Research On Microstructure And Mechanical Properties Of Ti40Alloy During Hot-Working Process

Being a burn resistant titanium alloy, Ti40(Ti-25V-15Cr-0.2Si) alloy has been researched at NIN for many years and many achievements have been obtained in the past10years, for example, elevated temperature deformation mechanism, burn resistant mechanism, relationship between mechanical properties and microstructures, thermal stability and creep characteristics under different conditions. However, being a typical alloy with difficult deformation ability titanium alloy, the dehiscence problem during hot-working process for Ti40large-scale bars is a very easy arisen trouble and which heavily limits the application of Ti40alloy. Aiming at dehiscence problem during hot-working process, in this paper, different condition samples were used which cut from the Ti40larger-scale ingot, the forged bar and the rolled ring, respectively. The microstructure evolution, solution characteristics, mechanical properties and stress-relief annealing treatment for different samples under various conditions were studied. The achievement obtained in paper will supply an instruction to the thermal working technology of the Ti40larger-scale bars and heat treatment of the Ti40forged ring.The results of the grain growth behavior for different samples at the hot-working temperature indicate that the microstructure evolution is mainly the recrystallization grain grown along the grain boundaries during the forging heating process. When the temperature is low, the lack of recrystalgrain growth as the result of the homogeneous organization, the degree of the structure homogeneity will be better on the condition of the recrystallization grain growth slowly as increasing temperature, however, an abnormal phenomena of the grain growth will appear at the high temperature region. The structure homogeneity and the average grain size, the heat temperature of1000℃and the heat treatment will not be larger than5hours is the best choice for the hot-working.The research results for the Ti40ingot, the final forging and the ring samples at hot-working temperature indicate that, the structure of the Ti40alloy is the single phase, the alloying element V and Cr is solutionizing in Ti40alloy resulting in the distortion of lattice and the decrease of the β lattice parameter, but the change of the lattice parameter is not obvious in different conditions. Then the results for the Ti40ingot and the Ti40ring at high temperature tensile experiment indicates that the Ti40alloy grain boundaries’s strength is extremely small at1000℃, the big strength difference between the inter-grains and the inter-grains is the chief reason for the dehiscence problem during hot-working process for the Ti40large-scale bars.Finally, the relief annealing treatment was carried out for the Ti40alloy after the conventional heat treatment, and misconstrue and mechanical were tested under different relief annealing condition. The results indicate that different relief annealing treatments produce the obvious influence on the mechanical properties of the Ti40alloy. When the temperature ranged from500℃and550℃, the Ti40alloy shows the best combination of the strength and plasticity, although there is a decrease endurance performance, which can meet the demands. When the annealing temperature is above600℃, the strength of the alloy shows microvariation, but the plasticity decreases sharply. The annealing treatment at550℃for4h is recommended for the relief annealing treatment of the Ti40alloy after the conventional heat treatment.