Repair Welding of Cracked Girth Gear of Cement Plant

BACKGROUND  FOR SUCH REPAIR REQUIREMENT :

• At one of the cement plant in North India, the Girth Gear was removed for the mill after observing cracks during the inspection. The spare Girth Gear was installed and this cracked one was given to WRTPL for welding.
• For a Cement Plant, Stoppage of the mill due to Girth Gear breakdown makes it to come to stand still. Such plants put up at enormous costs running into several 100 Crores of Rupees means hourly loss of more than 5 Lacs Rupees. This is unaffordable for being cost competitive.
• Plants normally have spare Girth Gear to cope up with such exigencies. However once the spare gear is replaced, to order a new one it will be costing more than 1.5-2 Crores Rupees and the delivery would be more than 1 Year. Also reconditioning costs only about 15-20% cost of new one and hence plants decide to keep the cracked one duly repaired as a spare.
• In-situ repairs are also done when cracks are not too many, but in this instance cracks were too many and hence the repair was done out side the mill after replacing the cracked one with the new one and starting the plant.
• The challenge was huge; as it had 36 cracks at various areas of the Girth Gear as shown below :

CONSUMABLES CHOSEN FOR THE REPAIR AND IT‘S UNIQUENESS :

FOR GOUGING : PRIME CUT ULTRATHERMIC CUTTING SYSTEM

• For crack removal, Primecut cutting system which is an exothermic cutting system
  was chosen.
• The Prime Cut Ultrathermic process offers speed in cutting or gouging a great diversity of materials; ease of operation; reduced noise, smoke and fumes; and portability.
• The Prime Cut Ultrathermic process is recommended for cutting materials oxyacetylene can not easily cut such as aluminum, cast iron, titanium, stainless steel, acetylene can not easily cut such as aluminum, cast iron, titanium, stainless steel, brass and refractory material. Even concrete and rock can be melted.
• Prime Cut Ultrathermicsystems use only oxygen and the consumable alloy is the fuel rod.
• There are no hazardous secondary fuels.
• Ignited by means of an electric spark from a 12v battery, the rods burn at temperatures in excess of 5537 deg C. Because Prime Cut operates at such high temperatures the target such high temperatures the target work material liquefies very quickly.
• However hardly any of above heat goes into base metal.
• Preheating is not required and radiated heat is reduced. Cutting is very fast.

GOUGING WITH PRIME CUT :

• Gouging can be performed with Prime Cut Ultrathermic equipment yet unlike air-carbon arc, it does not require a welding machine or air compressor.
• This makes for a safer, cleaner, quieter work environment. Equipment cost is reduced as well as the danger from high amperage, noise and smoke. is reduced as well as the danger from high amperage, noise and smoke.
• When gouging with the Prime Cut Ultrathermic process there is no carbon build up which reduces grinding or cleaning prior to welding.

MAINTENANCE ELECTRODE SELECTED FOR JOINING –XL –HTS NM :

XL-HTS NM IS DESIGNED TO GIVE EXCELLENT BENEFITS OF POST WELD HEAT TREATMENT
WITHOUT STRESS RELIEVING.

Advantages of using XL-HTS NM are as under :

• The weld metal has a similar thermal expansion coefficient compared to the base metal
• The weld metal acts as a hydrogen sink during welding and do not change the structure during the whole welding process.
• The weld metal does not generate brittle phases.
• XL-HTS NM welding can be accepted without PWHT where same is normally mandatory with conventional electrodes.
• Stresses are decreased by optimized yield strength, low thermal expansion coefficient, multipass stringer beads, peening etc…
• Low hydrogen inducement is avoided due to basic flux in electrode coating.

• Following the diagram, preheating upto 310° to 330°C and holding during the whole welding procedure is very important for homogeneous welding (composition of the weld metal very similar to the base metal) where cold cracks as under bead cracks in the base metal as well as root cracks, micro cracks or transverse crack in the weld metal are potential risks. It allows to avoid excessive martensite formation during welding even when welding thick and heavy sections (Preheating to core in the welding area).
  heavy sections (Preheating to core in the welding area).
• In case of homogeneous welding, the preheat and interpass temperature should be maintained during the whole welding procedure and even after welding for a sufficient time to allow transformation to a softer structure. Slow cooling of the component is recommended.
• Homogeneous repair welding (composition of the weld metal very similar to the base metal) is followed by an appropriate post heat treatment (in this case : stress relieving at 580° to 620°C / 8h is mandatory).
• As mentioned before, cold cracks usually appear during cooling below 200°C.
• However using XL-HTSNM through heterogeneous welding-the preheat and interpass temperature is maintained in the range 200 to 250°C , and no cracks occurred.(Preheating to core in the welding area).
• XL-HTS NM has an advantage that it has a similar thermal expansion coefficient compared to base metal, act as a hydrogen sink during welding and do not change the compared to base metal, act as a hydrogen sink during welding and do not change the structure during the whole welding procedure.
• This means in the case of heterogeneous welding with XL-HTS NM a preheat temperature range of 200 to 250°C can be sufficient in order to avoid cold cracking if certain rules are observed as follows:
    •  Heat input during welding must be a minimum (stringer bead technique).
    •  After welding, the preheating temperature must be maintained during one or several hours allowing to reduce the residual stresses.
    •  Slow cooling of the material is recommended.

• First clean the damaged area and checked DP.
• Before crack gouging welding of 20 mm round bars to avoid any distortions of teeth during welding.
• For crack removing, Prime cut cutting system was used.
• After gouging, grind the surface properly and make ‘U’ joint for welding and re-check DP to confirm all defect removed or not.

CONCLUSION :

The total welding took almost 20 days of round the clock working. The repaired Girth Gear was already installed by the customer when one of the mills Girth Gear broke down. It is now under working for last 1.5 years without any problem. This validates the consumables and process employed and open up a new world of opportunities for the core sector industry to successfully repair such critical components at fraction of cost of the new spares using the benefit of heterogeneous welding with XL-HTS (NM).