Question

Explain the best technique to increase the impact resistance and overcome the cracking problem of nails and bolts during the quenching process. With the aid of sketches, briefly describe how to conduct this procedure.

Answer 1

Usually, the bolts, nails, and fasteners are Through-hardened, which is a process in which the material gets rapidly quenched and tempered, to give a complete hardened product. The hardness of the part increases thorough out the material and then the strength of the part is recovered through the tempering process. This is comparatively less expensive and suitable for mass production of the bolts and nails.

But to improve the impact resistance and to overcome the cracking problem, we should use the carburizing process, in which the parts are kept inside a carbon-rich gas, liquid or solid medium, and heated to higher temperatures usually between 700^oC- 800 ^oC. The carbon present in the medium is gets diffused into the outer surface of the parts. The diffused carbon improves the hardness of the outer surface, keeping the inner core soft and ductile. Then the parts are quenched in oil or brine. The quenched part will be somewhat brittle therefore the part is tempered to improve its strength.  

Pack carburizing is the process in which the part is kept inside a sealed chamber with activated charcoal, then the medium is usually heated to 700^oC- 800 ^oC. The carbon present in the chamber is turned into carbon monoxide gas and the carbon gets diffused into the surface of the part increasing its hardness. The core of the part remains ductile and tough. Thus improving wear and crack resistance from the outside and also keeping its impact resistance from the inside. The hardened depth will be approximately equal to 0.1 - 1 mm. For mass production gas carburizing can be used instead of pack carburizing where a carburizing gas is passed through the part while heating the part and large components can be handled.