Utpal Borah

Assessing Constitutive Models for Prediction of High-Temperature Flow Behavior with a Perspective of Alloy Development

The utility of different constitutive models describing high-temperature flow behavior has been evaluated from the perspective of alloy development. Strain compensated Arrhenius model, modified Johnson–Cook (MJC) model, model D8A and artificial neural network (ANN) have been used to describe flow behavior of different model alloys. These alloys are four grades of SS 316LN with different nitrogen contents ranging from 0.07 to 0.22%. Grades with 0.07%N and 0.22%N have been used to determine suitable material constants of the constitutive equations and also to train the ANN model. While the ANN model has been developed with chemical composition as a direct input, the MJC and D8A models have been amended to incorporate the effect of nitrogen content on flow behavior. The prediction capabilities of all models have been validated using the experimental data obtained from grades containing 0.11%N and 0.14%N. The comparative analysis demonstrates that ‘N-amended D8A’ and ‘N-amended MJC’ are preferable to the ANN model for predicting flow behavior of different grades of 316LN. The work provides detailed insights into the usual statistical error analysis technique and frames five additional criteria which must be considered when a model is analyzed from the perspective of alloy development.

Hot Deformation and Microstructural Characteristics of Nitrogen Enhanced 316L Stainless Steel

Dynamic recrystallization (DRX) in 316LN austenitic stainless steel with 0.14wt% nitrogen has been studied using hot isothermal compression tests carried out in temperature range 1073-1423K and strain rate range 0.001 - 10 s-1. Critical strain and stress for DRX has been characterized using experimental data. Analysis of results shows that for the entire domain the critical stress is directly proportional to peak stress. However, no clear relationship between εc and εp prevails over the entire tested domain. Dynamic Recrystallized (DRX) grains are quantified by GOS and KAM maps. The four stages of DRX progression have been identified using the correlation between GOS and critical strain at different deformation conditions.

Microstructure Mapping: An Approach to Quantitative Interpretation of Microstructural Evolution in Indian Fast Reactor Advanced Clad Material during Hot Forging

In this paper, microstructural evolution of Indian Fast Reactor Advanced Clad (IFAC-1) steel during thermo-mechanical processing has been investigated. Hot isothermal forging has been simulated in a Gleeble® thermo-mechanical simulator in the temperature range 1173-1473K and true strain rate range 0.01-100s-1. Instability map has been developed using the stress-strain data obtained. Effect of major forging parameters on various microstructural features has been studied quantitatively. Results from this study have been used to construct various maps (‘μ-maps’) representing different aspects of microstructural evolution. An analogy between the μ-maps and instability maps provides essential insights into possible instability mechanisms operative in the material. The μ-map analysis shows potential as a tool for optimisation of industrial-scale forging parameters.

Atypical transitions in material response during constant strain rate, hot deformation of austenitic steel

Work hardening, dynamic recovery and dynamic recrystallization (DRX) occurring during hot working of austenitic steel have been extensively studied. Various empirical models describe the nature and effects of these phenomena in a typical framework. However, the typical model is sometimes violated following atypical transitions in deformation mechanisms of the material. To ascertain the nature of these atypical transitions, researchers have intentionally introduced discontinuities in the deformation process, such as interrupting the deformation as in multi-step rolling and abruptly changing the rate of deformation.In this work, we demonstrate that atypical transitions are possible even in conventional single-step, constant strain rate deformation of austenitic steel. Towards this aim, isothermal, constant true strain rate deformation of austenitic steel has been carried out in a temperature range of 1173–1473 K and strain rate range of 0.01–100 s−1. The microstructural response corresponding to each deform...

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.

Influence of Interfacial Oxides Formed During Dry Sliding Wear of NiCrBSiCFe Plasma Coating on AISI 316 Steel Substrates

Abstract Recent thermal spray processes such as plasma spraying are usually employed to deposit Nickel–Chromium coatings onto the AISI 316 steel substrates to improve the wear resistance. Ni-base alloys, which are widely used to obtain wear resistant coatings, are usually heat treated after thermal spraying to improve their tribological properties. The dry sling wear experiments were carried out in pin on disc tribometer at room temperature 35 °C (308 K), 150 °C (423 K), 250 °C (523 K) and 350 °C (623 K) against medium carbon steel (EN8). The worn surfaces were examined by X-ray diffraction and SEM (Scanning Electron Microscopy) with EDS (Energy dispersive spectroscopy). The applied load was 40 N and sliding speed was 2 m/s. In 350 °C, the wear rate was marginally lesser than the rest of the temperatures due to the initiation of oxide layer formation, which was also substantiated by SEM and EDS. Fe peaks and O peaks were found scarcely. Few Fe peaks were also predominantly noticed by EDS analysis. The presence of Fe3O4 in the tribolayers formed during sliding wear acted as a protective film to retard the wear rate in terms of mass loss.