Highly Stressed Parts





The highly stressed parts on the Liberty engine consisted of the

connecting-rod bolt, the main-bearing bolt, the propeller-hub key,

etc. The material used for parts of this type was selected at the

option of the manufacturer from standard S. A. E. steels, the

composition of which are given in Table 11.



TABLE 11.--COMPOSITION OF S. A. E. STEELS Nos. 2,330, 3,135 AND 6,130



Steel No 2,330 3,135 6,130

Carbon, minimum 0.250 0.300 0.250

Carbon, maximum 0.350 0.400 0.450

Manganese, minimum 0.500 0.500 0.500

Manganese, maximum 0.800 0.800 0.800

Phosphorus, maximum 0.045 0.040 0.040

Sulphur, maximum 0.045 0.045 0.045

Nickel, minimum 3.250 1.000

Nickel, maximum 3.750 1.500

Chromium, minimum 0.450 0.800

Chromium, maximum 0.750 1.100

Vanadium, minimum 0.150



All highly stressed parts on the Liberty engine must show, after

heat treatment, the following minimum physical properties: Elastic

limit, 100,000 lb. per square inch; elongation in 2 in., 16 per

cent; reduction of area, 45 per cent; scleroscope hardness, 40

to 50.



The heat treatment employed to obtain these physical properties

consisted in quenching from a temperature of 1,525 to 1,575 deg.F., in

oil, followed by tempering at a temperature of from 925 to 975 deg.F.



Due to the extremely fine limits used on all threaded parts for

the Liberty engine, a large percentage of rejection was due to

warpage and scaling of parts. To eliminate this objection, many

of the Liberty engine builders adopted the use of heat-treated

and cold-drawn alloy steel for their highly stressed parts. On

all sizes up to and including 3/8 in. in diameter, the physical

properties were secured by merely normalizing the hot-rolled bars

by heating to a temperature of from 1,525 to 1,575 deg.F., and cooling

in air, followed by the usual cold-drawing reductions. For parts

requiring stock over 3/8 in. in diameter, the physical properties

desired were obtained by quenching and tempering the hot-rolled bars

before cold-drawing. It is the opinion that the use of heat-treated

and cold-drawn bars is very good practice, provided proper inspection

is made to guarantee the uniformity of heat treatment and, therefore,

the uniformity of the physical properties of the finished parts.



The question has been asked many times by different manufacturers, as

to which alloy steel offers the best machineability when heat-treated

to a given Brinell hardness. The general consensus of opinion among

the screw-machine manufacturers is that S. A. E. No. 6,130 steel

gives the best machineability and that S. A. E. No. 2,330 steel

would receive second choice of the three specified.



In the finishing of highly stressed parts for aviation engines,

extreme care must be taken to see that all tool marks are eliminated,

unless they are parallel to the axis of strain, and that proper

radii are maintained at all changes of section. This is of the

utmost importance to give proper fatigue resistance to the part

in question.





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