Question

Explain Newton’s law of cooling.

Answer 1

Hint: We know that the effect of cooling rate is not limited to solid state transformations. Cooling rate of weldment is depending on the heat input by the welding arc to the weldment. Heat input is controlled by the three variables Current, Voltage, travel speed. With the higher heat input the cooling rate is slower and vice-versa. The thermal cycle of a metal (the change in the temperature of the metal in time) is determined by dimensions of the welded joint, initial and boundary conditions, thermophysical properties of the metal, parameters of the heating source and position of the metal to the source.

Complete step by step answer
Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings. The law is frequently qualified to include the condition that the temperature difference is small and the nature of heat transfer mechanism remains the same. As such, it is equivalent to a statement that the heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. This condition is generally met in heat conduction (where it is guaranteed by Fourier's law) as the thermal conductivity of most materials are only weakly dependent on temperature. In convective heat transfer, Newton's Law is followed for forced air or pumped fluid cooling, where the properties of the fluid do not vary strongly with temperature, but it is only approximately true for buoyancy-driven convection, where the velocity of the flow increases with temperature difference. Finally, in the case of heat transfer by thermal radiation, Newton's law of cooling holds only for very small temperature differences, and a more accurate description is given by Planck's Law.

Note: We know that the micro- hardness increases with the increasing cooling rate and carbon content due to solid solution hardening and formation of the martensite phase. Thus, heat treatment (heating and cooling) is used to obtain desired properties of steels such as improving the toughness, ductility or removing the residual stresses. The higher the cooling rate of the quenching, the smaller the size of the grain size. Hence, it will increase the hardness of the steel. When the cooling rate is very high, it will increase the strength of the steel but it will reduce the toughness and the ductility of the steel.