E. Hadasik

A new model describing the hot stress–strain curves of HSLA steel at high deformation

Abstract The torsion test is often applied for the determination of the hot flow stress of steels. An originally derived system of equations has been used to describe the basic parameters of the torsion test and has enabled precision to be obtained in the simulation of flat rolling. Because of the relatively low strain rates obtained with a torsion plastometer, it is necessary to extrapolate the laboratory results into the range of real working conditions. In this case, only a limited number of demanding and expensive interrupted tests have to be done. The more important role is then played by computer prediction based on equations that incorporate the results of simple, but mainly continuous tests. Examples of such mathematical models are presented, including the influence of dynamic or static softening in hot rolling.

Modelling of structure changes in TRIP type steel during hot deformation

Physical modelling of the changes occurring in the structure of low-alloy C-Mn-Al TRIP-aided steel during hot-rolling were presented in this paper. An influence of deformation and controlled cooling parameters on the microstructure of the tested steel was determined. Metallographic research showed continuous refinement of structure in consecutive stage of deformation. Applied parameters of cooling allowed obtaining multi-phase structure containing ferrite, bainite, and also retained austenite.

Research of selected properties of two types of high manganese steel wires

The article presents results of tests that aimed at establishing the impact of deformation on properties of wires made of two types of high manganese steels. The deformation process was carried out with the use of a draw bench machine at a speed of 0.5 m min−1. Mechanical properties and structure of strengthened and annealed wires for both steels at different levels of relative reduction in cross-section were determined. Strength of the tested materials was determined in the tensile test, while its hardness was measured with the Vickers hardness test method. Fractographic tests were performed using a scanning electron microscope. It was shown that at the beginning of tensile test, the investigated high manganese steels were characterized by very high plasticity and become stronger as the degree of deformation grows. Surfaces of fractures that were created in the areas where the sample was torn were analyzed. These fractures indicate the presence of transcrystalline ductile fractures.

Plasticity and Microstructure of Magnesium - Lithium Alloys

The article presents the results of tests connected with influence of strain parameters on the susceptibility to hot working of magnesium lithium alloys with lithium content of 7.5% mass. Plasticity tests were conducted in compression test from room temperature up to 400°C with strain rate of 0.1s-1. Conducted tests allowed for determination of susceptibility of magnesium alloys with different lithium content to hot working. The results were compared with typical alloy AZ31. The results of influence of deformation temperature on the microstructure of tested alloys are also presented in this paper.

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.

Resistance to Electrochemical Corrosion of Extruded Magnesium Alloy AZ31

The purpose of the study was evaluation of electrochemical corrosion resistance of extruded magnesium alloy AZ31. Corrosion tests were performed in NaCl solutions featuring chloride ions concentration from 0.01 to 2 M NaCl. Potentiodynamic tests enabled to register anodic polarisation curves. Immersion tests were performed in NaCl solution and time 1-5 days. Electrochemical impedance spectroscopy (EIS) was used to evaluate phenomena that take place on the surface of the tested alloy. Impedance spectra of the system were made in the tests, and obtained measurement data was matched to the equivalent system. The results of all performed tests prove explicitly deterioration of corrosion properties of extruded magnesium alloy AZ31 with the increase of molar concentration of NaCl solution. Options of magnesium alloy AZ31 application in aircraft and automotive industry are connected with the need for application of protective layers on elements made of the tested alloy.

Resistance to Electrochemical Corrosion of Extruded Magnesium Alloy AZ61

The purpose of the study was the evaluation of the electrochemical corrosion resistance of extruded magnesium alloy AZ61 in solutions with concentration of 0.012 M NaCl. Resistance to electrochemical corrosion was evaluated on the ground of registered anodic polarisation curves by means of potentiodynamic method. Immersion tests were performed in NaCl solution and time periods of 1-6 days. Scanning microscopy was used to obtain images of the alloy microstructure after immersion tests. Electrochemical impedance spectroscopy was used to evaluate phenomena that take place on the surface of the tested alloy. The results of all performed tests prove explicitly deterioration of corrosion properties of magnesium alloy AZ31 with the increase of molar concentration of NaCl solution. It was found that irrespective of molar concentration of NaCl solution, pitting corrosion can be detected on the surface of the tested alloy. Test results prove that it is necessary to apply protective layers on elements made of the tested alloy.

GEOMETRICAL PHENOMENA IN TUBE BENDING WITH LOCAL INDUCTION HEATING

A theoretical and experimental analysis of heat induction bending for tubes used in the power industry is performed. First, the design of the heat induction bending process for tubes is described and industrial application areas for this technology are presented. Next, the main methods for tube bending with local induction heating are discussed and the effect of the technology on geometrical parameters of bends formed is presented. Then, the heat induction bending process for tubes is modeled using numerical techniques (FEM). The simulations are performed in a three-dimensional strain state, where thermal phenomena are taken into account, using the commercial software package Simufact Forming v. 11.0. In the simulations, the changes in workpiece geometry in the region of the bend being made (cross section ovalization, darkening and thickening of walls, neutral axis position) are examined. Also, potential phenomena that could limit the stability of the bending process and cause shape defects are predicted. The results of the numerical modeling are then compared to those obtained under industrial conditions.

Forecasting of Corrosion Properties of Steel Wires for Production of Guide Wires for Cardiological Treatment

The study presents evaluation of the influence of strain in drawing process and of surface modification on resistance to electrochemical corrosion of wires made of stainless steel for production of guide wires used in invasive cardiology. The results of static tensile test enabled us to determine the course of flow curve of wires made of X10CrNi 18-8 steel as well as mathematical form of flow stress function. Resistance to electrochemical corrosion was evaluated on the ground of registered anodic polarisation curves by means of potentiodynamic method. The tests were performed in solution simulating human blood on samples that were electrolytically polished and samples that were polished and then chemically passivated. Exemplary anodic polarisation curves were given. It was proved that with the applied strain, corrosion properties decrease. It was found that chemical passivation improves wire corrosion characteristics. Statistical analysis showed that there is a significant dependence between corrosion properties (polarisation resistance ) and strain applied in drawing process. Functions that present the change were selected. The issue is of importance to guide wire manufacturers because application of the suggested methodology will enable us to forecast corrosion characteristics of wire with the required strength drawn with the applied strain.

Galvanic Corrosion Test of Magnesium Alloys after Plastic Forming

The purpose of this study was to evaluate resistance to galvanic and crevice corrosion of magnesium alloys AZ61 and AZ80. Resistance to galvanic corrosion was evaluated with additional application of aluminium alloy 2017A and 8Mn2Si steel as reference materials. The tests were carried out by means of potentiostat VoltaLab PGP 201 by Radiometer with application of Evans method. The tests were carried out in the solution with concentration of 0.01 M NaCl in ambient temperature. For comparison, the relations of the surface of magnesium alloys to aluminium alloys and steel (1:1, 5:1 i 10:1) was differentiated in the experiment. It was proved that AZ80 alloy features slightly higher corrosion resistance in contact with aluminium alloy and steel.