CRACKING RESISTANCE OF MODERN STEELS FOR RAIL
WHEELS AND RAILROAD TRACK
Alexander Balitskii
Karpenko Physico-Mechanical Institute of National Academy of Sciences of Ukraine
e-mail: balitski@ah.ipm.lviv.ua
http://www.ipm.lviv.ua/dep_11/balitski/Balitskii.htm
During last decades due to increasing axeloads and speeds large
quantity of fatigue cracks and wear damages after routine inspection and
failure accidents of rail wheels and railroad tracks [1-3] was founded.
Typical fatigue failure of railroad track (fig. 1 a) and rail wheels (fig.1 b)
shows, that crack initiation as well as crack propagation up to fatigue failure
was initiated by large inclusions or pre existing defects [3]. The light area on
fig. 1 a is the region of fatigue crack growth, and the surrounding darker area
is the region of fast fracture. The dark spot within the light area is the origin of
the failure, which is pre-existing defect due to a hydrogen flake [3].
Fracture of block-braked wheels has sharply increased recently,
because of the higher mechanical and thermal stresses, to the widespread
use of monoblock or solid wheels [1, 2].
Most of these fractures originate from a fatigue crack on the outer
circumference of the wheel tread (fig. 1 b). This fact leads to the deeper
understanding of the phenomenon of anisotropy of fracture toughness of
austenitic high nitrogen chromium-manganese steels for retaining rings [4-7].
Unfavourable residual stresses conditions in the wheel tread,
especially tensile stresses caused by heavy braking, promote crack growth
and thus the destruction of wheel.
Based on the results of a fracture-mechanics analysis of fractured and
non-fractured wheels of grade R7 material to UIC code 812-3, the paper [2]
indicates possible ways of improvement. The requirements to be met by
block-braked solid wheel to UIC standard in terms of a higher resistance of
fracture are discussed from the points of view of metallurgy and fracture
mechanics. Testing of smooth or notched specimens generally characterizes
the overall fatigue life of a specimen material. This type of testing, however,
does not distinguish between fatigue crack initiation life and crack
propagation life. With this approach, pre-existing flaws or crack-like defects,
which would reduce or eliminate the crack initiation portion of the fatigue life,
cannot be adequately addressed. Therefore, testing and characterization of
fatigue crack growth is used extensively to predict the rate at which subcritical
cracks grow due to fatigue loading.
b
Fig. 1. Fatigue failure of railroad track (a) and
rail wheels (b).
а
For components that are subjected to cyclic loading, this capability is
essential for life prediction, for recommending a definite accept/reject criterion
during non-destructive inspection [8], and for calculating in-service inspection
intervals for continued safe operation.
Problems can only be solved by developing a modern railroad track
and rail wheels materials with increased wear resistance and no tendency to
form hardening cracks and breaking cavities.
The chemical composition of traditionally used and novel materials for
wheel rims (Table 1) and some mechanical, crack resistance properties
(Table 2) shows a big difference due to initial structure of such materials.
The microstructure of KVR 600 (low alloyed carbon steel) is a fine
pearlite, homogeneous distributed in a ferrite matrix. The pearlite contributes
to good wear resistance and the ferrite gives the microstructure the desired
toughness, which is needed in regard to the deformation during service.
The idea to test a high nitrogen alloyed austenitic stainless steel as
wheel rim came from the main application of this material as retaining rings
for the trial wheel rims from P900 are shrunked on the spoke wheels. The
microstructure of P900 is fine austenite. Comparing the two materials
(Table 2) one great advantage of P900 can immediately be seen, namely the
high toughness, fatigue crack resistance and unique anisotropy of fracture
toughness [4-7].
Table 1.Chemical composition (wt%, Fe-balance) of the conventional steel
58CrMoV4 (KVR600) and high nitrogen 18Mn18Cr steels (P900, 12Kh18AG18Sh)
C
Si
Cr
Ni
Mn
P
S
N
V
KVR
0,55
0,28
0,99
0,85
0,012
0,011
0,09
P900
0.055- 0.3719.96- 0.119.92- 0.025- 0.001- 0.590,090.08
0.55
17.11
0,39
21.0
0.036
0,009
0.713
0.1
Ring
KVR
P900
Table 2.Properties of the conventional steel 58CrMoV4 (KVR600) and high
nitrogen 18Mn18Cr steels (P900, 12Kh18AG18Sh)18Mn18Cr steels
K1c,
HV
Charpy Kfc,
b,
0.2,
, %
, %
MPa m
J
MPa m
Mpa
MPa
910
500
16
36
60
1125102925-27.5 63-64
20540818695-105
1238
1192
390
420
212
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