Study On Martensitic Heat-resistant Steels Containing Multi-sized Carbonitrides

The thesis proposed a method to improve the microstructure stability at high temperature in order to increase the heat resistant steels. The new method was brought up based on the study of the failure mechanisms of the most currant used steels. Meanwhile, the concept of the new multi-size carbonitrides strengthened heat resistant (NS) steel was also, for the first time, brought up, which also provided the schematic of the multi-size carbontrides strengthened microstructure. The microstructure was experimentally gained through proper heat deformation and the following-up heat treatment. The NS steel with the desired microstructure showed better creep properties than the P92steel.Firstly, the chemical composition was modified by reducing the carbon content to decrease the volume fraction of M23C6particles and their coarsening rate through decreasing the driving force. Meanwhile, the boron was eliminated to avoid the formation of BN, which is brittle and would cause the cracking. Additionally, the Mo content was reduced to nearly zero in order to decrease the coarsening rate of Laves phase of Fe2Mo.During the hot deformation, the softening mechanisms, including dynamic recovery, dynamic recrystallization, metadynamic recrystallization, dynamic phase transformation and static recrystallization, were precisely located. The thesis also mentioned the conditions for the occurrence of all the above softening mechanisms and how these processes affected the microstructure evolution. Therefore, the desired microstructure could be formed by controlling the deformation conditions. For instance, at the low Zener-Hollomon (Z) value, i.e., high deformation temperature and low strain rate, both the dynamic recrystallization and the dynamic phase transformation took place, which resulted in the approximate equal-axial grains. With increase of the Z value, the process of dynamic recrystallization slowed down and the growth of strain-induced ferrite grains consumed most of the stored energy. Therefore, the growth of the ferrite made the most of the effort in maintaining the ductility. However, the growth rate of both dynamic recrystallization grains and the ferrite grains decreased, leading to the blend microstructure of ferrite and martensite with no obvious phase boundaries. Actually, the strain-induced ferrite played the most important role in the formation of precipitates, due to its low solute contents of alloying elements and high diffusion rates of alloying elements in it. Therefore, the volume fraction and distribution of ferrite determined the amount and the distribution of the precipitates. The optimum deformation for the formation of ferrite lied in the range of1000-1100℃with strain rate of0.01-1/s. Meanwhile, the ferrite preferred to form and grew along the prior austenite boundaries and inflated faster with increase of the temperature.In the stress relaxation curves, the stress abruption was determined to be the starting of the precipitation. However, different kinds of particles preferred to be precipitated under different conditions. For instance, the Nb(C,N) tended to formed at940℃under the condition of continuous deformation, while the M23C6particles preferred to precipitate during the relaxation after the second deformation at800℃followed by the primary deformation at900℃without interval relaxation. However, when200-second interval relaxation took place between the two passes, the (Nb,V)(C,N) particles bloomed at the temperature of750℃. Besides the factors mentioned above, the amount of reduction and the primary deformation temperature affected the precipitation behavior as well. The former one would prompt the precipitation by increasing the dislocation density with larger reduction. The number of nods of dislocations, which was the initial sites of precipitates, was exaggerated when the dislocation density was increased. The latter one would change the distribution and volume fraction of the precipitates through altering the distribution and volume fraction of the ferrite. As mentioned earlier, the nucleation and the growth of the strain-induced ferrite were closely depended on the deformation conditions. With extension of the relaxation time, the particles grew larger. As for the Nb(C,N) particles, which tended to precipitated at940℃, it would take them about1000s to achieve120nm maximum in size. While the M23C6particles at800℃grew up to230nm after1000s relaxation. However, the largest (Nb,V)(C,N) particles were only30nm after relaxation for1000s at750℃.The followed-up heat treatment, mainly austenizing and tempering, was to modify the microstructure. The dual phase of martensite and ferrite evolved into single martensite due to austenizing, while most of the precipitates resolved into the matrix except for the Nb(C,N) particles, which had small solution content in the austenite and would partially dissolved into the matrix. Therefore, the remained particles became the nuclei and the alloying element tended to integrate on them during tempering. The heterogeneous precipitation during the tempering led to uneven size of particles. The200nm-size precipitates among them were associated with the nucleus of remained particles and would contribute to the stabilization of the boundaries. Another one was that the under-20nm particles, which were attributed to the homogeneous formation of the precipitates without nucleus, would stump the dislocation movement effectively.The microstructure of muli-size carbonitrides strengthened heat resistant steel exhibited excellent stability at the aging temperature of600℃. Recrystallization took place in the microstructure after aging at650℃for3000h, latter than the single-size carbonitrides strengthened heat resistant steel at650℃for500h. The recrystallization was found to be associated to the resolution of the200nm-size particles at boundaries. The microstructure of the newly developed muli-size carbonitride strengthened steel got refined with the increase of stress at the creep tests. This refinement was mainly attributed to the augment of dislocations density. But when crept at650℃, with the increase of stress, besides the increase of the dislocations density, the speed of the dislocations movement was also advocated, resulting to the more refined microstructure. The new steel showed better creep resistance than the P92steel at600℃, as almost twice the creep rupture time as P92steel at210MPa.However, the200nm-size carbonitrides distributed unevenly and the Laves phase formed during aging/creeping tended to grow into chains, although the modified microstructure mainly met the required items. The coarse Laves phase could not abrupt the movement of boundaries efficiently and would highly stimulate the cracking. Therefore, the emphasis would be laid on the distribution of200nm-size particles and the growth behavior of Laves phase in the future work.