Industrial Specialty Nickel Alloys
The industrial demand for nickel and its alloys keeps increasing at the wide rate irrespective of its high price in the recent time. It is because of enduring need of metals that provide extensive corrosion resistance in the severe chemical media and stress. The equipments are developed using these metals that offer prolong life service. Additionally constantly increasing maintenance cost and higher protection needs have a significant role in decision making about nickel alloys. Nickel is normally utilized as an alloying agent such as in stainless steel and copper alloys.
The ore of nickel is found in the several places on earth’s surface however the economical mining is done in New Caledonia, Canada, China and Norway. The best Nickel ore contains only 1.2 to 5% of metal. For instance, a commonly occurring ore’s chemistry – 40 percent SiO2, 20 percent MgO, 15 percent Fe2O3, 10 percent water, 10 percent NiO, 1 percent Al2O3, 0.7 percent MnO2, 0.2 percent CoO and 0.1 percent CaO. The ore of nickel is found in green color which changes on the base of magnitude of iron oxide from yellow to brown. A commonly utilized technique of achieving nickel is smelting of nickel ore in cupola with coke and lime stone are included. The material received is pressed into bricks through a flux. The bricks are smelted in an oven subsequent to adding 30 percent coke mixture. In this way, a raw nickel oxide is received that contains significantly large magnitude of iron oxide which is purified with silicon based flux in a Bessemer converter. The received material consists of 80 percent nickel oxide and 20 percent sulfur. Then it is pulverized in a ball mil and roasted to remove sulfur. Then virtually pure nickel oxide is received, it is compressed with carbon into pill then heated for three days. While the heating process is conducted, a chemical reaction occurs between nickel oxide and carbon resulting in pure nickel product that is 99.25 percent pure. To obtain the maximum purity of nickel, electrolysis is followed.
By including any type of alloying agents the required nickel alloy can be received through smelting process. For the whole nickel alloys, the magnitudes of phosphorous and sulfur is kept nominal due to the risk of damage caused by them. From the element, sulfur should be prevented at any condition as it decreases the ductility of nickel and makes it brittle. While nickel’s smelting the sulfur can be limited by including nominal magnitude of manganese and / or magnesium.
Nickel takes 28th place in the periodic table and has face centered cubic lattice structure. It has high ductility. The special benefits of this metal, besides of excellent corrosion resistance are stable magnetic permeability, extremely slow thermal expansion and outstanding properties at high temperatures. It is also commonly utilized as an alloying agent to produce chromium, iron and copper alloys such as austenitic stainless steel, duplex steel, copper-nickel alloys, bronze, nickel-chromium alloys etc. Nickel is not just employed in the commercially pure form and also commonly used as nickel alloy in wire, plate, rod, bar, tube, strip, flanges, pipe, foil, sheet and others. It offers outstanding welding properties and accurate welding factors.
Nickel is also popular for offering great corrosion resistance, resulted by the extensive count of matrix twins and the production of precipitates that prevent dislocation moves to a large limit.
Pure Nickel 200 and Nickel 201
In the pure form, Nickel can be used to receive excellent outcomes in specific process device for which the two commonly used alloys are : Nickel 200 and Nickel 201 that describe the kinds that come in the class of pure nickel. Nickel 200 is a type with large ductility known as commercially pure nickel. Moreover, it offers high thermal conductivity and outstanding resistance to several corrosive conditions. The highly pure nickel has slight mechanical properties. Nickel 201 is almost similar to Nickel 200, having carbon content lower than 0.0025 percent. These grades provide outstanding corrosion resistance, particularly in de-oxidizing conditions. In the oxide conditions, a passive oxide layer is produced on the surface that makes nickel resistant to caustic soda, arid hydrochloric acid and arid bromine media. Commercial nickel grades offer great resistance to stress corrosion cracking in alkaline or chloride media. They can be worked in hot as well as cold forms. The stress relief annealing or soft annealing is important subsequent to cold forming above 5 percent. The hot treatment is essential for temperatures from 800oC to 1250oC. Prior the heat processing, the metal is cleared of oil, grease or any carbon or sulfur particles. The oven conditions should be mildly de-oxidized or neutral. If an operator is not confirmed about the presence of sulfur, it is widely preferred to verify that oven condition is nominally de-oxidized. Sulfur deposit should be prevented with nickel alloys to avoid the production of deleterious nickel sulfide.
To perform soft annealing, temperature limit about 700oC to 800oC should be kept, while stress relief annealing it should be 550oC to 650oC. The quenching should be conducted in water or in strained air. Thin parts can also be quenched in normal air.
Nickel 200/201 grades are easily weldable subsequent to greasing of products. The specific welding consumables are 2.4155 for TIG and MID wire and 2.4156 for electrodes.
Besides of commercially pure nickel, it is employed in the broad array of applications, several nickel alloys have been invented, the highly crucial alloying agents are chromium (Cr), Copper (Cu), Iron (Fe) and Molybdenum (Mo). In few cases, cobalt (Co), Aluminum (Al), boron (B), Niobium (Nb), vanadium (V), magnesium (Mg), silicon (Si) and others are added. These elements are included to enhance specific characteristics like oxidation stability, mechanical properties or creeping resistance, and they also leave an influence of enhancing the grain such as Nickel 200, Nickel 201, Monel 400, Monel K500, Inconel 600, Inconel 601, Inconel 625, Incoloy 800HT, Incoloy 825, Hastelloy C4, Hastelloy C22, Hastelloy C276, Hastelloy C2000, Hastelloy B2, Hastelloy B3 etc.
Monel 400 UNS N04400 is a copper (30 percent) – nickel (70 percent) alloy. It is extraordinarily versatile, as copper and nickel are fully miscible in each other. It has excellent mechanical properties as well as resistance to several corrosion based stresses. The mechanical properties of this alloy are sustained at the high temperatures and hence it is used in manufacturing pressure vessels for about 425oc applications. Nickel alloy 400 provides excellent corrosion resistance to de-oxidizing undiluted acids, organic acids, caustic solutions and salt solutions and to industrial dry gases like oxygen (O2), chlorine (Cl2) and hydrochloric acid (HCl), sulfur dioxide (SO2) and carbon dioxide (CO2). In the running sea water, it also provides outstanding performance even at conversion between seawater and air.
A specific benefit of Monel alloy 400 is its inhibition to stress corrosion in the wide range of applications. However it should not be employed in the conditions where materials come in contact of oxidizing media like iron or copper ions that are usually degraded from their salts.
Monel 400 is discovered to be extremely useful in offshore plant applications like condensers, piping, plate rings and valves. In the chemical plants, it also offers a suitable application in the production of different kinds of chemicals and salts. Its range of uses include in the production of shipping components, valves, fire extinguishing systems, pumps, propeller shafts and others. The power production units also use alloy 400 pipes in systems and heat exchanger devices. Latest, it is also found to provide significant performance in evaporators and disposal water crystallizers.
Monel 400 can be cold or hot processed. While forging, its temperature is kept among 930oC to 1160oC, for hot bending the limit varies from 1000oC to 1180oC. Subsequent to cold processing above five percent, stress relief annealing is essential that is highly preferred subsequent to hot processing. The oven media is made sulfur free and nominally de-oxidizing.
Welding can excellently be conducted with TIG or MIG procedures, however electrode welding is also completely applicable. Due to susceptibility to hot fracturing, the welding components used should be fully clean and free from dirt particles such as grease. Heating prior working is not preferred however heating after weld treatment is essential to avoid unwanted thermal stress. The most suitable materials for use as TIG and MIG welding wire are 2.4377 and suitable electrode is 2.4366.
Thus each nickel alloy can be evaluated. It is typically valid to state that nickel alloys consisting of chromium are made for use in oxidizing media while those without chromium are used in reducing conditions.
Since, Nickel is also an alloying agent and it is discovered normally in stainless steel copper alloys. It can also be discovered as an alloying agent in titanium. More about the advantages of nickel on such materials is stated following:
Nickel leaves effect on the configuration and mechanical characteristics of stainless steel to a large limit. Adding nickel in a large percent, stainless steel achieves an austenitic configuration that remains stable event at the cryogenic limits. It is because Nickel is a powerful austenite producer. As compared to chromium steel, combining with nickel makes considerable variations in the mechanical features. Moreover, it confirms suitable formability and hardness and also enhanced strength at the elevated temperatures. Nickel also enhances weldability and introduces variation in the physical characteristics of the material hence its magnetic properties vanish.
Second essential physical property of austenite structure over ferrite is lower heat conductivity and larger electric resistivity. In few conditions, the corrosion resistance is improved by the existence of nickel. Noticeably, nickel leaves minor or no variation when pitting attack is considered. It can be observed from the PREN or pitting resistance equivalent number that shows the absence of nickel.
Generally nickel is utilized to combine with iron and copper to produce their respective alloys. But, its significance as an alloying agent with titanium is not well popular. From the technical view, pure titanium metal prevents corrosion in the several chemical conditions. Although, at the elevated temperatures, it becomes sensitive to crevice corrosion so pure titanium is excellent for use in marine conditions and it is not sensitive to crevice attack at temperatures under 80oC. At increasing temperatures, the troubles occur, therefore a palladium alloyed (Pd 7% added) grade is used since palladium is very costly and it is in few cases is non- achievable, therefore 10 percent or higher molybdenum with 1 percent nickel has substituted this metal. It provides an alloy named titanium grade 12 that has very high resistance to crevice attack as compare to grade 2 alloy in marine conditions that is warmer than 80oC, however functionality is not up to grade 7.
Nickel based resistance heating alloys
Resistance heating alloys are utilized in several applications varying from typical household applications to extensive industrial process heating equipments and furnaces. These are used in helical coils of resistance wires installed with ceramic bushing in industrial process heating. In the furnaces, the service temperature reaches to 1300oC or 2350oF for furnace utilized in the metal processing industries and in kilns it reaches to 1700oC or 3100oF that are utilized for firing ceramics.
Nickel based Soft Magnetic alloys
Soft magnetic nickel-iron alloys comprising of 30 to 80 percent nickel are utilized broadly in applications that need properties:
- Large permeability
- Large saturation magnetostriction
- Small hysteresis energy loss
- Small eddy current loss in alternating flux
- Small curie temperature
- Stable permeability with variable temperature
The applications of these alloys include electromagnetic and radio frequency protection, transformers, amplifiers, tape recording head covering, lamination, ground fault interrupter cores, antishoplifting systems, torque motors and others.
The Nickel-Iron alloys are normally produced in sheet and strip forms, although wires are also produced. The strip forms are often delivered in a cold rolled condition for stamping lamination or thin foil for winding of tape toroidal cores. The strip and sheet products may also be delivered in a low temperature, mill-annealed, fine grain condition adequate for forming and deep drawing.
Commercial Nickel-Iron Alloys
The commercial Ni – Fe alloys are categorized on the base of nickel percentage – low nickel alloys and high nickel alloys. The low nickel alloys contain about 50 percent Ni and high nickel alloys contain about 79 percent Ni. Few alloys even contain lower nickel content from 29 to 36 percent, such materials are utilized in measuring equipments that need magnetic temperature compensation.
The influence of nickel magnitude in NiFe alloys on saturation induction and on initial magnetic permeability subsequent to annealing are shown in the following diagrams.
For nickel contents lower than 28 percent, the crystalline structure is body centered cubic low carbon martensite if quickly quenched, and ferrite and austenite structures are formed if slightly quenched and they are not counted significant for soft magnetic applications. For alloys containing nickel percent above 28 percent, the structure produced is face centered austenite. The curie point in this configuration is about room temperature for 28 % Ni content and increases quickly to 610oC or 1130oF for 68 percent Ni content. Hence austenitic alloys are ferromagnetic in nature.
The high nickel alloys comprising of 79 percent Ni possess large initial and highest permeability and extremely small hysteresis however they offer merely saturation induction about 0.8 T. The inclusions of molybdenum by 4 to 5 percent or copper and chromium to 79 Nickel-Iron alloys, offer accentuate specific magnetic properties. Commonly used high permeability alloys are Molypermalloy and Mumetal.
The magnetic characteristics of high nickel alloys are widely based on the treatment and heat work. The initial permeability in 78.8 NiFe alloy is small subsequent to furnace quenching or baking at 450oC or 840oF. However if it is quickly quenched from 600oC or 1110oF, the initial permeability increases abruptly. The high purity Ni – Fe alloys (containing 78.5% Nickel) can attain an initial direct current permeability of 5 x 10(4) and highest 3 x 10(5). These characteristics are received on ring laminations annealed in arid hydrogen at 1175oC or 2150of, quick furnace quenched to room temperature then reheated to 600oC or 1110oF and oil cooled. It is occasionally used on commercial scale due to the complicated heat processing, meanwhile its electric resistivity is merely 16 micro ohm-cm that permits large eddy current losses in AC applications.
The high permeability alloys should also be of extensive commercial purity. Air melting and vacuum melting procedures are utilized to develop low nickel alloys, however most of high nickel alloys are manufactured by vacuum induction melting. High Nickel alloys are used in laminations or components.
Low nickel alloys comprising of 45 to 50 percent nickel have smaller initial and highest permeability than high nickel alloys however they have larger saturation induction about 1.5 T. They have permeability about 1.2 x 10(4).
In these alloys, quenching effect is not extensive. The standard annealing process to create large permeability equivalent to the high nickel process except the quenching rate from 55oC per hour to 140oC per hour or 100of per hour to 252oF per hour is recommended.
Low expansion alloys
The room temperature coefficients of thermal expansion for major metals and alloys vary from 5 micro-m/m.K to 25 micro-m/m.K. In few applications, although alloys are chosen that offer extremely small thermal expansion or show consistent and expected expansion over specific temperature limits. The low expansion alloys are used in:
- Tapes for geodetic surveillance
- Balance pendulum and balance wheels for clocks and watches
- Running components that need controlled expansion like pistons for internal combustion engines
- Bimetallic strip
- Glass to metal seals
- Thermostatic strip
- Vessels for storing and shipping liquefied natural gases
- Superconductor systems in electricity transmission units
- Integrated circuit lead frames
- Parts for radios and various electronic equipments
- Structural parts in optical and laser devices
Low expansion alloys are also utilized in combination with high expansion alloys to create motion in thermoswitches and other temperature controlling equipments.
Nickel has a defined influence on the thermal expansion of iron. On the base of nickel percentage, nickel and iron alloys possess coefficients of linear thermal expansion from negative to highly positive value ranges. For nickel content from 30 to 60 percent, alloys with suitable expansion properties can be chosen. The alloys comprising of 36 percent nickel have very small coefficient of expansion that its size remains almost stable for normal temperature variations. It is called as Invar that refers to constant. The invention of invar gave rise to analysis of thermal and elastic characteristics of various similar alloys. Iron-Nickel alloys containing nickel magnitudes larger than that of invar possess some limit of expansion properties of invar. The alloys with nickel magnitude smaller than 36 percent possess larger coefficient of expansion as compare to alloys consisting of 36 or higher nickel. Invar 36, Invar 42, Invar 48 and Invar 52 are commonly used commercial low expansion alloys.
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