Ordering of Steel
Mill orders are normally prepared by the drawing room, with the plate sizes being lifted from ship plans or plating models or, alternatively, obtained directly from computer printouts. Each piece of steel is assigned an identifying mark on the plan and on the bill of material. Plates to be severely hot-formed are ordered somewhat thicker than the required size in order to allow for the thinning down that occurs during the forming process.
There are price extras for very narrow, very wide and odd gage plates. Plates in the 1830 to 2286 mm (72 to 90-in.) width range are the least expensive per unit weight. The most economical sizes for the shipyard are determined after considering all related factors including the number of welded seams. Selected even-gage plates cost less per unit weight. In English units, the even gages are normally every 1/32 in. for plates up to 1/2 in. thick and every 1/16 in. for plates over 1/2 in. thick. In metric units, even gages are normally every one mm for thin plates and every two or more mm for the thicker plates.
The exact called-for grade of steel may not be obtainable for some ship repairs or when a small amount of steel or a special grade of steel is required. It is then necessary to obtain a proper substitute. Specific classification society approval of the substitute might be required, especially when ASTM and other non-ship grades are involved.
There has been little effort to increase the amount of standardization of plate or shape sizes beyond that normally adopted because such increases have been considered to be of little, if any, economic value.
Design work on structures incorporating large castings or forgings, such as a stern frame, is started early due to the long lead time required for these large items./p>