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Consider the following statements: 1. Heat treatment is effective only in case of certain alloys. 2. Cooling rate is an important factor in any heat treatment process. 3. The temperature at which the change starts on heating the steel is called lower critical temperature. Which of the above statements are correct?

Consider the following statements: 1. Heat treatment is effective only in case of certain alloys. 2. Cooling rate is an important factor in any heat treatment process. 3. The temperature at which the change starts on heating the steel is called lower critical temperature. Which of the above statements are correct? Correct Answer 1, 2 and 3

Heat treatment: Heat treatment is the process of heating and cooling a metal in its solid state in order to obtain the desired changes in its physical properties.

Heat treatment or the amount of heat treatment an alloy can undergo essentially depends upon the alloying elements.

Once the metal is heated to certain temperature then it is the cooling rate which decides the properties obtained by the heat treatment, for example in the annealing process the steel it is allowed to cool in the furnace itself, the austenite changes into coarse pearlite, and in the normalising process the steel is allowed to cool in the open air then the austenite changes into fine pearlite.

Lower critical temperature: Steel is heat treated to mainly remove the internal stresses, reduce grain size or increase in toughness. For a steel alloy, the temperature below which, under equilibrium conditions, all austenite has transformed into ferrite and cementite phases. The heat treatment of the steel is done above the lower critical temperature, to convert it into the austenite.

Upper critical temperature: It is the highest temperature at which steel may be quenched in order to attain maximum hardness and the finest grain structure.

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