The following points highlight the seven main alloys of nickel used in industries. The alloys of nickel are: 1. Nickel-Molybdenum Alloys 2. Nickel-Copper Alloy 3. Constantan 4. Nickel-Silver or German Silver 5. Nickel-Iron Alloys 6. Nickel-Chromium Alloys 7. Inconel.
1. Nickel-Molybdenum Alloys:
There are several nickel-molybdenum alloys which are very much resistant to corrosion. These alloys are called Hastelloys A, C, D according to their composition.
(i) Hastelloy A:
It is nickel-molybdenum iron alloy. It has high strength and ductility. Its strength does not decrease even at high temperature and it can withstand a load of 110 kg/cm2 at 900°C with a creep of one per cent per year.
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It is easily forged and rolled into sheet. It can be machined and can form good castings. It can be welded by oxyacetylene or electric arc.
(ii) Hastelloy C:
It is nickel-molybdenum-chromium- iron alloy. Its malleability is less than that of Hastelloy A. It cannot be worked hot or cold. It is good for casting but cannot be machined at high speed.
(iii) Hastelloy D:
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It is cast alloy consisting of nickel, silicon, copper and aluminium. This alloy is strong and tough and has high transverse strength. It is good for casting but cannot be worked either hot or cold. Its machinability is poor.
It can be welded with oxyacetylene or electric arc but addition of a flux makes the operation easy.
2. Nickel-Copper Alloy:
Monel Metal:
It contains about 60% nickel, 33% copper and a small percentage of iron and manganese and a very small quantity of silicon and carbon.
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Monel Metal can be easily cast, forged, machined, silver soldered, brazed and can be drawn into wire. The alloy can be welded with oxyacetylene and electric arc. In rolled and annealed condition its tensile strength is 4650 kg/cm2. At a temperature of 600°C its tensile strength is 2850 kg/ cm2 while that of low carbon steel is 1100 kg/cm2.
Monel metal is not affected by atmosphere and sea- water. Monel metal resists the corroding effect of alkalies and acids other than nitric and sulphurous acids.
Monel metal is used for the following purposes:
(i) It is used for evaporators and other parts of chemical plants for the production and recovery of caustic soda, pump impellers used for handling corrosive liquors.
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(ii) It is used for pickling plants, drying plants and for pump contact parts where tar and tar products are handled.
(iii) Monel metal filter cloth is used extensively.
(iv) It is used for lining autoclaves used for the manufacture of dye-stuffs.
(v) Paints and varnish plants are made of monel. Contact points of colloid mills are also made of monel metal.
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‘K Monel’ Metal:
K monel metal can be made by adding aluminium into monel metal. The utility of K monel metal lies in the fact that by heat treatment it can be made hard and strong reaming the corrosion resisting properties of monel metal.
This K monel metal can be obtained in four different conditions by simple heat treatment:
(i) Hot rolled and softened.
(ii) Hot rolled and softened and thermally hardened.
(iii) Cold worked and rear y for thermal hardening after machining or fabrication.
(iv) Cold worked and thermally hardened.
Their strength, yield point and hardening progressively increase from (i) to (iv).
Uses of K Monel Metal:
It is used for applications where high strength and hardness combined with high corrosion resistance including immunity from rusting is required.
For these reasons it is widely used for the following purposes:
(i) As valves and seats in pumps working with oil containing brines and sodium sulphide.
(ii) As valves and seats on starting air bottles for diesel engines.
(iii) For blades in paper-making machinery.
(iv) For impulse blades of steam turbines working at high pressures and high temperatures.
(v) Due to its non-magnetic nature, high strength and corrosion-resistance qualities, it is used for aircraft and radio instruments etc.
3. Constantan:
It is a nickel-copper alloy having high electrical resistance and low temperature coefficient.
4. Nickel-Silver or German Silver:
It is a nickel, copper and zinc alloy. It contains 25% nickel, 60% copper and the rest zinc. It is very resistant to corrosion. It has tensile strength varying from 1700 to 1400 kg/cm2.
5. Nickel-Iron Alloys:
A continuous series of alloys are obtained by the addition of varying quantities of nickel into iron.
These alloys are very important with regard to the following properties:
(i) Magnetic Properties:
By adding increasing quantities of nickel in iron, the magnetic property of the alloy obtained gradually goes on decreasing. When the percentage of nickel reaches 28 to 30% the alloy obtained is practically non-magnetic. Iron generally contains a little carbon in it and thus the alloys obtained by addition of different quantities of nickel are high nickel steels and these possess very high mechanical strength.
Such alloys are commonly used for machines and plants which are put to fluctuating magnetic fields in order to reduce the energy losses due to electromagnetic influence because the interference with the magnetic field is reduced. Accordingly such alloys are used for making end plates of alternator rotors, switch gear parts and transmission system parts.
When percentage of nickel in iron is increased beyond 28 to 30%, alloys obtained have got high magnetic permeability and low hysteresis losses. The magnetic permeability goes on increasing by the addition of increasing quantities of nickel. It is maximum when nickel content is 78.5%.
(ii) Dilatation Properties:
With the addition of nickel into iron in different quantities, the thermal coefficient of expansion of the alloy so obtained fluctuates very much. The coefficient of expansion falls rapidly with the progressive addition of nickel above 25% and when the percentage reaches 35, the coefficient of expansion becomes practically zero. With the further addition of nickel, the coefficient of expansion again increases first rapidly and then slowly till the co-efficient becomes equal to that of pure nickel.
By utilising this property of these alloys, an alloy, having practically zero coefficient of expansion has been obtained. This alloy is called Invar and is used in making clock pendulums, measuring tapes and thermostats.
Alloys having special co-efficient of expansion are made and used for making glass metal joints and also for making bead in wires for electric lamps. Alloys having a wide range of coefficient of expansion are made by adding different quantities of nickel into iron.
6. Nickel-Chromium Alloys:
(i) By the addition of chromium to nickel, the resistance to oxidation of the resulting alloys increases. By adding increasing quantities of chromium to nickel, the property of resistance to oxidation goes on increasing progressively up to 20% of chromium. With such a composition (20%. chromium, 80% nickel) an alloy is obtained which resists oxidation upon 1000°C. This resistance to oxidation takes place due to the formation of a protective oxide film over the alloy.
(ii) By addition of increasing quantities of chromium to nickel the electrical resistance of alloy obtained goes on increasing progressively. An alloy containing 20% chromium and 80% nickel has electrical resistance which is eleven times the electrical resistance of nickel.
(iii) Tensile strength also goes on increasing by the addition of increasing quantities of chromium to nickel. Alloy containing 20% chromium and 30% nickel has tensile strength of 8000 kg/cm2. At high temperatures, the tensile strength remains high. Its limiting creep stress is far higher than any other non-ferrous alloy.
Uses of Nickel-Chromium Alloys:
Due to the above described properties these alloys are used in electrically heated appliances which have to work continuously above 850°C.
In electrically heated muffle furnaces, these alloys are used in the form of wire or tape for heating elements, as sheet or cast plates for the furnace bottom, as cast grids for protecting the heating elements, as tubes for thermocouple sheaths, and as wire for one of the thermo-couple elements. Nickel-chromium alloys are also used for making heating elements.
A nickel-chromium alloy called nichrome or chromel is used for heating coils for furnaces on account of its high resistance.
7. Inconel:
It is an alloy having 80% nickel, 14% chromium and 6% iron. The alloy has high mechanical properties coupled with corrosion and heat resisting properties. It can be made into sheet, strip, rod, wire or in cast form. It can be welded, riveted, brazed, pressed and soldered. It is highly resistant to oxidation even at high temperatures up to 900°C. The alloy is generally used for the manufacture of machinery for food processing industries, especially milk and milk products.