Boron-Containing Steel Structure and Properties at Room and Elevated Temperature

Abstract

A Gleeble 3800 thermomechanical process physical simulator unit is used to study the structure and mechanical properties of corrosion-resistant steel with a high boron content intended for production of hexagonal pipes for exhausted nuclear fuel storage. Compression tests at room and elevated temperature show that with an increase in test temperature from 20 to 600°C yield strength and flow stress decrease by a factor of two in the steady-state stage, and over the whole test temperature range the steel demonstrates good ductility in compression. The steel's structure does not undergo marked changes during deformation: stringing, formed during hot forging, is retained during subsequent plastic deformation.Corrosion-resistant steels with a high boron content are structural materials used most often for manufacturing compact storage racks for exhausted nuclear fuel due to the high absorbing capacity of boron with respect to neutrons. Due to a considerable amount of boride within the steel's structure, they exhibit good strength and hardness [1, 2], but low level of ductility both at room and elevated temperature [3]. In view of this, the boron content in steel used currently is limited to 1.8 wt.% (steel ChS82). In order to reduce embrittlement caused by the coarse shape of boride, it is necessary to perform steel deformation and heat treatment. Research has shown that the intensity of rapid plastic deformation (90-98%) followed by annealing in the range 600-700°C leads to the formation of an ultrafine grain structure and boride spheroidization [5,6], and this has a favorable effect on steel ductility. Additional alloying with titanium also improves ductility [7], and this is due to forming titanium boride (TiB 2 ) instead of iron and chromium boride (Fe, Cr) 2 B due to the greater thermodynamic stability of the first [8,9].Conversion of nuclear power plants to more enriched fuel, and correspondingly toughening of requirements for the absorbing capacity of material give rise to a requirement for increasing boron concentration within steels. The final stage of producing racks is shaping and straightening of hexagonal tubes in a rolling mill by cold (hot) deformation. In view of this, the search for steel thermal deformation treatment regimes with increased boron content is a very important task. The aim of this work is to study steel mechanical properties with increased boron content (3.15 wt.%) and changes in its structure during cold and hot plastic deformation in order to select the optimum hexagonal tube shaping regimes.