Petr Kawulok

Complex Flow Stress Model for a Magnesium Alloy AZ31 at Hot Forming

Abstract Compression tests of magnesium alloy Mg-3Al-1Zn (AZ31) at different temperatures and strain rate were made on plastometer Gleeble 3800. Deformation behaviour and particularly shape of stress-strain curves of the alloy AZ31 differ significantly at low and high values of Zener–Hollomon parameter Z. The border between these areas was determined mathematically as Z = 2.9E+13 s–1. While the calculated activation energy Q was for both these areas practically identical (157 or 155 kJ mol–1), mathematical description of coordinates of the peak stress differs considerably. Regression and statistical analysis of experimental data have confirmed unequivocally, that it was impossible to describe by a uniform equation the whole set of data (i.e. traditional stress-strain curves, as well as those with atypical initial stage, given by the massive twinning). Thats why two mathematical models were developed enabling prediction of the flow stress of investigated magnesium alloy in dependence on temperature, strain and strain rate, with inclusion of the influence of dynamic recrystallisation.

Homogenization of Microstructure of Castings from the Alloy Fe-40at.%Al-Zr-B

The samples intended for the study of static recrystallization on plastometer Gleeble were prepared from the laboratory castings of iron aluminide containing 24.6 Al 0.17 Mn 0.16 Zr 0.026 B 0.004 C (in wt. %, remainder Fe). Nevertheless, the structure analysis discovered that the results were excessively influenced by the huge heterogeneity of the as-cast microstructure, mostly of the grain size. Combination of the hot forming and recrystallization process during the long-term high-temperature annealing was selected for the necessary structure homogenization. As the tested intermetallic alloy is extremely brittle and susceptible to surface cracking, the original method was applied for its processing. The method consists in hot rolling in the protective capsules welded from the ferritic stainless steel sheet. The castings were rolled to 2/3 of their thickness by 4 reductions with the inter-stage heating, and then annealed at the temperature of 1200 °C in the vacuum furnace for several periods. Metallographic analysis revealed that annealing lasting 7 hours was essential for the uniform coarsening of the recrystallized grains. Material processed in this way proved successful for the subsequent metallographical study of static recrystallization.

Influence of Finish-Rolling Conditions on Microstructure and Mechanical Properties of Low-Alloy Mn-Ni-Cr-Mo Steel Grade

A physical simulation of the thermomechanical processing of the Mn-Ni-Cr-Mo low-alloy steel was performed in the laboratory rolling mill Tandem in the Institute of Modelling and Control of Forming Processes at VŠB – Technical University of Ostrava. The task was to determine the influence of the finish rolling temperature on the structural and mechanical properties of the rolled products. After different modes of rolling and slow cooling in the furnace, the final structure of the tested samples was in all cases composed of ferrite, bainite and islands of martensite. The finish rolling temperature markedly influenced a part by volume of the individual phases as well as the structure homogeneity. The results of the tensile tests at room temperature indicated that the studied steel did not show any pronounced dependence of the yield stress on the finish rolling temperature in the investigated range of values (750 – 1000 °C). On the other hand, the closely corresponding dependences of the ultimate tensile stress and elongation exhibited a considerable and very complex course, which can be explained mainly by the martensite fraction originating during the last stage of the final air cooling from temperature 600 °C.

Transformation diagrams of selected steel grades with consideration of deformation effect

The aim of this article was to assess the effect of previous plastic deformation on the transformation kinetics of selected steels with a wide range of chemical composition. Transformation (CCT and DCCT) diagrams were constructed on the basis of dilatometric tests on the plastometer Gleeble 3800 and metallographic analyses supplemented by measurements of HV hardness. Effect of previous deformation on transformation was evaluated of the critical rate of formation of the individual structural components (ferrite, pearlite and bainite) in the case of formation of martensite respect to Ms temperature. Previous plastic deformation accelerated especially diffuse transformations (ferrite and pearlite), temperature of Ms was lower after previous plastic deformation and bainitic transformation was highly dependent on the chemical composition of steel.

The combined effect of chemical composition and cooling rate on transformation temperatures of hypoeutectoid steels

The transformation temperatures Ar3 and Ar1 of four unalloyed hypoeutectoid steels with a carbon content of 0.029–0.73 % were determined using dilatometric tests. Unusually high cooling rates of 2 and 8◦C s−1 were used intentionally, corresponding to the conditions in the wire rod rolling mills. The developed regression models are phenomenological and allow a simple prediction of transformation temperatures, depending only on the cooling rate and the chemical composition of the steel represented by the carbon equivalent (in the case of Ar1), respectively by the Ac3 temperature (for Ar3). When calculating the Ac3 temperature, it was worth considering its non-linear dependence on carbon content. It has been verified that the derived equations are applicable even at relatively low cooling rates when the austenite decomposes exclusively on ferrite and pearlite. K e y w o r d s: hypoeutectoid steel, carbon equivalent, dilatometer test, cooling rate, decomposition of austenite, transformation temperature

Laboratory Controlled Rolling of Microalloyed Steel for Production of Seamless Tubes

Using the laboratory rolling mill with smooth rolls, piercing, as well as rolling in a pilger mill of the seamless tubes with diameter 273 mm from the HSLA steel microalloyed with vanadium steel was simulated. Influence of the wall thickness (6.3 – 40 mm) and finish rolling temperature on the final structural and mechanical properties was investigated. Necessary temperatures of the phase transformations in the course of cooling were determined by dilatometric tests. Based on the dilatometry results, finish rolling temperatures were reduced. Lower rolling temperatures yielded in a relative grain refinement. Effect of the finish rolling temperature did not have any marked impact on the tensile tests results. Strength properties decreased only slightly with the increasing wall thickness and the plastic properties were not influenced significantly by this parameter. The positive effect of the reduced finishing temperature appeared markedly in the results of impact tests performed at room temperature only. Notch toughness was increased by approx. 25 % in the case of the wall thickness of not less than 20 mm.

Study of Phase Transformation of Medium Carbon Steel after Hot Plastic Deformation

The CCT and DCCT diagrams of steel C60 (with approx. 0.6 % C) were constructed on the basis of dilatation tests with and/or without an influence of the previous deformation and they were then compared, order to make an evaluation of the influence of the previous deformation on the phase transformation kinetics. For the execution of the experiment, the dilatation module of the plastometer Gleeble 3800 was used. The accuracy of the diagrams was faced with metallographic analyses and measurements. The previous deformation expressly retarded a bainite transformation and slightly accelerated ferrite and pearlite transformations. The martensite start temperature was practically not influenced by the previous deformation; however, the applied deformation caused the creation of the martensite at lower cooling rates.

Defects and Particles in Laboratory Hot Rolled Steel of C-Mn-Cr-Nb Type

Susceptibility to cracking of the as-cast C-Mn-Cr-Nb steel was studied by laboratory rolling. The variable parameters were the heating temperature (1150 - 1340 °C) as well as the rolling temperature (950 - 1150 °C). Final microstructure of the free-cooled samples was constituted by bainite, pearlite and ferrite with different morphology and various contribution. Deformation temperature below 1000 °C yielded in the incomplete recrystallization of austenite. Surface cracks originated preferentially on the austenite grains boundaries. Size of the present particles (inclusions and precipitates) varied from 101 nm to 101 μm. SEM and EDS analysis revealed that the inclusions ware mostly of the MnS type. TEM analysis confirmed that the grain boundaries were not enriched by any particles. In addition to the Fe3C particles, the discoid niobium carbide particles with approximately 40 nm diameter and 10 nm thickness were detected. These small particles were not connected by any notable pinning of dislocations.

Influence of Cooling Rate on Final Properties of Laboratory Rolled Products

The objective of investigation was to determine the influence of chosen cooling rates after finish rolling on final microstructural and mechanical properties of the laboratory rolled products from steel 42CrMo4. Metallographic analysis showed that microstructure of rolled products, which were after finish rolling cooled down in accelerated manner by water sprays, was composed mostly by hardening phases and by smaller amount of ferrite. Microstructure of the rolled product cooled down in furnace in decelerated manner was formed by pearlitic blocks with minority occurrence of ferrite. Laboratory rolled products cooled down by more complicated modes, which consisted of combination of their cooling by water sprays with subsequent annealing in furnace, showed different phase morphology and distinct band structure. Results of tensile test have proved that the applied modes of cooling of rolled products by accelerated manner had only very small influence on their final mechanical properties. Considerably cooling the rolled product by decelerated manner in furnace resulted in drop yield strength, but on the other hand in an increase of ductility. More complicated modes of cooling the rolled products, which comprised also their annealing, showed the possibilities of significant influencing the strength, and particularly the plastic properties of the steel 42CrMo4.

Dynamic Recrystallization of the Alloy Fe-40at.%Al-Zr-B

On the basis of the uniaxial compression tests realized in the temperature range from 800 °C to 1200 °C and at strain rates from 0.05 s-1 to 30 s-1 the value of the activation energy at hot forming of coarse-grained as-cast iron aluminide Fe40at.%Al-Zr-B was calculated as Q = 502 kJ·mol-1. With use of this material constant, relatively precise mathematical description of the maximum flow stress and the corresponding strain in dependence on the Zener-Hollomon parameter was possible. As was discovered by metallographical analysis, at high values of the Zener-Hollomon parameter slight decrease in flow stress occurs even without significant course of dynamic recrystallization.