Charles Witness Siyasiya

Dynamic Globularization of α-Phase in Ti6Al4V Alloy during Hot Compression

Ti6Al4V samples were isothermally compressed using a Gleeble(TM) 1500D thermo-mechanical simulator. Differential scanning calorimetry (DSC), microstructural analyses, and thermodynamic calculations were used to investigate the sequence of transformation of β into α or vice-versa and the presence of different phases in the compressed Ti6Al4V sample. Globular alpha phase was revealed in the isothermally compressed sample in addition to martensitic and lamellar α/β structures. The transition temperature range of β into α-phase was determined using the DSC thermograms and thermodynamic calculated diagrams. The fraction of α-phase globulized increased as the strain rate decreased from 0.01s-1 to 10-3s-1, and the spheroidization of the α-phase is only possible in a specific range of deformation temperatures.

Effect of Ageing Treatment on the Microstructure and Hardness of the Ti6Al4V Alloy

The effects of ageing temperature, time and cooling medium on the microstructure and hardness of a solution treated Ti6Al4V alloy were investigated. The furnace cooling after ageing for 0.5 hours gave a homogenous structure with higher hardness values than the solution treated and water quenched Ti6Al4V alloy. Increasing the ageing time to 2 hours reduced the alloy hardness. Ageing at temperatures between 500 and 700°C, followed by furnace cooling, led to homogenously distributed α- and β-phases within a fully martensitic matrix leading to improved hardness. A heterogeneous structure with a high variation in microhardness was revealed when ageing at 800 and 900°C.

Influence of chemical composition on the evolution of texture in 441 ferritic stainless steels

The influence of addition of Nb+Ti (0.26 to 0.7wt%Nb and 0.15 to 0.3wt%Ti) to 441 on the development of annealing crystallographic textures was investigated by XRD and EBSD ODFs. It was found that addition of (Nb+Ti) to 441 improved the {111} texture to a limit while the resistance to ridging continued improving with more additions of Ti and Nb. It was found that the TiN nucleates heterogeneously first on the MgO.Al2O3 spinel and followed by Nb(C,N) on the TiN particle. The improvement in ridging was attributed to particle stimulated nucleation (PSN) of randomly oriented grains caused by these coarse compound particles.

Constitutive modelling of mill loads during hot rolling of AISI 321 austenitic stainless steel

Abstract A modelled constitutive equation derived from hot working tests to predict hot rolling mill loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel. Good correlation is found between the predicted mean flow stress, the Zener–Hollomon Z parameter and actual industrial mill load values from mill logs if allowances are made for differences in von Mises plane strain conversion, friction and front or back end tension. The multipass hot working behaviour of this steel was simulated through Gleeble thermomechanical compression testing with the deformation temperature varying between 1200°C down to 800°C and the strain rate between 0.001 s−1 and 5 s−1. At strain rates greater than 0.05 s−1, dynamic recovery as a softening mechanism was dominant, increasing the dynamic recrystallisation to dynamic recovery transition temperature to higher temperatures. This implies that through extrapolation to typical industrial strain rates of about 60 s−1, most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only dynamic recovery. Grain refinement by dynamic recrystallisation is, therefore, unlikely in this steel under plant hot rolling conditions. Finally, mill load modelling using the hot working constitutive constants of the near-equivalent AISI 304 instead of those specifically determined for AISI 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot working constants even for near-equivalent steels is, therefore, recommended.