Performance of TMCP steel with respect to mechanical properties after cold forming and post-forming heat treatment * David Porter a, * , Anssi Laukkanen b , Pekka Nevasmaa b , Klaus Rahka b , Kim Wallin b a Rautaruukki Oyj, Box 93, FIN-92101 Raahe, Finland b Materials and Structural Integrity, VTT Industrial Systems, P.O. Box 1704, Vuorimiehentie 5, Espoo FIN-02044, VTT, Finland Abstract 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 !K50 8C in the absence of blank welds and !K20 8C when blank welds are present. For PFHT DEs T 27J !K50 8C even for the blank welds. T 27J was found to correlate with the fracture toughness reference temperature T 0 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 8C. 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. q 2004 Elsevier Ltd. All rights reserved. Keywords: Dished ends; Cold forming; Heat treatment 1. Introduction In common with the other parts of the ECOPRESS project, the purpose of this part was to help demonstrate that high-strength grades of steel can be applied in pressure vessels. Here the properties of the thermomecha- nically processed grade P420ML2 (EN10028-5) were examined. The use of such steel should allow higher design stresses than can be reached with conventional normalised grades like P355N, while offering the advantages of superior weldability over the normalised grade P420N. Thermomechanical processing is known by the acro- nyms TM or TMCP, i.e. thermomechanical controlled process. Steels made in this way are characterised by excellent combinations of strength, toughness and weld- ability. The lower carbon equivalent of TMCP steels compared with conventional normalised grades of equiv- alent strength means easier welding with an increased safety against hydrogen cracking, lower preheat, less need for repair welding, etc. Also, the low carbon and impurity contents of TMCP steels mean that they are well suited to 0308-0161/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2004.07.006 International Journal of Pressure Vessels and Piping 81 (2004) 867–877 www.elsevier.com/locate/ijpvp * 29th MPA Seminar, Stuttgart, October 9 and 10, 2003-ECOPRESS Seminar. * Corresponding author. Tel.: C358(0)20 59 22 266; fax: C358(0)20 59 23 101. E-mail address: david.porter@rautaruukki.com (D. Porter).