Full text of DOI:10.1007/BF03187169

NOTES
severe plastic deformation. The welding thermal/ me-
chanical conditions are as follows: heating speed, 30℃/s;
heating temperature, about 1 400℃; deformation rate, 100
mm/s; deformation pressure, 70 MPa; cooling rate around
melt point temperature, 30℃/s. A Formarst-F model dila-
tometer was used to study the phase transformation pro-
cedure of the 0Cr16Ni22Mo2Ti steel and ZGMn13 steel
in the heat cycle of the flash welding. At the same time,
their melting points were determined. Structural investiga-
tions of the welding joint were performed on an EM420
transmission electron microscope operating at 175 kV
acceleration voltage and using selected area diffraction
patterns, bright field and dark field images.
Formation of nanocrystalline
during flash welding of
0Cr16Ni22Mo2Ti steel
ZHANG Fucheng, ZHANG Ming, HU Baitao
WANG Tiansheng
&
College of Materials Science and Chemical Engineering, Yanshan Uni-
versity, Qinhuangdao 066004, China
Correspondence should be addressed to Zhang Fucheng (e-mail:
zfc@ysu.edu.cn)
Abstract A nanocrystalline layer was fabricated in bond
area of 0Cr16Ni22Mo2Ti austenite steel using flash welding.
The mean grain size near bond line is about 20 nm, and the
farther the distance from bond line, the larger the size of the
nanocrystalline. The thickness of the nanocrystalline layer is
about 50 μm. The formation mechanism of the nanocry-
stalline may be that the metal in semisolid state is deformed
severely and its solid grains are fragmented.
2
Results and discussion
The microstructures of the 0Cr16Ni22Mo2Ti steel
forged are of recrystallized equiaxial grains, and its grain
size is about 60 μm. The results of thermal expansion
analysis indicate that the microstructure of 0Cr16Ni22-
Mo2Ti steel and ZGMn13 steel are always of single aus-
tenite in the heating cycle of the flash welding. The melt-
ing points of the 0Cr16Ni22Mo2Ti and ZGMn13 are
Keywords: nanocrystalline materials, flash welding, plastic defor-
mation.
1
285℃ and 1 370℃, respectively.
A typical microstructure of 0Cr16Ni22Mo2Ti steel
Materials with ultrafine grain (UFG) structures and
grain size in the range of nanometers or submicrometers
have attracted considerable attention of researchers in
various areas. Usually, the mechanical alloying, ultrafine
powder sintering and amorphous cystallining methods are
used to prepare block nano-materials. Fast heating can
refine the austenite grains. Recent investigations have
demonstrated that materials with UFG structure (nano-
and submicron crystalline) can be produced by severe
in the bond area is shown in fig. 1. It is found that the mi-
crostructure of 0Cr16Ni22Mo2Ti steel is of nanocrystal-
line with f.c.c. structure, its average size near the bond
line is about 20 nm. Using transmission energy spectrum
analysis, we found that in the nanocrystalline the contents
of alloying elements are Cr 16.8, Ni 21.2, Mo 2.3, Ti 0.51,
which correspond to the average chemical composition of
the 0Cr16Ni22Mo2Ti steel. In the bond area, the thickness
of the nanocrystalline structure zone along the bond line is
about 50 μm. The farther the distance from weld junction,
the larger the nanocrystalline size, as shown in fig. 2. The
structure in the heat affected zone (HAZ) is of austenite
with a large amount of dislocation. However, the micro-
structure of ZGMn13 steel in the bond area is of usual
austenite and there exists no nanocrystalline.
[
1]
plastic deformation . The nanomaterials can also be ob-
[
2]
tained through supercooling treatment . However, there
are no reports on the question of whether the nanocrystal-
line structure can be obtained or not by severe deforma-
tion of steel in the semisolid state. High-speed rail de-
mands the welding of ZGMn13 steel crossing with carbon
steel rail. Many materials, including 1Cr16ni22Mo2Ti
steel, were chosen as welding materials and the flash
The formation mechanism of the nanocrystalline of
0Cr16Ni22Mo2Ti steel in the bond area may be that, in
the process of the flash welding, the bond area in the
semisolid state is deformed severely, the solid grains are
fragmented. It is the same as the fact that the metals are
fragmented by severe plastic deformation at room tem-
[
3]
welding technology was employed for experiment . In
studying the microstructures of the bond area, we found
that the microstructure of 0Cr16Ni22Mo2Ti steel near the
bond line was the nanocrystalline. This note studies the
nanocrystalline structure and discusses its formation
mechanism.
[
4]
perature . In addition, compared with the microstr ucture
of ZGMn13 steel in the bond area subjected to the same
thermal/mechanical process it is indicated that the forma-
tion of the nanocrystalline in the 0Cr16Ni22Mo2Ti steel is
connected with its chemical composition and melting
point. Because the melting point of the 0Cr16Ni22Mo2Ti
steel is low, its bond area was severely deformed in the
process of the flash welding. The solid grains can be
fragmented fully. In addition, the 0Cr16Ni22Mo2Ti steel
contains Ti that can form TiN and hence effectively retard
1
Materials and methods
The experimental materials were 0Cr16Ni22Mo2Ti
austenite stainless steel and ZGMn13 austenite steel. The
Cr16Ni22Mo2Ti steel was smelted in a vacuum induc-
0
tion furnace, and then forged and cooled in air. A
GAAS80/700 model flash welder made in Sweden was
used to weld the 0Cr16Ni22Mo2Ti steel and ZGMn13
steel. During the flash welding the welding materials were
heated to melt state quickly and then welded together by
210
Chinese Science Bulletin Vol. 46 No. 3 February 2001

S
crystal growing. As in the area farther from the bond line
the deformation of the steel is smaller and the fragment
degree of the solid grains is lower, the nanocrystalline
structure becomes irregular and its size becomes larger.
Gradually it becomes usual austenite in structure, as
shown in fig. 2. Fig. 3 shows the grain size distributions
of the nanocrystalline of 0Cr16Ni22Mo2Ti steel.
Fig. 2 also indicates that the formation of the
nanocrystalline is not caused by fast solidification. In the
welding joint the temperature of the bond line is high, and
its solidification rate is low. If fast solidification is the
reason of the formation of the nanocrystalline, the size of
nanocrystalline in the bond line should be the largest, and
the size of grains far from bond line should be smaller.
The result is to the contrary. In fact, after the flash weld-
ing the cooling rate of the bond line is low (only about 30
℃/s around melt point temperature), which does not meet
the condition of the fast solidification.
Fig. 1. Light-field TEM image (a)
and dark-field TEM image (b) near the
bond line of the 0Cr16Ni22Mo2Ti
steel. The selected area is an electron
diffraction pattern, showing that the
microstructure is ultrafine crystalline
with f.c.c. structure.
Fig. 2. The microstructures of the bond area of 0Cr16Ni22Mo2Ti steel at the distances of 2 (a), 10 (b), 30 (c) and 50 μm (d) from
the bond line, respectively.
Chinese Science Bulletin Vol. 46 No. 3 February 2001
211

NOTES
Fig. 3. Grain size distributions of the nanocrystalline of 0Cr16Ni22Mo2Ti steel, with (a) corresponding to fig. 1 and (b) corre-
sponding to fig. 2(a).
A nanocrystalline layer with f.c.c. structure was fab-
ricated in bond area of 0Cr16Ni22Mo2Ti austenite steel
using flash welding. The formation mechanism of the
nanocrystalline may be that the metal in semisolid state is
deformed severely and its solid grains are fragmented.
In terms of mechanical behavior, steels with the
nanocrystalline structures exhibit ultrahigh strength and
[
5]
superplastic-like behavior , they can be used to make
very important parts. Based on defining the formation
mechanism of the nanocrystalline of 0Cr16Ni22Mo2Ti
steel, the precision parts whose microstructure is ultrafine
crystalline can be made by using the method of subjecting
the metal in semisolid state to pressure work.
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(
Received June 5, 2000)
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