Question

In resistance projection welding, two steel sheets of 1.2 mm thickness are used with a current of 32000 A for 0.005 seconds. The heat required for melting steel is to be taken as 12 J/mm${}^3$ and the effective resistance of the joint can be taken as 90 micro ohms. The projection welded joint can be considered as a cylinder of diameter 5 mm and 1.6 mm height. Then heat lost to the surrounding is ________ Joules.

• A. 75.63 J
• B. 83.01 J
• C. 67.55 J
• D. 99.45 J

Answer 1

The correct option is B 83.01 J

$\text{Resistance projection welding}$

$\text{Steel sheet thickness = 1.2 mm}$

$\text{Current, I = 32000 A}$

$\text{Time for which current is passing through the joint, T = 0.005 sec}$

$\text{Resistance of joint, R = 90}\times \text{10}{}^{-6}\text{ohms}$

$\text{Heat required for melting steel sheet = 12 J/mm}{}^3$

$\text{Weld joint diameter (d) = 5 mm,}$

$\text{Height (h) = 1.6 mm}$

$\text{Q}{}_{loss}\text{= ? Joules}$

$\text{Heat supplied = I}{}^2\text{RT}$

$\text{= (32000)}{}^2\times 90\times {10}^{-6}\times \text{0.005}$

$\text{= 460.8 J}$

$\text{Volume of the joint =}\frac{\pi }{2}\text{d}{}^2\text{h}$

$\text{=}\frac{\pi }{4}\times 5^2\times \text{1.6 = 31.416 mm}{}^3$

$\text{Heat required for melting the volume of the joint}=31.416\times 12=376.99J$

$\text{Heat lost to surrounding = 460.8 - 376.99}$

$\text{= 83.01 Joules}$