Gopa Chakraborty

Effect of dilution on GTAW Colmonoy 6 (AWS NiCr–C) hardface deposit made on 316LN stainless steel

Abstract Nickel based Colmonoy 6 (conforming to AWS NiCr–C) hardfacing alloy finds application in hardfacing of various components made of austenitic stainless steel (SS) used in fast reactors. Owing to considerable difference in melting points of the SS and Colmonoy 6 alloys, significant dilution from substrate occurs during hardfacing using gas tungsten arc welding process. Dilution has a significant effect on microstructure, hardness and wear resistance of the deposit. To overcome the adverse effects of dilution on the hardness and, hence, the wear resistance of the deposit, often, the minimum thickness specified for the deposit on hardfaced components is high, which in turn increases the susceptibility of the deposit to cracking during deposition. In the present investigation, microstructure of different layers of multilayer Colmonoy 6 deposits on 316LN SS is characterised by optical and scanning electron microscopy, and the correlation between hardness and microstructure of the individual layers with dilution from the base metal has been established. The dilution from the base material is the highest in the first layer, and it progressively decreases in the subsequent layers. With progressive decrease in dilution, the precipitate fraction increases from about 16 to 20% from the first to the fifth deposit layers. This is accompanied by hardness increase from about 480 to 800 HV. The precipitates in the deposit consist of both borides and carbides, with the boride content varying more with dilution than the carbide content. The boride fraction increased from 5 to 8% with a decrease in dilution; however, layer to layer variation in carbide fraction was only marginal at about 11–12%. High dilution from the base material suppresses the formation of borides in the deposit and is responsible for low hardness of the deposit diluted with the austenitic SS compared to those of the undiluted deposit.

Effect of brazing temperature on the microstructure of martensitic–austenitic steel joints

ABSTRACT Dissimilar joining of reduced activation ferritic–martensitic steel to AISI 316LN austenitic stainless steel is carried out by brazing in inert atmosphere at three different temperatures, i.e. 980, 1020 and 1040°C using AWS BNi-2 powder. The braze joints are characterised by scanning electron microscopy, X-ray diffraction, micro-hardness measurement. With increasing brazing temperature from 980 to 1040°C, the approximate width of the braze layer decreases from 350 to 80 µm and hardness reduces from 600 to 410 VHN. However, not much difference is found in microstructure and hardness between braze joints produced at 1020 and 1040°C. With increasing brazing temperature, morphology and volume fraction of intermetallics formed in the braze layer change, thereby reducing the hardness variation between the braze layer and the base metal.

Effect of Alloy 625 Buffer Layer on Hardfacing of Modified 9Cr-1Mo Steel Using Nickel Base Hardfacing Alloy

Dashpot piston, made up of modified 9Cr-1Mo steel, is a part of diverse safety rod used for safe shutdown of a nuclear reactor. This component was hardfaced using nickel base AWS ER NiCr-B alloy and extensive cracking was experienced during direct deposition of this alloy on dashpot piston. Cracking reduced considerably and the component was successfully hardfaced by application of Inconel 625 as buffer layer prior to hardface deposition. Hence, a separate study was undertaken to investigate the role of buffer layer in reducing the cracking and on the microstructure of the hardfaced deposit. Results indicate that in the direct deposition of hardfacing alloy on modified 9Cr-1Mo steel, both heat-affected zone (HAZ) formed and the deposit layer are hard making the thickness of the hard layer formed equal to combined thickness of both HAZ and deposit. This hard layer is unable to absorb thermal stresses resulting in the cracking of the deposit. By providing a buffer layer of Alloy 625 followed by a post-weld heat treatment, HAZ formed in the modified 9Cr-1Mo steel is effectively tempered, and HAZ formed during the subsequent deposition of the hardfacing alloy over the Alloy 625 buffer layer is almost completely confined to Alloy 625, which does not harden. This reduces the cracking susceptibility of the deposit. Further, unlike in the case of direct deposition on modified 9Cr-1Mo steel, dilution of the deposit by Ni-base buffer layer does not alter the hardness of the deposit and desired hardness on the deposit surface could be achieved even with lower thickness of the deposit. This gives an option for reducing the recommended thickness of the deposit, which can also reduce the risk of cracking.

Non-Destructive Characterization of Nickel-Base Hardface Deposit on Austenitic Stainless Steel Through Eddy Current and Magnetic Barkhausen Techniques

Nickel-base Colmonoy (AWS ER NiCr) alloys find extensive application for hardfacing of austenitic stainless steel (SS) components in Fast Breeder Reactors. Gas Tungsten Arc deposited Colmonoy alloys suffer from significant loss in hardness and wear properties due to dilution from the austenitic SS substrate. In a multilayer deposit, dilution in first layer is highest and dilution decreases progressively in the subsequent layers. Although, both Colmonoy alloys and austenitic SS are non-magnetic, the deposit of Colmonoy on austenitic SS is ferromagnetic. The susceptibility to magnetic attraction in the hardface deposits is highest for maximum level of dilution and reduces with decreasing dilution. In the present study, Colmonoy-6 alloy co-deposited with austenitic SS filler wire, to obtain deposits with different levels of dilution, have been non-destructively evaluated by eddy current (EC) and magnetic Barkhausen (MB) techniques. The deposits were also characterized by microstructural and hardness studies. The EC parameters viz. magnitude and phase angle of the induced voltage showed an increasing trend with increasing dilution of the deposits. MB root mean square (RMS) voltage and peak height also indicated similar trend with dilution. The results produced from nondestructive tests could be correlated with hardness and microstructure of the deposits. Thus, it is possible to develop a non-destructive technique to predict hardness of the Colmonoy hardfacing alloy, which can serve as quality control tool in estimating the hardness of the hardfaced coating to ensure that dilution of the deposit by the substrate does not bring down the hardness of the deposit below the acceptable minimum values.

Self-welding susceptibility of cold worked alloy D9 and 316LN austenitic steels in flowing sodium

Abstract Specimens of 20% cold worked alloy D9 and 316LN were tested for self-welding susceptibility in flowing sodium at 823 K. A contact stress of 24·5 MPa was maintained during the test, and tests were carried out for two different durations of 3 and 4·5 months for each. It was found that susceptibility to self-welding in flowing sodium is higher for cold worked alloy D9 than for cold worked 316LN. This is in contrast with no self-welding reported for annealed alloy D9 and high susceptibility to self-welding reported for annealed austenitic stainless steels like 316LN, 321 and 304. Dynamic recrystallisation of the cold worked structure at the location of self-welding, which does not occur for cold worked 316LN steel, is the reason attributed to the high susceptibility of cold worked alloy D9 to self-welding. It appears that carburisation of cold worked alloy surface, which leads to the formation of coarse TiC precipitates at the surfaces, assists dynamic recrystallisation.

Effect of dilution and cooling rate on microstructure and magnetic properties of Ni base hardfacing alloy deposited on austenitic stainless steel

Abstract Nickel base Colmonoy 6 alloy finds application in hardfacing of various components made of austenitic stainless steel used in fast reactors. Gas tungsten arc deposited Colmonoy suffers from significant loss in hardness and wear properties due to dilution from the substrate. Magnetic property of the deposit is also influenced by dilution, and variation in magnetic parameters with dilution can be employed for non-destructive assessment of hardness in the hardfaced coating of the finished components. During hardfacing deposition, cooling rate may vary with deposition technique and process parameters. Development of a procedure for measurement of hardness of hardface deposit, irrespective of cooling rate, can prove to be useful for practical application. In the present study, the effect of cooling rate on microstructure, hardness and magnetic parameters of the Ni–Cr–B–Si hardface deposits is evaluated, and for coating cooled at different cooling rates, good prediction of hardness from its magnetic property could be obtained.

Estimation of Hardness in Nickel-Base Hardafacing Deposits on 316LN Stainless Steel by Magnetic Techniques

Nickel-base Colmonoy-6 (AWS ERNiCr) alloy is the material chosen for hardfacing of austenitic stainless steel (SS) components used in Indias’s Prototype Fast Breeder Reactor. Gas Tungsten Arc deposited Colmonoy hardfacing alloys suffer from significant loss in hardness and wear properties due to dilution from the austenitic stainless steel substrate. Although both austenitic SS and the undiluted Colmonoy-6 alloy are non-magnetic, the hardfacing deposit diluted by SS becomes ferromagnetic. In a multilayer deposit, magnetism is highest in the first hardfacing layer and decreases progressively in the subsequent layers. As in actual hardfacing deposits, dilution is difficult to control for any deposited layer, it is not easy to study variations of magnetic properties as a function of dilution. Hence, a separate set of deposits (twin-deposits) were produced by co-deposition of Colmonoy-6 alloy rods and austenitic SS filler wire on a copper block. Magnetic properties of hardfacing and twin-deposits were examined using Magnegage and Feritscope equipments. The saturated magnetic moments and Curie temperature of the twin-deposits were measured from room temperature to 873 K. Further, optical, scanning electron microscopy, energy dispersive X-ray spectroscopy and hardness measurements were performed for both hardfacing and twin-deposits. Correlation between hardness, microstructure and bulk magnetic property of deposits with different dilution levels could be established. It was also possible to correlate hardness with the magnetic property of the deposits. Thus, the present study indicates a potential use of magnetic techniques for estimating hardness and dilution of the Colmonoy hardfacing deposit in a component, which cannot be subjected to destructive examination.