Arun Kumar Bhaduri

Improvement in Creep Resistance of Modified 9Cr-1Mo Steel Weldment by Boron Addition

The microstructural evolution in the heat-affected zones (HAZs) of boron-added modified 9Cr-1Mo steel (P91B) weldment is distinctly different from that of steel without boron addition (P91). This is accompanied by less reduction in hardness in the inter-critical heat-affected zone (ICHAZ) of the weldment of the boron-containing steel than what is observed in boron-free steel. Though, tensile properties of both the boron-containing steel weldments and boron free-steel weldments do not differ much at room and at high temperatures, there is a significant improvement in the creep strength of the weldment of the boron-containing steel over that of the weldments of boron-free steel. In fact, for creep tests so far completed for weldments of P91B, fracture occurred in the base metal, not in typical Type IV mode (fracture at the ICHAZ or fine grained heat- affected zone) as occurred for the P91 weldment for identical conditions of testing. This improvement is explained based on the role of boron in modifying the HAZ microstructure of the steel.

Study of Hot Cracking Behaviour of Nitrogen-Enhanced Austenitic Stainless Steels using Varestraint and Hot Ductility Tests

Hot cracking is a major problem in the welding of austenitic stainless steels (SS), particularly the fully austenitic grades. A group of alloys of high nitrogen stainless steel is being developed for structural components of the Indian Fast Reactor programme. Studying the hot cracking behaviour of this nitrogen-enhanced austenitic stainless steel is an important consideration during welding, as this material solidifies without any residual delta-ferrite in the primary austenitic mode. Nitrogen has potent effects on the solidification microstructure; hence, it is expected to have a strong influence on the hot cracking behaviour. Both Varestraint and hot ductility tests were used to evaluate its solidification and liquation cracking susceptibility. Different heats of this material were investigated, which included fully austenitic (high nitrogen stainless steels) containing 0.07–0.22 wt. (%) nitrogen. Varestraint tests were carried out on these alloys using specimens of 3 mm thickness at four strain levels between 0.5 and 4.0%. The Brittleness Temperature Range (BTR) was also evaluated from these tests. The Varestraint test results showed that the solidification cracking susceptibility is higher for 0.22 wt. (%) steel and the liquation (HAZ) crack significantly increases with increasing nitrogen content. Hot ductility tests were conducted on these alloys using a thermomechanical simulator and the Nil Strength Temperature (NST), Nil Ductility Temperature (NDT) and Ductility Recovery Temperature (DRT) were determined. The hot ductility test results showed that the nil ductility range (NDR), the difference between NST and DRT of the nitrogen-enhanced steel containing 0.22 % N, is higher (50 °C) than that of the alloys containing 0.07 % N (40 °C) and 0.14 % N (30 °C), indicating high susceptibility of the 0.22 % N alloy to liquation cracking. This paper presents and discusses the role of high nitrogen content on hot cracking susceptibility of this class of steels using Varestraint and hot ductility tests.

Weldability studies on borated stainless steel using Varestraint and Gleeble tests

Borated austenitic stainless steels are widely used in nuclear applications due to their higher neutron absorption efficiency. Weldability of these steels is a major concern due to the formation of low-melting eutectics enriched in iron, chromium, molybdenum and boron. Varestraint and Gleeble-based tests have been carried out to evaluate the solidification and liquation cracking behaviour of 304B4 Grade B stainless steel. The solidus and liquidus temperatures have been determined using differential scanning calorimetry and Scheil simulation for solidification of welds. The results indicate that this steel has good solidification cracking resistance due to the backfilling effects of eutectic borides. However, liquation cracking resistance in this steel is moderate due to its wide nil ductility range. This paper discusses the role of boron on hot cracking susceptibility of this borated austenitic stainless steel using both the Varestraint and Gleeble-based hot ductility tests.

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.

Characterization of Hot Workability of Boron-Added Modified 9Cr-1Mo Steel (P91B) Using Dynamic Materials Model

Elevated temperature workability of Boron added modified 9Cr-1Mo steel is studied in temperature range 1223-1473K and strain rates of 0.001-10s-1 using Dynamic Materials Model. Towards this end hot isothermal compression tests are carried out and the experimental results are used to obtain processing map. Extensive microstructural investigation is carried out to validate different domains of processing map. On the basis of the microstructurally validated processing map, parameters for the thermomechanical processing of P91B are recommended.

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.

Finite Element Modelling of the Creep Behaviour of Weldments

Numerical simulation of the creep behaviour of weldments was carried out using Finite Element (FE) modelling. Data of creep tests at a given temperature and over a range of applied stresses for the for base metal, weld metal and different thermally-simulated heat-affected zone (HAZ) regions were used to determine Garofalo’s equation for creep strain for each material zone, while creep strain at failure was used as the damage criterion. FE meshes for the transverse-weld (weldment) specimens were constructed after calculating the HAZ locations using Rosenthal’s heat flow equation. The efficacy of the FE model is demonstrated in this paper for predicting the creep behaviour, including the failure time and failure location, of 12Cr-1Mo-V steel weldments at 823 K at different stresses between 170–250 MPa. The stress and strain redistribution across the length of the transverse-weld specimens were also predicted.