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

It has been investigated the effect of tempering time at 780 degrees 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 degrees C and 190 MPa up to rupture. The results show that a tempering time to 780 degrees C 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 degrees C during different times and subjected to creep to 600 degrees 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.