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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G. Srinivasan ¹, Hemant Kumar ¹, V. Ramasubbu ¹, S. K. Albert ¹, A. K. Bhaduri ¹

Affiliations
1 Materials Technology Division, Indira Gandi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, IN

Abstract

Austenitic stainless steel has been widely used as an ideal structural material for nuclear, petrochemical and other industries, due to its very good resistance to corrosion and other required properties. Welding is an important, inevitable and reliable joining process in the fabrication of heavy structural components. Often weld repair becomes unavoidable due to the presence of unacceptable defects specified by various codes. However, as a good practice, repeated weld repair is limited to a maximum of two times during fabrication. In practice, often more than two weld repairs become inevitable, albeit in rare occasions. Hence, a study was undertaken to find out the effect of repeated weld repair upto four times on the structural integrity and mechanical properties of the joints of austenitic stainless steel. Variation in microstructure, 5-ferrite content, hardness, tensile properties and impact toughness with number of repairs was examined. Results show the properties of the weld joint are not significantly affected by the repeated weld repair upto four times. Hence, repeated weld repair upto four times can be permitted on austenitic stainless steel welds under rare occasions that are inevitable.

Keywords

Weld Repair, Austenitic Stainless Steel, Impact Toughness, Delta Ferrite Content.

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V. Shankar ¹, G. Srinivasan ¹, B. G. Muralidharan ¹, T. P. S. Gill ¹, V. K. Sethi ²

Affiliations
1 Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, IN
2 Nuclear Systems Division Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, IN

Abstract

Nickel base alloy 718, a precipitation-hardenable alloy, is used in the sodium-water heat exchanger of fast reactor steam generators. Although Alloy 718 is considered to be weldable, it is susceptible to fusion zone and heat affected zone (HAZ) cracking during welding. Two heats of 718 of thicknesses 2 mm and 4 mm were tested for hot cracking susceptibility using the moving torch Varestraint hot cracking test. A single bead technique was used to determine fusion zone and base metal HAZ cracking susceptibility. Alloy 600, a solid solution alloy, was also tested for comparison. The hot cracking tests showed that the Alloy 718 was much more susceptible to fusion zone and HAZ cracking than 600. The fusion zone cracking susceptibilities of the two heats of Alloy 718 are shown not to depend on linear heat input values used in the testing. This is presumably due to the differing cooling rates as slower cooling rates lead to a higher cracking tendency despite lower heat input.

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