The effect of microstructure of ferritic spheroidal graphite cast irons on intergranular fracture at intermediate temper
- PDF / 1,351,966 Bytes
- 6 Pages / 590.28 x 785 pts Page_size
- 68 Downloads / 202 Views
I.
INTRODUCTION
A L T H O U G H the phenomenon of brittleness of ferritic spheroidal graphite cast iron (ferrite S.G.I.) at elevated temperatures has been studied by many authors, [~-51 it still has been incompletely understood. O. Yanagisawa and T. S. Lui s report Ia suggested that the minimum ductility of ferritic S.G.I.'s at 673 K was a direct result of intergranular fracture influenced by dynamic strain aging and a triaxial stress field. The triaxial stress field developed in the ferrite matrix between graphite nodules. Our previous paper I61 investigated the effect of the triaxial stress field on intermediate temperature embrittlement of ferritic S . G . I . ' s . We found that the intergranular embrittlement of ferritic S.G.I. can be eliminated by decreasing the triaxial stress field around graphite nodules. However, another factor, such as the independent effect of ferrite grain size on the phenomenon, is not clear. In this study, we (i) changed the microstructure (ferrite grain size and graphite nodules) by varying the cooling rate with constant carbon content and (ii) changed ferrite grain size with the same graphite nodules which developed the same triaxial stress field around the nodules to investigate the effect of ferrite grain size on the intermediate temperature embrittlement o f ferrite S.G.I.'s.
II.
(i) Four kinds of Y-block shape castings were poured, as shown in Table I. All castings were annealed to a ferritic state by keeping at 1203 K for 3 hours, 993 K for 3 hours, and cooled in a furnace. (ii) A 30 x 90 • 240 mm sand mold of Y-block was poured. The casting was machined into round bars which were heated to 1193 K and rolled for an axisymmetric reduction of 50 pct, as previously reported, t6l After the rolling process, those round bars were annealed to a ferritic state by heating at 1243 K, 1223 K, 1203 K, and 1183 K, respectively, for three hours prior to cooling to 993 K and holding at this temperature for three hours before finally furnace cooling to room temperature, as shown in Figure 1.
,Specimen 1 2 4 3 k ~ 3hrS
/ ,223k,_3hrs\
[_LJ s
1203 k
3hrs
11 8 3 X r 3 h r s
~
5a
" - -
\ \ ~,
5c
\
5d
rY Ill {3_
I
993 k, 3hrs
LLJ t--
EXPERIMENTAL PROCEDURE
The raw materials were melted in a basic high frequency induction furnace, spheroidized with Fe-45 wt pct Si-4 wt pct Mg alloy, and innoculated with Fe-75 wt pct Si alloy. C.G. CHAO, Graduate Student, T.S. LUI, Associate Professor, and M. H. HON, Professor, are with the Department of Materials Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. Manuscript submitted May 19, 1988.
Table I.
Specimen 1 2 3 4 5
C 3.62 . . . 3.60
TIME Fig. 1--Heat treatment condition of specimen 5.
Chemical Composition (Wt Pct) and Casting Dimension of Specimens
Si 2.59 . . .
. . . 2.60
METALLURGICAL TRANSACTIONS A
Mn 0.18 . . . 0.19
P 0.020 . . . 0.031
S 0.011
Mg 0.053
0.006
0.035
. . .
Mold metal mold sand mold sand mold sand mold sand mold
Thickness (30 ram) (15 mm) (30 ram) (65 ram) (30 ram)
VOLUM
Data Loading...
