Amrita Kundu

Residual stresses in P91 steel electron beam welds

Abstract This paper describes the characterisation of residual stress in electron beam welded P91 ferritic–martensitic steel plates (9 mm thick) by neutron diffraction and contour measurement methods. The novelty of the work lies in revealing the residual stress profile at a fine length scale associated with a ∼1 mm wide fusion zone. A characteristic ‘M’ shaped distribution of stresses across the weld line is observed with high tensile peaks situated just beyond the heat affected zone/parent material boundary. Measured stresses close to the weld centreline are significantly less tensile than the adjacent peaks owing to martensitic phase transformation during cool down of the weld region. The effect of applying a second smoothing weld pass is shown to be undesirable from a residual stress standpoint because it increases the tensile magnitude and spread of residual stress. The results are suitable for validating finite element predictions of residual stress in electron beam welds made from ferritic–martensitic steels.

Grain Structure Development during Reheating and Deformation of Niobium-Microalloyed Steels

The present investigation looks into the effects of reheating and simulated rolling deformation on development of grain morphology and distribution in specimens taken from quarter thickness position of a 290 mm thick cast slab of Nb-microalloyed steel. Plane strain compression in a Gleeble 3500 thermomechanical simulator has been carried out at a strain rate of 10/s up to a maximum deformation of 30%. It was observed previously that, depending upon the reheat temperature, the grain structure of reheated specimens could be either unimodal or bimodal. The variation in the reheated grain structure has been related to the segregation of microalloying element (Nb) and stability of Nb(C, N). The effect of deformation on the reheated specimens towards development of final grain structure is found to depend upon the deformation temperature through the interaction between recrystallization and precipitation phenomena. The Dutta–Sellars model has been applied to the experimental condition and indicates that the segregated microalloying elements (particularly Nb) can promote bimodality during deformation by affecting the local recrystallization kinetics over a much wider range of temperatures than for a homogeneous material.

Pinning of Austenite Grain Boundaries by Mixed AlN and Nb(C,N) Precipitates

The use of Nb(C,N) to pin prior austenite grains during thermomechanical processing can give rise to bimodal structures linked to Nb segregation and subsequent variation in precipitate distribution and stability on reheating and deformation. The segregation tendency of Al is much less compared with Nb so that AlN may provide grain boundary pinning in regions of reduced Nb(C,N) volume fraction and stability. Quantification of precipitate and prior austenite grain size distributions after reheating has confirmed the governing mechanisms of precipitate dissolution / coarsening whilst identifying grain boundary pinning by AlN at temperature below 1125 °C, but controlled by Nb(C,N) at higher temperatures.

Asymmetric Cyclic Behaviour of Fine- and Coarse-Grained Commercially Pure Copper and Aluminium

In the present study, uniaxial ratcheting behaviour of annealed copper and aluminium with two different grain sizes has been investigated. Engineering stress-control tests have been performed at various combinations of stress amplitudes and mean stresses. To account for grain size variation, stress amplitudes were selected keeping the constant ratio of equivalent stress amplitude and tensile strength. It is found that ratcheting life of fine- and coarse-grained copper and aluminium is inversely related to both stress amplitude and mean stress. However, the effect of stress amplitude on ratcheting life is more as compared to mean stress for both the FCC metals. It is also find out that the ratcheting life of fine-grained FCC metals is more compared to coarse-grained FCC metals. The ratcheting strain rate follows three stages irrespective of stress combination and grain size of both FCC metals. It is observed that for both the FCC metals, the average steady state ratcheting strain rate follows perfect power relationship with cycles to failure.

Geometrically Necessary Dislocation Density Evolution in Interstitial Free Steel at Small Plastic Strains

Measurement of geometrically necessary dislocation (GND) density using electron backscatter diffraction (EBSD) has become rather common place in modern metallurgical research. The utility of this measure as an indicator of the expected flow behavior of the material is not obvious. Incorporation of total dislocation density into the Taylor equation relating flow stress to dislocation density is generally accepted, but this does not automatically extend to a similar relationship for the GND density. This is discussed in the present work using classical equations for isotropic metal plasticity in a rather straight-forward theoretical framework. This investigation examines the development of GND structure in a commercially produced interstitial free steel subject to tensile deformation. Quantification of GND density was carried out using conventional EBSD at various strain levels on the surface of a standard dog-bone-shaped tensile specimen. There is linear increase of the average GND density with imposed macroscopic strain. This is in agreement with the established framework.

Effect of Strain on Recrystallisation during Hot Deformation of Nb-Containing Microalloyed Steel

The Dutta-Sellars equations for predicting recrystallisation and precipitation in microalloyed steels have been assessed for accuracy over a range of strain levels (0.15 to 0.45) during hot deformation (975-1075 °C) of a homogenised 0.045 wt % Nb steel. It has been found that the model predicts the deformation state well at a strain of 0.3 whereas at both lower and higher strains i.e. 0.15 and 0.45, the agreement is less good. The differences between prediction and experimental measurement have been related to solute drag and precipitate potential of Nb.