Ultrahigh Strength Steels – ASTM A 538, Grade A, B, C -An Overview
When all the steels that we usually uses might do not meet desired strength requirements, then ultrahigh-strength steels can be employed. This includes alloy steels, tool steels, steels with a high concentration of alloying elements, and stainless steels. The through-hardening alloy steels that we recommended for general use, AISI 4140 and 4340, when heat treated to high strength levels, are considered to be ultra high strength steels. Two hot-work tool steels, AISI H11 and H13, have sufficient strength and toughness to be used for structural applications. The stainless steels will similarly be discussed in subsequent chapters. In the high-alloy category, a particularly useful group of steels in design is the 18% nickel maraging steels.
All these steels are available in some of the standard wrought forms, but the bulk of these steels are supplied as forged shapes. Many of these alloys are also melted using vacuum, VAR, and other special techniques aimed at improved cleanliness. The common denominator for ultrahigh-strength steels is high strength with measurable toughness. Most tool steels have high strengths, but they are so brittle that they are unusable except where bending is not expected. Their elongation in a tensile test is almost immeasurable. The strength and toughness characteristics of some ultrahigh-strength steels are shown in maraging nickel and stainless steels. These latter steels are usually supplied from mills in the martensitic or semiaustenitic condition, and hardening to maximum strength is accomplished on most grades by a low-temperature aging process (900 to 1150°F; 428 to 620°C). It is common to designate these alloys by tensile strength; a 300-ksi maraging steel is an 18% nickel steel with a minimum tensile strength of 300,000 psi. The more appropriate designation would be the ASTM specifications and grade: ASTM A 538, grade A, B, or C.
Experience in design shows that three alloys, 4340 and the 250- and 300-ksi grades of the 18% nickel maraging steels, will meet most needs. The 250-ksi maraging steel has solved innumerable breakage problems in injection-molding cavities. It also provides an extra measure of toughness for high-loaded shafts with unavoidable stress concentrations. It has solved problems on components with operating stresses over 100 ksi (689 MPa). The alloy steel 4340 is more available and cheaper to use when the service is not quite as severe.