Nr.2 - ALPA - Albanian Papers

Lamani0.53570.39), T 1810 T with R 2 = 0.998. Thecomparison of strain-hardening parameters forboth zones is illustrated in Fig. 5.Figure 5 Comparison of the strain-hardeningparameters, n and K, for the zones I and II.The same analysis, performed with the tensiletest results (Fig. 6), identifies almost the samebehavior of the material in the first zone, whilstin the second one, the results diverge. So, valuesof K and n for the second zone, obtained by thetensile test, are about 25% lower than those ofthe disk test.insufficient to cause a martensitictransformation, so there is a low strainhardening,as much as the austenitic structureitself enables. Meanwhile, hardening effect issignificantly amplified in the second zone by thecreation of martensite. The above reasoning isconfirmed by the diffractograms of ruptureddisks, which have revealed the presence ofmartensite. Tensile test, although confirming thedual steel behavior, differentiates less the twozones. Thus, if the disk test shows an increase ofthe exponent n by 2.31 times (in the first zoneversus the second one), according to the tensiletest, this increase is only 1.85 times. The sameconclusion is applied to the index K. Differentresults in the second zone, must be explained bythe main cause of the hardening effect (in thiscase), namely, the martensitic transformation. Itseems that the disk test, thanks to the biaxialstress conditions, favors more thistransformation, and therefore its effect turns outgreat.Influence of temperatureThe results of 10 tests conducted attemperatures of 18-650ºC and processed withthe analytical model, are shown in Fig. 7.Figure 6. Comparison of logσ T – logε T curves fordisk and tensile tests at 18ºCThe differentiation of the steel behavior can notbe explained by anything else, except by theeffect of a structural transformation, namely amartensitic one, induced by the plasticdeformation, when the last one exceeds a certainrate [8, 9]. In the first zone, deformation isFigure 7. Effect of the temperature on theultimate true stressesIt is obvious that the increase in temperaturereduces the rupture stress of the material, as wellas its ductility (see also Fig. 2). Such an evolutioncannot be explained simply by the normal'softening’ that materials experience with thetemperature rising (because it would beAKTET Vol. IV, Nr 2, 2011 191