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The effect of prior cold work on precipitation in a copper-bearing steel: Some metallographic observations
Metallography
253
The Effect of Prior Cold Work on Precipitation in a Copper-Bearing Steel : Some Metallographic Observations M. R. KRISHNADEV College of Engineering,
University
AND
I.
LE MAY
of Saskatchewan,
Cold working before aging of precipitation
Saskatoon,
Canada
hardening alloys has many applica-
tions and has been used extensively to increase the strength of such alloys and to prevent the formation of a denuded zone. l However, little systematic work on the effect of prior cold work on the aging of or-iron solid solutions has been done,2 and, although the use of copper as a major strengthening
element in low-
carbon steels is increasing rapidly, little has been done to study the effects of cold
FIG. 1. 60,000 x .
Electron micrograph
of copper-bearing
steel, cold-rolled 507”. Magnification
Metallography, Copyright
0
2 (1969) 253-256
1969 by American Elsevier Publishing Company,
Inc.
254
M. R. Krishnadev and I. Le May
working on the precipitation martensitic
of copper in such steels, particularly those with a
matrix.
In the present investigation, a commercial rimming steel (Nicuten3) was used, of base composition O.O5o/oC, 2.14% Cu, 1.45% Ni, all percentages being by weight.
Specimens
were austenitized
by heating
for 2 hours at 925” C and
quenched in water, the as-quenched structure being composed primarily of laths of martensite. The specimens, originally 0.125 inch thick, then were cold-rolled with a reduction of 50%
before being aged in neutral salt baths at temperatures
of 450” C and 500” C. In the cold-rolled
specimens,
the martensite
laths were
heavily distorted and dislocations were arranged in the form of cells as shown in Fig. 1, the cell walls being poorly defined. On aging, the walls became more sharply defined, and precipitation
of the c-phase (almost pure copper) began
both along cell walls and inside the cells, as shown in Fig. 2. This precipitation took place at somewhat
shorter times than for material which had not been
cold-reduced. Even after aging for 72 hours at 500” C there was no evidence of recrystallization or of extensive recovery;
the laths remained distorted and the cell walls were
stable, only the dislocation density within the cells decreasing
to some extent,
while further precipitation took place along cell walls and within the cells (Fig. 3).
FIG. 2. Material quenched, cold-rolled 500/b, and aged for 3 hours at 450°C. Precipitation of the c-phase is occurring both within the cell and along the cell boundaries. Magnification 160,000 x .
Some Metallographic
Observations
255
FIG. 3. Material quenched, cold-rolled 50%) and aged for 72 hours at 500°C. spread precipitation of the e-phase is apparent. Magnification 60,000 x .
The precipitation
Wide-
of copper within the cells as well as at cell walls was un-
expected, since clustering of solutes has been reported to occur in the cell walls of cold-worked
alloys.5 The
observation
might be explained
in terms of the
relatively high dislocation density within the cells. However, the nucleation of the e-phase in or-iron requires little strain energy as the specific volumes (volume per atom) in the matrix and precipitate
are approximately
equal, the energy
barrier to nucleation being dependent on the surface energy between the F.C.C. precipitates and the B.C.C. matrix; also, in samples which had not been coldreduced
before
aging, precipitation
took place along dislocations
as well as
uniformly within the matrix.4 This would indicate that the effect of dislocations in promoting precipitation in the material is small. Finally, the relative insensitivity of the microstructure be attributed to the retardation of grain boundaries
to recrystallization
and the stabilization
may of the
cell structure by the precipitated E-phase particles.
This work was supported by the Algoma Steel Corporation Ltd. and by the National Research Council of Canada under Grant-in-Aid A-2103. One of us (M.R.K.) is grateful for the award of a National Research Council Scholarship.
M. R. Krishnadev
256
and I. Le May
References 1. A. Kelly and R. B. Nicholson, Progr. Mater. Sci., 10 (1963) 243. 2. E. Hornbogen, in Precipitation from Iron-Base Alloys (G. R. Speich and J. B. Clark, eds.) Gordon and Breach, New York, 1965, p. 1. 3. W. E. Creswick, Canadian patent No. 763,897. 4. M. R. Krishnadev, Ph.D. Thesis, University of Saskatchewan, Saskatoon, 1969. 5. W. C. Leslie, J. T. Michalak, and F. W. Aul, in Iron and Its Dilute Solid Solutions (C. W. Spencer and F. E. Werner, eds.) Interscience, New York, 1963, p. 119.
Accepted
May 26, 1969
253
The Effect of Prior Cold Work on Precipitation in a Copper-Bearing Steel : Some Metallographic Observations M. R. KRISHNADEV College of Engineering,
University
AND
I.
LE MAY
of Saskatchewan,
Cold working before aging of precipitation
Saskatoon,
Canada
hardening alloys has many applica-
tions and has been used extensively to increase the strength of such alloys and to prevent the formation of a denuded zone. l However, little systematic work on the effect of prior cold work on the aging of or-iron solid solutions has been done,2 and, although the use of copper as a major strengthening
element in low-
carbon steels is increasing rapidly, little has been done to study the effects of cold
FIG. 1. 60,000 x .
Electron micrograph
of copper-bearing
steel, cold-rolled 507”. Magnification
Metallography, Copyright
0
2 (1969) 253-256
1969 by American Elsevier Publishing Company,
Inc.
254
M. R. Krishnadev and I. Le May
working on the precipitation martensitic
of copper in such steels, particularly those with a
matrix.
In the present investigation, a commercial rimming steel (Nicuten3) was used, of base composition O.O5o/oC, 2.14% Cu, 1.45% Ni, all percentages being by weight.
Specimens
were austenitized
by heating
for 2 hours at 925” C and
quenched in water, the as-quenched structure being composed primarily of laths of martensite. The specimens, originally 0.125 inch thick, then were cold-rolled with a reduction of 50%
before being aged in neutral salt baths at temperatures
of 450” C and 500” C. In the cold-rolled
specimens,
the martensite
laths were
heavily distorted and dislocations were arranged in the form of cells as shown in Fig. 1, the cell walls being poorly defined. On aging, the walls became more sharply defined, and precipitation
of the c-phase (almost pure copper) began
both along cell walls and inside the cells, as shown in Fig. 2. This precipitation took place at somewhat
shorter times than for material which had not been
cold-reduced. Even after aging for 72 hours at 500” C there was no evidence of recrystallization or of extensive recovery;
the laths remained distorted and the cell walls were
stable, only the dislocation density within the cells decreasing
to some extent,
while further precipitation took place along cell walls and within the cells (Fig. 3).
FIG. 2. Material quenched, cold-rolled 500/b, and aged for 3 hours at 450°C. Precipitation of the c-phase is occurring both within the cell and along the cell boundaries. Magnification 160,000 x .
Some Metallographic
Observations
255
FIG. 3. Material quenched, cold-rolled 50%) and aged for 72 hours at 500°C. spread precipitation of the e-phase is apparent. Magnification 60,000 x .
The precipitation
Wide-
of copper within the cells as well as at cell walls was un-
expected, since clustering of solutes has been reported to occur in the cell walls of cold-worked
alloys.5 The
observation
might be explained
in terms of the
relatively high dislocation density within the cells. However, the nucleation of the e-phase in or-iron requires little strain energy as the specific volumes (volume per atom) in the matrix and precipitate
are approximately
equal, the energy
barrier to nucleation being dependent on the surface energy between the F.C.C. precipitates and the B.C.C. matrix; also, in samples which had not been coldreduced
before
aging, precipitation
took place along dislocations
as well as
uniformly within the matrix.4 This would indicate that the effect of dislocations in promoting precipitation in the material is small. Finally, the relative insensitivity of the microstructure be attributed to the retardation of grain boundaries
to recrystallization
and the stabilization
may of the
cell structure by the precipitated E-phase particles.
This work was supported by the Algoma Steel Corporation Ltd. and by the National Research Council of Canada under Grant-in-Aid A-2103. One of us (M.R.K.) is grateful for the award of a National Research Council Scholarship.
M. R. Krishnadev
256
and I. Le May
References 1. A. Kelly and R. B. Nicholson, Progr. Mater. Sci., 10 (1963) 243. 2. E. Hornbogen, in Precipitation from Iron-Base Alloys (G. R. Speich and J. B. Clark, eds.) Gordon and Breach, New York, 1965, p. 1. 3. W. E. Creswick, Canadian patent No. 763,897. 4. M. R. Krishnadev, Ph.D. Thesis, University of Saskatchewan, Saskatoon, 1969. 5. W. C. Leslie, J. T. Michalak, and F. W. Aul, in Iron and Its Dilute Solid Solutions (C. W. Spencer and F. E. Werner, eds.) Interscience, New York, 1963, p. 119.
Accepted
May 26, 1969