The effect of austenitizing temperature on the fracture initiation toughness of As- quenched hp9- 4- 20 steel
- PDF / 3,360,135 Bytes
- 15 Pages / 594 x 774 pts Page_size
- 55 Downloads / 178 Views
I.
INTRODUCTION
B E G I N N I N G with the work of Zackay et al. Ill there have been a number of studies of the effect of austenitizing temperature on the fracture initiation toughness of ultra high strength steels. I2-28] These investigations have shown that increasing the austenitizing temperature can substantially increase the fracture initiation toughness (Krc) of ultra high strength steels in the as-quenched and lightly tempered conditions. [1-7,t0,13,19,21-25,27]A number of possible explanations for this behavior have been proposed. One is the improved hardenability associated with coarser grained steels E~'7]and the consequent elimination of mixed microstructures. Another explanation for this behavior I2'7'~~ has been that higher austenitizing temperatures should result in less segregation of impurity elements to prior austenite grain boundaries, and rapid quenching from these temperatures would therefore result in improved toughness. Other proposed explanations include an increase in the amount of retained austenite, 13'6'7'9'2~ a reduction in the amount of twinned martensite, [6'~7'24] and an increase in grain size. IH1 Other investigators have attributed the improved fracture toughness to the dissolution of alloy carbides and other void initiating particles 13'5'14'15'18'25"27]and to an increase in .the volume fraction of auto-tempered carbides. Iz21 Although considerable research has been conducted on the effects of austenitizing temperature on fracture initiation toughness, it is apparent that there is no agreement as to why the fracture toughness increases with increasing austenitizing temperature. There is no reason to expect agreement. Indeed, it is possible that some factors are important in some steels and not in others. It is also possible that in a given steel many factors contribute to the observed increases in fracture initiation toughness. However, we suggest that as long as the fracture is microvoid coalescence and is transgranular with respect to the prior austenite grain boundaries, the increased fracture initiation toughness associated with increased austenitizing temperatures will in KEVIN J. HANDERHAN is Vice President of Technology. Ellwood City Forge, Ellwood City. PA. WARREN M. GARRISON, Jr. is Associate Professor. Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted February 26, 1988 METALLURGICAL TRANSACTIONS A
most cases be the result of changes in the characteristics of the various distributions of void initiating particles. The purpose of the present study was to investigate the effects of austenitizing temperature on the fracture initiation toughness of an ultra high strength steel, to quantify the effects of austenitizing temperature on both inclusion and other particle distributions, and finally to assess the importance of changes in the particle distributions on the fracture initiation toughness. The ultra high strength steel HP9-4-20 was selected for this study because it possesses high harde
Data Loading...
