Jominy Or End Quenching Test

The Simplest and most reliable of these methods is the Jominy or end quench test. The standard Jominy specimen consists of a cylindrical rod 100 mm long and 25 mm in diameter. In making a test, the specimen is first heated to a suitable austenitic temperature and held there long enough to uniform austenitic structure. It is then placed in a jig and a stream of water is allowed to strike one end of the specimen. The experimental arrangement is shown in the image.

The advantage of the Jominy test is that in a single specimen one is able to obtain a range of cooling rates varying from a very rapid water quench at one end to slow air quench at the other end. Following the complete transformation of the austenite in the bar, two shallow flat surfaces are ground on opposite sides of the bar and Rockwell hardness values are taken at 1.5 mm Intervals along the bar. The results of the test are expressed by the hardenability number lc, in which l is the distance from the quenched end to the point with a semi-martensite structure, and c is the hardness value. The measured hardness values may also be plotted against the distance to obtain a hardenability curve. A typical example is shown in the image where it can be seen that the hardness is greatest where the cooling is most rapid - near the quench end.

After plotting such a curve, a horizontal line may be drawn at the corresponding hardness of the semi - martensite zone for the given steel. The intersection this line with the curve will indicate the length of the hardened zone ( distance from the quenched end ).

Comments

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.

Mechinopedia For Mechanical Engineers

Search This Blog