WELDING OPERATIONS I - OD1651 - LESSON 1/TASK 2
3.
Welding Armor Plate
a. Properties of Armor Plate.
Armor plate is hardened by
normalizing or heating it to its upper critical point and letting
it cool in still air.
The base metal quenching effect produced
next to a weld in heavy armor plate under normal welding
conditions is about halfway between the effects of air cooling
and oil quenching it.
During the welding of armor plate the
temperature of the weld metal ranges upwards of 3000 F from the
original temperature of the base metal. Therefore, a narrow zone
on each side of the deposited weld metal is heated above its
critical temperature.
This narrow zone is then quenched by the
relatively cold base metal and becomes a hard brittle zone known
as martensite. It is in this zone that cracks are most likely to
occur upon the application of a load. For this reason, special
precautions must be taken in all welding operations to minimize
the formation of hard zones. In addition, care must be taken to
prevent rapid cooling of the armor plate after welding in order
to avoid the formation of cracks in hard zones.
b.
Types of Armor Plate.
(1) General.
There are two types of armor that are used on
combat vehicles: homogeneous, which can be cast or rolled, and
face hardened, which is rolled.
It is essential that the armor
plate be specifically identified before any welding or cutting
operations are performed. This is important because the welding
procedures for each type of armor are distinctly different and
are not interchangeable.
(2) Homogeneous Armor Plate. Homogeneous armor is heat treated
through its entire thickness to develop good shock or impact
resisting properties. This type of armor is uniform in hardness,
composition, and structure throughout and can be welded on either
side. Aluminum armor plate is in the homogeneous class. Welding
procedures for aluminum armor plate are the same as for gas
metal-arc welding, which are discussed in the inert gas welding
operation subcourse.
(3) Face Hardened Armor Plate.
Face hardened armor plate has
an extremely hard surface layer which is obtained by carburizing.
(Carburizing is the process of combining carbon with another
alloy
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