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C. R. Das ¹, N. C. Majumdar ²

Affiliations
1 Botanical Survey of India, Calcutta, IN
2 Dept. of Botany, University of Calcutta, IN

Abstract

The paper deals with two important forest elements of Indian flora having some interesting restrictions in habitat and some significance on the position in which they are grouped. Proposals for the valid name have been made and the total change of the family name has been given with bases not yet taken into account by modern workers.

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C. R. Das ¹

Affiliations
1 Botanical Survey of India, Calcutta, IN

Abstract

No Abstract.

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C. R. Das ¹, A. Datta ¹

Affiliations
1 Botanical Survey of India, Howrah, IN

Abstract

No Abstract.

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C. R. Das ¹, A. K. Bhaduri ¹, V. Thomas Paul ¹, P. Krishnamraju ², S. K. Albert ¹, R. Balakrishnan ¹

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, IN
2 Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620 015, IN

Abstract

Modified 9Cr-lMo (P91) steel is used in fossil-fuel fired power plants due to its good thermo-physical, weldability, fabricability and high temperature mechanical properties. Toughness of the P91 weld metal, deposited by the shielded metal arc welding (SMAW) process, is reported to be lower than that deposited by the gas tungsten arc welding (GTAW) process. In spite of this, considering the higher deposition rate and economics, the SMAW process is very popular in industry. Thus, achieving adequate toughness in the P91 weld metal deposited using the SMAW process is an important requirement to qualify the weld joints.

Weld joints were prepared using the SMAW process and subjected to post-weld heat treatment (PWHT) at 760°C for durations of 3, 10 and 100 hours. Microstructural observations revealed coarsening of the lath martensite and the precipitates accompanied by an increase in toughness with increase in PWHT duration from 3 to 10 hours. The effect of PWHT duration on subzero toughness was found to be significant, with the subzero toughness increasing with increase in PWHT duration upto 10 hours and then decreasing on PWHT for 100 hours. Increase in sub-zero toughness could be attributed to adequate tempering, while the decrease in toughness on PWHT for 100 hours was attributable to the formation of fresh martensite during cooling from the PWHTtemperature. This variation of toughness of the SMA weld metal with duration of PWHT needs better understanding from the view point of the composition of the weld metal. Detailed microstructural analysis was carried out to understand the reasons for the variations in the mechanical properties. This paper presents and discusses the results of this experimental investigation.

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C. R. Das ¹, A. K. Bhaduri ¹, S. Mahadevan ¹, P. Mastaniah ¹, S. K. Albert ¹

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

Abstract

Modified 9Cr-lMo steel is used in high temperature applications due to its good thermo-physical, mechanical, weldability and formability. This steel is susceptible to Type IV cracking, in which failure of the weld joint occurs on creep exposure in the intercritical region of the heat- affected zone (ICHAZ). Improved resistance to Type IV cracking in modified 9Cr-1Mo steel weld joints can be achieve by addition of 100 ppm boron along with controlled nitrogen. However, in the absence of boron-added welding consumables, weld joints of this steel are presently being made using boron-free welding consumables. Consequently, during creep, the microstructure of boron-free weld metal is relatively less resistant to coarsening compared to the boron-added base metal and HAZ. The objective of the present investigation is to understand the effect of electron beam welding parameters on the microstructure and mechanical properties of the welds. Weld joints of the boron-free (P91) and boron-added (P91B) modified 9Cr-lMo steels were prepared by varying the welding parameters during electron beam welding. The soundness of the weld joints was confirmed by radiographic examination and bend testing. The tempering behavior of weld metal and base metal was studied and compared. The test results show that the ductile-to-brittle transition temperature of the P91B steel is comparable to that of the P91 steel but lower than that of boron-free weld metal. This paper discusses the influence of electron beam welding parameters on themicrostructure and mechanical properties of boron-added modified 9Cr-lMo steel welds.

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Lakshmi Suresh ¹, C. R. Das ¹, A. K. Bhaduri ¹, Sujay Chakravarty ¹, Sujoy Kar ¹, S. K. Albert ¹

Affiliations
1 Materials Development and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, IN

Abstract

Boron is reported to improve Type IV cracking resistance of 9Cr-3W-3Co-V-Nb steel, with this improvement being attributed to the presence of uniform microstructure in the heat affected zone (HAZ) and its thermal stability during high temperature service. Similar results have been observed recently in modified 9Cr-lMo steel; hence, the present work aims to understand the role of boron and nitrogen in the microstructural evolution of the HAZ in this steel. For this purpose, using a Gleeble thermal-mechanical simulator, MAZ microstructures were simulated in normalized (1100°C/lh) and tempered (760°C/3h) modified 9Cr-lMo steels with different boron and nitrogen contents. The Gleeble simulation was carried out by subjecting the specimens to heating cycle at different peak temperatures in the range 875 to 1200°C using a heating rate of 45°C.s^-1. The effect of heating rate on the transformation temperatures was also studied using dilatometry. The simulated HAZ specimens were characterized using optical and scanning electron microscope, x-ray diffraction (XRD) and hardness measurements.

Microstructural examination of the Gleeble-simulated specimens shows that typical lath martensitic structure is present in all specimens of the boron-containing steel. However, in the 910°C simulated specimens of the boron-free steel lath martensite is absent and fine prior austenite grains are present. It is also observed that hardness increases marginally with increase in peak simulation temperature in the boron-containing steel. To understand the microstructural evolution in the HAZ, the crystallite size, lattice strain and dislocation density were estimated by analysis of XRD data. While variations in crystallite size are marginal, the lattice strain and dislocation density vary with peak simulation temperature. Further detailed microstructural analyses have clearly shown the stability of microstructure in the HAZ depends on the boron and nitrogen contents in the steel. The present paper presents and discuss the results of this experimental investigation.

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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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K. V. Sajunath ¹, C. R. Das ¹, S. K. Albert ¹, S. Raju ¹, A. K. Bhaduri ¹

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

Abstract

Formation of delta-ferrite in the weld metal, during autogenous bead-on-plate welding of 9-12Cr steels, using gas tungsten arc welding (GTAW) process has been studied. The microstructure of 9-12Cr steel base metal consists of tempered lath martensite, where precipitates decorate the boundaries. In single pass bead-on-plate, the area fraction of delta-ferrite was found to be higher at the weld interface as compared to center of the weld metal at all preheat temperatures. Decrease the area fraction of delta-ferrite, with an increase in preheat temperature were observed. Area fraction of delta-ferrite was found to be 7.6 in P91B weld metal at room temperature compared to that of 6.1 at preheat temperature of 250°C, whereas it was 5.7 in AISI 410 weld metal at room temperature and 3.9 at pre heat temperature of 250°C. High cooling rate caused stability of delta ferrite at room temperature as well as preheat condition in the weld metal.

Keywords

Weld Metal, Cooling Rate, Area Fraction of Delta-Ferrite.

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