Grain boundary precipitation in 18Ni maraging steels
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AND
P. H. HOLLOWAY
Although 18Ni maraging steels, when compared to quench and temper steels, generally exhibit higher levels of f r a c t u r e toughness, they may be severely embrittled if exposed to certain heat treatment schedules. 1'2 Kalish and Rack 2 proposed that this thermal embrittlement phenomena originally o c c u r s as a r e sult of solute segregation to austenite grain bounda r i e s . Advanced stages of intergranular embrittmment were then associated with Ti (C, N) precipitation. Results of other studies 3'4 suggested that, while segregation may occur, the p r i m a r y cause of thermal embrittlement in 18Ni maraging steel is the actual presence of intergranular TiC particles. Johnson and Steiifs 3 observations also indicated that TiC f o r mation is a direct result of Ti and C segregation to the austenite grain boundaries, while Nes and Thomas a proposed that carbide formation is a heterogeneous nucleation and growth phenomena occurring in a supersaturated F e - T i - C solid solution. Finally, the f o r m e r investigators concluded that the further reductions in f r a c t u r e toughness observed following subsequent age hardening of thermally embrittled maraging steel could be associated with the s e g r e gation of B to the prior austenite grain boundaries. The purpose of the present communication is to review some recent scanning Auger electron spectroscopy data which reinforces the important role of TiC in the thermal embrittlement p r o c e s s . Moreover, these same results show that impurity s e g r e gation is not a general phenomena in maraging steels and cannot therefore be considered to be the principal cause of the additional reductions in fracture toughness observed in aged thermaIIy embrittled alloys. The chemical compositions of the two heats of 18Ni maraging steel used in this investigation were identical (wt pct): 18.43 Ni, 7.58 Co, 4.93 Mo, 0.47 Ti, 0.13 A1, 0.02 Mn, 0.04 Si, 0.01 C, 0.0042 N, 0.005 S, 0.004 P, balance Fe. Unembrittled samples from Heat No. 1 were prepared by solution treatment at 1595 K for 1 h, water quenching and aging for 3 h at 753 K. Embrittled samples from this heat were hot quenched to 1143 K following the solution treatment at 1595 K, held for either 20 min or 4 h, air cooled and then aged for 3 h at 753 K. Johnson and Stein a have shown that these hold times at 1143 K will result in a decrease of 25 and 75 pct in the annealed Charpy impact energy when compared with the unembrittled condition. Samples from Heat No. 2 were prepared from hot isostatieally pressed powder. The powder was hot p r e s s e d at 1367 K and furnace cooled. After cooling the alloy was double sotution
treated at 1259 K/I128 K for 1 h with an intermediate air cool. Finally, these samples were aged at 773 K for 3 h. Tensile tests of samples prepared in this manner showed nil ductility as measured by r e duction in area. ~ Following heat treatment, prenotched samples 25.4 mm long • 2.5 mm square were mounted in an ultrahigh vacuum system. After pumping to a vacuum of ~2 x I0 -~~ T
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