Influence of tempering time at 780°C on the creep resistance of ASTM A335 P91 steel

<p></p><p>ABSTRACT It has been investigated the effect of tempering time at 780°C on the creep strength of ASTM A335 grade P91 steel. The tempering temperature corresponded to the industrial tempering temperature and the times evaluated were chosen in such a way to accumulate, between the tempering prior as received (40 min) and the treatment carried out in the laboratory, times of 3; 4; 5; 5,5; 5,7; 6 and 7 hours, so that in practice a double tempering was applied. Subsequently, the samples were creep tested at 600 ºC and 190 MPa up to rupture. The results show that a tempering time to 780 ºC has a very significant impact on the creep strength of the P91 steel. Up to 3 hours of tempering, the P91 steel retains its creep strength, with a minimum creep rate of 7x10-9 s-1. This creep strength falls off sharply to the 5 hours of tempering (1.5x10-7 s-1), and retrieved to the 5.5 hours (3 x10-8 s-1). This creep behavior is probably related to the evolution of the MX particles during tempering. The average size of the particles of the second phase in the samples tempered to 780 °C during different times and subjected to creep to 600 °C, would indirectly indicate a state of dissolution and reprecipitation of MX particles, which occurs during the tempering. Creep rupture occurs by the nucleation, growth and coalescence of cavities, in regions close to the prior austenite grain boundaries, resulting in a crack and propagation up to fracture.</p><p></p

Effect of heat treatment on the mechanical strength, intergranular and exfoliation corrosion of 7075 aluminum alloy

<p></p><p>ABSTRACT In this study, the effects of heat treatments in the peak aged (T6), overaged (T73) and retrogression and reaging (RRA, retrogression at 200 °C for 10 min) on the microstructure, mechanical strength, exfoliation corrosion and intergranular corrosion of aluminum alloy 7075, has been investigated by optical microscope, scanning electron microscopy, tensile tests, ASTM G110 tests, ASTM G34 tests, polarization curve and electrochemical impedance spectroscopy (EIS). The results of intergranular corrosion tests (ASTM G110) and exfoliation corrosion tests (ASTM G34), applied to the three treatments defines an order of susceptibility to corrosion given by: T6 > T73 > RRA. The trends of the corrosion resistance were further confirmed by polarization curve and EIS test. The RRA heat treatment improved the intergranular-exfoliation corrosion resistance of aluminum alloy 7075 without sacrificing the mechanical strength compared to T6 temper. The effect of heat treatment on mechanical strength and corrosion resistance is explained based on the state of precipitation in the matrix and grain boundaries.</p><p></p