Hot continuous-rolling process for Ti alloy seamless pipe and its application
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Abstract:
The low specific heat and the fast descent of surface temperature for Ti alloy cause the difficulty in rolling temperature control. High-temperature friction and plasticity characteristics of Ti alloy cause surface defects happening in the hot processing, which limits the hot-rolling production for Ti alloy. The rolling process was investigated by thermal-mechanical coupling of finite element method, in which a φ204 mm×16 mm tubular billet of TC4 alloy was rolled into a φ185 mm×7 mm semi-finished pipe by a tandem of three roll mills of PQF type. The simulation results showed that the temperature differences between inside and outside of the titanium alloy tube was up to 250℃, with the start rolling temperature at 900℃ and a rolling speed of 2.93 m/s. The tensile stress was over 200 MPa at the outer surface in the second and fourth passes, which most likely had caused surface defects. TC4 and TA1 were produced according to the simulation results. The industrial fabrication indicated that the high-precision seamless pipe of titanium alloy could be produced by a tandem of hot rolling mills. The relative deviations of the outer diameter and the wall thickness alloy were less than 0.5% and 10% respectively for the industrial hot rolling fabrication of the TC4 and TA1 titanium seamless pipe, which agree with the simulation results. The grain size of TA1 seamless tube was about 100 μm, the yield strength was 237 MPa, the tensile strength was 321 MPa, the elongation was 47%, and the ambient CVN impact energy was 100 J. The yield strength of the TC4 titanium alloy seamless pipe was 780 MPa to 846 MPa, the tensile strength was 910 MPa to 960 MPa, the elongation was 14% to 16%, and the ambient CVN impact energy was 38 J to 52 J.
