Investigation of Clusters in Medium Carbon Secondary Hardening Ultra-high-strength Steel After Hardening and Aging Treat
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SECONDARY hardening ultra-high-strength (SHUHS) steels, containing significant amounts of cobalt and nickel, possess excellent combination of strength, fracture toughness (KIC), and stress corrosion cracking resistance (KISCC). These steels have been used for niche applications such as aircraft landing gear, armor, etc., due to their unique combination of properties. Some typical steels of this type are HY180, AF1410, and AerMet100.[14] These are essentially quenched and tempered steels that derive their strength and toughness from fine M2C carbides present in a highly dislocated lath martensitic structure.[5] The optimum heat treatment for these steels, corresponding to a slightly overaged condition, consists of austenitizing in the range 1098 K to 1173 K (825 C to 900 C), followed by quenching to room temperature and LN2 treatment and tempering in the range 758 K to 783 K(485 C to 510 C) for 5 hours.[1,6] The microstructure of these steels in the as-quenched condition consists essentially of lath martensite. DependR. VEERABABU, R. BALAMURALIKRISHNAN, and M. SRINIVAS, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad 500058, India. Contact e-mail: veeruiisc@ gmail.com K. MURALEEDHARAN, Scientist, Director Technical (Materials), is with the Technical Core Group, DRDO Head Quarters, New Delhi 110 011, India. Manuscript submitted January 24, 2014. Article published online March 18, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
ing on the austenitizing temperature, small amounts of undissolved primary carbides such as MC, M6C, and M23C6 may also be present.[3,6] The typical tempering behavior of these steels has been extensively described in the literature[1,3,7-11]: at lower tempering temperatures [~698 K (~425 C)], coarse cementite is formed, which is associated with a drop in toughness. At higher tempering temperatures [723 to 773 K (450 to 500 C)], needleshaped coherent M2C precipitates are formed along h100i direction of ferrite. Even though M6C and M23C6 are thermodynamically more stable carbides than M2C, the higher coherency of M2C with the matrix has been reported to be responsible for its precipitation prior to that of the other carbides.[6] At even higher temperatures, the needle-shaped M2C precipitates begin to coarsen resulting in a slight loss of strength but accompanied by a disproportionate increase in toughness. For this reason, all secondary hardening steels are put into service in this slightly overaged condition. It has been reported that the observation and confirmation of M2C in this optimally heat-treated condition is often difficult due to small size of the precipitates (in spite of slight coarsening) and the high dislocation density of lath martensitic microstructure. However, Ayer et al.,[3] Stiller et al.,[12] and Akre et al.,[13] using TEM of extraction replicas, field-ion microscopy, and 3D atom probe, respectively, have been able to show that at least some of the precipitates are of M2C type. In addition to M2C strengthening carbide, several fine clusters
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