High-Strength Steels for Aerospace Forgings

The selection of material for a forged part usually requires some compromise between opposing factors - for instance, strength vs toughness, stress-corrosion resistance vs weight, manufacturing cost vs useful load-carrying ability, production cost vs maintenance cost, and so on. Material selection also involves consideration of melting practices, forming methods, machining operations, heat treating procedures, deterioration of properties with time in service, as well as the conventional mechanical and chemical properties of the alloy to be forged.

Structural material for forgings is first appraised for selection on the basis of strength at room temperature - either yield strength or tensile strength. For airborne vehicles, a more meaningful comparison is strength per unit weight, commonly designated as strength-density ratio. To match a material to its design component, the material is first appraised for strength toughness, and then qualified for stability to temperature and environment.

Requirements of a forged component include:

1. Pattern of Applied Load
   • Uniaxial Loads. Tensile or compressive, or reversible with changes in operation conditions.
   • Multiaxial or Combined Loads. Tensile, compressive, shear, bending, torsion and bearing. Stress concentration should be minimized in design by specifying smooth, contoured fillets at changes of configuration. Where stress concentration cannot be avoided, notch toughness of the material is usually important in material selection.
   • Cyclic Loads. These may be either high-cycle or low-cycle loads.
   • Sustained Loads. If these loads are tensile, they may accelerate stress corrosion. Interference fits and residual stress may give rise to sustained loading.
   • Thermal Loads. These are caused by variations in temperature.
2. Load Magnitudes and Conditions of Loading
   • Magnitudes
   • Rate of Load Application. Gradual or impact.
   • Temperature. The major time accumulations should be estimated for minimum, normal and maximum temperatures.
   • Environment. Cyclic periods of atmospheric condensation, chemical composition of environment, circumstances of corrosion, abrasion, erosion or other wear.
3. Special mechanical, physical or chemical requirements
4. Life Expectancy or Reliability

High-Strength Steels are applied for aerospace structural forgings such as landing-gear components, rocket cases and airframe fittings. The dividing line above which a steel is designated "high-strength" is commonly regarded as 180 ksi (~1200 MPa) yield strength.

Alloy content in these steels ranges from a few percent up to about one-third the weight of the steel. Alloying elements are added to prevent or retard the formation of nonmartensitic microconstituents during quenching. The maximum attainable strength level is determined by the carbon content. In order to improve the ductility and toughness of hardened steel, it is reheated for a relatively short time at the moderate temperature.

Many high-strength steels are variations of 4340 (SAE). Small or light forgings are also produced from 4330 (SAE), but the strength required in large airframe forgings is often supplied by 4340 or its modifications that are formed by adding silicon (1.45% to 1.80%) and vanadium (min. 0.05%).

Steel 5Cr-Mo-V, quality version of H11 (ASTM), which contains alloying elements in amounts greater then in 4340, can be transformation hardened by air cooling instead of liquid quenching. The slower cooling rate permits improved control of distortion and residual stresses, compared with conventional oil or water quenching.

Heat treatment of steels with 9% Ni and 4% Cr depends on carbon content. Steels with 0.2%-0.3% C are heat treated by oil quenching followed by subzero cooling and double tempering. This treatment produces a tempered martensite microstructure.

Steels with 0.45% carbon are recommended for isothermal transformation to produce bainitic microstructure, which is nominally tougher than martensite at a given strength.

Stainless steels are used in high-strength applications where corrosion resistance is ta controlling factor. The martensitic, age-hardenable martensitic and semi-finished grades are used for small forgings. Austenitic stainless steels that depend on cold working for hardness are not useful high-strength forgings.

High-strength steels are especially sensitive to nonmetallic inclusions because these inclusions became stress raisers that reduce fracture toughness and related measures of ductility. To minimize the content of nonmetallic inclusions, high-strength steels are tipically remelted under vacuum. Microscopic examination confirms a decrease in nonmetallic inclusions in the vacuum-remelted steel.

Vacuum remelting of steel also permits improved deoxidation. In a vacuum furnace, carbon is an excellent deoxidizer, and the deoxidation product, carbon monoxide gas, is continuously removed under vacuum. When the cast electrodes of air-melted steel are remelted under vacuum and deoxidized with carbon, the product has significantly improved purity.

In high-strength steels, high strength is attained at the expence of reduced ductility. The most common mechanical tests to assure that forgings of high strength steel will meet ductility and toughness requirements are:

1. The reduction in area and percent elongation in a transverse tension-test specimen
2. Charpy impact tests on both standard and pre-cracked specimens taken from the forged material.

Flaws and/or cracks within a material often create complex stress state. Depending on the material and its relative strength, a "critical crack length" or "critical flaw size" is required at a particular stress level for crack propagation. This critical crack length has decreased with the development of the high-strength steels.

High-strength steels that contain a microscopic flaw or crack or other local stress concentration may fall in a brittle or catastrophic manner when the base metal has considerable ductility. Fracture toughness testing attempts to predict the behavior of a material under conditions in which a flaw or notch is present. The ultimate goal is to determine the critical flaw size below which brittle fracture will not be occur, for a structure of specified design, using materials with a specified strength level, and with a specified design load.

High-strength steels generally are suitably protected from exfoliation corrosion by the application of organic finishes or metallic plating. However, these steels are sometimes subject to stress-corrosion or hydrogen stress cracking; then other preventives are needed, in addition to coatings.

Both types of cracking occur in a plane normal to the direction of stress or load, thereby reducing the design section, and both types proceed with the enlargement of flaws to accelerate further cracking and eventual brittle fracture. Factors that affect the stress-corrosion susceptibility of high-strength steels are their composition, structure, strength level, applied stress, residual stress, environment and time.