Study On Hot Extrusion Process Of Large Caliber Thick Wall Tube Based On P91 Steel Hollow Cast Blank

Large diameter thick wall steel tube is the key component in nuclear?fire?electricity,petrochemical and other fields.Its performance directly affects the safety of industrial production.In recent years,domestic and foreign scholars have performed fruitful research on thick wall tube production technology,but still cannot fully meet the high performance requirements of industrial safety for thick walled tubes.In this paper,the as-cast P91 heat-resistant alloy steel is chosen as the research object.The large caliber thick wall steel tube is produced by the casting-extruding forming technology.The slab method,balanced work method and upper bound method were used to derive the calculation formula for the extrusion force of thick walled tubes in the extrusion process and compared with the results of finite element calculations.The influence of extrusion process parameters on the forming of thick wall tubes were analyzed by finite element software.The evolution mechanism of microstructure and mechanical properties were studied by heat treatment experiment.The results demonstrate that:Compared with the FEM results,the average relative errors of the calculated results obtained by slab method and upper bound method are about7.70%and 7.61%,respectivel.It indicates that these two formulae can be used in certain engineering fields.The average relative error of calculated result obtained by balanced work method is very large compared with that of FEM results.The main reason is that the effect of velocity discontinuity on forming is not considered.Extrusion speed v,initial billet temperature T₀ and extrusion ratio?have more obvious effects on the average grain size and the grain uniformity than other parameters.With the increase of initial billet temperature,extrusion speed and extrusion ratio,the average grain size is deteriorated and the grain uniformity is improved.Finally,the proper hot extrusion parameters are initial billet temperature is about 1250?,extrusion ratio is 7⁹ and extrusion speed is30⁵0mm/s.Before heat treatment,the microstructure of the tube was lath martensite,but after normalizing and tempering,it became the tempering microstructure.The strength and hardness decreased obviously,while ductility and toughness were improved significantly.Furthermore,tempering technology have more obvious effects on the comprehensive mechanical properties of thick wall tube than other factors.When the tempering temperature is 750?and tempering time is 180 min,the uniform and fine tempering structure was obtained.The tensile and impact fracture had a large number of dimples,wide diameter and deep depth,which are typical characteristics of ductile fracture.Finally,the proper heat treatment process involves normalizing temperature at 1060?,normalizing time of 120 min,tempering temperature at 750? and tempering time of 180 min.