VII. STRUCTURAL AND SHEET METALS 



1 . Types and Characteristics of Metals and Alloys . 



a. Steel . Steel has been an important construction material for marine 

 service since the late 1800's. Steel obtained this dominance at the expense 

 of wood and iron because of greater strength and availability. Although 

 other materials may have advantages, such as corrosion resistance, steel is 

 relatively inexpensive, strong, and available in various shapes and sizes 

 for marine application. To ensure the quality of the material used in 

 construction, materials are purchased to specifications. ASTM standard 

 specifications define the requirements to be satisfied by the particular 

 material and indicate the procedure by which it may be determined that these 

 requirements are met. ASTM Standard A6, "Standard Specification for General 

 Requirements for Rolled Steel Plates, Shapes, Sheet Piling, and Bars for 

 Structural Use," lists a number of specifications for materials that are 

 suitable for marine application. Other ASTM specifications cover pipe, 

 mechanical tubing, fittings, forgings, and other materials. ASTM specifica- 

 tions both from ASTM Standard A6 and other appropriate ASTM specifications 

 for steel materials, suitable for marine application, are included in 

 Appendix A. 



(1) Metallurgy . 



(a) Carbon Steel . Metallurgists define carbon steel as an 

 alloy of iron and carbon with the carbon content under 2 percent. Structural 

 steel specifications limit carbon to 0.35 percent or less for weldability 

 considerations. Manganese is added to improve strength and toughness, but 

 mostly to aid in the deoxidation of steel during refining and modify the 

 detrimental effects of sulfur. Sulfur causes steel to be "hot short," i.e., 

 to be brittle at high temperatures, which can lead to cracking during hot 

 rolling and forging. Manganese combines with sulfur in the molten steel to 

 form insoluable manganese sulfide, some of which is removed as slag, and the 

 remainder as well distributed inclusions throughout the steel. The shape of 

 these sulfide inclusions can be controlled by special processing during the 

 steelmaking process. Phosphorous is also present as an impurity. Most steel 

 specifications require the phosphorous content to be less than 0.05 percent 

 because larger percentages of phosphorous cause a decrease of ductility and 

 toughness, rendering the steel to be "cold short." Silicon is often added 



as a deoxidizing agent during the melting and refining of steel. Copper may 

 also be present up to 0.25 percent. 



(b) Carbon Steel Alloys . Depending on the alloy content, 

 carbon steel can be classified as low, medium, or high alloy. Carbon steel 

 contains only residual elements and elements, such as manganese and silicon, 

 added during the melting and refining stage to obtain a workable product. 

 Low alloy steel contains up to 1.5 percent of elements added to obtain 

 increased strength or heat treatment capability. Medium alloy steel contains 

 1.5 to 11 percent added alloy elements. Above 11 percent alloy element 

 content, the steel is classified as high alloy. The high alloy steels 

 include both the ferritic and austenitic stainless steels. 



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