FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 4. Metal Structure, Welding, and Brazing | 1. Identification of Metals | 4-2. Identification of Steel Stock
9/27/01
AC 43.13-1B CHG 1
CHAPTER 4. METAL STRUCTURE, WELDING, AND BRAZING
SECTION 1. IDENTIFICATION OF METALS
4-1. GENERAL. Proper identification of
the aircraft structural material is the first step
in ensuring that the continuing airworthiness
of the aircraft will not be degraded by making
an improper repair using the wrong materials.
a. Ferrous (iron) alloy materials are
generally classified according to carbon con
tent. (See table 4-1.)
TABLE 4-1. Ferrous (iron) alloy materials.
MATERIALS
Wrought iron
Low carbon steel
Medium carbon steel
High carbon steel
Cast iron
CARBON CONTENT
Trace to 0.08%
0.08% to 0.30%
0.30% to 0.60%
0.60% to 2.2%
2.3% to 4.5%
b. The strength and ductility, or tough
ness of steel, is controlled by the kind and
quantity of alloys used and also by
cold-working or heat-treating processes used
in manufacturing. In general, any process that
increases the strength of a material will also
decrease its ductility.
c. Normalizing is heating steel to ap
proximately 150 °F to 225 °F above the steel’s
critical temperature range, followed by cooling
to below that range in still air at ordinary tem
perature. Normalizing may be classified as a
form of annealing. This process also removes
stresses due to machining, forging, bending,
and welding. After the metal has been held at
this temperature for a sufficient time to be
heated uniformly throughout, remove the metal
from the furnace and cool in still air. Avoid
prolonging the soaking of the metal at
high temperatures, as this practice will cause
the grain structure to enlarge. The length of
time required for the soaking temperature de
pends on the mass of the metal being treated.
The soaking time is roughly ¼ hour per inch of
the diameter of thickness (Ref: Military Tech
Order (T.O.) 1-1A-9).
4-2. IDENTIFICATION OF STEEL
STOCK. The Society of Automotive Engi
neers (SAE) and the American Iron and Steel
Institute (AISI) use a numerical index system
to identify the composition of various steels.
The numbers assigned in the combined listing
of standard steels issued by these groups repre
sent the type of steel and make it possible to
readily identify the principal elements in the
material.
a. The basic numbers for the four digit
series of the carbon and alloy steel may be
found in table 4-2. The first digit of the four
number designation indicates the type to which
the steel belongs. Thus, “1” indicates a carbon
steel, “2” a nickel steel, “3” a nickel chromium
steel, etc. In the case of simple alloy steels, the
second digit indicates the approximate per
centage of the predominant alloying element.
The last two digits usually indicate the mean of
the range of carbon content. Thus, the desig
nation “1020” indicates a plain carbon steel
lacking a principal alloying element and con
taining an average of 0.20 percent
(0.18 to 0.23) carbon. The designation “2330”
indicates a nickel steel of approximately
3 percent (3.25 to 3.75) nickel and an average
of 0.30 percent, (0.28 to 0.33) carbon content.
The designation “4130” indicates a chromium-
molybdenum steel of approximately 1 percent
(0.80 to 1.10) chromium, 0.20 percent
(0.15 to 0.25) molybdenum, and 0.30 percent
(0.28 to 0.33) carbon.
Par 4-5
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