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.

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.

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.

The room-temperature properties of 5Cr-Mo-V aircraft steel are affected by extended exposure to temperatures near or above the tempering temperature. The limiting temperature to which the alloy may be exposed for extended periods without significantly affecting its room-temperature properties may be estimated at 100°F below the tempering temperature for the desired strength level. The effect of temperature on the physical properties is shown in Figure 2.4.1.0.

The effect of temperature on various mechanical properties for heat-treated 5Cr-Mo-V aircraft steel is presented in Figures 2.4.1.1.1(a) through 2.4.1.1.4.

The 9Ni-4Co-0.20C alloy was developed specifically to have excellent fracture toughness, excellent weldability, and high hardenability when heat-treated to 190 to 210 ksi ultimate tensile strength. The alloy can be readily welded in the heat-treated condition with preheat and post-heat usually not required. The alloy is through hardening in section sizes up to at least 8 inches thick. The alloy may be exposed to temperatures up to 900°F (approximately 100°F below typical tempering temperature) without microstructural changes which degrade room temperature strength.

The heat treatment for this alloy consists of normalizing at 1650 ± 25°F for 1 hour per inch of cross section, cooling in air to room temperature, heating to 1525 ± 25°F for 1 hour per inch of cross section, quenching in oil or water, hold at −100 ± 20°F for 2 hours within 2 hours after quenching, and double tempering at 1035 ± 10°F for 2 hours.

A material specification for 9Ni-4Co-0.20C steel is presented in Table 2.4.2.0(a). Room temperature mechanical and physical properties are shown in Table 2.4.2.0(b). The effect of temperature on thermal expansion is shown in Figure 2.4.2.0.

Effect of temperature on various mechanical properties is presented in Figures 2.4.2.1.1, 2.4.2.1.2, and 2.4.2.1.4. Typical tensile stress-strain curves at room and elevated temperatures are shown in Figure 2.4.2.1.6(a). Typical compression stress-strain and tangent-modulus curves are presented in Figure 2.4.2.1.6(b).

The 9Ni-4Co-0.30C alloy was developed specifically to have high hardenability and good fracture toughness when heat treated to 220 to 240 ksi ultimate tensile strength. The alloy is through hardening in section sizes up to 4 inches thick. The alloy may be exposed to temperatures up to 900°F (approximately 100°F below typical tempering temperature) without microstructural changes which degrade room temperature strength. This grade must be formed and welded in the annealed condition. Preheat and post-heat of the weldment is required. The steel is produced by consumable electrode vacuum melting.

The heat treatment for this alloy consists of normalizing at 1650 ± 25°F for 1 hour per inch of cross section, cooling in air to room temperature, heating to 1550 ± 25°F for 1 hour per inch of cross section but not less than 1 hour, quenching in oil or water, subzero treating at −100°F for 1 to 2 hours, and double tempering at 975 ± 10°F (sheet, strip, and plate) or 1000 ± 10°F (bars, forgings, and tubings) for 2 hours.

Material specifications for 9Ni-4Co-0.30C steel are presented in Table 2.4.3.0(a). The room temperature mechanical and physical properties are shown in Table 2.4.3.0(b). The effect of temperature on thermal expansion is shown in Figure 2.4.3.0.

Effect of temperature on various mechanical properties is presented in Figures 2.4.3.1.1 through 2.4.3.1.4. Typical stress-strain and tangent-modulus curves are presented in Figures 2.4.3.1.6(a) through (d). Notched fatigue data at room temperature are illustrated in Figure 2.4.3.1.8.