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Number of results: 13
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Abstract

Influence of the initial grain size on hot deformation behavior of the low-alloy Mn-Ti-B steel was investigated. The uniaxial compression tests were performed in range of the deformation temperatures of 900-1200°C and strain rates of 0.1-10 s–1. One set of samples was heated directly to the deformation temperature, which corresponded to the initial austenitic grain size of 19-56 μm; the other set of samples was uniformly preheated at the temperature of 1200°C. Whereas the values of activation energy, peak stress and steady-state stress values practically did not depend on the initial austenitic grain size, the peak strain values of coarser-grained structure significantly increase mainly at high values of the Zener-Hollomon parameter. This confirms the negative effect of the large size of the initial grain on the dynamic recrystallization kinetics, which can be explained by the reduction in nucleation density.

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Authors and Affiliations

P. Kawulok
I. Schindler
R. Kawulok
P. Opěla
R. Sedláček
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Abstract

Due to the large amount of binder and low water-cement ratio, high-performance cement composites have high compressive strength and a dense hardened cement paste microstructure. External curing is insufficient, as it cannot reach the interior parts of the structure, which allows autogenous shrinkage to occur in the inside. Lack of prevention of autogenous shrinkage and high restraint causes structural microcracks around rigid components (aggregate, rebars). Consequently, this phenomenon leads to the propagation of internal microcracks to the surface and reduced concrete durability. One way to minimize autogenous shrinkage is internal curing. The use of soaked lightweight aggregate to minimize the risk of cracking is not always sufficient. Sorption and desorption kinetics of fine and coarse fly ash aggregate were tested and evaluated. The correlation between the development of linear autogenous shrinkage and the tensile stresses in the restrained ring test is assessed in this paper. A series of linear specimens, with cross-section and length custom designed to match the geometry of the concrete ring, were tested and analyzed. Determination of the maximum tensile stresses caused by the restrained autogenous shrinkage in the restrained ring test, together with the approximation of the tensile strength development of the cement composites were used to evaluate the cracking risk development versus time. The high-performance concretes and mortars produced with mineral aggregates and lightweight aggregates soaked with water were tested. The use of soaked granulated fly ash coarse lightweight aggregate in cementitious composites minimized both the autogenous shrinkage and cracking risk.
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Authors and Affiliations

Adam Zieliński
1
ORCID: ORCID
Anton K. Schindler
2
ORCID: ORCID
Maria Kaszyńska
1
ORCID: ORCID

  1. West Pomeranian University of Technology, Faculty of Civil Engineering and Environmental, al. Piastów 50a, 70-311 Szczecin, Poland
  2. Department of Civil and Environmental Engineering, Auburn University, 237 Harbert Center, Alabama 36849, Auburn, USA
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Abstract

The aim of the performed experiments was to determine the influence of a cooling rate on the evolution of microstructure and hardness of the steel 27MnCrB5. By using dilatometric tests performed on the plastometer Gleeble 3800 and by using mathematical modelling in the software QTSteel a continuous cooling transformation diagram for a heating temperature of 850°C was constructed. Conformity of diagrams constructed for both methods is relatively good, except for the position and shape of the ferrite nose. The values of hardness, temperatures of phase transformations and the volume fractions of structural phases upon cooling from the temperature of 850°C at the rate from 0.16°C · s–1 to 37.2°C · s–1 were determined. Mathematically predicted proportion of martensite with real data was of relatively solid conformity, but the hardness values evaluated by mathematical modelling was always higher.
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Authors and Affiliations

I. Schindler
P. Kawulok
J. Mizera
S. Rusz
R. Kawulok
P. Opěla
M. Olszar
K.M. Čmiel
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Abstract

Nil strength temperature of 1062°C and nil ductility temperature of 1040°C were experimentally set for CuFe2 alloy. The highest formability at approx. 1020°C is unusable due to massive grain coarsening. The local minimum of ductility around the temperature 910°C is probably due to minor formation of γ-iron. In the forming temperatures interval 650-950°C and strain rate 0.1-10 s–1 the flow stress curves were obtained and after their analysis hot deformation activation energy of 380 kJ·mol–1 was achieved. Peak stress and corresponding peak strain values were mathematically described with good accuracy by equations depending on Zener-Hollomon parameter.

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Authors and Affiliations

I. Schindler
M. Sauer
P. Kawulok
K. Rodak
E. Hadasik
M.B. Jabłońska
S. Rusz
V. Ševčák
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Abstract

The aim of the performed experiments was to determine the influence of deformation and of austenitization temperature on the kinetics of phase transformations during cooling of high-carbon steel (0.728 wt. % C). The CCT and DCCT diagrams for austenitization temperature 940°C and DCCT diagram for austenitization temperature 1000°C were constructed with the use of dilatometric tests. On the basis of obtained results, a featureless effect of austenitization temperature and deformation on the kinetics of phase transformations during cooling of investigated steel was observed. Critical cooling rates for the transformation of martensite in microstructure fluctuated from 5 to 7°C · s–1 (depending on the parameters of austenitization and deformation), but only at cooling rates higher than 8°C · s–1 a dominant share of martensite was observed in the investigated steel, which resulted in the significant increase of hardness.

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Authors and Affiliations

P. Kawulok
P. Podolinský
P. Kajzar
I. Schindler
R. Kawulok
V. Ševčák
P. Opěla
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Abstract

The work deal with an assembling and comparing of transformation diagrams of two low-alloy steels, specifically 16MnCrS5 and 20MnCrS5. In this work, diagrams of the type of CCT and DCCT of both steels were assembled. Transformation diagrams were assembled on the basis of dilatometric tests realized on the plastometer Gleeble 3800, of metallographic analyses and of hardness measurements. In addition, for comparison, the transformation diagrams were assembled even with use of the QTSteel 3.2 software. Uniform austenitization temperature of 850°C was chosen in case of both steels and even both types of diagrams. In case of both steels, an influence of deformation led to expected acceleration of phase transformations controlled by diffusion and also of bainite transformation. In both cases, the kinetics of martensitic transformation was not significantly affected by deformation.

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Authors and Affiliations

R. Kawulok
P. Kawulok
I. Schindler
P. Opěla
S. Rusz
V. Ševčák
Z. Solowski
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Abstract

Suitable and complete sets of stress-strain curves significantly affected by dynamic recrystallization were analyzed for 11 different iron, copper, magnesium, titanium or nickel based alloys. Using the same methodology, apparent hot deformation activation energy Qp and Qss values were calculated for each alloy based on peak stress and steady-state stress values. Linear dependence between quantities Qp and Qss was found, while Qp values are on average only about 6% higher. This should not be essential in predicting true stress of a specific material depending on the temperature-compensated strain rate and strain.

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Authors and Affiliations

I. Schindler
ORCID: ORCID
P. Opěla
ORCID: ORCID
P. Kawulok
ORCID: ORCID
M. Sauer
ORCID: ORCID
S. Rusz
ORCID: ORCID
D. Kuc
K. Rodak
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Abstract

High-temperature plastic properties of heat-resistant stainless steel X15CrNiSi 20-12 were assessed on the basis of hot tensile tests and nil strength tests. The results were supported by metallographic analyses using SEM and EDX analysis. The formability of the investigated steel can be divided into roughly three temperature areas. In the temperature range of 900°C to about 1050°C, formability was negatively affected by precipitation of carbide particles at grain boundaries. As the temperature rose to 1200°C, these particles dissolved, resulting in an increase in formability. Further temperature increases resulted in a relatively steep drop in formability caused by overheating of the material. The nil ductility temperature of 1280°C and the nil-strength temperature of 1362°C were determined. The Plastic and strength properties of the investigated material were compared with the deformation behavior of the reference steel X5CrNi 18-10, which shows a significantly wider range of suitable forming temperatures.

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Authors and Affiliations

R. Kawulok
ORCID: ORCID
I. Schindler
ORCID: ORCID
H. Navrátil
ORCID: ORCID
V. Ševčák
J. Sojka
K. Konečná
ORCID: ORCID
B. Chmiel
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Abstract

A simple methodology was used for calculating the equivalent strain values during forming the sample alternately in two mutually perpendicular directions. This method reflects an unexpected material flow out of the nominal deformation zone when forming on the MAXStrain II device. Thus it was possible to perform two temperature variants of the simulation of continuous rolling and cooling of a long product made of steel containing 0.17% C and 0.80% Mn. Increasing the finishing temperature from 900°C to 950°C and decreasing the cooling rate from 10°C/s to 5°C/s led to a decrease in the content of acicular ferrite and bainite and an increase in the mean grain size of proeutectoid ferrite from about 8 µm to 14 µm. The result was a change in the hardness of the material by 15%.
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Authors and Affiliations

I. Schindler
1
ORCID: ORCID
P. Kawulok
1
ORCID: ORCID
K. Konečná
1
ORCID: ORCID
M. Sauer
1
ORCID: ORCID
H. Navrátil
1
ORCID: ORCID
P. Opěla
1
ORCID: ORCID
R. Kawulok
1
ORCID: ORCID
S. Rusz
1
ORCID: ORCID

  1. VŠB – Technical University of Ostrava, Faculty of Materials Science and Technology, Ostrava, Czech Republic

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