What is Hardenability in Heat Treatment ?

Hardenability is that property of a steel which determines the depth and distribution of hardness obtained by the quenching. It is not an indication of the hardness. Hardenability is usually interpreted as the ability to become uniformly hard or to harden in depth. The depth of hardening is usually taken as distance from the surface to semi-martensitic zone, i.e., 50 per cent martensite plus 50 percent pearlite.

Full hardening of carbon steels is observed in articles of a diameter or thickness up to 25mm. Alloy steels harden to considerably larger depth due to the high stability of the super cooled austenite and correspondingly lower critical cooling rate. Therefore, alloy steels may be effectively hardened by quenching in oil instead of water.

DETERMINATION OF HARDENABILITY :

Several methods are used to determine the hardenability of steels,

1. By appearance of fracture.

2. By the distribution of the hardness along the cross-section and

3. By the end quench test.

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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

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Various types of melting furnaces are used in different foundry shops, depending upon the quantity of metal to be melted at a time, and the nature of work that is carried out in the shop. The primary objective in cupola is to produce iron of desired composition, temperature and properties at the required rate in the most economical manner. Besides, this furnace has many distinct advantages over the other types, e.g., simplicity of operation, continuity of production, and increased output coupled with a high degree of efficiency. Description Of a Cupola : The Cupola Furnace consists of a vertical, cylindrical steel sheet, 6 to 12 mm thick, and lined inside with acid refractory bricks or acid tamping clay. The refractory bricks or the tamping clay used consist of silicon oxide acid (SiO2) and alumina (Al2O3). The lining is generally thicker in the lower where the temperatures encountered are higher than in the upper region. The shell is mounted either on a brick work foundation or on

Mechinopedia For Mechanical Engineers

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