Preventing Cracks In Hardening





The blacksmith in the small shop, where equipment is usually very

limited, often consisting of a forge, a small open hard-coal furnace,

a barrel of water and a can of oil must have skill and experience.

With this equipment the smith is expected to, and usually can,

produce good results if proper care is taken.



In hardening carbon tool steel in water, too much cannot be said in

favor of slow, careful heating, nor against overheating if cracks

are to be avoided.



It is not wise to take the work from the hardening bath and leave

it exposed to the air if there is any heat left in it, because

it is more liable to crack than if left in the bath until cold.

In heating, plenty of time is taken for the work to heat evenly

clear through, thus avoiding strains caused by quick and improper

heating, In quenching in water, contraction is much more rapid

than was the expansion while heating, and strains begin the moment

the work touches the water. If the piece has any considerable size

and is taken from the bath before it is cold and allowed to come to

the air, expansion starts again from the inside so rapidly that the

chilled hardened surface cracks before the strains can be relieved.



Many are most successful with the hardening bath about blood warm.

When the work that is being hardened is nearly cold, it is taken

from the water and instantly put into a can of oil, where it is

allowed to finish cooling. The heat in the body of the tool will

come to the surface more slowly, thus relieving the strain and

overcoming much of the danger of cracking.



Some contend that the temper should be drawn as soon as possible

after hardening: but that if this cannot be done for some hours, the

work should be left in the oil until the tempering can be done. It

is claimed that forming dies and punch-press dies that are difficult

to harden will seldom crack if treated in this way.



Small tools or pieces that are very troublesome because of peculiar

shape should be made of steel which has been thoroughly annealed.

It is often well to mill or turn off the outer skin of the bar,

to remove metal which has been cold-worked. Then heat slowly just

through the critical range and cool in the furnace, in order to

produce a very fine grain. Tools machined from such stock, and

hardened with the utmost care, will have the best chance to survive

without warping, growth or cracking.





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