| The work of this dissertation was done by combined with the key research project'Application and Exploitation of the Advanced Metallurgical Technology– Spray Forming'of Baosteel Group Cooperation. In order to obtain high mechanical properties, most of the cold work die steels (CWDS) which were produced by conventional cast technique must be treated by large strain deformation and complex heat treatment in order to refine the size and distribution of the carbides. However, high grade high alloyed CWDS are hard to be produced by conventional cast technology; only the complex and high cost powder metallurgical technology can be used. In this dissertation, a powder metallurgical Vanadis4 (V4) CWDS produced by UDDEHOLM, Sweden, was chosen as the material for comparison, and spray forming technique was firstly proposed to produce the V4 steel. It was found that macro-segregation was reduced, and the grain size, the morphology and distribution of the carbides were refined greatly. In addition, microstructure and forming characteristics of the as-sprayed V4 steel were analyzed; the refine mechanisms and characteristics which different to other materials were revealed. Microstructural evolution during hot working and heat treatment were studied, and optimum parameters for the production of V4 steel were provided. Mechanical properties of the spray formed and powder metallurgical V4 steels after quenching and tempering were compared, and the secondary carbides precipitated during tempering were studied also. Compared with powder metallurgical technology, this self-developed route is more simple and cheaper. It strongly proves the possibility that spray forming can replace powder metallurgy to produce high alloy CWDS.Microstructural study of the as-sprayed V4 steel shows that it was composed of martensite, retained austenite (amount to 33%), MC and M7C3 carbides. Fine, homogeneous and fully spheroidal grains ranging from 8 to 10μm, which was substantially finer than the conventionally cast equivalent, are found. No coarse net-work carbides and eutectic structures were found in the matrix. Spheroidal VC carbides ringing from 0.5-2μm were uniformly distributed along grain boundaries; however, most of the M7C3 carbides with the size of about 180 nm were distributed in the grains. Rapid solidification inherent in the spray forming is the key refinement factor of the microstructure, in addition, the facts that the pre-solidified particles acted as heterogeneous nucleate sites, and that the VC carbides which were precipitated at high temperature during solidification and then confined the time and space of the growth of the dendrites, are other two important factors.The austenitizing temperatures of the as-sprayed V4 steel are 845℃(A1) and 890℃(A3). An obvious flexure appears in the measured dilatation curves due to the matrix decomposition and the precipitation of a large number of carbides. Isothermal compression test was carried out on the as-sprayed V4 steel within the range of temperatures between 850 and 1150℃. The obtained true stress-strain curves showed that the true stress increase with the decrease of temperature at a given strain rate and the increase in strain rate at a given temperature. Results of the hot rolling test showed that, when the steels were rolled within the ranges of temperatures between 850 and 950℃, a large number of M7C3 and M3C carbides were precipitated, and the primary carbides would grow also, therefore, uneven microstructures were obtained after rolling. Attractive microstructure could be obtained when rolled at 1050℃, however, when the rolling temperature was elevated to equal or above 1100℃, irregular carbides would precipitated along the grain boundaries after rolling. Thus, the hot rolling temperature is the key factor in controlling the evolution of type, morphology and distribution of carbides. During annealing, spheroidization of the carbide was greatly influenced by the temperature. The diffusion ability of the elements was low at 850℃, and many undissolved carbide stringers were found; 900℃was proved to be an ideal annealing temperature; with further increasing annealing temperature, the diffusion ability of the elements increased greatly and therefore, obvious carbide coarsening behavior was found. The average carbide size in the V4 steel obtained by the new method is more finer than the equivalent in the powder metallurgical V4 steel, however, the hot rolling and forging are much simple and can be easily used in mass production, and this is the unique merit of spray formed high alloyed cold work steels.The hardness of the spray formed V4 steel is slightly higher than that of the powder metallurgical V4 steel when quenched and tempered by the same technology, and they have equal impact energies. Obvious secondary hardening was found during tempering of the V4 steel. It was confirmed by TEM that the very fine and dense nature of secondary VC precipitates are responsible for the secondary hardening peak at 500℃. The nano-sized VC particles are thermodynamically very stable and show little tendency to coarsen when aged for prolonged times. The decrease of the hardness when the steel was tempered at 550℃can be attributed to the precipitation of M3C, which had lower dispersion strengthen effect as that of MC. The matrix decomposed quickly, and the dislocation density greatly decreased when the steel aged at 700℃for 5 min. The decomposition initiated with the nucleation of fine M7C3 carbides preferentially at the martensite lath boundaries; M23C6, M6C, and MC carbides were also found when aged for prolonged times. During the restoring process of the matrix, the dislocations in the cell boundaries rearranged and neutralized, and then gradually became sharp and the cell translated into sub-grains. The sub-grains coalesce and then the polygonal ferrite came into being. The precipitates, especially a high density of fine precipitates, retard the coalescence of sub-grains and recrystallization of ferrite.Sliding wear tests showed that the wear process of the spray formed and powder metallurgical V4 steels can all be divided into running-in and steady state regimes. The V4 steel has the best wear resistance when tempered at 500℃. SEM morphologies of the worn surface indicate that the main wear mechanism of the V4 steel was abrasive wear. Adhesive wear morphologies were also found on the surface of the powder metallurgical V4 steel, however, this characteristic was not found on the surface of the spray formed V4 steel. This is the main factor which caused the difference of the friction coefficient and surface roughness between the two kinds of steels. The results showed that the friction coefficients of the spray formed V4 steel were smaller than those of the powder metallurgical V4 steel; and the worn surface roughness of the spray formed V4 steel were lower too, which was obtained by laser scanning confocal microscope. Formation of the adhesive wear correlated with the total amount, distribution, and space between the carbides. The carbides in the spray formed V4 steel are finer and more uniformly distributed than those of the powder metallurgical V4 steel, which prevents the directly contact of the matrix, and therefore, adhesive wear was being prevented. The wear resistance of the spray formed V4 steel is finer than that of the powder metallurgical V4 steel when treated by the same quenching and tempering technique.
|
PDF Full Text Request