Microstructural Factors Controlling Blunt- and Sharp-Notch Toughness in La-Added 8Ni-13Co Secondary Hardening Steel
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focused on high Co-Ni secondary hardening steels because of their superior toughness even at ultrahigh strength levels of 2000 MPa.[1–8] The high Co-Ni martensite matrix, which consists of finely divided dislocated laths with nanosized secondary carbides precipitated during aging (tempering), may have unique properties such as high resistance to fracture. However, the level of toughness considerably depends on the presence of coarse primary particles to initiate cracks or voids, which develop from melting, solidification, forging/rolling, and heat treatments. These primary particles typically include nonmetallic inclusions and undissolved carbides. In addition to primary particles, the prior austenite grain size (PAGS) in martensitic steels plays an important role in the fracture process. Grain refinement is a well-known technique to improve both the strength and toughness. However, the decrease in PAGS
KI-SUB CHO, Researcher, SUNG-SOO PARK and YOUNG-BU KIM, Students, and HOON KWON, Professor, are with the Center for Advanced Materials Technology (CAMT), Kookmin University, Seoul 136-702, Korea. Contact e-mail: [email protected] HEEKWON MOON, Researcher, is with the Human Resources Development Service of Korea, Seoul 121-757, Korea. Manuscript submitted November 6, 2012. Article published online August 13, 2013 4440—VOLUME 44A, OCTOBER 2013
frequently deteriorates the fracture toughness (KIC) under sharp-notch (qfi0) conditions, in contrast with the enhancement of impact toughness (Cv) under blunt Charpy V-notch (q = 0.25 mm) conditions. Even though these conflicting results have been thoroughly researched in terms of micromechanics by taking into consideration the plastic zone size and the characteristic distance for void initiation ahead of blunt and sharp notches, some problems remain not clearly understood in terms of the accompanying microstructural variations that occur when austenitization is used to control the PAGS.[3,5,7–12] The purpose of the current study was to elucidate these microstructural variations and then to find a way to simultaneously enhance Cv and KIC via a relatively simple process such as two-step austenitization. Thus, it is necessary to investigate size, spacing, volume fraction, and composition of the primary particles and the secondary precipitation-hardened particles, as well as the grain size. In addition to scanning electron microscopy (SEM), we utilized small-angle neutron scattering (SANS), in which the various particles were quantitatively analyzed in a large volume in three dimensions to overcome the limited information that is available in a very small region by transmission electron microscopy (TEM) or SEM. Steel with a composition of 0.23C-2.98Cr-1.17Mo8.04Ni-13.0Co-0.009La (wt pct) was prepared from a 30-kg ingot by vacuum induction melting. The rolled plates were austenitized at 1323 K (1050 C) for 1 hour and then oil quenched. Two-step austenitization was done at 1473 K/1323 K and 1473 K/1173 K (1200 C/ 1050 C and 1200 C/900 C). The hardness, tensile properties, Cv, and
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