This report may be updated and revised periodically in response to the needs of the technical community; up-to-date versions can be requested from the editors at the address given below or downloaded at http://www.ca.sandia.gov/matlsTechRef/ . The content of this report will also be incorporated into a Sandia National Laboratory report (SAND2008-1163); the most recent version can be obtained from the link above. The success of this reference depends upon feedback from the technical community; please forward your comments, suggestions, criticisms and relevant public-domain data to the authors C. San Marchi (cwsanma@sandia.gov) and/or B.P. Somerday (bpsomer@sandia.gov) at: Sandia National Laboratories Technical Reference on Hydrogen Compatibility of Materials MS-9404 7011 East Ave Livermore CA 94550.

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Technical Reference on Hydrogen Compatibility of Materials

This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni3Ti and Fe7Mo6 were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni3Ti particles at very short aging times. The Fe7Mo6 phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe7Mo6 μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging.

Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co–containing C–300 steel

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni3Ti, Fe7Mo6 μ phase, Fe2Mo Laves phase, ω phase, Ti6Si7Ni16 G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe7Mo6μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys.

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

The ultrahigh-strength steels used in aerospace applications are primarily alloys developed 25 or more years ago, which would seem to illustrate the conservatism of alloy producers. However, the now widespread use of AF1410, which was developed in the mid-1970s, suggests that new alloys will be adopted for aerospace applications if their mechanical properties are markedly superior to incumbent alloys. Given the excellent properties of the secondary hardening steels such as AF1410, it would appear that the greatest need is for alloys with yield strengths of 1,725 MPa and higher, which have improved fracture toughness and resistance to stress-corrosion cracking. Control of sulfide-type inclusions, coupled with low sulfur levels, may be a means of achieving quite reasonable toughness at high strength levels. Improving resistance to stress-corrosion cracking appears to be a more difficult problem, but it is hoped that a general solution for the case of intergranular fracture by stress-corrosion cracking will arise from ongoing fundamental studies of the effects of segregating impurities and alloying additions on grain boundary cohesion.

Ultrahigh-strength steels for aerospace applications

Modeling of precipitation hardening during the aging and overaging of 18Ni–Co–Mo–Ti maraging 300 steel

Cobalt-containing high-strength steels. A critical review of the physical metallurgy of cobalt-containing high-strength steels, and a survey of their processing, properties and uses

L. Habrakan

Papers

Cobalt-containing high-strength steels. A critical review of the physical metallurgy of cobalt-containing high-strength steels, and a survey of their processing, properties and uses