Enhancing the Weldability of C-Mn Pressure Vessel Steels — A Tale of Two Elements

Abstract

Traditionally, pressure vessel steels for the ambient or lower temperature regime have been as-rolled or normalised products (sometimes tempered) developed from a C-Mn base, their strength being increased, as required, using small additions of Cu, Cr, Ni or Mo. For enhanced lower temperature toughness judicious additions of nickel, usually of less than 0.5 percent, have been the preferred option. Alternatively, quenching and tempering, has also been employed. The metallurgy of pressure vessel steels has not evolved as quickly as in the linepipe or automotive sectors and, surprisingly, niobium which is widely recognised to be a remarkable asset particularly in normalised steels where it provides significant grain refinement and increased toughness, has often been ineffectively utilised. However, in some circumstances it has become obvious that the combinations of properties required cannot be economically achieved without its presence. It is unfortunate, therefore, that some national and international pressure vessel specifications are still written in such a way as to, apparently, dissuade the steelmaker from making the best use of this unique and effective element.

Fortunately, here in China, progressive thinking has ignored such negativity and niobium levels of up to 0.07 percent are already admitted in structural steels up to the Q420 strength level and levels up to 0.05 percent are being advantageously employed in the most widely used pressure vessel materials enabling higher yield strengths and improved toughness to be attained, through enhanced grain refinement, with reduced carbon and carbon equivalent levels.

Carbon equivalent has a dominating effect on the tensile properties of C-Mn pressure vessel steels, particularly following post weld heat treatment (PWHT), and carbon is the single element most likely to impair weldability.

This paper is presented as a ‘contribution for discussion’ and proposes that progressive steelmakers, end users and those concerned with the evolution of national and international standards should explore the additional benefits which may accrue from adopting compositions with significantly lower carbon content, which in turn would allow the more efficient and flexible use of higher niobium levels. This route, it is argued, presents more economical options for the production of many pressure vessel grades when lower temperature toughness is essential.