In situ fracture observations on tempered martensite embrittlement in an AlSl 4340 steel
- PDF / 2,183,513 Bytes
- 4 Pages / 613 x 788.28 pts Page_size
- 71 Downloads / 208 Views
In Situ Fracture Observations on Tempered Martensite Embrittlement in an AISI 4340 Steel B.C. KIM, S. LEE, D.Y. LEE, and N.J. KIM The ability to document and understand the micromechanics of fracture processes, especially as they relate to macroscopic failure mechanisms, along with the influence of chemistry, microstructure, and processing, now underlies a good deal of alloy design methodology. Recent advances in fracture mechanics, in particular for ductile alloys, have provided new means of studying the interaction of cracks moving under quasi-static conditions in various microstructures. A particularly interesting set of recent studies of this type has shown, for example, that when ultrahigh-strength steels, such as AISI 4340 steels, are tempered near 350 ~ a loss in toughness, called tempered martensite embrittlement (TME), is observed. This is usually characterized by a trough in the plot of Charpy impact energy as a function of tempering temperature and by the ductile-brittle transition temperature exhibiting a maximum at the temperature corresponding to the minimum in the Charpy toughness, t~-6] Since this embrittlement problem in the 4340 steel limits the service temperature of the steel, it is very important to identity the reason of the problem for the development of ultrahigh-strength steels with improved toughness. Attempts to explain the TME phenomenon have focused on the possible effects of carbide distributions and segregation of impurity elements on prior austenite grain boundaries as key metallurgical variables. Thomas and co-workers IL3] have suggested a mechanism of TME in which the drop in toughness can be correlated with the decomposition of lath-boundary retained austenite and the subsequent formation of interlath cementite films during tempering. These interlath carbides may provide crack nucleation sites or easy crack paths. Recently, the effects of impurity content and grain size on the percentage of intergranular fracture in high-strength steels tempered in the TME range have been discussed. Several researchers [4'5'6] have found that impurity elements, primarily phosphorus and sulfur, segregate to the grain boundaries during austenitization. The aim here is to gain a more fundamental understanding of how the microfracture processes are affected by microstructural changes and impurity segregation during tempering. In this article, particular emphasis has
B.C. KIM, Research Assistant, S. LEE, Assistant Professor, D.Y. LEE, Associate Professor, and N.J. KIM, Associate Professor, are with the Departmentof Materials Scienceand Engineering,Pohang Institute of Science and Technology, Pohang 790-600, Korea. Manuscript submitted September 18, 1990. Table I.
Chemical Composition (Weight Percent) of the AISI 4340 Steel Investigated
C
Mn
P
S
Si
0.38
0.79
0.018
0.018
0.46
METALLURGICALTRANSACTIONSA
Ni 1.73
Cr 0.79
Mo 0.29
Cu 0.01
V 0.002
AI 0.011
Ti
O
0.008
0.021
VOLUME22A, AUGUST1991-- 1889
40 ~120 e~
1 150~
AUSTENITIZING
50 100 5r Z
30
80
O
O [... 20 ~;
60
r 40
z
Data Loading...
