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P. Rodriguez ¹, T. P. S. Gill ¹

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, IN

Abstract

The weldability of any material can be classified into two categories namely, weldability for fabrication and weldability for service. While the weldability for service aspects have received considerable attention from research workers in India, the weldability for fabrication has been a neglected field. This paper reviews the above two aspects of weldability as applied to common fabrication materials. The available information has been critically analysed and areas where further research efforts should be directed are outlined.

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T. P. S. Gill ¹, J. B. Gnanamoorthy ¹

Affiliations
1 Materials Development Laboratory Reactor, Research Centre Kalpakkam-603 102, Tamilnadu, IN

Abstract

The presence of delta-ferrite (2 to 10%) is essential in austenitic stainless steel weld metals to avoid the problem of hot tearing during solidification. However, during elevated temperature service or post-weld heat treatment, the delta-ferrite transforms to various secondary phases. The kinetics of transformation of delta-ferrite depends mainly on the growth kinetics of secondary phases, which in turn is dictated by the alloy composition. In this paper the results of investigations for the transformation kinetics of delta-ferrite conducted on several type 316 stainless steel weld metals of different compositions are given. Based on these data, it is found that the ageing time required to transform a given fraction of ferrite varies directly with "normalized equivalent chromium content" (NECC) of the alloy and that high values of NECC lead to a rapid decay of delta-ferrite.

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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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Placid Rodriguez ¹, A. K. Bhaduri ¹, T. P. S. Gill ¹

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

Abstract

The management of power plant components is discussed from the perspective of the repair welding philosophy including applicable codes and regulations. A few case histories of repair welding of power plant components are discussed with special emphasis to steam turbine components. Details of indigenous repair welding of cracked steam turbine blades and shrouds in some of the Indian nuclear power plants are also presented.

Keywords

Repair Welding, Power Plant Components, Repair Philosophy, Codes and Regulations, Case Histories, Steam Turbine Components.

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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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N. Parvathavarthini ¹, R. K. Dayal ¹, T. P. S. Gill ²

Affiliations
1 Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, IN
2 Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, IN

Abstract

Weldments of 9%Cr-1%Mo steel may become prone to accelerated localized corrosion attack if inadequately post-weld heat treated. The phenomenon is mechanistically identical to that observed in austenitic stainless steels where Cr-depleted regions along the grain boundaries are attacked in preference to the grain interiors. This paper describes a systematic study carried out to investigate the influence of initial microstructure, heat input, PWHT, accelerated ageing and repair-welding on the sensitization behaviour of 9%Cr-1%Mo steel weldments. It was found that repair-welding without PWHT resulted in the sensitization of the weld metal and base metal heat affected zones. Ageing of such repaired weldments led to the sensitization of both weld metal and HAZ. This study has also established the increased susceptibility of the sensitized regions to hydrogen induced cracking thus indicating that the region is microstruci.’rally weaker, where cracking due to environment may initiate preferentially.

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S. K. Albert ¹, T. P. S. Gill ¹, P. Rodriguez ¹, S. L. Mannan ¹, S. D. Kulkarni ²

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam, IN
2 Indian Institute of Technology, Powai, Mumbai, IN

Abstract

Hardness variation in the Cr-Mo steel weld metal as a function of Cr content and weld cooling rate was studied Chromium in the weld metal was varied by adding known amount of high purity Cr into the weld pool produced by a stationary arc. The cooling rate of the weld was measured by plunging W-Rh thermocouple in to the weld pool just at the time of extinguishing the arc and measuring the variation in temperature as a function of time using a X-t recorder. The microstructural studies revealed that in the low Cr steels the structure varied from fully martensitic to a mixture of bainite and martensite as the weld cooling rate decreased while for weld metal with Cr content higher than 5 wt% the structure remained fully martensitic irrespective of the cooling rate. The measured hardness of the weld metal for different Cr contents and cooling rates was compared with that predicted by Yurioka's and Hart's formulae. It was found that Yurioka’s formula can be used to predict the hardness of the weld metal with reasonable accuracy. It was further noticed that there is a reduction in the maximum hardness at Cr content above 5wt% and it is attributed to the presence of delta ferrite in the weld metal.

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S. K. Albert ¹, T. P. S. Gill ¹

Affiliations
1 Materials Development Division Indira Gandhi Centre for Atomic Research, Kalpakkam, IN

Abstract

Hydrogen Assisted Cracking (HAC) is the most widely encountered cracking problem in the welding of ferritic steels. One of the methods to mitigate this type of cracking is to control and minimize the amount of diffusible hydrogen content in the welds. This, in turn, calls for standard test methods to measure diffusible hydrogen present in weldments made with different welding consumables, processes etc. This report describes test methods to measure diffusible hydrogen in the welds as per different International Standards. It also highlights the deficiencies in the present Indian Standard lor measurement of diffusible hydrogen and suggests revision of the standard to cover Submerged Arc Welding, Gas Metal Arc Welding and Flux Cored Arc Welding Processes in addition to Manual Meta! Arc Welding Process. It also recommends inclusion of Gas Chromatography Method and removal of glycerin method in the Bureau of Indian Standard (BIS) specifications.

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S. Chakravarty ¹, T. P. S. Gill ¹, A. K. Joseph ², Pennathur ², N. T. Sebastian ²

Affiliations
1 Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, IN
2 Kemtrode Pvt. Ltd,, Bangalore 560 001, IN

Abstract

Austenitic stainless steels are commonly used as structural materials for cryogenic temperature application. The components fabricated using welding require that the welds should have desirable properties at cryogenic temperatures. This necessitates' the selection of right type of electrodes to provide a weld metal with beneficial properties such as tensile strength and impact properties. The various factors which characterize the weld metal such as solidification mechanism, optimum control of delta ferrite, resistance to hot cracking, minimum inclusion content, type of coating, heat input and the addition of rare earths are discussed with a view to elucidating the effect of these m improving the weld metal pertormance.

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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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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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