Influence of Carbide Modifications on the Mechanical Properties of Ultra-High-Strength Stainless Steels
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INTRODUCTION
SECONDARY hardened martensitic steels are attractive for use in advanced structural components that require ultra-high-strength and high impact/fracture toughness.[1–3] Recently, QuesTek designed a new-generation, ultra-high-strength steel, Ferrium S53 (Fe-0.21C-1W-2Mo-10Cr-5.5Ni-14Co-0.3V-0.02Ti),[4] based on the composition of AerMet 100 (Fe-0.23C-3.1Cr-1.2Mo-13.4Co-11.1Ni).[5] With ultrahigh-strength and a good pffiffiffiffi fracture toughness, above 1.9 GPa and 70 MPa m, respectively, combined with corrosion properties similar to (713 K) 440 C and 15-5 PH, Ferrium S53 has been recognized as a representative secondary hardened steel that can replace advanced structural components for use under severe working environments.[6] The ultra-high strength of this alloy is obtained by the combination of a hardened steel matrix (highly dislocated lath martensite), with nanocarbide M2C induced by secondary hardening, also known as precipitation or age hardening. In this alloy system, large amounts of Co are necessarily added to achieve a good balance between the strength and toughness. It has been well known that in secondary hardening alloy steels, the Co addition retards the recovery of
JOO-YOUNG SEO, HOON KWON, and KI-SUB CHO are with the Center for Advanced Materials Technology (CAMT), Kookmin University, Seoul 136-702, Republic of Korea. Contact e-mail: [email protected] SOO-KEUN PARK is with the Korea Institute of Industrial Technology, Inchon 406-840, Republic of Korea. Soo-Keun Park and Ki-Sub Cho have contributed equally to this work. Manuscript submitted January 31, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
dislocation substructure.[7] That is, if the alloy carbides are dislocation-nucleated, the finer precipitate dispersion may retain, as the result of an unrecovered matrix. This effect can also be determined using short-range ordering (SRO) effect[8]; i.e., as the recovery of dislocations has to be accompanied by iron diffusion, the strain filed formed around Co atoms can play a role as a barrier for the diffusion, thus increasing the SRO recovery resistance. In thermodynamic aspect, adding Co can lead to the increase in driving force for the precipitation of M2C carbides,[1,4,8] as well as coarsening resistance of M2C carbide.[9] All of these results[1,4,7–9] show that keeping the alloy carbides as fine as possible at completion of precipitation would contribute to significant enhancing the strength without the loss of toughness. Apart from the effects of Co on mechanical properties, Tang et al.[10] determined that the Co addition enhances the strength, as well as corrosion resistance, with respect to the increased Cr activity within the bulk matrix, where they reported that the Co accelerates the formation of Cr-rich oxide layer passivating the alloy surface under corrosive environments. Also Kim et al.[11] showed that the effect of the addition of a small amount of Co on the corrosion properties of low alloy steel. In this study, they suggested that a higher Co content improves not only the active
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