Knowing What Takes Place

: The Working Of Steel

How are we to know if we have given a

piece of steel the very best possible treatment?

The best method is by microscopic examination of polished and etched

sections, but this requires a certain expense for laboratory equipment

and upkeep, which may prevent an ordinary commercial plant from

attempting such a refinement. It is highly recommended that any

firm that has any large amount of heat treatment to d
, install

such an equipment, which can be purchased for from $250 to $500.

Its intelligent use will save its cost in a very short time.

The other method is by examination of fractures of small test bars.

Steel heated to its correct temperatures will show the finest possible

grain, whereas underheated steel has not had its grain structure

refined sufficiently, and so will not be at its best. On the other

hand, overheated steel will have a coarser structure, depending

on the extent of overheating.

To determine the proper quenching temperature of any particular

grade of steel it is only necessary to heat pieces to various

temperatures not more than 20 deg.C. (36 deg.F.) apart, quench in water,

break them, and examine the fractures. The temperature producing

the finest grain should be used for annealing and hardening.

Similarly, to determine tempering temperatures, several pieces

should be hardened, then tempered to various degrees, and cooled

in air. Samples, say six, reheated to temperatures varying by 100 deg.

from 300 to 800 deg.C. will show a considerable range of properties,

and the drawing temperature of the piece giving the desired results

can be used.

For drawing tempers up to 500 deg.F. oil baths of fresh cotton seed

oil can be safely and satisfactorily used. For higher temperature

a bath of some kind of fused salt is recommended.