Ultrahigh strength, low alloy, steels with a medium carbon (0·25–0·50 wt-%) content and various amounts of chromium, molybdenum, nickel, silicon, and vanadium have been needed for high performance aerospace parts. The steels can be successfully employed at yield strengths of ≥1400 MPa, but their commercial use is often limited in practice by their poor fracture toughness. The present review surveys studies carried out over about the past 10 years to improve the fracture toughness of ultrahigh strength, medium carbon, low alloy steels, and the present understanding of this property is outlined. Particular emphasis is placed on improving the fracture toughness by microstructural control via thermal and thermomechanical treatments, modification of sulphide inclusions, and new alloying design. The major metallurgical factors controlling the fracture toughness are discussed for each of these techniques.

Development of fracture toughness of ultrahigh strength, medium carbon, low alloy steels for aerospace applications

A study has been made of the effect of the microstructure on the mechanical properties of commercial-aircraft quality 300M steel which has been isothermally transformed in various bainitic temperature regions (593–673 K) after austenitization at 1173 K. The results were compared with those obtained by conventional quenching and tempering. Isothermal transformation of the steel at and below 623 K produced a dispersion of retained austenite (12–18 vol.%) in a carbon-free upper bainite matrix. The treatment improved the K I c value, owing to an increase in the Charpy impact energy; however, no improvement in strength was observed. As aresult of the isothermal transformation at and above 648 K, the retention of a large amount of retained austenite (22–25 vol.%) was encouraged, in conjunction with carbon-free upper-bainite; however, this was found to have a very detrimental effect on the strength and toughness. The results are described and discussed in terms of the microstructure and fractography.

Effect of microstructure on mechanical properties of isothermally bainite-transformed 300M steel

Wear characteristics of nano TiAlN-coated carbide tools in ultra-high speed machining of AerMet100

The effects of modified heat treatment (MHT) on the mechanical properties of 300M steel have been studied to assess MHT steel for possible ultrahigh strength applications. The microstructure of MHT steel has variable amounts of martensite, carbide free upper bainite, and retained austenite. This is produced by partial isothermal transformation at 593, 623, or 673 Kfor the required times, followed by oil quenching and subsequent tempering at 473 K after 1173 K austenitisation. The optimum combination of plane strain fracture toughness with other relevant mechanical properties was obtained when 50 vol.-% bainite was associated with tempered martensite and retained austenite for MHT at 593 and 623 K. The MHT steel transformed at 593 K showed improved plane strain fracture toughness over conventional quenched and tempered steel at a similar tensile strength level, with little change of percentage elongation and Charpy 2 mm V-notch impact energy. Compared with conventional quenched and tempered steel, ...

Effect of modified heat treatment on mechanical properties of 300M steel

Using the first principles full-potential linear-augmented-Slater-type orbital technique, the energies and charge densities of aluminum and aluminum-lithium supercells have been computed. The experimentally observed increase in aluminum's shear moduli upon alloying with lithium is argued to be the result of predictable changes to aluminum's total charge density, suggesting that simple rules may allow the alloy designer to predict the effects of dilute substitutional elements on alloy elastic response.

Charge-density-shear-moduli relationships in aluminum-lithium alloys.

An evaluation is made of the development status and prospective performance advantages and cost reductions obtainable through the application of state-of-the-art Al-Li alloys to naval aircraft airframes and ultrahigh strength Ni-Co alloy steels to their landing gears. Structural weight fractions are expected to be reduced by 8-10 percent through substitution of high-stiffness Al-Li for conventional Al alloys. Ni-Co alloy steels enhance landing gear damage tolerance and resistance to environmental degradation, without associated weight penalty, when supplanting conventional low-alloy steels. Both materials are judged ready for immediate application to existing and next-generation U.S. Navy aircraft. 8 refs.

Al-Li alloys and ultrahigh-strength steels for U.S. Navy aircraft

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Al-Li alloys and ultrahigh-strength steels for U.S. Navy aircraft