Satish Kumar Shekhawat

Direct Experimental Observations on Concurrent Microstructure and Magnetic Property Developments in Non-Grain Oriented Electrical Steel

Non-grain oriented electrical steel, with minor in-grain orientation gradients, was subjected to interrupted tensile deformations and concurrent microtexture, magnetic property and residual stress measurements. After the upper yield point, clear signatures of mechanical stress relief were observed. Changes in orientation gradients led to annihilation of low-angle (1 to 3 deg) boundaries. Prior deformation compressive residual stresses became tensile and magnetic properties improved. Beyond an optimum true strain of 0.01, this boundary annihilation ceased, compressive stresses were generated, and magnetic properties degraded.

Degradation of Magnetic Properties in Transformer Steel: Role of Prior Elastic Deformation

Transformer steel, fully processed Hi-B (high permeability) grade of cold rolled grain oriented (CRGO), was subjected to relatively minor, 0 to -55 MPa, stresses. Detailed magnetic and microstructural characterizations were then carried out. Shift of hysteresis loop and clear degradation in magnetic properties, namely increase in core loss and drop in permeability, were noted. Though absence of mesoscopic plastic strains was confirmed, possibilities of minor local plastic deformation cannot be ruled out. Prior deformations: 1) modified residual strain/stress signatures in both CRGO laminates and Forsterite (Mg2SiO4) coating and 2) caused corresponding degradation in magnetic performance. Redistribution of residual stresses through local plastic deformation appears to be the plausible justification for these observations. This paper brought out clear correlations between degradation in magnetic properties with introduced residual stress/strain.

Improved prediction of strain distribution during mechanical and hydro-mechanical deep drawing processes using microstructure-based dynamic strain hardening and anisotropy

Interstitial free (IF) and drawing quality (DQ) steel sheets were subjected to conventional mechanical deep drawing and hydro-mechanical deep drawing operations. Detailed macroscopic strain measurements were made at different cup depths. These were simulated using PAM-STAMP, a commercial finite element–based software. For continuum-based finite element simulations, the required key material properties are strain hardening exponent (n) and anisotropy index ( r ¯ ). These were kept either constant or dynamically varied during the simulation. The constant properties were taken from conventional tensile tests of the original material, while the dynamic variation in properties was extrapolated from developments in the crystallographic texture and in-grain misorientation. Considering dynamic properties during simulation provided a superior prediction of macroscopic strains. This study clearly demonstrates the need for considering evolution of critical continuum properties, such as work hardening and anisotropy index, through appropriate microstructural inputs for more realistic macroscopic predictions.

A new method for automated reconstruction of pre-transformation microstructures

Abstract A new method, referred to as the cluster method, is proposed and tested for the reconstruction of pre-transformation microstructure in Ti–6Al–4V alloy. The cluster method begins by constructing clusters of daughter grains (room-temperature hcp α-phase) that have a high probability of being transformed from the same parent grain (high-temperature bcc β-phase). These clusters are then grown, and back-transformed to the parent grain orientations using the ‘summation of mutual misorientation angle method’ (SMMA method). The cluster method is validated by comparing the obtained results with the results of the SMMA and triplet methods.