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.
| 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°F., in oil, followed by tempering at a temperature of from 925 to 975°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°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.