Research On The High Temperature Oxidation And Corrosion Properties Of New Martensitic Heat Resistant Steel

Ultra-supercritical units with high steam parameters have became the main develop direction of thermal power units in the world due to their high efficiency,cleanliness,economy and safety.The key to their development lies in the exploit of new heat-resistant materials.Among the boiler steels,ferritic heat-resistant steel has excellent comprehensive performance and cost advantages,and was widely used in ultra-supercritical coal-fired thermal power generating units.In addition,T91 steel was used as a benchmark for developing high-velocity parameter steel grades.In this investigation,T91 steel was used as the benchmark to design a new martensitic heat-resistant steel which resistant to high temperature oxidation and corrosion.The results were verified by JMatPro material performance simulation software.The microstructure of the test steel was lath martensite structure,and there were second phases such as intermetallic compounds?Al₃Fe?Al₆Fe?Laves?and carbides?Al₃C?.The mechanical properties at room temperature are:as-cast hardness 360HB,hardness after heat treatment 242HB,impact work42J,yield strength 813MPa,tensile strength 873MPa,elongation 17%,shrinkage 48%,the degree of strength and toughness match,good comprehensive mechanical properties,and the fracture mechanism is quasi-cleavage fracture.The high temperature oxidation test was carried out in an atmospheric environment at650?/200h?700?/200h.The research found that the test steel has excellent high temperature oxidation resistance,and the high temperature oxidation activation energy was 131KJ/mol,which belongs to the first oxidation resistance level.XRD phase analysis showed that the oxide was mainly Fe2O3,and EDS analysis revealed a small amount of Cr oxide.The surface morphology of the oxide film was irregularly distributed,which was divided into two layers.The inner layer accounts for a large proportion which was dense and continuous distribution,and the outer layer accounts for a small proportion was distributed in a loose porous state.During the high temperature oxidation of the test steel in the atmospheric environment at650?,the surface oxide film morphology evolved from a complex diamond shape,needle-like and granular shape to a spherical shape,and then evolved from a spherical shape to a relatively uniform irregular large block.The evolution of the structure of the oxide film was changed from one layer to two-layer structure,which means it was gradually evolved from a protective film with a high density to a"non-protective"oxide film with inner layer dense outer layer loose porous distribution.A high temperature corrosion test was carried out in an atmospheric environment at650?/100h?700?/100h with a composite salt composed of 75%Na₂SO₄ and 25%NaCl as a corrosive medium.The investigation showed that the high temperature corrosion resistance of the test steel was 50%higher than that of the same type of T91 heat resistant steel.After high temperature corrosion,the oxide film of the test steel was denser,and there were fewer voids and cracks inside,in addition,the bonding strength with the matrix was stronger,which has a certain protective effect on the material.