T) condition is treated at the cooling rate of 0.25°C/s, the hardness value achieved is 196 HV. The sample cooled at the rate of 5°C/s achieves the hardness value of 226 HV as illustrated in Figure 2(b).

[065] The test values from the impact toughness test conducted in normalised and tempered (N&T) condition are plotted against the different cooling rate conditions as illustrated in Figure 2(c). The specimen treated at a cooling rate of 0.25°C/s and 0.5°C/s are insufficient to meet the toughness requirement of 27 Joules at room temperature. The impact toughness of 47Joules is obtained for the cooling rate of 1°C/s and it raises to more than 250 Joules on increasing the cooling rate to 2°C/s and above. A charpy impact test which is a standardized high strain-rate test that determines the amount of energy absorbed by a material during fracture is performed. The absorbed energy is a measure of the materials notch toughness. This test is widely used in industries, since it is easy to prepare and conduct. Moreover, the results can be obtained quickly and cheaply.

[066] Also, a fractography imaging at the magnification of 3000x is conducted after the Charpy impact test. The capturing of the micrograph images of the specimens using optical microscope for analysing the presence of phase.

Working Example

[067] In the present disclosure the method of heat treatment is performed on a low alloy steel boiler tube panel. A hot bending operation at high temperature is conducted on the pre-normalised and pre-tempered low alloy steel containing 0.04-0.10% carbon, 0.5% (max) Silicon, 0.1-0.6% Manganese, 1.9-2.6% chromium, 0.2-0.3% Vanadium, 0.02-0.08 Columbium, 0.0005-0.006% Boron, 0.03% (max) Nitrogen, 0.03%(max) Aluminium, 0.05-0.30% Molybdenum and 1.45-1.75% Tungsten. In the hot bending process when the required shape is obtained the panel is further subjected to a normalisation process at a temperature of 1050 degree centigrade followed by a tempering treatment at a critical temperature of 750 degree centigrade.

[068] The accelerated cooling with the help of an external mist forming air-coolers is performed, to achieve the required cooling rate and in some cases, water quenching is conducted. Hence, the above process helps to achieve the required mechanical strength and toughness of the material.

Advantages of the invention
[069] The method (100) described in the present disclosure is having the following advantages:
a) The fundamental advantage of the method (100) is to strengthen steel.
b) The method (100) improves mechanical properties of the steel.
c) The method (100) is cost effective.
d) The method (100) can be performed instantly.

It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various systems that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.

Although embodiments for the present subject matter have been described in language specific to package features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/device of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.).

In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that “consist essentially of” or “consist of” the recited feature.