Papers by Author: Martin Strangwood

Abstract: Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. Plates with different thicknesses (up to approximately 100 mm) are produced for applications in the energy and power or lifting and excavation sectors. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.In this study the starting microstructure of a low alloy 0.17 wt% C Q&T steel has been varied using isothermal heat treatment at 430 °C to create mixed martensite and lower bainite microstructures (nominally 25:75; 50:50 and 75:25 percentages). The effects of tempering at 600 °C for times between 0.5 and 16 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single phase properties. The tempering response of the mixed microstructures is discussed.

Abstract: Low alloy quench and tempered (Q&T) steels plates up to 100mm thick are used in applications such as cranes and earth movers due to their combination of high strength and toughness. In order to ensure that appropriate tempering conditions are used to give optimum properties through thickness in Q&T steels it is desirable to be able to predict the effect of composition and tempering conditions (time and temperatures) on the microstructure and hence the hardness evolution. In this paper, the types and coarsening rates of carbides formed in a low alloyed Q&T steel have been investigated on tempering at 600 °C. It has been found that in the as-quenched condition auto-tempered martensite is present with needle-shaped epsilon and cementite particles, whilst after different tempering times (1 - 16 hours) cementite becomes the stable phase with an elliptical shape, which coarsens with time. Besides, the coarsening of inter-lath cementite with a faster rate is independent from that of intra-lath ones.

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 crystallisation behaviour of Vitreloy 105 during different thermo-mechanical exposure conditions has been studied in a Gleeble 3500 thermo-mechanical simulator (TMS). Strains up to 0.1 applied at rates of 0.001 – 0.01 s^-1 in the supercooled liquid region (SLR) from 420 – 435 °C resulted in no change in the crystallisation kinetics compared with purely thermal exposure for the same times and temperatures. Separate deformation studies on the amorphous bulk metallic glass confirmed that the deformation conditions used corresponded to Newtonian flow conditions. It remains to be confirmed whether the lack of influence of permanent deformation persists for deformation at higher strain rates in the non-Newtonian regime.

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: Melt spun Ni-Cr-P alloy ribbons were used as filler metal to join Al2O3 substrates together using a vacuum brazing process. The influence of the brazing load, brazing temperature and holding time on the shear resistance of brazed joints was evaluated. The experimental results showed that the holding time was the most important parameter. At a brazing temperature of 1060°C, the shear resistance value increased from 6.17 to 93.7 MPa with increasing brazing time from 20 to 100 min, respectively.

Abstract: AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.

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.