Performance of TMCP steel with respect to mechanical properties after cold forming and post-forming heat treatment

The paper describes the results of work done in the Finnish part of the ECOPRESS project on the mechanical properties of pressure vessels made from the TMCP steel grade P420ML2. Dished end (DE)-cylinder assemblies with diameters of 2500 mm and thicknesses of 15 and 30 mm have been examined using DE in both the cold-formed (CF) and CF and post-forming heat treated conditions. The blanks for the DEs contained welds to enable the effect of cold forming on weld metal to be evaluated.

Cold forming increases both the transition temperature and the strength of the DEs. Nevertheless, toughness against brittle fracture in the CF state is good for all parts of the DEs and girth weld with T_27J<−50 °C in the absence of blank welds and <−20 °C when blank welds are present. For PFHT DEs T_27J<−50 °C even for the blank welds. T_27J was found to correlate with the fracture toughness reference temperature T₀ which can be used to determine the minimum operating temperature. The impact toughness of the CF DE can be determined by compressing plate specimens 15% and ageing 30 min at 250 °C. Such a procedure can form the basis for an additional requirement on PxxxM/ML grades when high cold forming strains are involved in vessel manufacture. Ductile fracture is not of concern, as upper shelf toughness remains high in all parts of the DE.

The yield and tensile strengths of a CF DE are much greater than those of the cylinder, whereas the membrane stresses on the cylinder are greater than those on the DE. Consequently, design can be safely based on the properties of the nominally unformed cylinder. Furthermore, tensile instability will be first reached in the cylinder before it is reached in the DE, even though Y/T is at its lowest in the cylinder. Secondary bending stresses are greatest at the DE knuckle, but the CF DE has more than sufficient ductility in bending to accommodate the bending strain. The high Y/T of CF DE is combined with high material ductility and is therefore fully acceptable.