Isothermal hot tube material characterization – Forming limits and flow curves of stainless steel tubes at elevated temperatures

A new isothermal hot-tube-bulge test as well as a modified pressurized-tube-tensile test are presented to characterize forming limits of tubular materials at elevated temperatures – up to 1000 °C. By controlling temperature, internal pressure, and axial force the complete forming limit curve is determined under isothermal, frictionless conditions and nearly linear strain paths. In the hot-tube-bulge test, a tangential localization forms before failure, marking a forming limit. In the pressurized-tube-tensile test, an axial localization occurs. For the former, a new method for estimating the forming limit was developed whereas the later can be evaluated by standardized methods. The resulting forming limits are at comparable levels to those known from literature, while the new hot-tube-bulge test overcomes the uncertainty which arises from friction in other test setups. The materials used for the investigations are the ferritic and martensitic stainless steels X2CrTiNb18 and X12Cr13, respectively. The hot-tube-bulge test is examined for its suitability in determining flow curves. An empirical-analytical model is introduced to determine the stress components. It is based on tangential and meridional force equilibria, taking the internal pressure and the projected effective areas into account. Flow curves up to effective plastic strains of 0.5 are determined, exceeding the maximum strain of common hot tensile-test by a factor of 2. The strain-rate dependent stress-strain data was used to identify the parameters of a simplified Hensel-Spittel approach. This allows the prediction of isothermal flow curves with varying strains and strain rates at a temperature of 1000 °C.