Three-Dimensional (3-D) Atom Probe Tomography of a Cu-Precipitation-Strengthened, Ultrahigh-Strength Carburized Steel
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I.
INTRODUCTION
ADVANCES in high-power-density gear applications relies on the development of new materials, demanding very high surface hardness while maintaining sufficient levels of flaw tolerance. One family of materials that has shown promise for these applications are carburizing steels that display secondary hardening during tempering. This hardening response arises from the precipitation of nanometer-scale M2C carbides that are much more efficient strengtheners compared with the micron-scale transition carbides found in traditional gear steels. This gain in strengthening efficiency enables these alloys to achieve equivalent hardness values at almost half of the carbide phase fraction or carbon content, resulting in a high-toughness lathe martensite microstructure. To achieve maximum secondary hardening response, these steels depend on substantial alloying additions of Co, which maximizes the driving force for heterogeneous M2C precipitation on dislocations by retarding dislocation recovery during tempering. Because of the high cost BENJAMIN L. TIEMENS, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, is now Senior Research Scientist, UOP LLC, A Honeywell Company, Des Plaines, IL 60017-5017. Contact e-mails: [email protected]; [email protected] GREGORY B. OLSON, Professor, is with the Department of Materials Science and Engineering, Northwestern University. ANIL K. SACHDEV, Laboratory Group Manager, and RAJA K. MISHRA, Staff Researcher, are with the Materials & Processes Laboratory, General Motors Research & Development Center, Warren, MI 48090-9055. Manuscript submitted May 2, 2010. Article published online June 3, 2012 3626—VOLUME 43A, OCTOBER 2012
of Co, alternative means of maximizing secondary hardening in these carburizing steels is of great interest. In the first part of this two-part series, the authors[1] demonstrated the design and development of an ultrahard secondary hardening steel that incorporates bodycentered cubic (bcc) Cu precipitates as a means to achieve high hardness levels without significant amounts of costly Co alloying additions. The Cu was added to provide strengthening in two ways, the first of which is through precipitation strengthening by bcc Cu precipitates. Bcc Cu precipitation strengthening in steels is commonly attributed to so-called ‘‘modulus strengthening’’ as outlined by Russell and Brown.[2] Peak strengthening occurs as the metastable, spherical bcc Cu precipitates reach a critical diameter of 2.3 to 3.0 nm.[3] After further aging and growth, they begin to lose coherency through intermediate martensitic transformations to 9R and then 3R structures,[4,5] ultimately evolving into the incoherent rod-shaped morphology of the equilibrium face-centered cubic e-phase.[6] BCC Cu precipitation is known to occur at stage IV of tempering, i.e., 723 K to 873 K (450 C to 600 C), making it fully compatible with the heattreating regime used in M2C precipitation for secondary hardening as shown by
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