Indian Welding Journal

P. Ashok Kumar ¹, Nikhil Shajan ¹, Kanwer S. Arora ¹, M. Shome ¹

Affiliations
1 Material Characterization and Joining (MCJ) Group, Research and Development Tata Steel, Jamshedpur, Jharkhand - 831001, IN

Abstract

P460NH steels are extensively used in steel plants as ladle fabrication material. However, fusion welding of these steels leads to the problems such as cold cracking, residual stress, distortion and fatigue damage, these steels may require welding of the cracks that can develop during fabrication, handling and transportation stages, or during the service life of the plant. Present work explains repair welding procedure used for P460NH steel ladles welded by using shielded metal arc (SMA) welding and flux cored arc (FCW) welding processes and evaluation of its weld metal and heat affected area (HAZ) microstructure. Butter bead tempered bead technique followed to fill the joint groove and simulated repair welding of SMA and FCA welds was carried out at the weld/base metal interface, i.e. at the location at which cracks are usually reported to occur during fabrication. SMA repair welding procedure conforming to the ASME Boiler and pressure Vessel Code were used followed by post heating of welds at 300°C for 2 hours per 25mm thickness of the weld. Tensile properties, bend tests, hardness profiles, impact tests and metallography studies using scanning electron microscope (SEM) were determined for both SMA and FCA welds to evaluate simulated repair welds. Analysis of the test results showed that the cooling rate maintained by means of preheating, interpass and post heating plays an important role in repairing of P460NH steel weld cracks.

Keywords

Repair Welding, Ladles, Welding Procedure, P460NH Grade Steel, Microstructure.

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Nikhil Shajan ¹, Kanwer Singh Arora ¹, Mahadev Shome ¹, Atanu Roy ², US Patnaik ², Niranjan Behera ³, V. Santhanagopalan ³

Affiliations
1 Research and Development, TATA Steel, Jamshedpur- 831001, IN
2 PLTCM, TATA Steel, Jamshedpur- 831001, IN
3 Product Technology Group, TATA Steel, Jamshedpur- 831001, IN

Abstract

Flash butt welding (FBW) is extensively used for joining front end of incoming hot rolled coil to the tail end of the previous coil in pickling and tandem cold rolling mill (PLTCM). Improper welding causes failure in the cold rolling strands in the later stages. Hence, it is of paramount importance to establish a sound weld line by selecting the right welding process parameters. Optimizing the process parameters largely depends on the thickness and chemistry of the steels because slight variations in steel chemistry can cause weld failures. Recently, during flash butt welding of hot rolled Interstitial Free (IF) steel from TSK (Steel A) in the cold rolling mill (CRM) of TSJ (Steel B), adjustments had to be made in the welding parameters to enable smooth cold rolling. The flash butt welded TSK coils showed oxides of Fe, Mn, Al and Ti on fractured surfaces with the standard TSJ welding parameters. Back up study suggested that higher Mn, Al and Ti levels in the TSK coils were possibly responsible for greater volume fraction of oxides at the weld zone. Since, flash butt welding is done without the use of shielding gas, oxidation of the molten pool happens at the abutting edges. Improper process parameters can lead to the entrapment of oxides at the fusion line causing failure initiation. Therefore, this called for a change in welding parameter from the set value. Increasing the flashing length and upset length enabled in removing the excessive oxides formed during the flashing stage and prevented failure of the welds. The downstream end has hence been stabilized.

Keywords

Flash Butt Welding, Oxidation, Interstitial Free Steel, Cold Spot/penetrators.

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