PRESINTERING & SINTERING - POWDER METALLURGY

PRESINTERING :

Presintering is the process of heating the green compact to a temperature below the sintering temperature. This is done to remove the lubricants and binders added during blending and to increase the strength of the compact. All metals do not require presintering. But some metals like tungsten carbide are easily machined after presintering. After sintering they become so hard that they cannot be machined.

SINTERING :

After being compacted into a briquette having the shape of the finished workpiece, the cold-welded aggregate of metal particles is heated in a furnace to a temperature close to the melting point of the basic metal which goes into the mixture. This is carried out in controlled atmosphere furnaces. It may also be carried out under protective gas normally hydrogen or in a vacuum if material tends to react with the protective gas. The heating causes the metal particles to sinter, that is, a proportion of them partly melt and by so doing cement the remaining particles together in a cellular structure. From the economic point of view, the sintering time should be as short as possible , but the time must be long enough to obtain the required properties in the workpiece.

Sintering is performed to achieve all possible final strength and hardness needed in the finished product. The three most important variables governing the sintering process are temperature, time and sintering atmosphere. The workpiece dimensions change during sintering. Such changes may be either a shrinkage or growth. In general, bronze tends to expand and iron and brass to contact.

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THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

ELECTRICAL COMPARATORS AND ITS WORKING

Electrical Comparators are used as a means of detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a roll. The object to be checked is place

Advantages And Disadvantages Of Air Refrigeration System :

Advantages Of Air Refrigeration : 1. The refrigerant used namely air is cheap and easily available. 2. There is no danger of fire or toxic effects due to leakages. 3. The weight to ton of refrigeration ratio is less as compared to other systems. Disadvantages Of Air Refrigeration : 1. The quantity of Refrigerant used per ton of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content.

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