The term “Nickel-Base Alloys” is used for those alloys wherein, with very few exceptions, nickel is the principal element. These nickel alloys are used where corrosion resistance is of the greatest importance, and metal is exposed to cyclical heating conditions and electrolytic attack. Most commercially available nickel-base hard facing alloys can be divided into three groups: boride containing alloys, carbide containing alloys and laves phase containing alloys. The largest numbers of nickel alloys are those containing boron. These alloys were first commercially produced as sprayed and fused powders. Currently they are marketed by most manufacturers as other hard facing products as well. This group of alloys is primarily composed of Ni-Cr-B-Si-C, the boron content typically ranging from 1.5% to 3.5% depending on chromium content which varies from 0 to 15%. High chromium containing alloys generally contain a large amount of boron which forms very hard chromium borides with a hardness of 1800kg/mm2. Usually other borides high in nickel with a low melting point are also present to facilitate the fusing operation subsequent to spraying. The abrasion resistance of those alloys is a function of the amount of hard borides present. Alloys containing Ni – 14 Cr – 3.4 B – 4.0 Si are extremely abrasion resistant but very poor in resisting impact. Since most of these alloys do not contain high amounts of solid solution strengtheners, considerable loss of hardness occur at elevated temperatures.
The laves phase alloys contain an inter-metallic compound of nickel – molybdenum – silicon in a nickel chromium – molybdenum matrix. The cobalt laves phase alloys are similar in that the cobalt replaces the nickel in the system. Due to the fact that laves are inter-metallic compounds, they are believed to be less abrasive to mating materials than carbide or boride containing alloys. These alloys like Nickel 200. Monel alloy 400, Inconel alloy 800 etc are most likely to found in Agriculture, the Chemical, Petroleum, Space and Aircraft industries. Specifically for Agriculture, certain powder materials of nickel-base alloy composition have been produced by Castoline Company in Switzerland, for hardening new parts and reconditioning worn parts of machine by gas-thermal spraying. The company produces granulated powders (GP), which have been classified into three groups, base on their deposition techniques and service applications.
At this point it will be worth while to consider some of the characteristics of these powder alloy materials.
GP – 10 Ni – 01 is a spheroidized powder alloy based on the system Ni – Cr – B – Si – C – Fe. It is usually deposited on mild stainless steels and irons. This alloy has a low melting point and high yield point, which provides smooth surfaces after deposition. The coated layer is characterized by a very low friction coefficient and high resistance to wear, corrosion and oxidation at normal and high temperatures. Hardness of the coated layer is HRC 55 – 62 and it offers a wear resistance performance, even at a high temperature of about 700oC. It’s available in coating thickness up to 2mm, and can usually be finished by grinding. This alloy material is commonly used for hardening and reconditioning of tools, crank pins, eccentrics, ratchet and pawl gear, transporting chains, mixers, pushers and various spots.
PM – 12 NiWC – 01 is a composite powder material (PM) consisting of a mechanical mixture of the spheroidized powder GP – 10Ni – 01 of the system Ni – Cr – B Si – C – Fe and the very small particles of tungsten carbide WC (35%). It is deposited on mild and stainless steel and irons. It has a high resistance to wear at normal and high temperatures. Coating hardness is HRC 57 – 64, capable of operating under a high temperature up to 700oC. The coating thickness is usually up to 2mm while the surface finish is done by grinding. It is used for hardening and reconditioning of transporting chains, mixer blades and scrapers, fan blades, cutting tools, cutting auger conveyors etc.
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