MIL-HDBK-5H 1 December 1998

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1 The high-alloy steels in this section are those steels that are substantially higher in alloy content than the intermediate alloy steels described in Section 2.4 but are not stainless steels. The 18 Ni maraging and AF141 steels are in this category. 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.3 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, 9F. AF141 is an iron base alloy with nominally 14 percent cobalt, 1 percent nickel, 2 percent chromium, 1 percent molybdenum, and.15 percent carbon. When quenched from austenitizing temperatures, AF141 forms a highly dislocated lath martensitic structure with very little twinning or retained austenite. At aging temperatures ranging from 9 to 1F, 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 (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..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 2 ksiin. 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 1 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 M 2 C type carbides; alloys AF141 and AerMet 1. The effect of low carbon is indicated between the AF141 and.2af141 where the carbon levels are.15 and.2%, 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 434 and 3M. A slight anisotropic effect was observed for Hy-Tuf (TL vs LT); however, the effect was not apparent for AF141. 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). 2-92

2 12 1.2C mod AF 141 AF 141 (plate and billet) K Ι (ksi-sqrt in.) 8 6 Hy-Tuf(LT) AerMet 1 AF 141 (3.75" x 5.25" Bar) 4 3M Hy-Tuf(TL) 2 434ESR Time to Failure (hours)! 2-93

3 The 25 and 28 (3) maraging steels are normally supplied in the annealed condition and are heat treated to high strengths, without quenching, by aging at 9F. The steels are characterized by high hardenability and high strength combined with good toughness. The 25 and 28 (3) 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-vacuummelted quality grades are considered in this section. Manufacturing Considerations The 25 and 28 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 25 and 28 grades is not unlike 433 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 9F 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 25 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 (a). The room temperature properties for material aged at 9F are shown in Tables (b) and (c), and the effect of temperature on physical properties is shown in Figure " # $ %& #' $ % Grade Specification Form 25 AMS 652 Sheet and plate 25 AMS 6512 Bar 28 (3) AMS 6521 Sheet and plate 28 (3) AMS 6514 Bar Effect of temperature on 25 and 28 grade maraging steel is presented in Figures through Figures (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 (c) and (d). Figure (e) is a full-range stress-strain curve at room temperature for 28 grade maraging steel. 2-94

4 Specification... AMS 652 AMS 6512 Form... Sheet Plate Bar Condition... Maraged at 9F Maraged at 9F Thickness or diameter, in >.25 < Basis... S S S S S Mechanical Properties: F tu ksi: L T F ty, ksi: L T F cy, ksi: L T F su, ksi F bru, ksi: (e/d = 1.5) (e/d = 2.) F bry, ksi: (e/d = 1.5) (e/d = 2.) e, percent: L T... a a a 4 3 RA, percent: L T E,1 3 ksi E c, 1 3 ksi: L T G, 1 3 ksi µ Physical Properties:, lb/in C, K, and... See Figure a Elongation properties vary with thickness as follows:.9 2.5% % % % % 2-95

5 " # ( $ )* )& ' $ % Specification... AMS 6521 AMS 6514 Form... Sheet Plate Bar Condition... Maraged at 9F Maraged at 9F Thickness or diameter, in >.25 < Basis... S S S S S Mechanical Properties: F tu ksi: L T F ty, ksi: L T F cy, ksi: L T F su, ksi F bru, ksi: (e/d = 1.5) (e/d = 2.) F bry, ksi: (e/d = 1.5) (e/d = 2.) e, percent: L a a a T RA, percent: L T E,1 3 ksi E c, 1 3 ksi: L T G, 1 3 ksi µ Physical Properties:, lb/in C, K, and... See Figure a Elongation properties vary with thickness as follows:.9 2.5% % % % % 2-96

6 . MIL-HDBK-5H 7 6 K, Btu/[(hr)(ft 2 )(F)/ft] K 25 & 28 α α, 1-6 in./in./f.3 α - Between 7F and indicated Temperature 28 K - At indicated Temperature C - At indicated Temperature 2 C, Btu/(lb)(F) C Temperature, F # + & &* && ' 2-97

7 2 18 Strength at temperature Exposure up to 1/2 hr Percentage of Room Temperature Strength F tu F tu 6 F ty Temperature, F # # # + & * ' & 2-98

8 1 8 F cy F su Strength at temperature Exposure up to 1/2 hr Percentage of Room Temperature Strength 6 4 F cy Temperature, F # # + & & * ' & 1 F bru Strength at temperature Exposure up to 1/2 hr 8 F bry Percentage of Room Temperature Strength Temperature, F # # + & " " * ' & 2-99

9 2 18 Modulus at temperature Exposure up to 1/2 hr TYPICAL Percent E & E c at Room Temperature Temperature, F # #, + & & + + ' 2-1

10 3 Longitudinal 1/2-hr exposure -1 F RT 24 3 F 6 F 8 F 18 Stress, ksi F Ramberg - Osgood n (-1 F) = 24 n (RT) = 26 n (3 F) = 29 n (6 F) = 26 n (8 F) = 11 n (1 F) = 11 TYPICAL Strain,.1 in./in. # # - *& & " 36 3 Longitudinal 1/2-hr exposure RT -1 F 24 3 F 6 F 8 F Stress, ksi 18 1 F 12 6 Ramberg - Osgood n (-1 F) = 19 n (RT) = 22 n (3) = 17 n (6) = 17 n (8) = 12 n (1) = 11 TYPICAL Strain,.1 in./in. # # -" *& & ' " 2-11

11 35 3 Longitudinal 25 Stress, ksi Ramberg - Osgood n = 22 TYPICAL Strain,.1 in./in Compressive Tangent Modulus, 1 3 ksi # # - *& & & " & 2-12

12 35 3 Longitudinal 25 Stress, ksi Ramberg - Osgood n = 21 TYPICAL Strain,.1 in./in Compressive Tangent Modulus, 1 3 ksi # # - *& & & ' " & 2-13

13 3 Longitudinal 25 2 Stress, ksi TYPICAL Strain, in./in. # # - *& ' " & 2-14