Quenching is a rapid cooling process by which many metallic alloys are hardened. Brass (a copper alloy: 70 Cu, 30 Zn) plates that are 4 mm thick are quenched in a water bath at a heat treating plant. If a brass plate is initially at 4500C determine the quench time needed for the plate to attain 900C. The water bath temperature is 800C and has an average convective heat transfer coefficient of 45 W/(m2 K), determine how long the plate has to remain in the oven.
GIVEN
Brass alloy plate of thickness L=4 mm=0.004 m
Initial temperature of the plate (To) = 450°C
Water bath temperature (T?) = 80°C
Convective heat transfer coefficient ( ch ) = 45 W/(m2 K)
FIND
The quench time required for the plate to reach 900C.
ASSUMPTIONS
Constant thermal conductivity
End effects are negligible
SKETCH
Thus the flat plate can be considered as lumped capacitance.
Therefore, the internal resistance of the plate is negligible.
The temperature-time history of the plate, is
Solving for the time
The quench time required to reach 90°C for the plate is
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