Study of the behavior of the reduced activation martensitic ferritic steel F82H under continuous cooling

<p></p><p>ABSTRACT 9% Cr martensitic-ferritic steels are currently the privileged candidates to manufacture structural components of the so-called Generation IV nuclear fission reactors, because they exhibit very good thermophysical and mechanical properties under neutron irradiation. Along with them, the so-called Reduced Activation Ferritic-Martensitic steels (RAFM’s) have been in the same way selected for the future nuclear fusion reactors. In this contribution we introduce results involving the transformation behavior under continuous cooling of the F82H RAFM steel, obtained by applying the differential scanning calorimetry (DSC) technique. The metal-lurgical state of the as-received material was normalized and tempered. The applied thermal cycles were as follows: heating at 5 ºC/min up to 1050 ºC/min (austenite phase field), austenite holding for 15 min. and cooling at fixed rates between 1.5 and 50 ºC/min under constant pressure. After DSC test, samples were pre-pared by conventional techniques to be observed by optical microscopy. An algorithm under Matlab language was satisfactorily developed which allowed the simultaneous determination of the baseline and the transformed fraction as a function of temperature on the basis of a recursive procedure proposed in the previous literature. The transformation to martensite for the F82H steel under the prescribed thermal cycle conditions showed to be strongly dependent on the applied cooling rate, giving rise to a splitting phenomenon -or a multi-step transformation mechanism- as the cooling rate is lowered.</p><p></p

Study of the microstructural evolution during austenitization of an ASTM A335 P91 steel.

<p></p><p>ABSTRACT Quenched and tempered 9%Cr grade 91 steels (9Cr1MoNbVN) display a lath martensitic matrix with M23C6 (M = Cr, Fe) carbides and fine precipitates named MX (M = Nb, V; X = C, N). MX particles provide the key to control the size and size distribution of austenite grains, which is significantly important in designing materials with specific mechanical properties. In previous works it was reported that samples of a T91 steel austenized at 1050ºCfor times between 0 and 40 minutes following heating at a rate of 50 ºC/s exhibit a heterogeneous austenitic grain size distribution after austenite holding for 20 to 30 minutes from the austenite plateaus tart. Besides, it was observed that all the second phase particles coming from the as-received state are present at the beginning of the austenite holding and that the dissolution of the MM23C6 carbides and a change of the chemical identity of MX precipitates occur within the first 5 minutes of holding. In the present work the detailed evolution of second phase precipitates over the 5 first minutes of austenite holding, obtained by means of a high speed, high resolution dilatometer is studied and followed by scanning and transmission electron microscopy. M23C6 precipitates were not observed from the first minute of austenitization. MX precipitates change progressively their character from V-rich to Nb-rich. The observed diminution of the Ms temperature values would be related to the M23C6 carbides and V-rich MX carbonitrides dissolution.</p><p></p

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