RMZ – MATERIALI IN GEOOKOLJE

Calculation of stress-strain dependence from tensile tests at high temperatures using ...333specimen as well as in radial direction.Especially due to the latest weaknessstrain and strain rate can be expressedas equivalent strain and strain rate. Athot tensile testing also non-homogeneityof deformation occurs on specimenworking length that is expressedby its necking. It is well known, thatfor tensile test homogenous deformationtakes place only untilappearanceof necking which occurs at strains ofapproximately between 0.2 and 0.3,depending on precision of manufacturingof the tensile samples, onmaterialsinhomogeneity, on temperature gradientalong the tensile axis, etc.When the neck appears, the stress statechange from uniaxial to multi-axialmustbe taken into account. The correctiondue to multi-axiality is usuallydoneby Bridgman correction, whichpresumes that portion of the contourinclose neighborhood of the minimalcross-section of the neck may be characterizedby a single parameter, theradius of curvature of the circle osculatingthe profile at the neck. [1] But obtainingreliable data on stress – strainrelation from tensile tests after appearanceof necking is very difficulttask especially if testing is conductedat elevated temperatures, which is exactlythe case if one wants to study hotworkability of metallic materials.The aims of this study are therefore toevaluate the possibility of applicabilityof tensile test for determination of hotworking properties of metallicmaterialsalso after the appearance of neckingand to introduce appropriatemodel forcalculation of stress-strain dependencefrom the final shapes ofcontours of tensileloaded specimens.Experimental procedureA commercially produced Ni alloyAlloy201 was supplied by Thyssen-Krupp Gmbh as hot drowned rods withchemical composition given in Table1 and equiaxed grain structure with anaverage grain size of about 16 μm.Cylindrical specimens with dimensionsof 25 mm of effective length andof 8 mm in diameter where machinedfrom supplied rods for hot tensile tests(see Figure 1 for specimen’s geometryand dimensions).Table 1. Chemical composition of the commercially pure Ni (Alloy201) tested in massfractions w/%Mo Cr Si S Mg Co Cu P0.001 0.004 0.05 0.001 0.03 0.07 0.006 0.006Mn Ti Fe Sn C V Al Ni0.13 0.04 0.13 0.01 0.02 0.002 0.023 restRMZ-M&G 2012, 59