Study On The Mechanism Of Internal Surface Crack Occurrence Of T91 Steel Tubes Rolled By Mandrel Mill And Its Application

As a new kind of heat resistant steel, T91 is used to manufacture high pressure boiler tubes, a key material of the boiler for thermal power plant at present. Inner surface crack occurs frequently on seamless T91 steel tubes rolled by the Mandrel Mill. Since that it is difficult to deform, which leads to rejects and very lower yield of qualified product. Therefore, it is significant both in theory and practice to study on the mechanism of inner surface crack formation so as to take effective measures to prevent or alleviate the occurrence of inner surface crack.Taking 140mm Mandrel Mill with 8 stands as the object of the study, the aspect of the crack, microstructure near the periphery of the crack, were analyzed respectively by microscope inspections from the samples. With the aid of commercial FE code MSC.SuperForm 2005, Continuous tube rolling process on 8-stand Mandrel Mill was simulated 3-D coupled thermo-mechanically. Based on the mechanical property of T91 steel at high temperature, tested on Gleeeble at different strain rate and strain intensity, a material model of T91 steel was established together with its thermal property, which is used in the FE model. A special effort was made to investigate the metal flow, strain status and their development during rolling process, analyze the tendency of inner surface crack, and also to the influence of process parameters on the deformation of hollow shell. The main reasons for the occurrence of inner surface crack were approached from both T91 material property and rolling conditions.According to the results, a so-called groove theory was put forward, upon which a criterion taking the equivalent plastic strain distribution as the index for optimizing roll pass design of the Mandrel Mill was formulated. In the light of the criterion improved was the roll pass system design of the Mandrel Mill. By putting the new pass system into practice, the yield of qualified T91 tubes is increased notably. From the study above mentioned drawn the main conclusions are as follows:(1) The mechanical property of T91 steel, specifically the plasticity, is far inferior to that of low-alloy steels such as 12Cr1MoV etc, and with high sensitivity to strain rate. It is one of basic reasons why the probability of inner surface crack occurrence is high for T91 steel tubes rolled by Mandrel Mill under the same process conditions.(2) The occurrence of inner surface crack of T91 steel tubes is closely related to the deformation conditions during continuous rolling. The plastic deformation is easily localized due to unsteady metal-flow in conventional roll pass system of the Mandrel Mill. Consequently so-called groove forms near to the roll gap and leads to stress concentration. It is just the reason in mechanics for the occurrence of inner surface crack of T91 steel tubes(3) As T91 steel tube rolling on Mandrel Mill, the strain rate of metal closely adjacent to the inner surface groove is excessively high comparing with that of outer surface near to the roll gap. It is the factor in thermodynamics for the occurrence of inner surface crack of T91 steel tubes(4) During continuous rolling of T91 steel tubes, where the peak value of equivalent plastic strain occurs at the bottom of the inner surface groove is just the place where the peak circumferential stress follows in the next stand, and the inner surface crack easily arises.(5) Optimizing the roll pass design of the Mandrel Mill on the criterion of equivalent plastic strain distribution can effectively reduce the inner surface crack occurrence of T91 steel tubes.