Research On Cracking Analysis And Process Optimization Of P91Steel For The Supercritical Pressure Boiler Tube Billet

With the rapid development of electric power and boiler industry in China, thedemand of boiler tube billet continues to increase these years, because the P91(10Cr9Mo1VNb) steel possesses the excellent characterizations of high oxidationresistance and corrosion resistance to high temperature steam, good impact toughnessand durable plastic et al, the material market demand keeps increasing. However, P91steel smelting process is poor in quality stability, and the understanding of inadequate ofP91steel is inadequate, the processing is not mature, the composition of C, P, Al and Nis often overweight. Moreover, a number of worse phenomena occur, including theoverweight of non-metallic inclusion, subcutaneous bubbles, steel ingot overheating,surface crack, crack after annealing of subcutaneous, white spots and so on, whichcauses the useless products. Based on the investigation conducted in recent years, thementioned problems have been nearly solved except for the cracking problem.Therefore, how to improve the forming quality of P91steel forging billet has becomethe key problem which should be resolved as soon as possible.In this work, the cracking on the surface of P91steel tube is systematically studied.Various heat treatment experiments with different processing parameters have beencarried out. The microstructure of the research material has been observed and analyzed,and the mechanical property of hardness has been measured. Moreover, based on theFEM technique, the simulated software of ABAQUS has been employed to study theheat treatment of P91steel. It was found that the cracking of P91steel tube is caused bythe uneven temperature of the furnace, which makes the P91steel tube far away fromthe appropriate temperature. The martensitic structure is obtained to increase theresidual stress, which thus causes the cracking occurred on the surface of the P91steeltube. Therefore, the temperature can be consider to be increased over720℃in order tocompensate the uneven temperature in the furnace so that the speed of transaction ofaustenite structuring to ferritic structure can be improved. By this way, the crackingphenomenon would be avoided. In addition, it can be known from the C-curve of P91steel that the Ac1is from800-830℃, Ms temperature is about380℃. The final annealing temperature is determined to be720±10℃. When the tube is outside of thefurnace, the temperature should be less than300℃.