A comparison of the fracture behavior of two heats of the secondary hardening steel AF1410
- PDF / 5,972,992 Bytes
- 19 Pages / 590.28 x 785 pts Page_size
- 6 Downloads / 242 Views
I.
INTRODUCTION
THE fracture
initiation toughness of ultra high strength steels can be influenced by the inclusion type and spacing, the microstructural parameters established during austenitizing, and how the microstructure evolves during aging or tempering. The theoretical treatment of Rice and Johnson tll suggests that the critical value of the crack tip opening displacement, 8ic, will scale with the inclusion spacing, X0, at least over some range of X0. This prediction appears to be in accordance with experimental studies, ]2'31at least as long as certain requirements are met. These requirements appear to include that the inclusions can be regarded as pre-existing voids and that the microstructure remains otherwise constant. The austenitizing treatment will, for a given alloy composition, largely determine the characteristics of the undissolved particles (such as undissolved carbides) and the prior austenite grain size. These undissolved particles can affect the fracture behavior directly if they actually nucleate voids and indirectly through their influence on prior austenite grain size. Aging can influence the fracture behavior directly if particles precipitated on aging nucleate voids and indirectly by altering the flow characteristics of the matrix. The work of Speich et al. [4] provides an example of how, by control of composition and aging temperature, microstructures with very high toughness can be obtained. Explicitly it was shown that microstructures achieved by replacing cementite with very fine dispersions of alloy carbides precipitated during aging possess extremely high levels of toughness. It was suggested that the high toughness was possible because the fine alloy carbides were too small to nucleate voids. This work led to the development of HY180 steel (0.11C/10Ni/8Co/2Cr/1Mo), in which the alloy carbides precipitated on aging are M2C type carbides K.J. HANDERHAN is Vice President of Technology, Ellwood City Forge, Ellwood City, PA. W. M. GARRISON, Jr. is Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. N.R. MOODY is Member, Technical Staff, Sandia National Laboratories, Livermore, CA 94550. Manuscript submitted July 8, 1988. METALLURGICAL TRANSACTIONS A
where the M is Cr and Mo. Cobalt additions were used to enhance the secondary hardening strengthening. The carbon, chromium, and molybdenum levels are such that on aging at 510 ~ only fine alloy carbides are precipitated. At this aging temperature yield strengths of _ 1250 MPa and fracture toughnesses of over 200 MPaX/m have been achieved. This basic microstructure has been exploited in the development of the steel AF1410, I5'61which is identical to HY180 except for increased carbon and cobalt contents (0.16C and 14Co). The increased cobalt is used to enhance further the secondary hardening reaction. This steel is presently commercially available and on aging at 510 ~ yield strengths of 1550 MPa and fracture toughnesses of over 200 MPaV~m have been achieved.
Data Loading...
