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Difference Between Coated and Uncoated Cutting Tool in Metal Cutting

Pintu — Sun, 26 Jan 2020 06:34:35 +0000

Cutting tool is a part and parcel in every conventional machining process. Tool material and geometry are two active parameters that influence process capability and machinability. For uninterrupted material removal, the tool material should be harder than the workpiece material. In addition to the hardness, tool material should ideally possess certain common properties, such as high strength, high toughness, high fatigue strength, shape retention capability at high temperature, high thermal conductivity, good formability, etc. A single material may not offer all required properties. In order to harness most of the intended properties, tools are sometimes coated with suitable material so that the substrate material provides few properties and the coating material provides rest of the intended preparties. Sometimes multiple layers of coating are also deposited on the tool material so that each layer fulfils one desired property. Common coating materials for cutting tool include Titanium Nitride (TiN), Titanium Carbo-Nitride (TiCN), Titanium Aluminum Nitride (TiAlN), Diamond, etc.

Coating material deposition on the tool substrate is usually carried out either by Physical Vapor Deposition (PVD) method or by Chemical Vapor Deposition (CVD) method. Accordingly, coated tools become costlier as compared to uncoated tools. However, a coated tool can offer high material removal rate (MRR) and extended service life. These, in turn, reduce the frequency of tool replacement and cut down the associated cost for the machine idle time. Thus the overall production cost may reduce significantly with a coated tool. In addition, a coated tool has low wear rate and breakage tendency. This helps in machining features with tight tolerance for a prolonged period. Coating also helps in improving lubricity between flowing chips and rake surface of the tool. This can directly reduce the rate of heat generation, which helps in achieving better machinability. Despite several benefits, a worn-out tool cannot be re-used easily. In such case, an uncoated tool can be re-sharpened simply by grinding and can be re-used easily. Several similarities and differences between coated tool and uncoated tool are given below in table form.

Similarities between coated tool and uncoated tool

Irrespective of presence or absence of coating, the tool material provides necessary mechanical strength during metal cutting.
All intended geometrical features (inclinations and radius) are incorporated on the tool material only. Coating does not usually alter such features.
Heat generation, residual stress, burr formation, etc. are common problems in every conventional machining regardless of the usage of a coated or uncoated tool. However, the degree of such factors can be altered by selecting suitable coating material.

Differences between coated tool and uncoated tool

Coated Tool	Uncoated Tool
During machining with a coated tool, the coating material comes in physical contact with the workpiece material.	During machining with an uncoated tool, the tool material comes in physical contact with the workpiece material.
A coated tool typically offers extended tool life.	Uncoated tools usually have lower tool life.
Presence of coating on the tool substrate can reduce the rate of tool wear (adhesion, abrasion, oxidation, etc.)	Uncoated tools are prone to various wears.
Coated tools are usually expensive.	For same material, dimension and feature, an uncoated tool is cheaper.
Very less tool changing is required as each tool offers long life. So the idle time associated with tool replacement is also less.	As each uncoated tool has comparatively shorter tool life, so frequent tool changing is required. Accordingly, associated idle time is also high.
A coated tool cannot be re-sharpened easily once it is worn out.	The biggest benefit of uncoated tool is that it can be re-sharpened easily by grinding to re-use the worn out tools.
Some coating materials help improving lubricity (i.e. reduces sliding friction between flowing chip and rake surface of the tool).	For same workpiece material and cutting environment, coefficient of friction between chip and rake surface of the tool is usually higher.
Due to low coefficient of friction, the rate of heat generation during machining is also less.	For the same workpiece material and process parameters, the rate of heat generation is comparatively higher.
Coating can also restrict the rate of heat conduction into the tool substrate. This, in turn, discourages plastic deformation of the cutting edge caused by high cutting temperature.	Edges of the uncoated tools are susceptible to plastic deformation as the rate of heat conduction is not restricted.
Coated tools enable usage of high velocity, feed and depth of cut. So such tools offer high MRR.	High velocity, feed and depth of cut cannot be used with uncoated tool due to the inherent risk of accelerated wear and plastic deformation.
Coated tools can also offer high toughness, which restricts catastrophic breakage of the edges.	Uncoated tools usually have low toughness, and thus such tools are vulnerable to catastrophic breakage.
Coating can also improve fatigue resistance, so same tool can be used continuously for a longer duration.	Uncoated tools typically have low fatigue life.
Proper coating material also discourages built-up edge (BUE) formation on the edges of the tool.	At high temperature and mismatched compatibility, BUE formation on uncoated tool is a common incident.

References

Machining and Machine Tools by A. B. Chattopadhyay (Wiley).
Manufacturing Process for Engineering Materials by S. Kalpakjain and S. Schmid (Pearson Education India).
Saha et al. (2020); An investigation on the top burr formation during Minimum Quantity Lubrication (MQL) assisted micromilling of copper; Materials Today: Proceedings; https://doi.org/10.1016/j.matpr.2020.02.379

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Difference Between PWR and BWR – Pressurized Water Reactor & Boiling Water Reactor

Pintu — Mon, 14 Oct 2019 09:36:31 +0000

Nuclear reactor is one integrated part of every nuclear fission power plant where nuclear fuel is made to undergo nuclear fission reaction and the same is allowed to continue in a controlled chain reaction. Thermal energy (heat) derived from the exothermic nuclear fission is transferred to the coolant within this reactor. This coolant, in turn, drives the steam turbine (either directly or indirectly). Bases on the type of neutron used to initiate fission, the nuclear reactors can be broadly classified as thermal reactors and fast reactors. Thermal reactors, the most common one, are such reactors where thermal neutrons (having 0.025eV energy and 2.2km/s velocity at 20°C) are bombarded on the nuclear fuel to initiate fission. On the other hand, fast reactors employ fast neutrons (having 1 – 10MeV energy and about 50,000km/s velocity at 20°C) to initiate fission. Thermal reactors can again have various derivatives, namely Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), Pressurized Heavy-Water Reactor (PHWR), Advanced Gas Cooled Reactors (AGCR), Light Water Graphite Reactor (LWGR), etc. Although the PWR and BWR both utilize regular water as coolant as well as moderator, they have different working principle in driving the turbine to generate electricity.

In PWR, the thermal energy derived from nuclear fission is first transferred to the coolant (water). The coolant pressure is maintained in such a way that it does not boil, rather it remains in liquid phase even at very high temperature. A dedicated pressurizer is employed for such purpose. This high temperature coolant then transfers heat in a heat exchanger to another working fluid (again water) without physically mixing. This working fluid is allowed to change its phase to rotate the turbine. The coolant, after transferring heat to the working fluid, is returned back to the reactor to complete the cycle. Thus PWR power plants consists of two different loops – the primary loop where heat is taken from the reactor and is transferred to the working fluid, and the secondary loop where turbine is rotated. In the primary loop, the water is maintained at high pressure to restrict it from boiling, and thus the name “Pressurized Water”. On the other hand, the coolant (water) is allowed to boil (or change its phase from water to steam) in the Boiling Water Reactor (BWR). Thus the steam can be directly fed to the turbine without utilizing an intermediate heat exchanger. Hence BWR consists of only one loop. Although it offers higher thermal efficiency owing to elimination of intermediate heat exchanger, it is associated with the risk of radioactive contamination in case of any leakage. Various similarities and differences between Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) are given below in table format.

Similarities between PWR and BWR

Both Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) employ nuclear fission reaction to generate thermal energy, which, in turn, is utilized to drive the turbine for generating electricity.
Both PWR and BWR are thermal reactors, which indicate that the nuclear fission reaction is initiated by the thermal neutron (it has energy of 0.025eV and corresponding speed of 2.2km/s at 20°C). On the contrary, fast reactors utilize fast neutrons (1 – 10 MeV energy).
Both PWR and BWR require 3 – 5% enriched uranium fuel. An enriched fuel has higher percentage of U-235 isotope. In the naturally available uranium, U-235 isotope is only about 0.7%, and the rest is U-238 isotope. But this U-238 isotope is not fissile material (thus cannot be used as nuclear fuel). Thus only 3 – 5% U-235 isotope available within the entire fuel can undergo nuclear fission reaction to generate thermal energy, rest remains intact.
Both PWR and BWR employ only normal water or light water (H₂O) as moderator, as coolant and also as working fluid. On the contrary, heavy water reactors, gas cooled reactors and graphite reactors can employ other materials (like heavy water, carbon dioxide, graphite) for such purposes.

Differences between PWR and BWR

Pressurized Water Reactor (PWR)	Boiling Water Reactor (BWR)
Pressurized Water Reactor (PWR) power plants consist of two loops—(i) primary loop or coolant loop that takes away heat from reactor, and (ii) secondary loop or working fluid loop that drives the turbine. A heat exchanger (HE) is employed to transfer heat from primary loop to the secondary loop.	Boiling Water Reactor (BWR) power plants consist of a single loop where the coolant that takes away heat from the reactor is directly fed to the turbine. Thus no heat exchanger is desired.
In the primary loop, normal water (H₂O) acts as coolant-cum-moderator. In the secondary loop, the normal water acts as working fluid. However, water from one loop is not allowed to mix with the water of other loop.	Since it has only one loop, so normal water (H₂O) serves all three purposes – cooling, moderation, and working fluid.
Normal water in the primary loop, that acts as moderator-cum-coolant, is not allowed to boil. That means the water remains in liquid phase throughout the cycle of primary loop. However, the water in the secondary loop is allowed to boil.	Here the normal water (H₂O) is allowed to change its phase. Thus the water (liquid phase) is first converted into steam (gaseous phase) within the reactor, and then the steam is again condensed to water before pumping back to reactor.
Here steam is generated in a heat exchanger outside the nuclear reactor.	Here steam is generated within the reactor itself.
Here the water in the primary loop is maintained at high pressure (15 – 17 MPa) to avoid boiling at reactor exit.	Here water pressure remains comparatively low (7 – 8 MPa) as it is allowed to boil.
A pressurizer is required to use mandatorily to maintain water pressure in such a way that it does not evaporate even at very high temperature.	No such pressurizer is employed as evaporation of the water is desired.
The temperature of the water at the reactor exit is kept around 310°C (corresponding to the working pressure to avoid boiling).	Steam temperature at reactor exit remains comparatively low (around 285°C).
PWR has comparatively low thermal efficiency owing to two different loops.	BWR offers higher thermal efficiency.
In PWR, the control rods are inserted from the top of the nuclear reactor.	In BWR, the control rods are inserted from the bottom of the nuclear reactor.
Since the fluid is maintained at high pressure, so the PWR core volume is less.	For the same power generation, core volume of the BWR is comparatively larger.
Since the working fluid loop is separated from the primary loop, so PWR is less risky in spreading of radioactive materials owing to leakage.	Since same fluid passes through the reactor and turbine in BWR plants, so any leakage in the turbine can spread radioactive elements into the atmosphere.

References

Introduction to Nuclear Reactor Physics by R. E. Masterson (2017, CRC Press).
Fundamentals of Nuclear Reactor Physics by E. E. Lewis (2008, Academic Press).

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Difference Between PWR and PHWR – Pressurized Water Reactor & Pressurized Heavy Water Reactor

Pintu — Mon, 14 Oct 2019 09:33:29 +0000

In the nuclear power plant, the thermal energy generated by nuclear reaction (fission or fusion) is indirectly used to rotate the steam turbine to generate electricity. Nuclear fission power plants gained popularity owing to the easiness of initiating and controlling the fission reaction as compared to that of fusion. In fission power plants, nuclear fuel (mostly uranium fuel) is made to undergo fission reaction by bombarding it with high velocity neutrons. Reactor is the heart of nuclear power plant where the nuclear reaction takes place. It is a large enclosure where the fuel pallet or rod, its holder and necessary controlling elements are kept. A low temperature coolant (usually a liquid, such as normal water, heavy water, liquid sodium, etc.) is pumped into the reactor where the heat obtained from nuclear reaction is transferred to this coolant. Accordingly, the temperature of the coolant increases. Sometimes this coolant is allowed to change its phase (i.e. from liquid to vapour), else the pressure at the exit of the reactor is increased in such a way that the coolant remains in liquid phase even at very high temperature. If the reactor output is gaseous then it can be directly fed to the steam turbine. If the reactor output is liquid then a secondary loop is employed to obtain gaseous fluid for driving the turbine. In case of thermal reactors (where fission is initiated by thermal neutrons that has energy of 0.025eV and velocity of 2.2km/s), a moderator is mandatorily required to reduce energy of the prompt neutrons.

Even though the basic working principle is same for every fission power plants, thermal reactors can be classified in several categories based on the moderator and coolant fluid, namely, Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), Pressurized Heavy-Water Reactor (PHWR), Advanced Gas Cooled Reactor (AGCR), etc. Both in PWR and PHWR, the coolant pressure at the outlet of the reactor is maintained in such a way that the coolant does not boil. A dedicated pressurizer unit is employed for this purpose. Thus the reactor output is high temperature coolant in liquid phase, and hence a secondary loop is employed where the heat from this hot coolant is transferred to the working fluid (water) of the secondary loop to obtain high pressure steam for driving turbine. After the heat transfer, the coolant is pumped back to the reactor to complete cycle of the primary loop. Even though the working principle of PWR and PHWR are quite similar, normal water (H₂O) is used as coolant in PWR, while heavy water (D₂O) is used as coolant in PHWR. The respective coolant also serves the purpose of moderator in both the cases; however, PHWR reactors are designed in such a way that the moderator is not allowed to physically mix with the coolant (though both are heavy water). One advantage offered by heavy water moderator is the increase in fission cross-section and thus low enriched uranium can be used as reactor fuel. While PWR requires 3 – 5% enriched uranium to sustain chain reaction, the PHWR reactors can be operated without enrichment (i.e. at natural concentration of about 0.7% uranium-235). Various similarities and differences between PWR and PHWR are given below in table format.

Similarities between PWR and PHWR

Both PWR and PHWR fundamentally consist of two loops – (i) primary loop where heat from the nuclear reaction in transmitted to coolant, and (ii) secondary loop where heat from the coolant is transferred to working fluid for driving turbine.
Pressurizer is used in both the cases to restrict the coolant from boiling by maintaining a very high pressure.
Control rods are used in both type or reactors for controlling the rate of nuclear fission reaction and thus the rate of heat generation.
Due to presence of two different loops, there exists a less chance of radioactive element spreading for leakage in turbine.
Both the reactors work on the thermal neutrons. This indicates that the nuclear fission is initiated by a thermal neutron, rather than a fast neutron.

Differences between PWR and PHWR

Pressurized Water Reactor (PWR)	Pressurized Heavy Water Reactor (PHWR)
In PWR, the coolant also serves the purposes of moderator. So the same fluid acts as coolant-cum-moderator.	In most prevalent design of PHWR (i.e. at CANDU design), the coolant is kept separated from the moderator. Thus the moderator fluid don’t mix with the coolant.
In PWR, normal water or light water (H₂O) is used as coolant-cum-moderator.	In PHWR, heavy water (D₂O) based on deuterium is used as coolant, and also as moderator (but they are not allowed to mix).
Enriched uranium with around 3 – 5% U-235 isotope is used as fuel in the PWR reactors.	Mostly natural uranium that has around 0.7% U-235 is used as fuel in PHWR reactors as D₂O has negligible neutron absorption cross-section.
Uranium enrichment is costly and time consuming process. Thus the fuel for PWR reactors is costlier.	PHWR reactor fuel is cheaper as it utilizes naturally available uranium as reactor fuel.
The regular water (H₂O) that is used as coolant-cum-moderator in PWR is abundantly available and is cheaper.	The heavy water (D₂O) that is used as coolant and moderator in PHWR is highly expensive.
Fluid in the primary loop or coolant loop of PWR is maintained at higher pressure (around 15 MPa).	Fluid in the primary loop of PHWR is maintained at comparatively lower pressure (8 – 10 MPa).

References

Introduction to Nuclear Reactor Physics by R. E. Masterson (2017, CRC Press).
Fundamentals of Nuclear Reactor Physics by E. E. Lewis (2008, Academic Press).

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Difference Between Gaseous Diffusion and Gas Centrifuge Techniques

Pintu — Mon, 14 Oct 2019 05:12:12 +0000

Naturally available uranium consists of mainly uranium-238 isotope (about 99.3% of the natural uranium is U-238 isotope, 0.7% is U-235 isotope and trace amount of U-234). Among the three naturally available isotopes of uranium, only U-235 is fissile material and can sustain chain reaction as it offers higher cross-section towards thermal neutron. So only U-235 can be used as fuel in nuclear reactors, but its availability in the natural uranium is just about 0.7%. Due to lack of natural availability, the proportion of U-235 is required to increase artificially to prepare nuclear fuel. The process of increasing the proportion of U-235 in a uranium mass is called enrichment. Typically, thermal reactors of the nuclear power plant require 3 – 5% enriched uranium, fast reactors require 15 – 20% enriched uranium, while weapon grade fuel require above 90% enriched uranium. There are several techniques for enriching the uranium fuel, some of them include (i) Gaseous diffusion, (ii) Gas centrifuge, (iii) Laser based processes – AVLIS, MLIS and SILEX, (iv) Aerodynamic enrichment, (v) Electromagnetic separation, (vi) Chemical separation, (vii) Plasma separation, etc. In 20^th Century, the gaseous diffusion process was the most popular method for uranium enrichment; however, it gradually became obsolete with the development of cheaper and less time consuming processes. In today’s world, most of the separations are carried out by Gas Centrifuge process, while the other processes are either economically not viable or still under development.

Gaseous diffusion process works based on the Graham’s Law of effusion (the rate of effusion of a gaseous substance is inversely proportional to the square root of its molecular mass). Metallic uranium is first converted into uranium hexafluoride (UF₆) gas that has natural concentrations of U-235 and U-238. It is then kept in a large cascade under pressure and the lighter isotope is allowed to pass via a semi-permeable membrane. U-235 being about 0.85% lighter than U-238, the former one passes the semi-permeable membrane more rapidly compared to the later one. Thus UF₆ with slightly higher concentration of U-235 can be obtained inside the membrane chamber. Multi-stage separation can ultimately provide significantly enriched UF₆. This enriched UF₆ can then be converted into uranium dioxide (UO₂) to use as nuclear fuel. On the other hand. Gas Centrifuge process is also based on the slight mass difference among U-235 and U-238; however, here a rotating centrifuge is employed rather than a semi-permeable membrane. The UF₆ gas is first introduced in the rotating centrifuge. U-238 being heavier experiences higher centripetal acceleration and thus flows towards the outer wall of the centrifuge, whereas U-235 remains close to the axis of the centrifuge. Accordingly, enriched uranium can be extracted by sucking out the UF₆ gas from the centrifuge axis. This enriched UF₆ gas is then converted to uranium dioxide (UO₂) as usual to use as nuclear fuel. Various similarities and differences between gaseous diffusion and gas centrifuge processes for enriching uranium are given below.

Similarities between gaseous diffusion and gas centrifuge

Working principle of both the processes is based on the slight mass difference between U-235 and U-238 isotopes. U-235 isotope is about 1.26% lighter than U-238 isotope.
Both the processes require uranium hexafluoride (UF₆) gas for separation. No separation is possible from the metallic uranium gas. Thus the oxide of uranium (U₃O₈, also called yellow cake) is first chemically converted to uranium hexafluoride (UF₆) gas. Then separation is carried out. Notably, ²³⁵UF₆ gas is only about 0.85% lighter than ²³⁸UF₆ gas, even though the metallic U-235 is about 1.26% lighter than U-238. The enriched UF₆ gas is once again converted to uranium dioxide (UO₂) and sintered to rod or pallet to use as fuel.
None of the process can give highly enriched uranium in single stage. Thus multiple stages are require to obtain substantially enriched uranium.

Differences between gaseous diffusion and gas centrifuge

Gaseous diffusion	Gas centrifuge
A semi-permeable membrane is used to sieve UF₆ gas having higher concentration of U-235 from the natural concentration UF₆ gas.	A centrifuge (rotating cylinder) is used to separate heavier molecules (U-238) from the lighter molecules (U-235).
Isotope separation is obtained by means of atomic diffusion following the Grahams Law of effusion (U-235, being the lighter molecule, passes the membrane more rapidly).	This technique is not based on the Grahams Law; rather separation is obtained owing to varying rate of centripetal acceleration experienced by gas molecules of different masses. U-235, being the lighter molecule, remains close to the centre of the centrifuge.
Enriched uranium is obtained inside the membrane enclosure, while depleted uranium is obtained outside the enclosure.	Enriched uranium is obtained close to the axis of the centrifuge, while depleted uranium is obtained towards the wall of the centrifuge.
Here the UF₆ gas is maintained at high pressure (higher than atmospheric pressure).	Here the UF₆ gas is maintained at low pressure (usually lower than atmospheric pressure).
The chance of UF₆ gas leakage to the atmosphere is high. Thus the process is inherently associated with the risk of spreading radioactive elements in the air.	The chance of UF₆ gas leakage to the atmosphere is low (as the gas in the pipeline is maintained at a pressure lower than atmospheric pressure, so any leakage will lead to the inward flow of air into the pipeline).
Gaseous diffusion is time consuming and costly process.	Gas centrifuge is a productive and cost efficient process.
Earlier, gaseous diffusion technique was popular and had been used extensively for uranium enrichment.	Now-a-days, gas centrifuge technique is used overwhelmingly across the world for uranium enrichment.
Gaseous diffusion plants are usually of large size and can process tonnes of fuels in one step.	Gas centrifuge plants can be small scale that can process few tens of kilogram fuel in one step.
Owing to its large size, certain activities of these plants can be monitored remotely. Thus these plants are less risky for proliferation perspective.	Gas centrifuge facilities can be operated in in-house mode, and thus it becomes difficult to remotely monitor any activity of such plants. This possesses the risk of nuclear proliferation.
Gaseous diffusion plants typically require large amount of electrical power for its operation.	Power consumption per unit mass of enriched uranium is significantly less in gas centrifuge plants.

References

SIPRI Yearbook 2007 – Armaments, Disarmament and International Security by Stockholm International Peace Research Institute (Oxford University Press, 2007).
Uranium Enrichment by United States Nuclear Regulatory Commission, available at https://www.nrc.gov/materials/fuel-cycle-fac/ur-enrichment.html

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