This chapter contains the engineering properties and related characteristics of steels used in aircraft and missile structural applications. General comments on engineering properties and other considerations related to alloy selection are presented in Section 2.1. Mechanical and physical property data and characteristics pertinent to specific steel groups or individual steels are reported in Sections 2.2 through 2.7. Element properties are presented in Section 2.8.
The selection of the proper grade of steel for a specific application is based on material properties and on manufacturing, environmental, and economic considerations. Some of these considerations are outlined in the sections that follow.
The steel alloys listed in this chapter are arranged in major sections that identify broad classifications of steel partly associated with major alloying elements, partly associated with processing, and consistent generally with steel-making technology. Specific alloys are identified as shown in Table 2.1.1 (see the Steel Alloy Index at the top of this page).
One of the major factors contributing to the general utility of steels is the wide range of mechanical properties which can be obtained by heat treatment. For example, softness and good ductility may be required during fabrication of a part and very high strength during its service life. Both sets of properties are obtainable in the same material.
All steels can be softened to a greater or lesser degree by annealing, depending on the chemical composition of the specific steel. Annealing is achieved by heating the steel to an appropriate temperature, holding, then cooling it at the proper rate.
Likewise, steels can be hardened or strengthened by means of cold working, heat treating, or a combination of these.
Cold working is the method used to strengthen both the low-carbon unalloyed steels and the highly alloyed austenitic stainless steels. Only moderately high strength levels can be attained in the former, but the latter can be cold rolled to quite high strength levels, or “tempers.” These are commonly supplied to specified minimum strength levels.
Heat treating is the principal method for strengthening the remainder of the steels (the low-carbon steels and the austenitic steels cannot be strengthened by heat treatment). The heat treatment of steel may be of three types: martensitic hardening, age hardening, and austempering. Carbon and alloy steels are martensitic-hardened by heating to a high temperature, or “austenitizing,” and cooling at a recommended rate, often by quenching in oil or water. This is followed by “tempering,” which consists of reheating to an intermediate temperature to relieve internal stresses and to improve toughness.
The maximum hardness of carbon and alloy steels, quenched rapidly to avoid the nose of the isothermal transformation curve, is a function in general of the alloy content, particularly the carbon content. Both the maximum thickness for complete hardening or the depth to which an alloy will harden under specific cooling conditions, and the distribution of hardness can be used as a measure of a material’s hardenability.
A relatively new class of steels is strengthened by age hardening. This heat treatment is designed to dissolve certain constituents in the steel, then precipitate them in some preferred particle size and distribution. Since both the martensitic hardening and the age-hardening treatments are relatively complex, specific details are presented for individual steels elsewhere in this chapter.
Recently, special combinations of working and heat treating have been employed to further enhance the mechanical properties of certain steels. At the present time, the use of these specialized treatments is not widespread.
Another method of heat treatment for steels is austempering. In this process, ferrous steels are austenitized, quenched rapidly to avoid transformation of the austenite to a temperature below the pearlite and above the martensite formation ranges, allowed to transform isothermally at that temperature to a completely bainitic structure, and finally cooled to room temperature. The purpose of austempering is to obtain increased ductility or notch toughness at high hardness levels, or to decrease the likelihood of cracking and distortion that might occur in conventional quenching and tempering.
The strength properties presented are those used in structural design. The room-temperature properties are shown in tables following the comments for individual steels. The variations in strength properties with temperature are presented graphically as percentages of the corresponding room-temperature strength property, also described in Section 9.3.1 and associated subsections. These strength properties may be reduced appreciably by prolonged exposure at elevated temperatures.
The strength of steels is temperature-dependent, decreasing with increasing temperature. In addition, steels are strain rate-sensitive above about 600 to 800°F, particularly at temperatures at which creep occurs. At lower strain rates, both yield and ultimate strengths decrease.
The modulus of elasticity is also temperature-dependent and, when measured by the slope of the stress-strain curve, it appears to be strain rate-sensitive at elevated temperatures because of creep during loading. However, on loading or unloading at high rates of strain, the modulus approaches the value measured by dynamic techniques.
Steel bars, billets, forgings, and thick plates, especially when heat treated to high strength levels, exhibit variations in mechanical properties with location and direction. In particular, elongation, reduction of area, toughness, and notched strength are likely to be lower in either of the transverse directions than in the longitudinal direction. This lower ductility and/or toughness results both from the fibering caused by the metal flow and from nonmetallic inclusions which tend to be aligned with the direction of primary flow. Such anisotropy is independent of the depth-of-hardening considerations discussed elsewhere. It can be minimized by careful control of melting practices (including degassing and vacuum-arc remelting) and of hot-working practices. In applications where transverse properties are critical, requirements should be discussed with the steel supplier and properties in critical locations should be substantiated by appropriate testing.
The elongation values presented in this chapter apply in both the longitudinal and long transverse directions, unless otherwise noted. Elongation in the short transverse (thickness) direction may be lower than the values shown.
Steels (as well as certain other metals), when processed to obtain high strength, or when tempered or aged within certain critical temperature ranges, may become more sensitive to the presence of small flaws. Thus, as discussed in Section 1.4.12, the usefulness of high-strength steels for certain applications is largely dependent on their toughness. It is generally noted that the fracture toughness of a given alloy product decreases relative to increase in the yield strength. The designer is cautioned that the propensity for brittle fracture must be considered in the application of high-strength alloys for the purpose of increased structural efficiency.
Minimum, average, and maximum values, as well as coefficient of variation of plane-strain fracture toughness for several steel alloys, are presented in Table 2.1.2.1.3. These values are presented as indicative information and do not have the statistical reliability of room-temperature mechanical properties. Data showing the effect of temperature are presented in the respective alloy sections where the information is available.
| Alloy | Heat Treat Condition |
Product Form |
Orientationb | Yield Strength Range, ksi |
Product Thickness Range, inches |
Number of Sources |
Sample Size |
Specimen Thickness Range, inches |
KIc, ksi √in. | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. | Avg. | Min. | Coefficient of Variation |
|||||||||
| AerMet 100 | Anneal, HT to 280 ksi | Bar | L-R | 236–281 | 2.75–10 | 1 | 183 | 1 | 146 | 121 | 100 | 7.9 |
| AerMet 100 | Anneal, HT to 280 ksi | Bar | C-R | 223–273 | 2.75–10 | 1 | 156 | 1 | 137 | 112 | 90 | 8.5 |
| AerMet 100 | Anneal, HT to 290 ksi | Bar | L-R | 251–265 | 3–10 | 1 | 29 | 1 | 110 | 99 | 88 | 6.5 |
| AerMet 100 | Anneal, HT to 290 ksi | Bar | C-R | 250–268 | 3–10 | 1 | 24 | 1 | 101 | 88 | 73 | 9.7 |
| Custom 465 | H950 | Bar | L-Rc | 229–249 | 3–12 | 1 | 40 | 1–1.5 | 104 | 89 | 76 | 7.4 |
| Custom 465 | H950 | Bar | R-Lc | 231–246 | 3–12 | 1 | 40 | 1–1.5 | 94 | 82 | 73 | 6.4 |
| Custom 465 | H1000 | Bar | L-Rc | 212–227 | 3–12 | 1 | 40 | 1–1.5 | 131 | 120 | 108 | 5.2 |
| Custom 465 | H1000 | Bar | R-Lc | 212–225 | 3–12 | 1 | 40 | 1–1.5 | 118 | 109 | 100 | 3.7 |
| D6AC | 1650°F, Aus-Bay Quench 975°F, SQ 375°F, 1000°F 2+2 | Plate | L-T | 217 | 1.5 | 1 | 19 | 0.6 | 88 | 62 | 40 | 22.5 |
| D6AC | 1650°F, Aus-Bay Quench 975°F, SQ 400°F, 1000°F 2+2 | Plate | L-T | 217 | 0.8 | 1 | 103 | 0.6–0.8 | 92 | 64 | 44 | 18.9 |
| D6AC | 1650°F, Aus-Bay Quench 975°F, SQ 400°F, 1000°F 2+2 | Forging | L-T | 214 | 0.8–1.5 | 1 | 53 | 0.6–0.8 | 96 | 66 | 39 | 18.6 |
| D6AC | 1700°F, Aus-Bay Quench 975°F, OQ 140°F, 1000°F 2+2 | Plate | L-T | 217 | 0.8–1.5 | 1 | 30 | 0.6–0.8 | 101 | 92 | 64 | 8.9 |
| D6AC | 1700°F, Aus-Bay Quench 975°F, OQ 140°F, 1000°F 2+2 | Forging | L-T | 214 | 0.8–1.5 | 1 | 34 | 0.7 | 109 | 95 | 81 | 6.7 |
| 9Ni-4Co-0.20C | Quench and Temper | Hand Forging | L-T | 185–192 | 3.0 | 2 | 27 | 1.0–2.0 | 147 | 129 | 107 | 8.3 |
| 9Ni-4Co-0.20C | 1650°F, 1–2 Hr, AC, 1525°F, 1–2 Hr, OQ, −100°F, Temp | Forging | L-T | 186–192 | 3.0–4.0 | 3 | 17 | 1.5–2.0 | 147 | 134 | 120 | 8.5 |
| PH13-8Mo | H1000 | Forging | L-T | 205–212 | 4.0–8.0 | 3 | 12 | 0.7–2.0 | 104 | 90 | 49 | 21.5 |
| a These values are for information only. | ||||||||||||
| b Refer to Figures 1.4.12.3(a) and 1.4.12.3(b) for definition of symbols. | ||||||||||||
| c L-R also includes some L-T; R-L also includes some T-L. | ||||||||||||
The stress-strain relationships presented in this chapter are prepared as described in Section 9.3.2.
Axial-load fatigue data on unnotched and notched specimens of various steels at room temperature and at other temperatures are shown as S/N curves in the appropriate section. Surface finish, surface finishing procedures, metallurgical effects from heat treatment, environment and other factors influence fatigue behavior. Specific details on these conditions are presented as correlative information for the S/N curve.
The physical properties (ω, C, K, and α) of steels may be considered to apply to all forms and heat treatments unless otherwise indicated.
The effects of exposure to environments such as stress, temperature, atmosphere, and corrosive media are reported for various steels. Fracture toughness of high-strength steels and the growth of cracks by fatigue may be detrimentally influenced by humid air and by the presence of water or saline solutions. Some alleviation may be achieved by heat treatment and all high-strength steels are not similarly affected.
In general, these comments apply to steels in their usual finished surface condition, without surface protection. It should be noted that there are available a number of heat-resistant paints, platings, and other surface coatings that are employed either to improve oxidation resistance at elevated temperature or to afford protection against corrosion by specific media. In employing electrolytic platings, special consideration should be given to the removal of hydrogen by suitable baking. Failure to do so may result in lowered fracture toughness or embrittlement.