Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel
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I.
Table I.
INTRODUCTION
THE segregation of group IV B to VI B elements to prior austenite grain boundaries causes temper embrittlement in alloy steels, and the ductile-brittle transition temperature is a widely-accepted tool used to measure the embrittlement susceptibility. An alternative measure is the area fraction of intergranular facets on the fracture surface; both measures are directly related to the amount of segregated impurities on prior austenite grain boundaries. However, the occurrence of grain boundary fracture is sensitive not only to the grain boundary composition, but also to the matrix hardness, ~ grain size, 2 test temperature, 3 etc. A fractographic survey of partially embrittled specimens may show an inhomogeneous distribution of intergranular facets on the fracture surface, and the fracture mode may vary systematically along the fracture path. For example, in the ICrMoV steel used by Viswanathan and Joshi 3 the local fracture mode changed as a function of distance from the notch root, and the average area fraction of intergranular fracture was highest at a test temperature which depended on microstructure and matrix hardness. The reason w h y the maximum amount of intergranular fracture occurred at other than the lowest test temperature was not well understood. However, the result suggested that the test temperature and the location of fractographic observations should be selected in a consistent manner if the percent intergranular fracture (pct IG) is to be a useful measure of embrittlement susceptibility. The present report is intended to clarify this matter further.
II.
EXPERIMENTAL P R O C E D U R E
Circumferentially notched cylindrical bars of a phosphorus-doped 2.25 Cr-1 Mo steel, 6.3 mm diameter, heat treated as shown in Table I and embrittled to achieve nearJIN YU is Assistant Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 150 Chongryang, Seoul, Korea. C.J. McMAHON, Jr. is Professor, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Manuscript submitted October 21, 1983. METALLURGICAL TRANSACTIONS A
C 0.2*
Composition and Heat Treatments
Composition: (Wt Pct) Cr Mo 2.58 1.0"
P 0.036
Group A
Group B
Austenitization Grain size Microstmcture Tempering
1443 K, 2 h ASTM No. 5 100 pct martensite 923 K, 10 h
Hardness Embrittlement Auger PHR (pct)
Rc20 step cooling
1223 K, 4 h ASTM No. 7 100 pct martensite 923 K, 10 h or 963 K, 13 h Rc20 or Rcl3 793 K, 1000 h {19.5r 3.5 (Re20) -(Rc 13)
Plzo/Fe7o3
*Nominal value
equilibrium segregation of phosphorus, were tested in slow bending over the temperature range from 77 K to room temperature; the lower temperatures were obtained with a liquid N2 spray. ~,4,5The end of the cantilever beam specimen was displaced at the rate of 0.085 mm per second, and the loading point was 1.1 cm from the notch root, as reported previously. 4. Scanning electron micrographs at • 300 mag*This specimen configuration, originally int
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