Materials Science Forum Vol. 879

Abstract: Influences of dynamic strain ageing and long term ageing on deformation, damage and fracture behaviors of Alloy 617 material have been studied. Dynamic strain ageing can occur in this alloy at temperature from 400 to 700°C, which leads to a strain hardening and also an increase in fracture strain due to plastic deformation caused by twinning. Long term ageing at 700°C for up to 20 000 hours can cause different precipitation such as γ ́, M₆C (Mo-rich) and M₂₃C₆ (Cr-rich) carbides. These carbides are both inter-and intra-granular particles. The long term ageing reduces the fracture toughness of the material, but the alloy can still have rather high impact toughness and fracture toughness even with an ageing at 700°C for 20 000 hour. The mechanisms have been studied using electron backscatter detection and electron channeling contrast imaging. It shows that besides dislocation slip, twinning is another main deformation mechanism in these aged Alloy 617 materials. At the crack front, plenty of micro or nanotwins can be observed. The formation of these twins leads to a high ductility and toughness which is a new observation or a new concept for this type of material.

Abstract: In this research evolution of austenite grain size in Nb-microalloyed steels X65÷X120 grades during slab reheating and roughing rolling was studied. A mathematical model has been development to obtain the target temperature and soaking time in furnace, which ensure a uniform austenite structure and maximum possible dissolution of the carbonitride particles prior to roughing rolling. The Hot Rolling Recrystallization Model (HRRM) has also development to predict the austenite microstructure evolution during roughing rolling. The model is based on empirical equations and organized following a tree-structure. A validation of the model has been carried out in the laboratory by multipass compression tests. The models jointly have been used to create new strategies of processing technology of rolled plates on rolling mill 5000 for the South Stream pipeline. The industrial application has confirmed a great benefit of the models in point of cold resistance of rolled plates.

Abstract: Two intermediate thermal mechanical treatment (ITMT) Processes were designed for investigation the influence of multi-scale precipitated particles on microstructure evolution during thermo-mechanical processing of Al-7.6Zn-1.5Mg-1.75Cu-0.12Zr alloy by hot compressive experiments and microstructure testing of OM and EBSD. It is found that the size and distribution of precipitated particles preprocessed by over-aging at 400°C for14h can meet the particle stimulated nucleation of recrystallization. Refined and uniform grains present in the sample after hot deformation at about 20 of LnZ up to 80% reduction and subsequent final solid solution treatment. But for samples preprocessed by solid solution at 435°C for 2h and aging at 200°C for 12h, Refined uniform recrystallized grains or recovery sub-grains in elongated grains present in the samples after hot deformation at about 25 of LnZ up to 60% reduction followed by annealing at 350°C for 0.5h and final solution treatment.

Abstract: The deformation and internal defect in the flat-wedge cross-wedge rolling (CWR) of GH4169 superalloy were investigated numerically using a coupled thermo-mechanical finite element analysis (FEA) model. The simulation analysis showed that the temperature distribution in the work piece was non-uniform during the flat-wedge CWR. When the initial temperature of the work piece was relatively low, the work piece temperature increased, a heating effect of the plastic deformation, while relatively high initial work piece temperatures resulted in cooling the work piece, caused by the work piece contact with the tools. It is noted that the increase of tools moving speeds is helpful to reduce the generation of internal defects in the flat-wedge CWR of GH4169 alloy. An important tendency is that both increasing the tools temperature and decreasing friction coefficient all can contribute to avoiding the center defects in the flat-wedge CWR of GH4169 alloy

Abstract: A strong research effort has continued to determine how selective laser melting (SLM) process parameters relate to the amount of defects present in SLM products. However, how these parameters affect the track profiles which should geometrically affect the amount of lack of fusion (LOF) during SLM has not been studied. This study is needed as severe LOF should be a severe quality and property issue. In this work, how laser power (P) and thus energy, as other build rate related parameters are kept unchanged, affects the size of SLM tracks and the formation of LOF during SLM of CoCrMo alloy has been studied. Through examining the track profiles, track sizes have been geometrically demonstrated to insufficiently overlay and overlap for LOF free when the recommended condition (P = 180W) was used. A further contributing factor for LOF is the irregularity of track shape. Increasing P increased the sizes and improved the stability of the melt and the shape regularity of the tracks, reducing the amount of LOF. It will be shown that there is a rapid decay in the amount of LOF as P increases from 180 W, predominantly as the result of geometrical effect of the track profile on track coverage.

Abstract: Fe-Mn-Al steels with low density have the potential to substitute for TRIP (transformation induced plasticity) steels. For the development of Fe-Mn-Al TRIP steels, phase transformations play an important role. Our methods of studying the phase transformations of the Fe-16.7 Mn-3.4 Al (wt%) austenitic steel include heating and cooling. We have studied the martensitic transformation of the ternary Fe-Mn-Al steel. Single austenite phase is the equilibrium phase at 1373 K, and dual phases of ferrite and austenite are stable at low temperatures. It is noteworthy that lath martensite forms in the prior austenite grains after cooling from 1373 K via quenching, air-cooling, and/or furnace-cooling. The crystal structure of the martensite belongs to body-centered cubic. The formation mechanism of the ferritic martensite is different from the traditional martensite in steels. Ferrite is the stable phase at low temperature.

Abstract: In order to produce reliable welded product quality accumulated experiments are basic requirements.But nowadays production conditions are changing and quality requirements are becoming more and more severe. In these cases using achievement of computer in hardware and software can be a nice tool for improving welding production in efficiency and time saving. In this study three samples are shown in order to show the improvement in welding products with the aid of computer. (Repair welding, Tack welding and High frequency electric resistance welding)

Abstract: With the purpose of fabricating equiaxed and bimodal Ti-6Al-4V alloy with different grain/primary α (α_p) sizes, thermomechanical processing and additional annealing were carried out on samples with martensite initial microstructure. Deformation at 700°C with a strain rate of 0.01s^-1 to a true strain of 0.8 could effectively break the martensite initial microstructure into ultrafine-grained (UFG) equiaxed microstructure (mean grain size of 0.51μm) with reasonable uniformity. Subsequent annealing at 930°C with different periods were conducted to change the equiaxed microstructure into bimodal microstructures. The holing time proved to be more critical than heating rate for determining the α_p size. An UFG bimodal Ti-6Al-4V with the average α_p size of 0.55μm was successfully obtained for the first time by annealing the UFG equiaxed Ti-6Al-4V at 930°C for 2 seconds. The mechanical properties of the equiaxed and bimodal Ti-6Al-4V with different grain/α_p sizes were evaluated by tensile tests at room temperature. The bimodal Ti-6Al-4V showed superior balance between strength and uniform elongation than that of the equiaxed Ti-6Al-4V. Moreover, the uniform elongation in the bimodal Ti-6Al-4V was nearly unaffected by reduction of the α_p size.

Abstract: In this study the effects of heating rate on the sharpness and size of Goss oriented ({110}<001>) grains during secondary recrystallization annealing at 900 °C was observed. The results show that, at the same annealing temperature, rapid heating of the samples to this temperature generates a higher drag force compared to a slower heating rate (5°C/min). The two groups of samples show different growth kinetics for Goss grains, in which at the longest annealing time, the rapid heating sample exhibits larger maximum Goss grain size compared to the slower heated samples.

Abstract: A new generation of low-carbon microalloyed High Strength Low Alloy (HSLA) steels has been developed to utilize a combination of single-phase ferritic microstructures and optimized interphase precipitation to provide high level strength and exceptional formability. The interphase precipitation reaction is a transient process lending itself strongly to take advantage of in-situ characterization techniques. The austenite/ferrite interface kinetics during isothermal transformation at 1003 K is measured using HT-CSLM, the pre-exponential effective mobility constant was found to be mobility 0.822 (m J)/(mole s). The V interphase precipitation is characterised using TEM at isothermal transformation temperatures of 923 and 973 K as having inter-sheet spacing of 22±7 and 32±9 nm respectively. Interphase precipitation inter-sheet-spacing is simulated using a revised Quasi-Ledge model and qualitatively predicts the observed trends observed for inter-sheet spacing. The results of in-situ characterisation and modelling suggest that it is possible to optimize the strengthening potential of the precipitation processes by controlling the thermal processing of microalloyed HSLA.