Note: Descriptions are shown in the official language in which they were submitted.
CA 0221~941 1997-09-19
WO 96/29 ~69 PCT/GB96/00458
ov~ments in and relating to Steel Rails and
Methods of ProA~c; n~ the same
This invention relates to steel rails and to methods
of producing the same. More especially, the invention
relates to the production of high integrity long welded
steel rails.
Railway tracks have traditionally comprised a
plurality of rails connected together by bolts and fish
plates. In use, such tracks have proved to be noisy,
uneven and require considerable maintenance. More
recently, individual rails have been welded together in
continuous lengths thereby enabling higher train speeds to
be achieved at relatively low noise and vibration levels.
The welds between adjoining individual rails have, however,
previously been subject to occasional bending fatigue
failures from the foot of the rail caused inter alia by
CA 02215941 1997-09-19
positi~-e -_n~ ~oading s~esses experienc-~ ~-~ ~~e ~ail
foot dur--~ S~-'i'C5.
One ~bj_-- o- the present invention i~ rs~t--e a
high i~tc -i -~ lc-.g welded steel rail wh--- -~ _ s a
bendinc a__5~ e commensurate with that ~ nor-we~ded
rail and an o-~_~al_ improved residual stress _G__ern ac_oss
the we;a :az --o--le. Another object is ~ e a
welded st_el -G' 1 in which the bending LatiG__ -_~e.c~~ or
each welG is at ~~ast equal to if not gr___~- t:-a~ the
fat-gue c_~enc_:~ c the parent rails joinec _~f ~'~a_ ~eld.
A further - J e~~ is to produce a welded stee~ ~a_~ w~ich
the p-~se-ce o t:e welds is not apparent --.,. a v-âual
inspectic- (_'-G- _S to say an lnvisibiy we:~_ ~a__). A
furtne- ~~ _c_ -'s to provide a method -- -.et:~~cs of
manufact_--~g w_~ced steel rail by which the ~~ es set
out above ~â _~ â~hieved.
T~~ ~â _ i _ ~~g weldec. rail is a lens ~~ _:~e c-de~
of :OOm .~ 30~- c~ longer.
ACCC_~iLS ~c _he present invention in o _ â~?ec~ .:~ere
is ?rcvi-e- a s~eel rail which comprises a _-_ra --! of
individua: ~a-:s welded together with any r~__~--~c e~-ess
material -~mcve~ -om the rail by a strip~ CCeSâ in
which we:- s _~s at and below the foot o_ _â~~ ~â ~ are
subjec_ec ~c --~.G- ng.
Doc~ e-_ e---_led Esve_d: "Mcdern Railw_-,-~-ac~ g89
diSCUSSeC ~;C~ 5~S welding tecnniques -_-- ,_- ing
neichbou~ -~s of a continu-us trac~ ---~ of
weld mate-'-~ -~_r. .he rail fcc~ and foot u-~~~ s,
howeve-, -e~ lated.
_-c'~ w_~~ e may be c_ared wi~ ~_--or
i~,_bi~i-~ -~-_-- --:. Such COG_' n~ ma~ be a~ c -- -he
rai~ fJ-G _-~ ~- _:~e rail web a-d/cr the ~__: '~-_-. The
coa~ - __u-se, al_e~~a~ivel~ be _-_: -~ -~ 'he
) S~
CA 02215941 1997-09-19
enti-e we~c s ._.
Welc~ng ,.a-y be by flash butt welding v_~.-_ we'cing
processes may however be employed.
Tn anot:e- aspect, the present inventicn -rov des a
method of prcGucing a welded steel rail which c~~.~rises the
steps of we;~- ?5 individual rails together, _-~.cving any
resu~tinc ex-ess material from the weid s tes and
subsecuer _iy SL~ ecting the weld sites at a-G ~el 5W the
foot of each -G' 1 ~0 air cooling, grinding an~ _een n,g,
ste?s of we ~i~g individual rails together, r-~ov_~g any
resultinc ex_ess material from the wel~ s ~es and
subse~uently s~Djecting the weld sites to -a~urG air
and/or i-aepe-~ent forced cooling and grin~ -~ 5~ c~her
SUr_GCe ~lla'_e-' G technique and/or peening.
As-r~ ~ai' grades such as 700 and 90~ crmal
cual ty a-.G C-~a~e A/AREA) achieve weld :~AZ ka-~-_ss _-vels
simi ar -_ ~h5Ce O~ the parent rails fo_low--~ -a.ur_l air
coo'_ns o~ _:~e we as in still air. These r~ fo~:owing
welc-ng a~e, the-efore, not normally _~_ ec_-~ to
acce~e_a_ed c~cling. It has, however, ~25~ ~ound
aavartageous _o increase the hardness/s_---gt:~, in
partlcula-, o- the welded rail foot by accele-~_-d cocling
with a view ~~ improving the bending fatigue â__eng_h of
the weldG~ ra- .
Acc-ler__ed cccling may be achieved by a_?~v g air,
air mist o- wa_er under pressure independent~~ _~ t:-- head
and/o- t-e we~ a.c/or the foot cf the rai a eac:- weld
site. ~ y, forced cooling is applie- --r~u h the
a?pro?r--~e -:-_se transformation temperat~ an -, eg
a-ls.--._t_ _~ __a_: te, austeni.e t~ bainite cr c_a_e~i_- to
mar~ s--e. T~ cally cooling is in- ~ c a. a
te~?cra_~_r_ w~ . the high tem?eratu~e aus~ :-a~- of
CA 0221~941 1997-09-19 PCT/GB96/00458
VVO 96/29~69
the steel and stopped at a temperature deemed s-aitable for
the completion of a given phase tra..sformation.
Accelerated cooling may typically be effected or a period
of between 30 and 120 seconds. Forced cooi ng may be
effected by either individual hoods or nozzles positioned
above and to the sides of the head of the rai , opposite
each web of the rail, and below the foot sur_ace of the
rail, each independent hood or nozzle being supplied with
air, air mist or water under pressure from a common or a
different source. Thus cooling may be effecled to, for
example, only the rail head,foot or web, or to the entire
rail.
Fine grinding or surface material removal by another
technique of the welded regions may be applied o the head
and/or the underside of the rail foot and/or to the we~ of
the rail. Invisibly welding of a rail entails, however,
grinding or other surface material removal all around the
rail profile after welding of the indiviaual rails
together.
Robotic material removal techniques may be employed.
Surface material removal may be effectéd either cold or at
a temperature below the austenite temperature o. the steel
from which the rail is produced. Sensors may be provided
to ensure that the required material removal depth and
surface finish are achieved.
Peening may, for example, be by a shot or a h~er
peening process and may be applied to the underside of the
foot of the rail and/or to the head and/or web of the rail
CA 0221~941 lss7-os-ls
W096/29469 PcT/Gsg61004~8
or to the entire welded or unwelded (ie parent rail) rail
profile.
Peening may be achieved by directing a peening medium
under pressure independently to the ~oot and/or head and/or
web of the rail at each weld site. The depth of the
residual compressive stress layer may be, for example,
between 0.75mm and lmm and the compressive stresses
achieved may be, for example, of the order of 60% to 80% of
the yield strength of the rail material in compression.
Peening is generally carried out at a temperature below the
stress relieving ~emperature of the steel from which the
rail is produced, typically ~elow 250~C.
The invention will now be described by way of example
with reference to the accompanying diagrammatic drawings in
which:-
Figure 1 is a transverse section ta~en through a steelrail and shows substantially typical fatigue initiation
sites in the foot of the rail associated with bending
fatigue failures.;
Figures 2 and 3 are side views of accelerated cooling
apparatus in accordance with the present invention;
Figure 4 graphically illustrates the effect of forced
cooling on flash butt weld HAZ hardness of fully pearlitic
plain carbon rails;
Figure 5 graphically illustrates the generation of
residual stress in a shot peened steel after surface
grinding and showing beneficial compressive stress produced
following fine grinding;
CA 0221~941 1997-09-19
W O 96129469 PCTIGB96/004S8
Figure 6 is a side view of peening apparatus in
accordance with the invention;
Figure 7 graphically illustrates induced compressive
surface stresses and tensile stresses in a shot peened
steel material;
Figure 8 graphically illustrates longitudinal residual
stress distributions in a roller straightened steel rail,
a flash butt welded rail and a ground and shot peened
welded rail; and
Figure 9 graphically compares bending fatigue
strengths of a parent rail, a normal production weld, a
weld ground all around, a weld ground all around and shot
peened and a normal production weld after shot peening.
The welded rail illustrated in Figure 1 has a head 1,
a web 2 and 2 foot 3. Typical bending fatigue initiation
sites in the lllustrated welded rail are indicated by
reference numbers 4, 5 and 6. As will be seen, all of
these fatigue initiation sites are located in the foot
region of the rail. Of the sites illustrated, site 4
located in the base of the foot 3 is found to be the most
prevalent, and is generally caused by maximum tensile
stresses occasioned by the bending forces generated during
service.
As mentioned previously, it is an object of this
invention to provide a high integrity long welded steel
rail having bending fatigue life and wear performance at
least as good, if not better than, those of rails
manufactured currently. This objective is currently
CA 0221~941 1997-09-lg
W096l29469 pcTlGss6loo4s8
achieved by flash butt welding (otherwise called electrical
resistance welding) individual steel rails together,
stripping the resulting flash from the weld sites and then
subjecting the weld sites to one, more than one or indeed
all of the process steps of natural air or forced cooling,
fine grinding and peening. These process steps will now be
discussed in more detail.
Flash butt welding is a process in which the rail ends
to be joined together are held between water cooled copper
grips, which act as both clamps and electrodes. The first
stage in the flash butt welding sequence is generally
termed the "Burn-Off" or the "Pre-flashing" stage. During
this stage, the rail ends are separated slightly and
arcing/flashing is initiated between them in order to
square up the rail ends. "Burn-Off" times are typically
between ~ and 10 seconds, with 1-5mm of platten movement.
The next stage in the welding sequence is tAe preheating
stage. The main aim of the preheating stage is to heat the
rail ends to a sufficient temperature so that flashing
initiates easily, and a suitable temperature gradient is
achieved in the rail ends, prior to the onset of the final
flashing sequence. Preheating is generally achieved by
bringing the rail ends into contact, and allowing a high
current to flow across the rail ends. The rail ends are
brought together and allowed to heat by means of resistance
heating.
In between each preheating cycle a short period of
flashing is generally allowed to occur in order to maintain
CA 0221~941 lss7-os-ls
W096/29~69 pcT/GBs6loo458
rail end squareness. This is followed by the final
"flashing" stage. During this stage the rail interfaces
become molten, and the correct conditions for the final
upset or the forging stroke are achieved. The moving head
of the welding machine during this stage is accelerated
parabclically, with the resultant increase in the freguency
and number of flashing ruptures or arcs across the weld
interface. This ensures that the oxygen content at the
weld interface is reduced sufficiently to give a semi-
protective atmosphere. The primary purpose of the final
flashing stage is to generate enough heat to produce a
plastic zone that permits adequate upsetting. A total
flashing distance of between 9 and 15 mm, over a period of
to lO seconds, is typically employed for rails.
Immediately following the final flashing stage, the movable
platten of the welding machine is accelerated so that the
rail ends are butted together either under a constant
platten speed or under impact loading of up to 600 kN. The
load is calculated to give a pressure at the weld interface
of approximately 50-60 N/mm2, to ensure adequate weld
consolidation. The welding current is generally supplied
during the initial part of the forging operation to avoid
oxidation at the weld interface. During welding of rails,
a typical forging distance of between 12 and 20 mm is
generally employed. Following the completion of the
forging stroke, the molten and soft steel is forced out of
the weld joint. This extruded material, generally termed
"flash" is then removed quickly by means of an automatic
CA 0221~941 1997-09-lg
W096t29469 PcT/Gsg6/004sB
weld upset removal tool.
One recognised advantage of flash butt welding is that
good quality welds requiring no external weld metal can be
produced in a relatively short period of time, typically
within 45 to 90 seconds.
Other welding techniques may be employed, e.g. squeeze
welding.
Accelerated forced cooling is a process for rapidly
and controllably reducing the temperature of a product.
When used in the context of the present invention, the weld
sites, for example, of heat treated, plain carbon pearlitic
rail are cooled through the austenite to pearlite
transformation temperature range (typically from 700~C to
500~C) at, for example, at up to 7~C/second. Enhanced
cooling of the weld sites at the correct rate produces weld
HAZ hardness levels which match those of the parent plain
car~on heat treated rail. The effect of different forced
cooling rates on HAZ hardness can be seen from Figure 4.
Thus, the effect of independent forced air cooling of the
welded rail head, web and foot weld sites is effected with
a view to increasing the hardness and wear resistance of
the rail head, to improve the rolling contact fatigue life
of the welded rail head and bending fatigue strength of the
rail foot, and to improve the overall residual stress
pattern across the weld HAZ profile.
As-rolled rail grades such as 700 and 900 (BS11 Normal
quality and Grade A/AREA) achieve weld HAZ hardness levels
similar to those of the parent rails following natural air
CA 022l~94l lgg7-o9-l9 PcT/Gss6/00458
096/29469
cooling of the welds in still air. These rails following
welding are, therefore, not normally subjected to
accelerated cooling. It has, however, been found
advantageous to increase the hardness/strength, in
particular, of the welded rail foot by accelerated cooling
with a view to improving the bending fatigue strength of
the welded rail.
Apparatus for effecting forced cooling is illustrated
in Figures 2 and 3. In Figure 2 the cooling medium is air
and in Figure 3 water. As shown in Figure 2, cooling hoods
7 supported within a frame 9 and each connected by conduits
8 to a source of air under pressure are positioned above
and to each side of the rail head, alongside each rail web
and below the rail foot. The arrangement is such that one
or more (or indeed all) of the hoods may be connected to
direct cooling air under pressure onto the adjoining rail
surface. The flow of air through the conduits is
individually controlled by means of valves positioned, for
example, within the conduits.
In the arrangement illustrated in Figure 3, water is
supplied under pressure via nozzle ll to a manifold 12.
Cooling water is selectively projected onto the head, web
and/or foot of the rail by nozzles 14.
The effect of forced cooling on HAZ hardness of fully
pearlitic plain carbon rails is illustrated graphically in
Figure 4.
Surface grinding of the welded rail head is carried
out as a routine production procedure to match the profile
CA 0221~941 lss7-os-ls
WO96l29469 PcT/Gss6/004sg
of the parent rail. Alternatively, surface material
removal f-om the rail head may be carried out by another
technique. Additional surface grinding (or surface
material removed by another technique) to the web and foot
of the rail may be effected in the present invention with
a view to rendering the weld invisible to the eye.
Beneficial grinding (or other method of surface material
removal) of the weld sites may be effected to reduce the
number of potentia~ fatigue initiation sites, eg surface
pitting and by removing all traces of weld flash and
decarburized layer from each weld site, particularly that
from the rail foot which experiences tensile loading
stresses durins service.
Su_face material removal, for example, by grinding may
be carried out by use of a robotic grinder which
automat_cally fine grinds selected parts or each entire
weld si.e of tAe rail.
Grinding may be effected either cold or at a
temperature below the austenite temperature of the rail,
typically 700~C. Sensors may be provided to ensure that
the required grinding depth and surface finish are
achieved.
A graph illustrating the generation of residual stress
in a 4340 steel after surface grinding is shown in Figure
5. The beneficial compressive stress introduced by fine
grinding is to be noted.
In addition fine grinding or other surface material
removal technique particularly of the base of the welded
CA 0221~941 1997-09-19
PCT/GB96/004~8
WO 96/29469
rail foot also enables a full in-line automatic non-
destructive testing (NDT) and/or additional alternative
manual inspection of the welded rail to be c2rried out.
Thus, the rail head, web and foot can readily be
ultrasonically tested to a required specification, the rail
foot base can be eddy current tested, and the flatness of
the rail running surface and the rail head sides can be
inspected. Also, manual ultrasonic inspection of each weld
site for transverse defects can be effected with
considerable ease.
Peening may be effected by, for example, a shot or
~m~er peening process.
Shot peening is a cold working process in which the
surface of a part is bom~arded with small spherical media
called shot. Each piece of shot striking the material acts
as a tiny peening hammer, imparting to the surface a small
indentation or dimple. In order for the dimple to be
created, the surface fibres of the material must be yielded
in tension. Below the surface, the fibres try to restore
the surface to its original shape, thereby producing below
the dimple, a hemisphere of cold-worked material highly
stressed in compression. Overlapping dimples develop an
even layer of metal in residual compressive stress. It is
known that a compressively stressed zone increases the
initiation time required for a crack to form for a given
applied tensile stress range. Since nearly all fatigue and
stress corrosion failures originate at the surface of a
part, compressive stresses induced by shot peening provide
CA 022l~94l lgg7-o9-l9
W096129469 PCT/GB96/00458
considerable increases in part life. The maximum
compressive residual stress produced at or under the
surface of a part by shot peening is at least as great as
half the yield strength of the material being peened. Many
materials will also increase in surface hardness due to the
cold working effect of shot peening.
Shot peening apparatus for use, for example, with the
invention is schematically illustr~ted in Figure 6. This
apparatus comprises a plurality of nozzles 15 positioned
above, below and to the sides of the rail. Each nozzle is
mounted on a frame 16 and is connected to receive a source
of gas under pressure and shot. Each nozzle can be
independently controlled whereby the head, web and foot of
the rail can be together or selectively peened.
Benefits obtained by shot peening are the result of
the effect of the compressive stress and the cold working
induced. In the present invention, peening is effected to
welded rail and/or unwelded rail, i.e. parent rail, thereby
creating compressive stresses which act to mi~imise the
onset of crack initiation during service.
In one example, shot peening to the specification MIL-
S-1316~C was applied to the rails in the present invention.
This peening process employed a shot size of 1.375mm (MI
550) at an intensity of 0.012-0.014C. A range of shot
sizes and intensity levels can, nevertheless, be employed
to obtain beneficial compressive stresses.
Typical compressive stresses achieved by the process
are, for example, of the order of 60 to 80~ of the yield
CA 0221~941 lss7-os-ls
W096/29469 PcT/Gss6l00458
strength of the rail material in compression. The depth of
the residual compressive layer produced by peening is, for
example, typically between 0.75mm and 1.Omm. Peening is
generally effected at a temperature below the stress
relieving temperature of the steel from which the rail is
produced, typically below 250~C.
The effect of induced compressive and tensile stresses
in a shot peened material with no external load is
illustrated in Figure 7 of the drawings and ~igure 8
graphically illustrates longitudinal residual stress
distribution in a conventional rail, a weld and a shot
peened weld.
A comparison of the results of 4-point bend fatigue
testing of conventional rails and welded rails in
accordance with the present invention can be seen from
Figure 9. This Figure shows that the high integrity welds
produced in accordance with this invention and labelled "3"
"4" and "5" possess bending fatigue strength levels
significantly in excess of that of the parent or unwelded
rail labelled "1".
Bending fatigue data for as-rDlled welded rails which
have been either ground or ground and shot peened have been
found to be at least as good as, if not better than, those
of the parent rails. These rails have shown similar
overall trends to those observed in the case of mill heat
treated rails (MHT) as exemplified in Figure 9.
The techniques mentioned above concerning the
production of high integrity invisible welds may also be
CA 022l~94l lss7-os-ls
W096/29469 . PCT/GB96/00458
applied readily to all grades of as-rolled and heat treated
pearlitic rails and to any other additional rail grades
developed in the future.
In addition, the techniques mentioned above may be
applied readily to the bainitic and martensitic steels
disclosed in our co-pending patent applications 950097.1
and 9313060.
It will be appreciated that the foregoing is merely
exemplary of rails in accordance with the invention and
that modifications can readily be made thereto without
departing from the true scope of the invention.
