Selection and welding of niobium bearing structural steels in some recent UK shipbuilding contracts

Within the UK shipbuilding sector the use of niobium (Nb) bearing steels tend to have been limited to high stress areas, where minimum strengths of 350MPa were commonly applied. As thicker gauge steels were used in these areas, the steel always contained Nb to develop the desired strength/toughness combinations. In traditional naval contracts strengths of 270MPa were the norm, but recent vessel designs have been very sensitive, which has resulted in these steels being displaced by the higher strength/superior toughness steel grades. The reasons of which are discussed in this paper, as is the chemical composition of the steels used. The paper presents some of the recent Nb-bearing steels that have been used in the build and fabrication recent naval vessels. The welding of these, predominantly Nb-bearing, steels is also described particularly with reference to the heat affected zone toughness. An evaluation of laser welding these steels is described, as a significant proportion of these steels used have been thin plate (<8mm). The selection of a 460MPa strength steel for the flight deck and hanger of an upcoming aircraft carrier contract is described as is the evaluation of the submerged arc welding of steel processed by two different manufacturing routes. (AU)