Study On The Effect Of Grain Boundary Engineering On The Corrosion Properties Of Domestic Super304H Heat-resistant Stee

Reducing carbon emissions and improving the efficiency of ultra-supercritical boilers is the future trend of thermal power generation units,domestic Super304H heat-resistant steel as an important structural component material in the unit,high intergranular corrosion susceptibility is a major potential threat in service,closely related to intergranular corrosion,pitting corrosion and uniform corrosion performance deterioration.Grain boundary engineering(GBE)has recently achieved considerable research results in austenitic stainless steels and nickel-based alloys to improve corrosion and mechanical properties,both in terms of intergranular corrosion resistance and tissue thermal stability,low-energy grain boundaries are better,so GBE is expected to become a new means of solving the high intergranular corrosion susceptibility of super heat-resistant steels.However,the process of implementing GBE is not clear and remains at the experimental stage,with most studies focusing on 304 and 316 stainless steels,and less systematic research on the evolution of the corrosion performance of new heat-resistant steels containing stabilised alloying elements by GBE treatments.In this paper,we will search for a grain boundary characterisation optimisation scheme suitable for the domestic Super304H austenitic heat-resistant steel based on the theory of coincidence site lattice(CSL)and the idea of GBE,and comprehensively evaluate the effect of GBE treatment on the corrosion performance,and obtain the following research results:Firstly,to investigate the effect of different deformation methods on the susceptibility of heat-resistant steel to intergranular corrosion after high-temperature short-time annealing of GBE treatment.0.5MPa shot blasting 1 min or 8 min of severe deformation combined with high-temperature short-time annealing,after aging at 650℃for 10 h,in which the heat-resistant steel sensitization at 1100℃with the annealing time extended and rapid decline and stabilised at about 5min,but the final sensitization all exceeded the size of the solid solution reference sample,producing a negative effect.The deformation introduced by the severe deformation method is not uniformly distributed,and the deformation storage energy is too large in the local area,which rapidly recrystallises at high temperatures,making it difficult for the grain boundaries to migrate and generating a large number of low-energy annealed twin boundaries,thus failing to achieve the goal of optimising the grain boundary characteristics.When 5%cold rolled small deformation,if not annealed or annealing time is insufficient,resulting in higher susceptibility to intergranular corrosion,so need sufficient annealing time to make strain-induced grain boundary migration.While annealing at 1100℃for more than 5min,no intergranular corrosion susceptibility was observed after aging at 650℃for 2 h.When the aging time was extended to 10 h,the improvement effect remained stable and a large number of thermally stable annealed twin boundaries could be observed in the tissue.Secondly,the effect of the GBE optimization process on the evolution of grain boundary characteristics and intergranular corrosion of heat resistant steels was investigated.5%cold rolled deformation combined with annealing at 1100℃for different times(1-30 min),after aging at 650℃for 10 h,the percentage of low-ΣCSL grain boundaries of the samples was improved compared to the solid solution samples.The most important contribution is from the annealing twin(Σ3 grain boundary),the growth mechanism of the low-ΣCSL grain boundary is mainly through the multiple twinning of the existingΣ3 grain boundary in the heat resistant steel tissue and the continuous generation of newΣ3,Σ9 andΣ27 grain boundaries.A high proportion of low-ΣCSL grain boundaries reduce precipitation of chromium containing carbides,thus avoiding the creation of Cr-poor regions.At the same timeΣ3 grain boundaries and the remainingΣ3~n are interconnected into large clusters,reducing the network connectivity of high-energy random grain boundaries and inhibiting the expansion of corrosion cracks,both of which work together to avoid intergranular corrosion phenomena.Finally,the influence of the evolution of grain boundary characteristics and second phase precipitation on the uniform corrosion and pitting ability of heat resistant steels is investigated.Uniform corrosion performance and passivation performance are closely related to low-ΣCSL grain boundaries and intergranular corrosion susceptibility,with the presence of Cr-poor regions severely affecting passivation film generation,reducing passivation film protection and increasing corrosion rates.The CR5%-1100℃/5min sample has the smallest self-corrosion current density and the largest polarisation resistance value,with the best resistance to uniform corrosion in an acidic environment.The high Cr and Fe oxide content in the surface passivation film of samples with a high proportion of low-ΣCSL grain boundaries in a chloride ion-containing environment helps to improve the denseness and stability of the passivation film and reduce the number of pitting crater nucleation sites,while the low-ΣCSL grain boundaries resist corrosion in an acidic environment and slow down the autocatalytic effect of the steady-state pitting process,thus improving the overall resistance to pitting.In addition,the prolonged high temperature annealing time leads to the precipitation and growth of a large number of Nb-containing second phases,and has a detrimental effect on the uniform corrosion performance and pitting performance of the heat-resistant steel.