Fundamental Research On The Fabrication Technique Of Hot Deformation Induced Powder Solid-consolidation For High Borated Stainless Steel Composite Plate

High borated stainless steel has the capacity of shielding neutron,?-,?-and ?-rays and maintains the properties of excellent corrosion resistance,higher service temperature and lower cost.And therefore it has being widely utilized in internal-basket structure,storage container and transport container for spent nuclear fuel.The conspicuous incompatibility between austenitic matrix and boride particles as well as the low melting point eutectic of borides with iron makes hot forming operations of high borated stainless steel complex and leads to low availability of material and poor mechanical properties,which are the key obstacles for its fabrication and application.Up to now,previous works on high borated stainless steel mainly focus on the boride effects on mechanical property,corrosion behavior,welding workability,shielding capacity and so on.However,the densification behavior and deformation behavior of powder metallurgy high borated stainless steel as well as the microstructure characteristics therefrom has not been reported.Therefore,the present study proposed a technique based on powder hot deformation induced solid-consolidation for high borated stainless steel composite plate fabrication.Meanwhile,a new method was first developed to realize the simulation of powder hot deformation induced solid-consolidation on Gleeble.For these two purposes,lots of experiments and research works were carried out to investigate the densification behavior,deformation behavior,and powder direct forging of high borated stainless steel.The main research contents and results are as follows:Instead of hot isostatic pressing,high borated stainless steel was fabricated through hot-pressing sintering at different conditions.Microstructure characteristics of gas-atomized powders and microstructure evolution during sintering were investigated.There was no phase transformation and the present phases were orthorhombic structure M2 B and austenite.Eutectic boride particles suffered Ostwald ripening during sintering and as-sintered microstructure consisted of two distinct regions for different transformation mechanisms:(a)regions of inter-particle contact areas consisted of equiaxed grains for recrystallization;(b)and the regions exhibited elongated grains for dendrite arms coarsening and boundary migration.In addition,dense high borated stainless steel was obtained at 1150°C for 30 min under 30 MPa at argon atmosphere.Its strength and hardness were measured to be 862 MPa and 305 HV,respectively,and the plasticity could be enhanced by longer sintering circles.To investigate borides effect on the hot deformation behavior and microstructure evolution of powder metallurgy high borated stainless steel,hot compression tests at temperatures of 950~1150 °C and strain rates of 0.01~10 s-1 were performed.Flow stress curves indicated that borides increased the stress level at low temperature while sacrificing the strength as temperature higher than 1100 °C.The apparent hot deformation activation energy and apparent stress exponent were determined to be 355 kJ/mol and 3.2,respectively.Processing maps showed that flow instability mainly concentrated at 950~1150 °C and strain rate higher than 0.6 s-1.In instability region cracks were generated in addition to cavities.Interestingly,borides maintained a preferential orientation resulting from particle rotation during compression.A novel method was first proposed and successfully carried out to realize the simulation of powder hot deformation induced solid-consolidation on Gleeble at accurate deformation conditions.Near dense high borated stainless steel can be obtained through direct forging in a single strike with large scale deformation at 1150 °C and 0.01~10 s-1 with 50 % reduction.Enhanced boride coarsening at inter-particle contact zone was found to be attributed to the large scale deformation.Interestingly,strain rate had no obvious influence on densification but significantly affected the deformation of powder particles.Furthermore,mechanism of particle deformation enhanced by higher strain rate was discussed.Based on the above results,high borated stainless steel composite plate was successfully obtained by powder hot deformation induced solid-consolidation(i.e.modified powder direct forging).High borated stainless steel and 316 L cover layer were well bonded,which impeded the formation of macro-cracks and improved the plasticity of high borated stainless steel to 20%.The coarsening rate of borides was dominated by the diffusion of Cr at grain boundaries and was followed t1/4 time dependence.The proposed hot deformation induced powder solid-consolidation technique can enrich the fabrication methods for high borated stainless steel product.And the related research results deepen the understanding of microsturture evolution,densification characteristic and hot deformation behavior of high borated stainless steel during fabrication,which facilitates the development and fabrication of high borated stainless steel and has signification influence on engineering practice.