Informatics Publishing Limited

Girish Kumar Padhy ¹, V. Ramasubbu ¹, N. Murugesan ², C. Remash ², S. K. Albert ²

Affiliations
1 Material Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, Tamilnadu, IN
2 Material Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, Tamilnadu, IN

Abstract

Diffusible hydrogen (H_D) measurement in steel welding consumables having cellulose, rutile and basic coating has been carried out using a Proton Exchange Membrane Based Hydrogen Sensor (PEMHS). The sensor is an electrochemical fuel cell based device which uses Nafion@117 as proton exchange membrane electrolyte. This can detect hydrogen in an Ar+H₂ mixture with detectable limit of 1 ppm. Further, HD measurements have also been carried out on basic coated electrodes of modified 9Cr-1Mo steel, with very low levels of H_D content. Results obtained have been compared with those obtained from H_D measurement using mercury manometer as per standard ISO 3690. One to one correlation has been obtained between these two different methods of measurements. This sensor has shown good sensitivity, accuracy and precision hence is reliable for H_D measurement. In addition to the above measurement, this method was used to study hydrogen evolution from the weldments as a function of time. The paper presents and discusses the principles of H_D measurement using this sensor, its applications for H_D measurements in weldment, the results obtained, its application to study the hydrogen evolution from weldment as a function of time and the possibility of using this sensor for measurement of hydrogen evolved from the weld specimens at high temperatures.

Keywords

Diffusible Hydrogen, Nafion Hydrogen Sensor, Hot Extraction, Hydrogen Diffusivity.

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S. K. Albert ¹, T. P. S. Gill ¹, V. Ramasubbu ¹, S. D. Kulkarni ²

Affiliations
1 Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, IN
2 Indian Institute of Technology, Mumbai-400 076, IN

Abstract

In this paper results obtained from diffusible hydrogen (HJ measurement and hydrogen assisted cracking susceptibility (HAC) of three different Cr-Mo steel welds are discussed. H₀ measurements were carried out using a gas chromatograph and HAC susceptibility studies using the ur-modified hydrogen sensitivity test (UT-Modified HST). Specimens for both these studies were prepared by employing identical welding parameters. Results showed that with increase in alloy content the cracking susceptibility of the steel increased while H₀ content in the weld decreased. The decrease in the H₀ content is explained based on decrease in the apparent diffusivity and increase in the apparent solubility of hydrogen with alloy content. These are in turn attributed to increase in the density of hydrogen traps with alloy content. Regression analysis of the results showed that it is possible to predict the safe preheat temperature for the steel from the composition if H₀ content in the weld is known.

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M. Divya ¹, C. R. Das ¹, S. K. Albert ¹, V. Ramasubbu ¹, A. K. Bhaduri ¹, P. Sivaraman ¹

Affiliations
1 Materials Joining Section, Materials Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, IN

Abstract

Cracked shrouds of the 3^rd stage of a Low Pressure turbine was in-situ repaired by removing the cracked pieces of the shroud and welding new shroud pieces to the existing shroud. The shroud material was made of AISI414 martensitic stainless steel (SS), and the repair welding was carried out using ER 410NiMo filler wire. The tenon heads of the blades, which were removed for carrying out the in-situ repair, were also built-up by weld deposition. The repair welds were subjected to in-situ two-stage post weld heat treatment (PWHT) as required for the 414 SS material and the 410NiMo weld metal. For prior to simulation of the constraints of actual in-situ repair, a mock-up piece was made using the same blade material, welding consumable, welding procedure and PWHT as were to be used for the actual repair. After successful completion of the repair, the mock-up piece as well as separate weld pads prepared using the shroud material and ER 410NiMo consumable were subjected to detailed microstructural characterization and mechanical properties testing to generate data on the properties of the repair weld now in service. The paper discusses the details of the in-situ repair and results of the characterization of the weld joints. Results confirm the repair weld has adequate strength and ductility. The turbine with repair welded shrouds has been performing satisfactorily since 2008.  

Keywords

Repair welding, Turbine Shroud, PWHT, Supermartensitic Stainless Steels.

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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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S. K. Albert ¹, V. Ramasubbu ¹, T. P. S. Gill ¹

Affiliations
1 Materials Joining Section, Materials Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam - 603102, IN

Abstract

Hydrogen Assisted Cracking (HAC) susceptibility of modified 9 Cr-1 Mo (T91/P91) steel has been studied using UT-Modified Hydrogen Sensitivity Test. Autogenous bead-on-plate welds were made on specimens clamped in a copper fixture using Gas Tungsten Arc Welding process. Hydrogen was introduced through the shielding gas and pre-heating of the specimens was done by heating the copper fixture to the desired temperature. Immediately after welding, specimens were strained to a known strain level in a straining jig for 24Eh and subsequently checked for cracking. The critical preheating temperature, above which no cracking occurred was measured for different hydrogen levels and strain levels. The results indicated that without preheating cracking occurs in this steel even at hydrogen levels as low as 0.25 vol.% in the shielding gas. The critical preheating temperature was found to be a strong function of strain level, while for a given strain level the critical preheating temperature did not vary appreciably with hydrogen content in the shielding gas. Microstructural observation of the cracked samples showed that in this steel cracking occurs both in the weld metal and coarse-grained heat affected zone, in contrast to that obtained in the 9Cr-1Mo steel where cracking is predominantly confined to weld metal. Thus, the present study showed that modified 9Cr-1Mo steel is highly susceptible to HAC and cracking can occur even at very low hydrogen levels.

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S. K. Albert ¹, T. P. S. Gill ¹, V. Ramasubbu ¹, F. C. Parida ¹, S. D. Kulkarni ²

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, IN
2 Dept, of Met, Engg. and Mat, Sci., l.l.T. Powai, Mumbai 400 076, IN

Abstract

Hydrogen Assisted Cracking Susceptibility of 2.25Cr-1Mo steel was studied using UT-Modified Hydrogen Sensitivity Test (UT-Mod. HST). Welding was carried out using GTAW process and hydrogen was introduced into the weld metal by mixing it with argon shielding gas. Autogenous bead-on-plates were made on specimens fixed on a copper fixture and the specimens were strained for 4% strain immediately after welding for 24 hrs and checked for cracks. Preheating during welding was achieved by heating the copper fixture and the minimum preheat temperature required to prevent cracking for known vol.% of hydrogen in the shielding gas found out. Diffusible hydrogen content (H₀,) in the specimen prepared under identical conditions employed for testing was measured using Gas Chromatograph Weld cooling cun/es were obtained by plunging a thermocouple into the molten weld pool and cooling time (t_8/5)was estimated from the curves. Variation in weld metal hardness and microstructure was also studied. The results showed that both minimum preheat temperature and (H₀) content increased with hydrogen in the shielding gas. Preheating results in reduction in (H₀) content but does not affect the hardness indicating that cracking is prevented due to reduction in (H₀) content in the weld prepared with preheating. The results show that it is possible to choose an optimum preheat temperature to prevent cracking if (H₀) level in the weld is known.

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