The Effect of Heat Treatment on the Fracture Toughness and Subcritical Crack Growth Characteristics of a 350-Grade Maraging
The heat treatment response of a 350-grade maraginf steel, with the nominal composition 18.5 Ni, 12 Co. 4.6 Mo, 1.4 Ti, balance le. has been determined in billet and bar form. When ■ged at temperatures below 90(rF, the material was very susceptible to subcritica! crack growth, and premature brittle Iracture occurred in unnotched tension specimens loaded at a slow strain rate in laboratory air. Fracture mechanics was used to interpret this behavior. The introduction of reverted austenite signiticantly decreased the strength level but had little effect on Iracture toughness. The resistance to brittle Iracture of this material is contrasted with that ot high-strength steels currently used by the airplane industry. INTRODUCTION F'he continuing demand lor ultra-high-strength steels prompted the recent development ol two 3S0-grade maraging steels. The development and properties of one alloy nominal composition P.5 Ni. i:.5Co. .VK Mo. 1.7 Ti. 0.15 Al. balance le were recently described ( I ). Vasco Metals, a subsidiary of I eledyne. Inc.. introduced a second alloy that is lower in titanium than the first to avoid segregation problems and has slightly higher nickel and molybdenum contents. Fn this paper, the effect of heat treatment on the fracture toughness and subcritical crack growth resistance of this second alloy is described.
The physical metallurgy of maraging steels
AbstractMaraging steels are a class of high-strength steels characterised by very low carbon contents and the use of substitutional elements to produce age-hardening in iron–nickel martensites. The term 'maraging' was coined from a combination of martensite and age-hardening.
Electron microscopic investigation of the aging of Fe−Ni−Al Alloys
Conclusions1.Aging with precipitation of (Ni, Fe) Al and Ni3Al occurs in the α-phase which has a martensitic structure and in the ferrite which has a structure close to the equilibrium structure. The state of the initial α-phase has an effect on hardening resulting from aging.2.Hardening resulting from aging at 350–450°C is not accompanied by any visible change in the structure of the alloy. Only after aging at 500°C can one see areas (less than 20A in size) with an ordered (Ni, Fe) Al structure. This result leads us to conclude that these areas are also formed at lower temperature, but their size and number are probably much smaller. At temperatures above 500°C not only areas of the (Ni, Fe) Al phase are formed, but also large precipitates of Ni3Al. These precipitates do not play any significant role in the hardening of the α-phase resulting from aging.3.The increase in hardness as the result of aging is due to the initial stage of the formation of areas with (Ni, Fe) Al structure which are coherent with the matrix. It is probable that it is the ordering of the structure which is responsible for the hardening of the alloy resulting from aging.