Trends In The Steels Used In Merchant Shipbuilding

ABSTRACT With the increase in the size of modern merchant vessels, as well as changes in design and increased plate thicknesses, there has been a growing interest in the use of higher strength steels for ship construction. The history of hull steel specifications following the advent of welded construction is discussed, together with present American Bureau of Shipping requirements for both ordinary and higher strength steels. Some precautions that are considered particularly necessary when welding on higher strength steels are also presented. Since mobile offshore drilling units are now being classed by the Bureau, the owners and designers of such structures may find the discussions included in this paper of interest. INTRODUCTION As everyone closely connected with the shipbuilding industry is aware, the present trend toward the use of higher strength hull steel is increasing and it appears to be increasing at a rather rapid rate. As the size of merchant ships continues to grow and as the size and number of hatch openings in cargo vessels and ore carriers increase, it has become necessary to use heavier plating of ordinary strength steel, which in some cases approaches 2", in order to meet strength requirements. There has been reluctance to use plating of excessive thickness since investigations on the brittle fracture problem have indicated that, in addition to metallurgical considerations, there are "size effects" which adversely influence notch toughness as the thickness of the plating is increased. The primary advantage of higher strength steels is that they allow a reduction in plate thickness which tends to minimize "size effects" and which results in a ship of lighter weight and greater load-carrying capacity for a given size vessel. Depending on the needs or wishes of individual owners and the size or type of vessel they choose to build other advantages may be (1) greater speed (2) lower fuel consumption(3) a smaller size propulsion unit or (4) improved stability through reduction of top-side weight. The use of lighter gage plating during construction offers the further advantages of (1) lighter sub-assemblies to handle (2) less man-hours of welding and (3) lower electrode consumption. Some of these economic advantages are offset however to some degree by the higher cost of the steel and electrodes, possible replacement difficulties and the refinements or improvements that may be necessary in design, workmanship and inspection. It is, therefore a decision of individual owners and designers whether to use higher strength steels and to what extent. Although higher strength steels are now being used rather extensively they are not entirely new in merchant ship construction and have been used in Naval vessels for a number of years. Higher strength steels were used with satisfactory results on such vessels as the SS "United States" built in 1952 on the SS "France" and several ore-oil carriers built in 1962, and on a few vessels strengthened for navigation in ice.