The 18 Ni maraging steels are iron base alloys with nominally 18 percent nickel, 7 to 9 percent cobalt, 3 to 5 percent molybdenum, less than 1 percent titanium, and very low carbon content, below 0.03 percent. Upon cooling from the annealing or hot-working temperature, these steels transform to a soft martensite which can be easily machined or formed. The steels can be subsequently aged (maraged) to high strengths by heating to a lower temperature, 900°F.

AF1410 is an iron base alloy with nominally 14 percent cobalt, 10 percent nickel, 2 percent chromium, 1 percent molybdenum, and 0.15 percent carbon. When quenched from austenitizing temperatures, AF1410 forms a highly dislocated lath martensitic structure with very little twinning or retained austenite. At aging temperatures ranging from 900 to 1000°F, a precipitation of extremely fine alloy carbide containing chromium and molybdenum occurs, which simultaneously develops strength and toughness properties.

The stress corrosion cracking resistance of high strength steels is of concern for highly loaded structural components such as landing gears and wing attach fittings that are subjected to corrosive environments such as sea spray or water. Figure 2.5.0.2(a) indicates the relative stress corrosion cracking resistance of several high-strength steel alloys. The data in this figure were obtained from Reference 2.5.0.2. The stress corrosion cracking threshold stress intensity (K_Issc) for each steel was defined as the value at which cracking did not occur. For most of these alloys, this value is about 20 ksi√in. As indicated, there is a definite difference in the stress corrosion resistance between the alloys.

In general, the high-strength steels do not reach a true threshold stress intensity until after 1000 hours of exposure. The highest stress corrosion cracking resistance in high-strength steels is associated with low carbon levels and lath martensite microstructure containing a fine distribution of M₂C type carbides; alloys AF1410 and AerMet 100. The effect of low carbon is indicated between the AF1410 and 0.20AF1410 where the carbon levels are 0.15 and 0.20%, respectively. The lower stress cracking corrosion resistance is associated with higher carbon and the martensite is of plate morphology that exhibits a twinned structure; alloys 4340 and 300M. A slight anisotropic effect was observed for Hy-Tuf (TL vs LT); however, the effect was not apparent for AF1410. The differences in anisotropic properties may be due to differences in the cleanliness of the steels since Hy-Tuf was an air melted product and the others were either vacuum induction melted (VIM) or electroslag remelted (ESR).

The 250 and 280 (300) maraging steels are normally supplied in the annealed condition and are heat treated to high strengths, without quenching, by aging at 900°F. The steels are characterized by high hardenability and high strength combined with good toughness. The 250 and 280 (300) designation refers to the nominal yield strengths of the two alloys. The two alloys are available in the form of sheet, plate, bar, and die forgings. Only the consumable electrode-vacuum-melted quality grades are considered in this section.

Manufacturing Considerations. The 250 and 280 grades are readily hot worked by conventional rolling and forging operations. These grades also have good cold forming characteristics in spite of the relatively high hardness in the annealed (martensitic) condition. The machinability of the 250 and 280 grades is not unlike 4330 steel at equivalent hardness. The 18 Ni maraging steels can be readily welded in either the annealed or aged conditions without preheating. Welding of aged material should be followed by aging at 900°F to strengthen the weld area.

Environmental Considerations. Although the 18 Ni maraging steels are high in alloy content, these grades are not corrosion resistant. Since the general corrosion resistance is similar to the low-alloy steels, these steels require protective coatings. The 250 grade reportedly has better resistance to stress corrosion cracking than the low-alloy steels at the same strength.

Specifications and Properties. Material specifications for these steels are shown in Table 2.5.1.0(a). The room temperature properties for material aged at 900°F are shown in Tables 2.5.1.0(b) and (c), and the effect of temperature on physical properties is shown in Figure 2.5.1.0.

Effect of temperature on 250 and 280 grade maraging steel is presented in Figures 2.5.1.1.1 through 2.5.1.1.4. Figures 2.5.1.1.6(a) and (b) are room and elevated temperature tensile stress-strain curves. Typical compressive stress-strain and tangent-modulus curves at room temperature are presented in Figures 2.5.1.1.6(c) and (d). Figure 2.5.1.1.6(e) is a full-range stress-strain curve at room temperature for 280 grade maraging steel.

AF1410 alloy was developed specifically to have high strength, excellent fracture toughness, and excellent weldability when heat treated to 235 to 255 ksi ultimate tensile strength. AF1410 has good weldability and does not require preheating prior to welding. The alloy maintains good toughness at cryogenic temperatures, as well as high strength and stability at temperatures up to 800°F. The alloy is available in a wide variety of sizes and forms, including billet, bar, plate, and die forgings. The alloy is produced by vacuum induction melting followed by vacuum remelting.

Heat Treatment. The heat treatment for this alloy consists of heating to 1650 ± 25°F for 1 hour, forced-air cooling to room temperature, reheating to 1525 ± 25°F for 1 hour, forced-air cooling to room temperature, cooling to −100 ± 15°F, holding at temperature for 1 hour, warming to room temperature, and aging at 950 ± 10°F for 5 hours, and air cooling. A forced-air cool from austenitizing temperatures should be used for section thicknesses up to 2 inches. For sections of greater thickness, an oil quench should be utilized. A single austenitizing treatment (1525 ± 25°F) can be used to minimize heat treating distortion with a resulting slight decrease in fracture toughness.

Environmental Considerations. AF1410 has general corrosion resistance similar to the maraging steels. It should not be used in the unprotected condition. The alloy is highly resistant to stress-corrosion cracking compared to other high-strength steels.

Specification and Properties. A material specification for AF1410 is presented in Table 2.5.2.0(a). Room temperature mechanical properties are shown in Table 2.5.2.0(b).

Typical stress-strain curves at room temperature are shown in Figures 2.5.2.1.6(a) and (b).

AerMet 100 is a higher strength derivative of AF1410. The Ni-Co-Fe alloy can be heat treated to 280–300 ksi or to 290–310 ksi tensile strength while exhibiting excellent fracture toughness and high resistance to stress-corrosion cracking. AerMet 100 has good weldability and does not require preheating prior to welding. AerMet 100 is available in a wide variety of sizes and forms including billet, bar, sheet, strip, plate, wire, and die forgings. The alloy is produced by vacuum induction melting followed by vacuum-arc remelting.

Heat Treatment. This alloy can be heat treated to several strength levels. Consult the applicable materials specification for specific procedures.

Environmental Considerations. AerMet 100 is not considered corrosion resistant; consequently, parts should be protected with a corrosion resistant coating. The alloy is highly resistant to stress corrosion cracking compared to other high-strength steels of the same strength level. This alloy displays good toughness at cryogenic temperatures as well as high strength and stability at temperatures up to 800°F.

Specification and Properties. A material specification for AerMet 100 is shown in Table 2.5.3.0(a). Room temperature mechanical properties are presented in Table 2.5.3.0(b) for both heat treated conditions.

Typical stress-strain curves at room temperature are shown in Figures 2.5.3.1.6(a) and (b). A full-range tensile stress-strain curve is presented in Figure 2.5.3.1.6(c).

Typical tensile and compression stress-strain curves and compression tangent-modulus curves at room temperature are shown in Figures 2.5.3.2.6(a) and (b). A full-range tensile stress-strain curve is presented in Figure 2.5.3.2.6(c).