Developed primarily for high-strength applications, these steels are usually heat-treated alloys that provide strengths at least equal to those of as-rolled steel. Heat-treated constructional alloy steels and the ultrahigh-strength steels are used in applications where high strength can be converted to a weight-saving advantage over other steels.

High-yield-strength, quenched-and-tempered constructional alloy steels are usually heat treated at the mill to develop their properties so they require no further heat treatment by the fabricator. Although these heat-treated alloy steels are available in all conventional product forms, they are most common in plate products. Some grades are also available as abrasion-resistant (AR) modifications. In these conditions, high hardness is the desired property, with some toughness being sacrificed. Over 20 types of these proprietary high-strength alloy steels are produced. Some have been developed to combine improved welding characteristics along with high strength. Most have good impact properties in addition to high strength. An example of the high-yield-strength grades in this class is HY-80/100, which is used for naval vessels. This material combines high strength and toughness with weldability.

Ultrahigh-strength steels start with grade 4340 and are modifications of this alloy. When these steels are used for aerospace components, they are usually produced by the vacuum-arc-remelt (VAR) process. Steels commonly considered to be in the ultrahigh-strength category have yield strengths greater than 180,000 psi. They are classified into several broad categories based on chemical composition or metallurgical-hardening mechanisms.

Medium-carbon alloy steels are generally modifications of grade 4330 or 4340 (usually with increased molybdenum, silicon, and/or vanadium). These grades provide excellent hardenability in thick sections.

Modified tool steels of the 5% Cr, 1% Mo, 1% V hot-work die-steel variety (H11 modified, H13) provide the next step in increased hardenability and greater strength. Most steels in this group are air hardened in moderate to large sections and, therefore, are not likely to distort or quench crack. Structural uses of these steels are not as widespread as they once were, mainly because of the development of other steels costing about the same but offering greater fracture toughness.

Maraging steels contain 18% nickel, along with appreciable amounts of molybdenum, cobalt, and titanium, and almost no carbon. These alloys can be strengthened significantly by a precipitation reaction at a relatively low temperature. They can be formed and machined in the solution-annealed condition but not without difficulty. Weldability is excellent. They can be heat treated to 250 to 300-ksi yield strength with a simple 900癋 aging treatment. Fracture toughness of the maraging steels is considerably higher than that of the conventional high-strength steels.

Maraging steels are used in a variety of high-performance applications, and most extensively in aircraft and tooling components.

The 9% Ni, 4% Co alloys were designed to provide high strength and toughness at room temperature as well as at moderately elevated temperatures -- to about 800癋. Weldability and fracture toughness are good, but the alloys are susceptible to hydrogen embrittlement. These steels are used in airframes, gears, and large aircraft parts.