Indian Welding Journal

G. Srinivasan ¹, H. C. Dey ¹, A. K. Bhaduri ¹, S. K. Albert ¹, T. Jayakumar ¹

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, IN

Abstract

India has embarked on a mission program for setting-up a coal-fired 800 MWe Advanced Ultra Super Critical (AUSC) power plant with steam parameters of 710°C/ 720°C temperature 310 kg/cm² pressure. For the boiler tubes in the final stages of the superheater and reheater, the material chosen is 304HCu stainless steel (SS), a 18Cr-9Ni-3Cu-Nb-N austenitic SS (UNS S30432) as ASME code case 2328 and VdTUV 510 specifications. The 304HCu SS is a variant of the conventional 18Cr-8Ni austenitic SS, containing ~3% copper, increased carbon and controlled amounts of niobium and nitrogen, for enhanced elevated temperature strength, especially creep. India-specific version of 304HCu SS boiler tubes, of 52 mm diameter with 9.5 mm wall thickness, produced in collaboration with Indian manufacturers has been used for welding procedure qualification trials.

The objective of the present study is to choose the appropriate gas tungsten arc (GTA) welding consumable, from among ER304HCu (matching chemistry) and ERNiCrMo-3 and ERNiCrCoMo-1 (both nickel-base) filler wires, for welding of 304HCu SS. This involved qualifying the welding consumables, development of welding procedures for 304HCu SS tubes, and producing weld coupons for evaluation of mechanical properties and microstructural characterisation. Mock-up tube weld joints, with single-V groove joint geometry, were prepared using all the three filler wires to optimize the welding parameters. Weld defects like ischolar_main crack; hot crack and crater cracks were observed during the initial trials in weld joints made with the nickel-base filler metals due to the poor fluidity of the weld metal. These problems were overcome by suitably altering the weld joint design and optimisation of heat input. Subsequently, the weld joints were qualified, in the as-welded condition, by liquid penetrant and radiography examinations followed by tensile tests and bend tests as per ASME section IX requirements for preparing the Welding Procedure Qualification records. Based on detailed evaluation of mechanical properties, including creep, ER304HCu filler metal was chosen for welding of ER304HCuSS.

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H. C. Dey ¹, Alpesh Patel ², A. K. Bhaduri ¹, Shiju Sam ², S. K. Albert ¹, G. G. Roy ³

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, IN
2 Institute for Plasma Research, Gandhinagar 382428, IN
3 Indian Institute of Technology, Kharagpur 72130, IN

Abstract

The impact toughness of various regions of heat-affected zone (HAZ) of modified 9Cr-1Mo steel has been carried out on physically simulated specimens. For this, modified 9Cr-1Mo steel specimens were subjected to simulated thermal cycles in a Gleeble thermo-mechanical simulator at four different peak temperatures, viz. 860°C and 900°C for the inter-critical HAZ (ICHAZ), at 975°C for the fine grain HAZ (FGHAZ), and at 1200°C for the coarse grain HAZ (CGHAZ). HAZ simulated specimens were subjected to simulated post weld heat treatment (PWHT) at 760°C for 4h. Charpy V-notch impact toughness, hardness measurement and microstructure studies were carried out in the as-simulated and after simulated PWHT. Impact toughness after PWHT for all the four different peak temperatures is comparable to base material toughness. Microhardness measured across the simulated specimen showed substantial reduction in hardness after PWHT and hence recovery of ductility and toughness. This paper presents and discusses the results of this experiment investigation on the microstructure, hardness distribution and impact toughness of different regions of HAZ in modified 9Cr-1Mo steel.

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