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RAIL WELD FAILURE

Now a days cases of rail fracture or failure is increasing day by day due to growth in rail traffic.It is very difficult to control for P.Way Engineer.Mostly it is occurring early in the morning hours.


Existence of the rail joint has been considered as necessary evil since the beginning of railway era. The presence of a gap between two rail ends was generally thought to be the prime cause of impact at the joints. Although the provision of long rails had the effect of reduction in the number of joints, it was disfavored as it increased the intensity of impact, if proportionately wider gaps where to be provided at the ends. Welding of rails in long lengths did not , therefore, appear to be technically interesting. later it was realized that the intensity of impact is not directly proportional to the width of the gap, and the gap itself need not be exactly proportional to the length of the rail, as the rail could not really behave as like a free rail, when laid in track. with this new knowledge, any reduction in the number of joints was welcome as it had the prospects of minimizing , if not completely eliminating, the problems connected with the jointed rail track. SO, before describing rail weld failures , welding philosophy and its techniques need to be studied. The welding techniques used in India with their pros and cons can be seen here....

Rail Weld Failures in India

Rail and weld failures are more numerous in the winter months, and during severe cold weather the Trackman should be especially watchful for signs of failures. With the increasing speed and heavier axle loads the failure rate also increases. Amongst weld failures are approximately 66%, with rail failures contribute 34%. The causes of Rail and weld Failures and the remedies constitute a problem which requires the most careful study by every permanent way engineer of railways. The elimination of Rail and weld failures in a practical way is the subject of careful investigation by the permanent way engineers. To study it intelligently one must have adequate knowledge about the different types of rail and weld failures. Though it is an internationally accepted fact that rail and weld failures cannot be eliminated fully due to various reasons, it can be controlled through proper welding and maintenance practices.

The theory of stresses in rails and the reasons for various types of defects can be seen at this page

Failures in AT Welds

In India the population of AT Weld is very high. the At weld is quiet inferior in quality and strength as compared to FB welds. initially 13 m rails were welded , but now a days 10 or even 20 rails FB welded panels are in use.and the emphasis is on complete replacement of AT welds with FB welds, doing the field welding using mobile flash butt welding plants.
Conditions and parameters affecting the strength and durability of AT Welds:

1.Gaps at the rail joint may not be proper,

2.Edges may not be straight and leveled

3.Pre-heating may not be upto the desired temperature

4.Portion used may be outdated

5.Chemical composition may vary

6.Welding staff may nit be competent

7.Removal of excess material may be done by chisel

8.Crucible and moulds may be defective

9.Weather conditions may affect the quality

10.Time allowed to gain the strength of weld may not be sufficient

so, all the above listed parameters are prone to vary in the field.

Failures and their causes in different types of welding

Alumino-thermic welds

The thermite welding method is widely applied as in-track welding method. here most of the damages are in form of transverse fissure initiated in the rail base. Welding discontinuities, that cause transverse fissure in the rail base, are mainly “lack of fusion (no fusion between weld metal and rail)” and center-line shrinkage (crack occurred by rail movement during solidification).” As the other causes of transverse fissure in the rail base, there are “porosity (a lot of large low holes in the weld metal caused by bad welding material)” and “stress concentration (excess reinforcement of the rail base in old type thermite weld).”

most of the transverse fissures in the rail web are caused by “solder cracks” occurred when a remaining part of solder for rail bond connection was reheated. Transverse fissures by solder cracks account for about 20% of he whole. On the other hand, horizontal fissures caused by “blow (addition of impact stress)” and “pit (blowhole in the surface of weld metal)” rarely occur in the rail web.

The solidification processes in alumino-thermic weld metal advances in the following order:

(1) Rail base

(2) Outer part of web

(3) Inner part of web

(4) Transition area from base to web

(5) Rail head

It was found that solidification cracking occurs when rails move under the conditions of a time of 90 − 160 s after pouring. Moreover, the same form as solidification cracks occurring at the transition area from the rail base to the web in failed alumino-thermic welds was simulated under the conditions of a time of 100 − 110 s after pouring and a rail movement amount of 0.4 mm. As a result, it is considered that the main factor behind solidification cracking is rail movement.

Precaution during ATW

It is necessary to apply a rail tensor on alumino-thermic welding execution to prevent rail movement. In addition, it is desirable to check the tensioning force of the rail tensor to monitor rail movement, because rails may move even if a rail tensor is applied.

Enclosed/submerged arc welds

Enclosed arc welds are damaged by fatigue failure. Initiations of fatigue cracks are liquidation crack (hot crack in heat affected zone)”, “solidification crack (hot crack in weld metal)” and “lack of fusion.” All transverse fissures in the rail web are initiated by liquidation cracks, and all transverse fissures in the rail base are initiated by lack of fusion. The main cause of formation of lack of fusion in the rail base is narrow root opening. Moreover, magnetic arc blow is one of the factors to produce a bad weld in installed rails; however, this is an inevitable factor. Most of transverse fissures in the rail head are generating liquidation cracks and/or lack of fusion as initiation points of damage. The damage by solidification cracks rarely occurred in high carbon enclosed arc weld metals applied to high carbon covered electrode.

Gas pressure welds

The transverse fissures in the rail base accounts for 57% and the failure in the rail web is 38%. The ratio of failure in in the rail web is larger than that in the head. Damages of transverse fissure in the rail base caused by “trimming cracks” and “repair” account for more than 90%. The “trimming crack” is a kind of hot crack, and occurs at the weld interface. If the metallic bonding strength is low, the interface is not able to endure the stress caused by plastic deformation in the trimming process just after welding. The “repair” means the remained discontinuity in spite of the execution of welding in order to repair the discontinuity. Damages of horizontal fissure in the rail web were caused by a lot of minute cracks from overheating, the high tensile residual stress and the addition of impact stress by hammer blow. On the other hand, it is thought that damages of transverse fissures in the rail web are caused by high tensile residual stress and corrosion of welded part.

Flash butt welding

Horizontal fissures in flash butt welds accounts for 50%. Most of rail web failures are caused by “solder cracks” like in the thermite weld. A transverse fissure in the rail web is caused by a striking blow in some track work at a welding discontinuity. On the other hand, all damage of transverse fissure in the rail head were developed by fatigue cracks initiated at the “flat spot” that was caused by oxide inclusions. Moreover, it is thought that the tensile residual stress affected these damages. Transverse fissures in the rail base were caused by “arc strikes” between electrode tip and rail base.

In nut-shell:

1) The form of damage is mainly the transverse fissure,and this accounts for 90% of the whole. Over 50% of total damage are the transverse fissure initiated in the rail base.

In thermite welds, welding discontinuities, that cause transverse fissures in the rail base, are mainly “lack of fusion” and “center-line shrinkage.” On the other hand, most of transverse fissures in the rail web are caused by “solder cracks.”

2) The enclosed arc welds are damaged by fatigue failure. Initiation of fatigue cracks is “liquidation crack”and “lack of fusion.” on the other hand, all transverse fissures in the rail base are initiated by lack of fusion.

(8) In gas pressure welds, damages of transverse fissure in the rail base are mainly caused by “trimming cracks” and “repair.” On the other hand, in flash butt welds, most of rail web failures caused by “solder crack,” and all damages of transverse fissure in the rail head were developed by fatigue cracks initiated at “flat spot.”

It is considered that solidification cracks are caused by the pulling apart of welded rails in the final stage of the solidification process. Solidification cracking is also known as “center-line shrinkage” and “hot tearing” in many countries. Approximately 80 per-cent of alumino-thermic weld failures due to solidification cracking occur within half a year of installation because solidification cracks are usually formed at the rail base and are relatively large.

Damage caused due to faulty rolling stock

1)Flat tyre in the wheel,continuously hammers the rail during the run and may cause Rail Failure/Weld Failure especially in winter.

2)Worn wheel will have hollow tyre and deep flange. Hollow tyres will hammer on outer side of Stock rail of SEJ,X-ing after ANC and make them to break.

Deep flange hammer the Jogglle Fish plates,fitted on weldings and make them to break.

Before on set of winter the rakes running in a section should be invariably tested for these defects and reported for rectification.

USFD testing of Rail-weld as per USFD manual of Indian Railways

for manual follow the link http://wiki.iricen.gov.in/doku/lib/exe/fetch.php?media=track_related_manuals:usfd.pdf

In addition to routine testing by USFD tester, hand testing is also done at welds using zero degree probe of 2 MHz

Classification of rail-weld defects

Classification Marking on both faces of web
IMR(W) Three cross with red paint
OBS(W) One cross with red paint
DFW Two cross with red paint

Action to be taken

1.IMR(W)

The flawed portion should be replaced by sound tested rail piece of not less than 4 m length within 3 days of detection.

Interim Action

PWI/USFD shall impose speed restriction of 30 kmph or stricter immediately and to be continued till flawed rail is replaced. He should communicate to sectional PWI about the flaw location who shall ensure that clamped joggled fish plate is provided within 24 hours.

2.OBS(W)

The rail/weld to be provided with clamped joggled fish plate within 3 days. PWI/USFD to specifically record the observations of the location in his register in subsequent round of testing.

Interim Action

PWI/USFD to advise sectional PWI within 24 hours about the flaw location. Keyman to watch during his daily patrolling till it is joggled fish plated.

3. DFW

PWI/USFD shall impose speed restriction of 30 kmph or stricter immediately. And sectional PWI will ensure-

(a) Protection of defective weld by joggled fish plates using minimum two tight clamps or two far end tight bolts, one on each side after which speed restriction can be relaxed upto 75 kmph for goods train and 100 kmph for passenger trains on BG and 30 kmph for goods train & 50 kmph for passenger trains on MG.

(b) In case the protection has been done using joggled fish plates and clamps, the defective weld shall be replaced within 15 days. However in case the protection has been done using joggled fish plates with two far end tight bolts, the speed restriction imposed in (a) above shall continue till the defective weld is replaced which should not be later than 3 months. The defective weld with speed restriction as (a) above may be continued in track if the track is to be renewed within 12 months.

RECTIFICATION OF RAIL FRACTURES (as per Manual of Instructions on LWR-1996 of Indian Railways)

(refer the manual http://wiki.iricen.gov.in/doku/lib/exe/fetch.php?media=track_related_manuals:lwrm1.pdf for all under mentioned paras, annexures and figures) Equipment required

i) Special 1 metre long fishplates with screw clamps and joggled fishplates with bolted clamps(for ractures at welded joints)

ii) Steel tape capable of reading upto one mm

iii) Alumino-thermic welding and finishing equipment

iv) Equipment for destressing

v) 6.5 metre long sawn rail cut piece of the same section as LWR duly tested by USFD

vi) Rail closures of suitable lengths

vii) Equipment for protection of track

viii) Equipment for night working

Procedure for repairs

If any fracture takes place on LWR/CWR, immediate action shall be taken by the official who detected the fracture to suspend the traffic and to protect the line. He shall report the fracture to the Gangmate/Keyman/PWI, who shall arrange for making emergency repairs to pass the traffic immediately. Repairs shall be carried out in four stages as described below:-

a) Emergency repairs to pass the traffic immediately

b) Temporary repairs

c) Permanent repairs

d) Destressing.

Emergency repairs

The fractured rails shall be joined by using the arrangements shown in Fig. 4.4.3(a) & (b) or ©. If the gap at fracture does not exceed 30 mm, insertion of any closure rail piece is not necessary. The traffic may then be resumed at a speed of stop dead and 10 km/h for the first train and 20 km/h for subsequent trains.

Temporary repairs

If a welding party is not readily available, the fracture shall be repaired by using a cut rail (not less than 4 metre long) and clamped/bolted as per arrangement shown in Fig. 4.4.3 (a) & (b) or ©.

i) A traffic block shall be taken as soon as possible preferably when the rail temperature is within the range specified for td in para 1.11.

ii) a) Two points on either side of the fracture shall be marked on the rail such that the length of closure rail (not less than 4 metre) to be inserted is equal to the total length of the rail pieces removed from the track minus allowances for two welds and saw cut (normally 51 mm). See Fig. 7.2.4.

b) Alternately two points on either side of the fracture shall be marked on the rail at a distance equal to the length of the available closure rail.The length of closure rail should not become less than 4 metre at the time of permanent repairs. See Fig. 7.2.4.

iii) The rails shall then be cut through at these points simultaneously,if possible. The closure rail shall then be inserted and joined as per para 7.2.3 of manual. After joining, the traffic shall then be resumed at restricted speed in accordance with Annexure-III. In case closure rail as per para (ii) (a) above is inserted, one of the joints may have to be provided with closure piece of adequate width and joined by one metre fish plate and clamps.

Permanent Repairs

i) If the fracture is such that, wide gap AT welding can be adopted, then the total length of fractured ends to be cut shall be equal to the gap required for wide gap welding. Once the two ends are cut, a gap required for wide gap welding will be created by using rail tensors and joint welded by wide gap AT welding technique.

ii) In case rail closure as per para 7.2.4(ii)(a) has been provided for temporary repairs, one joint of the closure rail shall be welded without rail tensor after setting correct gap for welding. However, to ensure correct gap during welding of the other joint, tensor shall be used.

iii) In case rail closure as per para 7.2.4(ii)(b) has been provided at the time of temporary repairs, the rail closure shall be suitably cut such that the length of the rail to be finally inserted in track is equal to length of rail removed from track after fracture minus allowances for two welds considering saw cut i.e. 49 mm (50 mm-1 mm (for saw cut)). Once the closure rail is cut, the closure rail will be welded as given in para 7.2.5 (ii).

iv) After welding of joints, a length of track equal to breathing length or about 125 metres on either side be unfastened and tapped to ensure equalisation of stress and then refastened.

References:

(i) The book on ‘ Welding Techniques’ march-2006, by IRICEN, Pune
(ii) thermit_welding_-_old_or_new_technology.doc a synopsis by R.S. Johnson - Eur Eng - Sen Mem TWI Director - Technical Sales, Thermit Welding (GB) Ltd
(iii) occurrence_conditions_and_preventive_methods_for_solidification_cracks.pdfa synopsis by Hajime ITO Researcher, Yoshihiro TERASHITA Assistant Senior Researcher,Mitsumasa TATSUMI Senior Researcher, Laboratory Head,Ryu-ichi Yyu-ichi AMAMOTO Senior Researcher,Hideki SHITARA Senior Researcher,Rail Welding Laboratory, Track Technology Division, Japan railways
(iv) analysis_of_damaged_rail_weld.pdf a synopsis by Yoshihiro TERASHITA Mitsumasa TATSUMI Researcher, Assistant Senior Researcher, Rail welding, Track Technology Div.
(v)quality_evaluation_methods_for_rail_welds_in_japan.doc a synopsis by Mitsumasa Tatsumi, Yasuto Fukada, Katsuyoshi Ueyama, Hideki Shitara and Ryu-ichi Yamamoto:Engineer, :Senior Engineer, :Chief Engineer Railway Technical Research Institute, Kokubunji, Japan
(vi)'Manual of Instructions on LWR-1996' of Indian Railways
(vii)'MANUAL FOR ULTRASONIC TESTING OF RAILS AND WELDS' of Indian Railways

 
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