Effects of Processing Parameters of Casting-extruding on Microstructure and Mechanical Properties of P91 Thick-wall Steel Tube

Casting-extruding forming technology is a new technology to produce large caliber-thick-wall steel tube. It possesses many advantages such as short process, saving energy and materials. Taking P91 steel commonly used in thick-wall tube for example, microstructure evolution mechanism and laws during deformation is revealed by means of thermal simulation experiments and microstructure analysis. On the basis of virtual orthogonal test, the effect of different processing parameters on the average grain size, grain uniformity and the maximum extrusion force is obtained. Forming process parameters and their influence on metal deformation and microstructure evolution during extruding forming of cast tube are investigated by theoretical analysis and numerical simulation. The effect of heating treatment parameters on the structure and mechanical property of the material is also studied. The results show that extrusion speed v, initial billet temperature T_0 and extrusion ratio λ have more obvious effects on the average grain size and the grain uniformity than other ones. When the initial billet temperature is 1 250 ℃, the extrusion speed from 30 to 50 mm/s, and the extrusion ratio from 7 to 9, the average grain size is smaller and the grain uniformity is improved. Before heat treatment, the microstructure of the tube is acicular martensite and lath martensite, but after normalizing and tempering, it becomes the tempered martensite. The strength and hardness decrease obviously, while ductility and toughness are improved significantly.