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Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses.

Suppressing hydrogen embrittlement in martensitic steels is a longstanding challenge. Here, we studied the effects of prior austenite grain (PAG) shape and size with a 0.25C steel utilising novel in situ H-charging, constant-displacement Tuning-fork testing (TFT) and H-permeation tests. Anisotropic elongated PAG structure has enhanced HE resistance transverse to the rolling direction (RD) with slower crack propagation rate (CPR) and quasi-cleavage fracture. Larger elongated grains are prone to intergranular fracture when crack propagates in RD. Reaustenitised equiaxed PAGs fail with intergranular cracking, which accelerates max CPR up to threefold compared to quasi-cleavage. All the microstructures have similar H-diffusion ∼5 × 10−7 cm2/s and density of reversible H-traps NT ∼ 3 × 1016, irrespective of PAG surface area, indicating that PAG boundaries are not effective diffusion paths. Deformed PAG boundaries mitigate susceptibility to intergranular cracking.

A novel in-situ scanning electron microscope (SEM) characterization technique for observing phase transformation is developed and its results studied and compared to well-founded experimental ex-situ methods as well as numerical modelling of bainite formation. Phase transformation kinetics are compared between the in-situ SEM, dilatometry test as well as mean field and Cellular Automata (CA) models. Microstructural evolution during phase transformation is compared between in-situ SEM, ex-situ EBSD and CA model results. The in-situ method allows for direct observation of bainite sheaf growth and impingement. The in-situ results in combination with EBSD data can be used to parameterize the CA model sheaf nucleation and growth equations.

Digitalization is becoming increasingly common in the steel industry. Formerly developed models of individual phenomenon or separate sub-processes are being further developed into wider complexes where multiple models are coupled together. Virtual rolling automation, which can be used to control a finite-element rolling model, is a new element in these complexes. The automation enables to model the variations caused by the process adjustment. It must be taken in the account that neither the model nor the industrial process are ideal, but there are limitations in the attainable accuracy in both cases. Inclusion of the new automation control in the FE-model introduces new requirements: the setup calculations for all six rolling stands and the automation logic adjustments must perform within the model. The focus of the current article is prediction of the roll force and the virtual rolling automation of six stand finishing mill.

Controlling the hot rolling process requires a deep understanding of the underlying metallurgical phenomena. Quantitative methods are of paramount importance for achieving the capability of controlling microstructural evolution. Since the final mechanical properties of steel result from microstructural evolution in the whole process, analysis of the microstructure provides an important input for numerical simulations that can be used for tailoring the mechanical properties of steel. The evolution of grain size distribution of a low-carbon CrNiMnB ultrahigh-steel in austenitic state is studied in hot forming and annealing using experimental data obtained with the Gleeble 3800 thermo-mechanical simulator. A general method is described that can be utilized to systematically compare the grain size distributions obtained from the experimental studies. The experimental data has been obtained from laser scanning confocal microscopy images using the mean linear intercept method. A custom-made semi-automatic software has been utilized to process the data rapidly and reliably.

Prior austenite grain (PAG) structure is an important factor influencing hydrogen embrittlement (HE) susceptibility of ultrahigh-strength steels. In this study, the effect of PAG shape and size on HE behaviour is investigated using a novel tuning-fork testing method and hydrogen thermal desorption spectroscopy (TDS). Different PAG structures were acquired via re-austenitization (860°C = A860, 960°C = A960) and rapid quenching of an as-received 500 HBW direct-quenched (DQ) steel, which has an auto-tempered lath-martensitic microstructure and elongated PAG morphology. Fractography reveals different crack propagation mechanisms depending on the PAG shape. With the elongated PAG structure, hydrogen-induced crack propagation transverse to elongated PAGs was transgranular quasi-cleavage. Propagation was partially intergranular with the equiaxed PAG structures, regardless of the PAG size, leading to equally faster fracture. The TDS results show that there are no significant differences between the total hydrogen contents, but re-austenitized A860 and A960 steels contain a higher fraction of weakly trapped hydrogen. This indicates that the PAG boundaries are not the dominant hydrogen traps, and the different crack propagation mechanisms are rather linked to the geometrical shape of the grain structure.

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t50%) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain‘s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from − 1.9 to − 2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t50%. The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains.

Koli Calling '21, Contains fulltext : 240180.pdf (Publisher’s version ) (Open Access)

A combination of experimental and numerical approaches was applied for constructing a dynamic model for thermomechanical processing, which was used for simulating laboratory rolling and cooling, and for designing a cooling path to enable phase transformation from austenite to ultrafine (~ 50–100 nm) bainitic laths. Physical thermomechanical simulation experiments were used for calibrating the numerical models. Hot rolling and water cooling experiments were conducted and they were numerically simulated. The calibrated numerical models were used for simulating the main processing stages affecting the final microstructure evolution during a laboratory scale processing, i.e. the low temperature (500 °C) ausforming and subsequent cooling schedules leading to the decomposition of austenite into bainite and martensite. The fitted model parameters and simulation results are presented for the laboratory rolling and two different cooling paths: (i) air cooling to 350 °C temperature with subsequent holding for 1–1.5 h, and (ii) water cooling close to martensite start temperature, and furnace holding for 1–1.5 h. Microstructural analysis was carried out using scanning electron microscopy combined with electron backscatter diffraction as well as X-ray diffraction and the structures were corroborated with mechanical properties evaluated in respect of hardness, tensile and impact toughness properties. The achieved mechanical properties and microstructures were further interpreted with the numerical simulation results. The results show that the calibrated numerical simulations provide an effective tool for designing suitable thermomechanical processing paths leading to desired microstructure.

Dual-phase steels are generally used in the car industry due to high tensile strength and good formability, which are obtained by a mixture of bainite and ferrite phases. This microstructure is achieved through slow rate coil cooling. However, the manufacturing of dual-phase steels introduces various challenges such as the instability of the cold rolling process. An important factor affecting this is the non-uniform coil cooling of a hot rolled strip. In coil cooling the cooling rates are not controlled and there are different thermal contacts during coil conveyance causing unequal cooling of the steel coil. Unequal cooling rates lead to non-uniform coil cooling, producing irregular phase transformations on different sides of the coil, which causes periodical variations of the phase fractions in the steel strip. Varying phase fractions cause thickness deviations in the strip during the cold rolling process. A three-dimensional transient heat transfer finite element model was developed and used for modeling the complete coil conveyance chain and coil field cooling of the coil on an industrial scale. A coupled phase transformation model is implemented as a subroutine into the finite element model for calculating the resulting phase fractions. It was found that the different thermal contacts during the coil conveyance produce uneven cooling rates causing length- and widthwise variations in the phase fractions. The heat transfer model is validated by comparing temperature profiles between the simulated and measured coil edges. The phase transformation model is fitted into experimental data and verification is carried out in industrial conditions by comparing the modeled phase fractions and test samples from a cooled and unwound steel coil.

Hydrogen embrittlement is a well-known problem with high-strength steels. An important aspect of hydrogen embrittlement research is the effect of the prior austenite grain (PAG) structure on hydrogen-induced fracture. The microstructural anisotropy of PAG structure depends on the steel manufacturing process. In this study, 500 HBW martensitic steels with different PAG structures are investigated with a novel tuning-fork test that utilizes an integrated loadcell system. The loadcell clamping system is used during hydrogen charging, allowing tracking of the applied force throughout the tests, which enables detection of separate phases of cracking and time-to-fracture. The elongated PAG morphology produces different results depending on the crack path direction in relation to the rolling direction, whereas the equiaxed PAG morphology does not manifest an orientation dependence. Depending on the PAG shape, also the fracture morphology differs. Time-to-fracture results show that elongated grain morphologies with transgranular quasi-cleavage crack propagation are more beneficial against hydrogen-induced fracture than equiaxed grain structure with intergranular crack propagation. These results demonstrate that the shape of the PAG structure plays an important role in the crack propagation mechanism and that it is important to consider the possible direction of hydrogen-induced cracks in the final structural applications.

Coil cooling process is an important step in production of certain steel grades. Phase transformations for dual phase steels and precipitations for precipitation hardened steels occur mainly during the coil cooling. Generally, a coil goes through a coil conveyance chain before arriving at the final cooling storage at a steel plant. This conveyance chain contains various thermal contacts with different types of conveyors. Ambient temperatures and weather conditions may also change considerably. Those variables are relatively easy to measure and define in a simulation model whereas internal stresses and contact pressure inside the coil are very challenging to measure in industrial scale process. Thermal conductance between adjacent strip revolutions is dependent of contact pressure. In addition, thermal conductance is influenced by the combined thermal conductivity of steel and oxide layer of contact interfaces as well as thickness profile. In this paper the internal contact pressure between strip revolutions due to strip coiling and gravity are solved and considered when defining thermal conductance. Heat transfer is computed using FE-model, and GAPCON subroutine in Abaqus is utilized to calculate thermal contact conductance, taking into consideration the contact pressure between the strip revolutions. Also, the whole coil conveyance chain commencing from downcoiler mandrel to coil field cooling is implemented..

The growth of prior austenite grains (PAG) of low alloyed martensitic steel is proven to be one of the key attributes contributing to the mechanical properties of ultrahigh-strength steels. The mean linear intercept -method (MLI) is traditionally used to acquire average PAG sizes from light optical microscopy images, which are from experimental test samples. The MLI -method is arduous and time-consuming as well as a highly generalizing method, where you lose information about the grain size distribution. Therefore, a more sophisticated and computerised method is in high demand among metallurgists. A program has been developed that encompasses an importing, digitalizing and calculating tool, which provides grain sizes and their distribution from multiple images. The tool mimics the workflow of manual MLI -method so the user sets the measure lines and marks all the linear intercepts. After this the tool calculates the MLI grain sizes and their 95 % confidence limits. Additionally, the tool provides the size of each intercepted grain and combines them to create a distribution. This information has been used to study the effects of holding temperature and time on grain sizes throughout the test samples in a case where abnormal grain growth at the centreline was expected. In the present study, PAG sizes were studied before and after deformation at ¼ and ½ thicknesses at various temperatures and holding times using the grain size calculation tool. The average MLI grain sizes show very little differences between temperatures and holding times, so information about grain size distribution is needed. Traditional presentation of the grain size distributions also shows too much variation to interpret the data properly. Instead, using the grain size distribution information and grouping grains to small, medium and large instances gives more profound data, especially in cases where grain size variation is significantly large. Distribution data from the test series also

A finite element (FE) simulation model illustrating the stress relaxation test was established with the Abaqus TM software. The microstructural evolution of steel during relaxation includes the complex phenomena of recrystallization. While the compression introduces the planned deformation and stress into the test piece, subsequent softening relieves the stress and at the same time creates microstructural reconstitution and refinement. In this study, a model was developed to simulate the kinetics of static recrystallization taking place during holding, using a technique based on FE-simulation. The simulation results have been compared to the experimental stress relaxation data obtained on a Gleeble TM 3800 thermo-mechanical simulator. The model can be used to estimate the recrystallization kinetics throughout the test piece. In the future, these results can be used for estimating the required rolling forces for multi-pass roughing with reasonable accuracy, for instance. The modelling methodology can be extended to other steels too, with or without microalloying additions.

Computer simulations of steel microstructural development provide a powerful tool, which can form the basis of mechanical property predictions. However, in order to create detailed understanding of the factors affecting the properties, the model should predict microstructural evolution during cooling. The present study compares the results of cellular automata simulations with experimental data for two distinct austenite conditions, recrystallized and deformed. Detailed microstructural features were studied using a laser scanning confocal microscope, FESEM and FESEM-EBSD. The two-dimensional cellular automata (CA) model for simulating the formation of lath martensite was parameterized using fitted Johnson-Mehl-Avrami-Kolmogorov and Koistinen-Marburger equations. The parent austenite microstructure for the CA model was determined from the final martensitic microstructure using austenite grain reconstructions based on the use of MATLAB software and the MTEX toolbox. The results of this cellular automata simulation can be used to estimate the shapes and sizes of martensite blocks, which offers new possibilities for the qualitative estimation of the mechanical properties of high-strength steels formed from recrystallized or deformed austenite.

A model for static recrystallization by Zurob et al. [1] has been fitted to experimental stress relaxation [2] data obtained on a low-alloyed steel using a Gleeble thermomechanical simulator. The model has been implemented as an algorithm that calculates the stress relaxation as a function of time, including physical descriptions of the recovery and recrystallization processes. The activation energy and volume were used as fitting parameters for recovery, and the activation energy of diffusion and nucleation site density were used as the fitting parameters for recrystallization. The four fitting parameters were determined from the experimental data by applying the Nelder-Mead algorithm within Matlab software. It can be concluded from the preliminary results that Zurob’s model can be successfully fitted to the stress relaxation data in order to illustrate the static restoration characteristics and kinetics in carbon steels using these fitting parameters.

In order to study the coupled effect of rolling deformation, heat transfer and water cooling on microstructure formation of a steel strip, a multiscale and multiphysical FE-model is developed. The deformation state and temperature distribution of the steel strip after the last rolling pass are required to calculate the final microstructure. The sub-model predicting final microstructure on the steel strip after accelerated water cooling is implemented numerically. Final phase fractions are calculated with the Johnson-Mehl-Avrami-Kolmogorov equation for different parts of the object, taking into account non-uniform temperature distribution.

Manipulation of host phenotype (e.g. behaviour, appearance) is suggested to be a common strategy to enhance transmission in trophically transmitted parasites. However, in many systems, evidence of manipulation comes exclusively from laboratory studies and its occurrence in natural host populations is poorly understood. Here, we examined the potential for host manipulation by Diplostomum eye flukes indirectly by quantifying the physiological effects of parasites on fish. Earlier laboratory studies have shown that Diplostomum infection predisposes fish to predation by birds (definitive hosts of the parasites) by reducing fish vision through cataract formation. However, occurrence of cataracts and the subsequent potential for host manipulation in natural fish populations has remained poorly explored. We studied the occurrence of eye fluke-induced cataracts from 7 common fish species (Gymnocephalus cernuus, Rutilus rutilus, Leuciscus leuciscus, Alburnus alburnus, Osmerus eperlanus, Coregonus lavaretus and Gasterosteus aculeatus) from the Bothnian Bay in the Baltic Sea. We found that the parasite-induced cataracts were common in fish and they also reached high levels which are likely to predispose fish to predation. However, we observed such cataracts only in species with the highest parasite abundances, which suggests that only certain hosts may be strongly affected by the infection

Trophically-transmitted parasites frequently alter multiple aspects of their host's phenotype. Correlations between modified characteristics may suggest how different traits are mechanistically related, but these potential relationships remain unexplored. We recorded 5 traits from individual isopods infected with an acanthocephalan (Acanthocephalus lucii): hiding, activity, substrate colour preference, body (pereon) coloration, and abdominal (pleon) coloration. Infected isopods hid less and had darker abdominal coloration than uninfected isopods. However, in 3 different experiments measuring hiding behaviour (time-scales of observation: 1h, 8h, 8 weeks), these two modified traits were not correlated, suggesting they may arise via independent mechanisms. For the shorter experiments (1h and 8h), confidence in this null correlation was undermined by low experimental repeatability, i.e. individuals did not behave similarly in repeated trials of the experiment. However, in the 8-week experiment, hiding behaviour was relatively consistent within individuals, so the null correlation at this scale indicates, less equivocally, that hiding and coloration are unrelated. Furthermore, the difference between the hiding behaviour of infected and uninfected isopods varied over 8 weeks, suggesting that the effect of A. lucii infection on host behaviour changes over time. We emphasize the importance of carefully designed protocols for investigating multidimensionality in host manipulation

For trophically transmitted parasites, transitional larval size is often related to fitness. Larger parasites may have higher establishment success and/or adult fecundity, but prolonged growth in the intermediate host increases the risk of failed transmission via natural host mortality. We investigated the relationship between the larval size of an acanthocephalan (Acanthocephalus lucii) and a trait presumably related to transmission, i.e. altered colouration in the isopod intermediate host. In natural collections, big isopods harboured larger worms and had more modified (darker) abdominal colouration than small hosts. Small isopods infected with a male parasite tended to have darker abdominal pigmentation than those infected with a female, but this difference was absent in larger hosts. Female size increases rapidly with host size, so females may have more to gain than males by remaining in and growing mutually with small hosts. In experimental infections, a large total parasite volume was associated with darker host respiratory operculae, especially when it was distributed among fewer worms. Our results suggest that host pigment alteration increases with parasite size, albeit differently for male and female worms. This may be an adaptive strategy if, as parasites grow, the potential for additional growth decreases and the likelihood of host mortality increases

Many complex life cycle parasites exhibit seasonal transmission between hosts. Expression of parasite traits related to transmission, such as the manipulation of host phenotype, may peak in seasons when transmission is optimal. The acanthocephalan Acanthocephalus lucii is primarily transmitted to its fish definitive host in spring. We assessed whether the parasitic alteration of 2 traits (hiding behaviour and coloration) in the isopod intermediate host was more pronounced at this time of year. Refuge use by infected isopods was lower, relative to uninfected isopods, in spring than in summer or fall. Infected isopods had darker abdomens than uninfected isopods, but this difference did not vary between seasons. The level of host alteration was unaffected by exposing isopods to different light and temperature regimes. In a group of infected isopods kept at 4°C, refuge use decreased from November to May, indicating that reduced hiding in spring develops during winter. Keeping isopods at 16°C instead of 4°C resulted in higher mortality but not accelerated changes in host behaviour. Our results suggest that changes in host and/or parasite age, not environmental conditions, underlie the seasonal alteration of host behaviour, but further work is necessary to determine if this is an adaptive parasite strategy to be transmitted in a particular season