Reconditioning of Points and Crossing

• The point and crossing components gets worn out due to passage of traffic and the wear reaches permissible limits in due course of time.

• It is neither economical nor practical to replace them straight away. The life of these components can be prolonged by reconditioning and thus these can be utilized to maximum extent.

• Heat Treated/welded crossings will have same composition as that of 90 UTS rails but hardness of 330-340 BHN.

• MM rails are generally not used in manufacturing of points & crossings now

SUITABILITY FOR RECONDITIONING

• Only suitable components should be taken up for reconditioning.

• Component should be in sound structural condition.Components containing cracks on the worn-out portion having depths more than 3 mm beyond condemning size should not be taken up for reconditioning .

• Wear should be within prescribed limit. 

• Component should be tested ultrasonically to decide service ability.

TECHNIQUES FOR RESURFACING

• Single Electrode Welding

– Only one type of electrode is used to make up the entire worn out position. This system is standardized and in vogue on IR.

• Double Electrode Welding

– A buffer layer having material properties intermediate between the base metal and the top layer is laid using a different kind of electrode. Presently not adopted on IR.

• Flux Cored Continuous Wire Welding

– In this technique, the feeder wire (flux cored) is continuously fed into the molten metal pool. Has several advantages but not suitable for CMS

crossing.

ADVANTAGE OF CONTINUOUS WIRE WELDING

• Faster welding as higher current can be used and rate of deposition is more.

• More arcing hours are possible per shift as there is no stop and start and slag cleaning time also gets reduced.

• Wastage of stub end is eliminated.

• Welder fatigue is reduced and weld of superior appearance is obtained.

• Less operator skill is required.

• Better penetration is achieved

SYSTEMS FOR RECONDITIONING

• In-situ Reconditioning

– Is carried out on track either under block or under caution order

Advantage

Traffic block; transportation not required

Reconditioning can be done even if spares are not available (Imp in case of Special. Layouts) .

Disadvantage

Difficult to ensure quality. Not suitable for CMS Crossings.

• Depot Reconditioning : Components are brought to welding depot and then they are reconditioned.

Advantage – Better Quality

Disadvantage – Spares, traffic block, transportation arrangement are required. Cost is more.

WELDING ELECTRODES

• Only H3 class of electrodes (H3, H3A and H3B) manufactured by firms approved by RDSO are to be used. RDSO to circulate list every July.

• Traffic carrying capacity of electrodes (of H3 class)

– H3 – Min. service life of 15 GMT

– H3A – Min. service life of 25 GMT

– H3B – Min. service life of 35 GMT

• Electrodes having cracked and damaged flux covering to be discarded.

• Damp Electrodes not to be used. Either packing of electrodes should be absolutely intact and all the electrodes should be consumed within six hours of opening of pack.

• Or electrodes to be dried at 130º- 170º for at least one hour.

• Only 4 mm dia electrodes to be used

IMPORTANT STEPS OF RECONDITIONING PROCESS

• Surface Preparation

Grind the surface by pneumatic or electrical grinder to remove adherent scales, deformed and work hardened metal and surface cracks. Normally grinding depth should be upto 2 mm. Depth of worn out area should not be less than 3 mm in case of single electrode system. 

After grinding the area should be tested for crack by Dye Penetration Test in case of CMS crossing and by DPT or magnaflux for others.

• Storage of electrodes – As per para 11.2 of IS : 814-1991

• Welder – Only trained welder should be used

• Reconditioning of Switch : Recondition the Stock Rail first. A wornout tongue rail shall be reconditioned in closed position i.e. resting against the stock rail. In case of broken tip the TR is to be built up initially in the open condition and thereafter it should be closed with SR to attain final profile.

• Welding Current – The current range as recommended by the manufacture of electrode should be adhere to.

• Electrical Appliances – All appliances should be properly earthed. Generator should be capable of supplying 200 amperes at 60% duty cycles OCV (Open Circuit Voltage) should be in the range 65V to 85V.

• Welding - to be done in flat position in proper sequence. Care to be taken to fill the crater to the full weld size before breaking the arc. Weld metal to be deposited to provide an excess of 3 mm to be finished by grinding. Normally thickness of weld deposits should not exceed following limits (mm):

• Grinding: The reconditioned area should be ground to the original contour. Grinding wheel should be continuously moved back and forth and not stopped at one location to avoid high localized heating and subsequent cooling.

• Passage of Trains:

Trains can be passed at normal speed over the weld metal on crossing even before completion of welding. After passage of train, welding can be restarted. After passage of train, welding can be started again. However, weld metal should be allowed to cool for 2 to 3 minutes before allowing passage of train.

• Testing and Inspection

1. Visual Inspection: Check for surface defects – undercut, slag inclusion, porosity, crack etc.

2. Dimensional measurement: By straight-edge and template

3. DPT or Magnaflux test to detect surface cracks

4. Rectification

If cracks found, the portion should be gouged either by pneumatic gouging or grinding and remaining portion should be reexamined for crack. If not cracks are found, the portion should be rewelded, ground and inspected.

• Periodical Inspection

Resurfaced Point & Crossing should be inspected quarterly. Wear on crossing should be recorded at ten specified locations and on TR at seven locations starting from toe towards heels at distance of 100 mm each.

STEPWISE RECONDITIONING PROCEDURE FOR MM/90UTS P&C

• Suitability for reconditioning

• History of crossing : History should be collected and recorded on a card or in a register

• Surface preparation

• Selection , Inspection and Drying of electrodes

• Preheating: The component should be preheated on the surface by to and fro play of oxy-acetylene torch to obtain 2500C to 3000C.

• Welding operation: In flat position following the recommended sequence.

Welding to start immediately after preheating. Craters to be filled up. During restart arc should be stuck ahead of crater and then drawn back. Slag should be removed thoroughly in between the passes. Interpass temperature of 2500 – 3000C should be maintained.

• Grinding

• Testing

• Inspection.

SEQUENCE FOR RESURFACING

STEPWISE RECONDITIONING PROCEDURE FOR CMS CROSSING

• Suitability for reconditioning to be ascertained.

• History of crossing: should be recorded on a card or in a register.

• Surface preparation : Magnaflux cannot be used for detecting crack as material is non-magnetic

• Selection, Inspection & Drying of Electrode

• Welding plant: DC generator with reversed polarity is recommended.

• Preheating not required.

• Welding operations –In flat position following recommended sequence. The temp during welding process not to go up beyond 150 ºC. This is to be ensured by proper intervals between the runs and by means of compressed air jet or water quenching after welding. Or, the Xing may be kept submerged in water bath with only top 1 cm above water level. At a time only 7-8 cm length to be deposited. Electrode should be held at 450 to the direction of welding. Arc length should be equal to electrode diameter (4 mm). Welding cycle should be short (2 min).

• Grinding – During grinding water should be sprayed frequently to prevent

heating.

• Testing & Inspection.

PRECAUTIONS IN METAL ARC WELDING

• During metal arc welding harmful ultraviolet and infra-red rays and fumes are emitted. Hot metal, slag and spatter can also be hazardous to the operators.

• Precautions should therefore be taken to safeguard from

– Electric shock

– Radiation

– Scattering hot particles & globules

– Flying pieces of slag

– Heats and Fumes