The Microstructure And Phase Transformations Of The New High Chromium Ferritic Heat-resistant Steel

Ferritic heat-resistant steel, has become the first choice of material used in Boiler Tube Steel and been widely used in high temperature structural components such as main steam pipe,because its performance is superior to that of austenitic heat resistant steel and it could more effectively meet the requirements of the use of Boiler Tube Steel. Furthermore, under the pressure of energy shortage and environment pollution, the study on the thermal efficiency of generating station and heat-resistant temperature of the boiler tube is almost imperative.In this thesis, two different components of new steel have been designed:the steel A(10Cr-1.5W-0.05Ti-B) and steel B(10Cr-1.5W-0.3Mo-1.6Co). Using heat treatment and linear thermal expansion, the author has systematically investigated the microstructual evolution and phase transition of the steel A and steel B, and provided a preliminary evaluation of the performance of two new steel.To begin with, the microstructure of the steel A consists of ferrite and martensite, and the ferrite accounts for 25%. Titanium-rich carbonitride precipitates and large particles of BN constitute the defects in the microstructure of the steel A. Excluding the problems in the design, the defects are possibly related with the composition segregation and the control of the nitrogen content during smelting.Appropriate heat treatment can reduce the content of ferrite, just because of the forming ofδ-ferrite, the result of the fact that the content of ferrite increases with the increasing normalizing temperature. It is also found that the grain is coarsening with the increasing normalizing temperature.Then the author using the linear thermal expansion to investigate the austenitization and the transformation of super-cooled austenite of the steel A. The Ac 1 temperature for the onset ofaustenization and the A c3 temperature for the end of austenization increase with heating rate increasing. The austenization rate evidently increases, and the periods of austenization shorten remarkably. The quenching rate has a significant effect on the onset temperature of Martensite transformation MS , which falls when cooling rate reduces. With the quenching rate increasing, the structure has a tendency to fine.Finally, the content of ferrite in the steel B increases with the increasing normalizing temperature, and the microhardness increases firstly and then decreases. After tempering, carbonitride precipitates(M23C6) start to form in grain boundary and lath boundary. The steel B begins to recrystallize after tempered at 800℃. After tempering at high temperature, we can find great amount of carbonitride precipitates in the steel B, which are coarsening with the increasing of the tempering temperature. At the same time, nano-sized dispersing MX carbonitride precipitates have formed in the matrix, tangling with the dislocations to produce a strong inhibition against the movement of dislocations, which significantly improve the properties of ferritic heat-resistant steel. In addition, the microhardness of the steel B decrease with the tempering temperature increasing.