Papers by Author: Claire L. Davis

Abstract: The continuous drive toward higher operating efficiency, greater reliability and longer life of steam turbines has introduced a need for higher integrity components to operate at higher temperatures and pressures. This poses several material and processing challenges to ensure that the components have metallurgical stability and the required mechanical properties in the high temperature environment. Modelling the open-die press forging process, used to manufacture steam turbine discs from cast ingots, is complex due to the variation of strain, strain rate and temperature within the ingot. These variations mean that recrystallisation and grain growth do not occur uniformly throughout the ingot. Severe plastic deformation is used to promote recrystallisation in order to refine the grain size and improve strength and toughness properties. A major part of the modelling described in this paper involves prediction and validation of strain, strain rate and temperature distributions during open-die forging. A sensitivity study has confirmed the requirement for accurate thermal and physical data such as Interfacial Heat Transfer Coefficient (IHTC), work-piece emissivity, specific heat and friction coefficient. In this paper experimental determination of these data for the grade of heat resistant steel being modelled, over process parameter ranges appropriate to open-die forging operations, is described. Incorporation of these data into a finite element-based model for strain variation within an ingot is reported with consideration and measurement of dead zone for thermo-mechanical simulation trials.

Abstract: 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.

Abstract: 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.

Abstract: The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.

Abstract: Bimodal grain size distributions were found in continuously cast slab and thermomechanical controlled rolled (TMCR) samples of Nb-microalloyed steel. Scanning electron microscopy (SEM) revealed inhomogeneous distributions of Al- and Nb-containing precipitates, which were found to pin prior austenite grain boundaries during reheating. An effort has been made to establish parameters to quantify the extent of bimodality of reheated and rolled microstructures. Quantification of bimodality using peak grain size range, (PGSR) and peak height ratio, (PHR), is found to match closely with the visual observation of bimodality. Thermo-Calc software was used to predict the sequence of precipitation for different compositions and that could explain the formation of bimodality during reheating.