2.4.0
COMMENTS ON INTERMEDIATE ALLOY STEELS — The intermediate alloy steels in this section are those steels that are substantially higher in alloy content than the alloy steels described in Section 2.3, but lower in alloy content than the stainless steels. Typical of the intermediate alloy steels is the 5Cr-Mo-V aircraft steel and the 9Ni-4Co series of steels.
2.4.0.1
Metallurgical Considerations — The alloying elements added to these steels are similar to those used in the lower alloy steels and, in general, have the same effects. The difference lies in the quantity of alloying additions and the extent of these effects. Thus, higher chromium contents provide improved oxidation resistance. Additions of molybdenum, vanadium, and tungsten, together with the chromium, provide deep air-hardening properties and improve the elevated-temperature strength by retarding the rate of tempering at high temperatures. Additions of nickel to nonsecondary hardening steels lower the transition temperature and improve low-temperature toughness.
2.4.1 5Cr-Mo-V
2.4.1.0 Comments and Properties — Alloy 5Cr-Mo-V aircraft steel exhibits high strength in the temperature range up to 1000 °F. Its characteristics also include air hardenability in thick sections; consequently, little distortion is encountered in heat treatment. This steel is available either as air-melted or consumable electrode vacuum-melted quality although only consumable electrode vacuum-melted quality is recommended for aerospace applications.
The heat treatment recommended for this steel consists of heating to 1850°F ± 50, holding 15 to 25 minutes for sheet or 30 to 60 minutes for bars depending on section size, cooling in air to room temperature, tempering three times by heating to the temperature specified in Table 2.4.1.0(a) for the strength level desired, holding at temperature for 2 to 3 hours, and cooling in air.
| Ftu, ksi | Temperature, °F | Hardness, Rc |
|---|---|---|
| 280 | 1000 ± 10 | 54-56 |
| 260 | 1030 ± 10 | 52-54 |
| 240 | 1050 ± 10 | 49-52 |
| 220 | 1080 ± 10 | 46-49 |
Material specifications for 5Cr-Mo-V aircraft steel are presented in Table 2.4.1.0(b). The room-temperature mechanical and physical properties are shown in Tables 2.4.1.0(c) and (d). The mechanical properties are for 5Cr-Mo-V steel heat treated to produce a structure containing 90 percent or more martensite at the center prior to tempering.