Abstract: The forming feature of open die extrusion without mandrel for thick wall tube is analyzed. Based on the concepts of hydrodynamics and field theory the kinematically admissible velocity field is established. The peculiarity of the billet to form given part is analyzed. Based on upper bound method the prediction of the billet to form given part by open-die cold extrusion of thick wall pipe is constructed. The billets set can be obtained by determining the diameter of the inner hole firstly and predicting outer diameter. The billets set is the group of different billets can be extruded for the specific part. The curve of the relationship between the billets set and the parts shows wall thickness and D1/d1 decrease with the inner hole diameter d0 increasing for the parts whose outside diameter is same and inner diameter is different；For the same part, the billet’s inner and outer diameter ratio is not a simple linear relationship among the billets set. The D1/d1 reduces with the inner hole diameter d1 increasing. The experiment is finished based on the he prediction. And the prediction results and the experiment results are contrasted and analyzed. Prediction error is less than 2%.

Key words: thick wall pipe;open die extrusion;stream function;upper bounding theory;billets set;prediction of reversed process