Materials Science Forum Vol. 879

Abstract: Mathematical modelling of phase transformations and hardness distribution in non-monotonic quenched steel specimen was developed based on the results of simple experimental test i.e. Jominy test. The hardness in specimen points was estimated by the conversion of cooling time results to hardness by using both, the relation between cooling time and distance from the quenched end of Jominy specimen, and by using the Jominy hardenability curve. The cooling curve at the specimen point was predicted by numerical modelling of cooling by using the finite volume method. Developed numerical model for computer simulation of quenching was also experimentally verified. Limitations of proposed numerical model were found out as well. It has been shown that proposed numerical model can be successfully applied for purposes of simulation of continuous and interrupted quenching of carbon and low alloyed steel specimens.

Abstract: Recent interests in developing novel super-high strength steels have led to extensive research efforts in direct quenching with or without tempering (DQ, DQT) or combined with partitioning (DQP). Both strip and plate products have been targeted for different applications. For boron-microalloyed DQ/DQT steels, the ASTM A255 approach for predicting the hardenability was considered inapplicable. Fresh attempts were made to develop new hardenability models through non-linear regression analysis by dynamically varying both the boron factor and multiplying factors of most elements in the alloy factor. Based on the recent concept of quenching and partitioning (Q&P), a novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Examples of recent advances made in DQ processing and associated challenges, such as those related to the bendability of low carbon martensitic-bainitic steels and influence of boron on the toughness of Nb-bearing martensitic steels are presented.

Abstract: Ti-6Al-3Sn-3Zr-3Mo-3Nb-1W-0.2Si (BTi-6431S) alloy is a novel two-phase high temperature titanium alloy for short-term using in aerospace industry up to 700°C. The effects of heat treatment on the microstructure evolution of BTi-6431S alloy bar were investigated through optical microscopy (OM), X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The results show that solution treatment in β region at 1010°C followed by water quenching results in the formation of orthorhombic martensite α′′ phase, while air cooling leads to the formation of hexagonal martensite α′ phase. When solution-treated in α+β phase field at temperatures from 900°C to 980°C following by water quenching, the content of primary α phase decreases with the increase of heat treatment temperature. For the alloy subjected to identical heat treatment, the content of Al in α phase is much higher than that in β phase, while the contents of Nb, Mo and W elements in α phase are much less than those in β phase.

Abstract: Extensive efforts are underway worldwide to develop new steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. It is likely that microalloying can provide product enhancements in these emerging products, such as Q&P, TBF, medium-Mn TRIP, etc. and this paper examines the expected behavior of niobium using inferences based on published AHSS literature and principles of Nb microalloying. Some benefits of Nb in terms of microstructure refinement and precipitation strengthening have been reported. The potential influences of Nb are complex due to the sensitivity of Nb dissolution and precipitation to chemical composition and processing; differences in the expected role of Nb are pointed out with respect to different product forms produced via hot-rolling or annealing after cold-rolling, and microstructures with or without substantial quantities of primary ferrite. Some issues that warrant further examination are identified, as a deep understanding of Nb microalloying and other fundamental behaviors will be needed to optimize the performance of these next-generation steels.

Abstract: The comprehensive characterization of the change in metallic materials’ microstructure due to an applied load is of prime importance for the understanding of basic fatigue mechanisms or more general damage evolution processes. If those mechanisms and processes are to be understood to a much greater extent, advanced fatigue life calculation methods being far away from linear damage accumulation models, have to be realized providing more than “classic fatigue data” only. Among others the PHYBAL (physically based fatigue life calculation) method including current enhancements and a thereon-based development named SteBLife (step-bar fatigue life approach) have been developed over the last 10 years. These methods allow the efforts in experimentation to be reduced by more than 90 % and therefore offer the possibility to take further fatigue relevant parameters into account. This therefore allows a variety of S,N-curves dependent on those fatigue relevant parameters to be generated with those methods easily establishing a multidimensional dataset. To just name a few examples of those parameters such as the influence of temperature, loading conditions, geometry as well as thermal and mechanical ageing processes on the fatigue behavior can now be calculated in accordance to a process being straightforward leading to an important step with regard to improving the efficiency of assessing structural components. Consequently, safety factors can be defined more in accordance to structural needs, being of highest interest with respect to the increasing number of ageing infrastructure such as highways, bridges or others. A lot of this ageing infrastructure has a strong need to be managed with respect to its structural integrity and the engineering community therefore tries the residual life of this infrastructure to be determined as appropriate as possible. In that context non-destructive testing parameters are increasingly considered to characterize a metallic material’s microstructure allowing more precise information to be obtained regarding the actual damage condition and the integrity of a component. The paper will address the high capability of non-destructive testing techniques for the evaluation of damage evolution processes also with respect to mechanism based fatigue as well as residual life calculations according to PHYBAL and SteBLife.

Abstract: The effects of the intercritical annealing temperature and initial microstructure on the stability of retained austenite were investigated for a 0.1C-6Mn (wt-%) steel. Medium-Mn transformation-induced plasticity (TRIP) steels exhibit a strong dependence of their mechanical properties on the variation of intercritical annealing temperature. This behavior is strongly linked to the amount and stability of the retained austenite. Thus, interrupted tensile tests were used to examine the effect of annealing temperature on the stabilization of the retained austenite. Detailed microstructural investigations were employed to elaborate the effects of its chemical and mechanical stabilization. Furthermore, the final microstructure was varied by applying the batch annealing step to an initial non-deformed and deformed microstructure respectively. Retained austenite stability along with resulting mechanical properties of the investigated medium-Mn TRIP steel was significantly influenced as the amount and morphology of the respective phases altered as a consequence of both initial microstructure and applied intercritical annealing temperature.

Abstract: The effect of plastic deformation under various conditions of the equiatomic CoCrFeNiMn alloy with single face-centered cubic phase structure was studied. The alloy was rolled at room and cryogenic temperatures, and uniaxially compressed at room temperature and temperatures of 600-1100°C with different height reductions. In addition, multiaxial forging at 900-1000°C was performed. Scanning and transmission electron microscopy, including EBSD analysis, was widely employed to characterize microstructure of the deformed alloy. At room and cryogenic temperatures, mechanical twinning and shear banding plays play dominant role in microstructure evolution. Extensive refinement of the microstructure occurs as the result of rolling with reduction of 80%. During deformation at 600-1100°C, discontinuous dynamic recrystallization takes place. The recrystallized grains size and their volume fraction increases with increase of deformation temperature. Multiaxial forging at 900-1000°C was used to produce fully recrystallized structure with average grain size of 6.7 μm. The alloy in the initial condition had low yield strength of 160 Mpa but remarkable tensile ductility of 68%. Rolling substantial increases yield strength to 1120-1290 MPa, but results in loss of ductility. After multiaxial forging the alloy has balanced combination of properties – yield strength of 280 MPa and elongation of 56%.

Abstract: An essential part of cell cultivation via cell culture technology is the determination and monitoring of culture parameters. Such parameters refer to the vitality or mutual mutations of the cell culture, while the actual number of living cells in each batch indicates the correct growth rate rather than stagnation or an overgrowth of the cell culture. Today such parameters are determined by applying light microscopy methods or by staining specific constituents of the cells. Commonly such methods are a stressful procedure for the studied cells. Most applied dyes are toxic over a certain period of time and thus they are used in low concentrations only when necessary. Within this work a new kind of measurement device prototype was designed to address these problems. This device is based on the Electronic Speckle Pattern Interferometry (ESPI). ESPI is an optical high-resolution method combined with a photonic analysis system capable of analyzing cellular deformations and oscillations. In this approach the combination of a greatly modified microscope together with ESPI method is presented. The apparatus allows the determination of cellular deformation (i) at very high magnifications, (ii) with high lateral resolution. Furthermore the system studies (iii) contact free, (iv) in vitro cells, in a non-invasive and non-destructive way. A co-developed cultivation system allows monitoring the culture parameters in real time minimizing the stress for the cell culture. Since no additional substances are needed, the presented prototype is automated to a large extent and can be operated by a special control-and regulation system (CRS) based on a microcontroller development board (Arduino Mega).

Abstract: In the scope of this work, 2 mm thick TZM sheet metal is butt welded by electron beam welding (EBW) without filler material and a systematic investigation of the most relevant welding parameters to improve the weld quality is conducted. With the aid of design of experiment (DoE), it is shown that with careful selection of the welding parameters it is possible to considerably reduce the size of the fusion zone and the heat affected zone and the grain size of both. Furthermore, the influence of the parameters on the quality of the weld and the characterizing values ultimate tensile strength and hardness of fusion zone is presented. It is concluded, which parameters influence the quality of the weld and suppress pores and cracks.

Abstract: The application of functional layers has increased constantly over the last decades [1]. Coating processes like plasma spraying allow efficient processing of metal or oxide particles, and have already found their application in various sectors of industries. Ultra fine cleaning, surface activation or surface modification with the plasma arc are currently also under investigation. In the scope of this work the influence of four different main parameters - current, working distance, feed speed and gas flow - on the arc temperature field was investigated. Due to the complex and different interactions of these parameters on the temperature field, the temperature distribution in steel and aluminium sheets was systematically examined. Furthermore, the relationship between the measured surface temperatures and the wettability of the substrates is being discussed. To generate the required data, two different experimental setups were used. First, the spatial heat distribution of the plasma arc was measured with a special arrangement of thermocouples. Second, the temperature fields during the plasma surface treatment of DC01 and Al 6082 substrates was measured. In addition to measurements with NiCr-Ni thermocouples an investigation with an IR-Camera was performed. After the plasma surface treatment, the resulting wettability was determined by contact angle measurement. The obtained results and especially the measured temperature fields will be used in a next step to validate numerical simulations with SYSWELD and ANSYS CFX, which will be used for process optimization.