Critical Assessment of the Effect of Atmospheric Corrosion Induced Hydrogen on Mechanical Properties of Advanced High Strength Steel
Darya RudomilovaTomáš ProšekInes TraxlerJosef FaderlGerald LuckenederGabriela Schimo‐AichhornAndreas Muhr
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Hydrogen absorption into steel during atmospheric corrosion has been of a strong concern during last decades. It is technically important to investigate if hydrogen absorbed under atmospheric exposure conditions can significantly affect mechanical properties of steels. The present work studies changes of mechanical properties of dual phase (DP) advanced high strength steel specimens with sodium chloride deposits during corrosion in humid air using Slow Strain Rate Test (SSRT). Additional annealed specimens were used as reference in order to separate the possible effect of absorbed hydrogen from that of corrosion deterioration. Hydrogen entry was monitored in parallel experiments using hydrogen electric resistance sensor (HERS) and thermal desorption mass spectrometry (TDMS). SSRT results showed a drop in elongation and tensile strength by 42% and 6%, respectively, in 27 days of atmospheric exposure. However, this decrease cannot be attributed to the effect of absorbed hydrogen despite the increase in hydrogen content with time of exposure. Cross-cut analysis revealed considerable pitting, which was suggested to be the main reason for the degradation of mechanical properties.Keywords:
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Pitting Corrosion
The effects of excess Si and Mg2Si content and the heat treatment process on the microstructure and mechani-cal properties of the Al-Mg-Si alloy automotive body sheets were investigated.The results showthat,the tensile strength ofthe sheets can increase obviously about 30% before simulated paint baking,and the elongation increases approximately15% with increasing the excess Si content.After simulated paint baking,however,the tensile strength of the sheets in-creases less than 15% and the elongation does not change significantly.With the increase of Mg2Si content,the strength of the sheets before and after simulated paint baking increases,but the effect on the elongation of the sheets is trifling.When the Mg2Si content increases from 1.25% to 1.97%,the tensile strength of the sheets increases from 316MPa to383MPa in T4P condition after simulated paint baking.The effect of pre-aging on the yield strength and elongation ofsheets is unnoticeable before simulated paint baking,which means that it cann′t affect the deformation ability of sheets.After simulated paint baking,however,its effect is remarkable.The value of383 MPa of tensile strength of the sheets canbe obtained,which is close to that of the tensile strength in T6 condition.The elongation of the sheets increases simulta-neously,which is beneficial to the improvement of performance of the sheets.
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Tensile tests of copper foil with various thickness and grain sizes were performed at room temperature to investigate the size effects on tensile strength and elongation.The results show that the tensile strength and elongation are affected by the thickness and grain size together,and then the ratio of thickness to average grain diameter(T/D ratio) must be introduced to represent the size effects rightly.The relationships between the tensile strength and T/D ratio are different in distinct T/D ratio intervals.The elongation decreases with the decreasing thickness while the T/D ratio is constant.The tensile fracture surface was observed by SEM.The analyses indicate that the sharp decline of elongation with the decreasing thickness is attributed to the transformation of fracture mechanism.
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The effects of the tensile rate and samples preparation method on tensile strength and elongation after fracture for PP and PS were studied.The results showed that tensile strength of PP and PS increased with the increasing of tensile rate.But the elongation after fracture of these two materials presented different changes.The tensile strength and elongation after fracture of injection molding samples of PS were much higher than the compression molding samples of PS.The elongation after fracture of injection molding samples of PP increased 69% than the compression molding samples,but the tensile strength did not change obviously.
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Effects of different technological parameters such as stirring temperature, stirring speed and stirring time on the microstructure of semi-solid Y112 alloy were investigated using a self-developed mechanical stirring device in this paper. The results show that the higher of stirring temperature and stirring speed, the finer and more homogeneous the spherical grains, the higher the tensile strength and elongation. If the temperature is too high, the solid grains will be melted, the tensile strength and elongation will be reduced. With the increasing of stirring time, the grains firstly become fine, uniform and round, and then grow up. The tensile strength and elongation firstly increase, and then reduce.
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The dynamic tensile properties of TRIP steel with reduced silicon(Si=0 62%) were concluded with high speed tensile impact tester.The results illustrated that the elongation and tensile strength have been all elevated in a temperature range(50℃～80℃) at the strain ratio =1350s 1 ±50s -1 .The largest uniform elongation e u was 15 6%,total elongation e t was 32 1% respectively and tensile strength σ b reached 938 MPa.Good combination of strength and toughness can be achieved under such conditions.
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To achieve the objective of both high strength and plasticity in TRIP(transformation induced plasticity) steels,the influences of alloying elements and isothermal treatment temperatures on mechanical properties were investigated by tensile test,SEM,TEM and XRD techniques.The results indicated that with the isothermal treatment temperature increasing,the yield strength and the tensile strength decrease initially then increase.Nevertheless,the elongation and the combination of strength and elongation follow the countrary trend of the strength.The maximum tensile strength of over 1 000 MPa and combination of strength and elongation of 22 080 MPa·% are achieved when the bainite isothermal temperature is below 320 ℃ or above 480 ℃.When the isothermal treatment temperature is within the bainite region of 400 ℃,the highest elongation and combination of strength and elongation are 31% and 27 150 MPa·%,respectively.If Si is substituted partially by Al,the tensile strength decreases.As 0.5% copper is added,the tensile strength increases while the total elongation value decreases.The combination of strength and elongation under such a condition is 25 085 MPa·%.Thus,under certain heat treatment process,high tensile strength and elongation of steels containing proper alloying elements can be obtained.
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The mechanical properties of Mg alloy AZ91 with different content of Nd at room temperature and 150℃,such as tensile strength and elongation are investigated by the tensile tests at room temperature and high temperature together with fracture analysis and microstructure observation,and the relationship between Nd content and the mechanical properties of Mg alloy AZ91 is found out.The results show that the tensile strength is increased firstly and then decreased after the peak appears at 1%Nd and the elongation is gradually decreased with the increase of Nd content.With the suitable content of the Nd addition,the tensile strength and elongation of Mg alloy AZ91 are improved and the appliance requirements at high temperature are reached.
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The microstructure and mechanical properties of AZ31 magnesium alloy sheet produced by differential speed rolling process at the conditions of different rolling temperature, reduction in pass and rolling route were studied. The results show that when the rolling temperature increases from 623 K to 723 K, the grains grow up and twins disappear, and the ultimate tensile strength decreases from 275 MPa to 250 MPa,while the elongation increases from 14.5% to 18%. The ultimate tensile strength increases from 265 MPa to 300 MPa and the elongation decreases from 18% to 15% with increasing reduction in pass from 5% to 20%. The rolling routes affect the quantity of twins in the sheets. Twins in the sheet processed by route A are more than that by other routes, which induces higher ultimate tensile strength and lower elongation. On the other hand, the sheet prepared by route D exhibits higher elongation and lower ultimate tensile strength.
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