Study On The Delta-ferrite In P91 Heat Resistant Steel

In order to make the development of the electric energy and the environmental protection get coexistence and development at the same time,the development of large capacity turbine is urgent needly.The key is whether the heat resistant material meets the high temperature and high pressure required by the power industry boiler.P91 heat-resistant steel has solves the problem,its excellent performance makes the development of the electric power industry produced a qualitative leap.However,the presence of delta ferrite in the P91 heat resistant steel during hot working(heat treatment,welding)may affect the properties of the material.The effect of direct heating,different heat treatment processes and different TIG processes on the phase transformation,morphology and volume fraction of delta ferrite in P91 heat resistant steel and its influence on the properties of the material were investigated.It provides a theoretical basis for solving the problem of delta ferrite in P91 heat resistant steel,and provides a reference for the optimization of heat treatment process and welding process.Firstly,laser scanning confocal microscope(LSCM)was used to observe the solid-state phase transformation in heating processes.The results showed the phase transformation processes and the transformation temperature from ferrite(?)to austenite(?)and from austenite(?)to high-temperature ferrite(?).By observing the transformation of ?-ferrite in heating and cooling,it can provide reference to determine the parameters of the subsequent heat treatment processes.Secondly,a set of heat treatment processing parameters were formulated and the study of the delta-ferrite obtained at different heat treatment temperatures were carried on.In order to analyze the influence of quenching temperature to the quantity of delta ferrite in P91 steel,using metallographic microscope(OM)to observe the microstructure of samples corroded by Vilella reagent.The results showed that the microstructure was bright white and presenting massive polygonal morphology.With the increase of quenching temperature,the volume of delta-ferrite increased.The results also showed that the ferrite forming component elements of Cr and Si in delta-ferrite was a little higher than those in martensite and the austenite forming elements Mn in delta-ferrite was slightly lower than that in martensite.And more,after material performance tests,the delta-ferrite has great influence on toughness of P91 steel and with the increase of delta-ferrite volume,the impact toughness of P91 greatly decreased and the impact fracture was shifted from ductile and brittle mixed fracture to brittle fracture.Presence of delta-ferrite in P91 steel caused selective corrosion and with the increase of delta-ferrite volume,the corrosion resistance of P91 steel reducese.Finally,welding process was used to research the ferrite in P91.Using TIG arc as heat source to change the heat input during welding.The results indicated that with the heat input increasing,the volume fraction of delta-ferrite increased and then tended to be constant,and the delta-ferrite distributed non-uniformly.The delta-ferrite of polygonal morphology was largely aggregated in heat effect zone(HAZ)and trace strip morphology presents at weld zone(WZ).By micro Vickers hard-ness,the microhardness of bright white polygon ferrite was 235HV0.2,the gray area was 373HV0.2.The content of delta-ferrite had a remarkable effect on impact toughness and impact toughness of joint and with the increase of delta-ferrite volume,the impact toughness decreased and impact fracture mode was shifted from ductile and brittle mixed fracture to brittle fracture.The crack propagation in the longitudinal section of the fracture showed that the crack mainly generated at the interface between martensite and delta ferrite,and extended to the interior of the material along the polygon delta ferrite.