rain than the low-copper.
The quenched and drawn specimens of high-copper steel were found
to be slightly more martensitic.

HIGH-CHROMIUM OR RUST-PROOF STEEL

High-chromium, or what is called stainless steel containing from
11 to 14 per cent chromium, was originally developed for cutlery
purposes, but has in the past few years been used to a considerable
extent for exhaust valves in airplane engines because of its resistance
to scaling at high temperatures.

Percentage
Carbon 0.20 to 0.40
Manganese, not to exceed 0.50
Phosphorus, not to exceed 0.035
Sulphur, not to exceed 0.035
Chromium 11.50 to 14.00
Silicon, not to exceed 0.30

The steel should be heated slowly and forged at a temperature above
1,750 deg.F. preferably between 1,800 and 2,200 deg.F. If forged at temperatures
between 1,650 and 1,750 deg.F. there is considerable danger of rupturing
the steel because of its hardness at red heat. Owing to the
air-hardening property of the steel, the drop-forgings should be
trimmed while hot. Thin forgings should be reheated to redness
before trimming, as otherwise they are liable to crack.

The forgings will be hard if they are allowed to cool in air. This
hardness varies over a range of from 250 to 500 Brinell, depending
on the original forging temperature.

ANNEALING can be done by heating to temperatures ranging from 1,290
to 1,380 deg.F. and cooling in air or quenching in water or oil. After
this treatment the forgings will have a hardness of about 200 Brinell
and a tensile strength of 100,000 to 112,000 lb. per square inch.
If softer forgings are desired they can be heated to a temperature
of from 1,560 to 1,650 deg.F. and cooled very slowly. Although softer
the forgings will not machine as smoothly as when annealed at the
lower temperature.

HARDENING.--The forgings can be hardened by cooling in still air
or quenching in oil or water from a temperature between 1,650 and
1,750 deg.F.

The physical properties do not vary greatly when the carbon is
within the range of composition given, or when the steel is hardened
and tempered in air, oil, or water.

When used for valves the following specification of physical properties
have been used:

Yield point, pounds per square inch 70,000
Tensile strength, pounds per square inch 90,000
Elongation in 2 in., per cent